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The Hishikari gold deposit: high-grade epithermal veins in Quaternary volcanics of southern Kyushu, Japan
Journal of Geochemical Exploration, 36(1990) 1-56
1
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
The Hishikari gold deposit: high-grade epithermal veins in Quaternary volcanics of southern Kyushu, Japan EIJI IZAWA1, YUKITOSHI URASHIMA2, KENZO IBARAKI3, RYOICHI SUZUKI3, TAKE0 YOKOYAMA3, KIYOSHI KAWASAKI3, AKITO KOGA4 and SACHIHIR0 TAGUCHI5
1Department of Mining, Kyushu University, Higashiku, Fukuoka 812, Japan 2Department of Environmental Science, Kagoshima University, Japan 3Sumitomo Metal Mining Co. Ltd., Japan 4Geothermal Research Centre, Kyushu University, Fukuoka, Japan ~Department of Earth Science, Fukuoka University, Fukuoka, Japan (Received March 31, 1989; revised and accepted July 13, 1989)
ABSTRACT Izawa, E., Urashima Y., Ibaraki, K., Suzuki, R., Yokoyama, T., Kawasaki, K., Koga, A. and Taguchi, S., 1990. The Hishikari gold deposit: high-grade epithermal veins in Quaternary volcanics of southern Kyushu, Japan. In: J.W. Hedenquist, N.C. White and G. Siddeley (Editors), Epithermal Gold Mineralization of the Circum-Pacific: Geology, Geochemistry, Origin and Exploration, II. J. Geochem. Explor., 36: 1-56. The Hishikari epithermal gold-silver deposit is located in northeastern Kagoshima Prefecture, Kyushu, Japan. Geological and geophysical surveys played important roles in the discovery, made in 1981. Subsequent mine development has proved Hishikari to be one of the major gold deposits in the western Pacific. The production from July, 1985, to December, 1988, totaled 21.7 metric tons of gold and 14.3 metric tons of silver. Ore reserves are estimated to be 1.4 million metric tons at an average grade of 70 g/metric ton gold (98 metric tons of contained gold) in the Honko ore zone and approximately 2 million metric tons at 20 to 25 g/metric ton gold in the newly discovered Vamada ore zone. The geology of the mine area is composed of basement sediments of the Cretaceous Shimanto Supergroup, and volcanic rocks of Quaternary age. The deposit is of the quartz-adularia vein type, with associated electrum, naumannite-aguilarite, pyrargyrite and smectite. K-Ar age dating of adularia bearing ore indicates a Pleistocene age of 0.84 _+0.07 to 1.01 _+0.08 Ma for gold mineralization. Fluid-inclusionstudies indicate that the representative temperature of gold deposition was 210 ° C in the basement and less than 200 ° C in the overlying volcanic rocks. J~s0 values of quartz veins range from + 8.8 to + 6.8%o. Chlorite and sericite alteration is directly associated with high-grade gold mineralization. Alteration zones of interstratified clay minerals and quartz-smectite envelope the mineralized center and form a near-horizontal layer of intense argillization, located 50-100 m above the Hishikari vein system. These alteration zones are surrounded by a zone of cristobalite-smectite and essentially unaltered rocks. Subsurface structures were recognized by a detailed gravity survey, with uplifted basement blocks
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© 1990 Elsevier Science Publishers B.V.
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E. IZAWAET AL.
represented by gravity highs. Schlumberger vertical soundings are useful in estimating the depth to the basement and to identify the approximate resistivity structure, while resistivity and IP surveys define the areal distribution of the hydrothermal alteration related to mineralization. Geochemical exploration using Hg, C02 and radon in soil gas has been effective in tracing fracture zones. The extremely high-grade gold mineralization is focused near the unconformity between basement sediments and overlying volcanic rocks of the Honko area. The high grades of gold may be explained as a combination of two processes. As the higher-temperature fluids ascended, boiling resulted in gold deposition; further mineralization was favored by subsequent mixing of the deep fluids with steam-heated groundwaters near the unconformity, causing rapid cooling and oxidation.
INTRODUCTION
The recent development of the Hishikari mine following the discovery of gold veins in 1981 has proven Hishikari to be one of the major gold deposits in the Western Pacific. Geologic and geophysical surveys played important roles in the discovery of the rich veins beneath an old, small mining site (MITI, 1982). Initial mineralogical and fluid-inclusion studies using core samples were made by Urashima and Izawa (1982). There was a subsequent detailed description of the veins by Abe et al. (1986), and the geophysical surveys were discussed by Kawasaki et al. (1986); these studies were conducted during the early state of mine development. The regional geology has been newly studied by the Sumitomo Metal Mining Co., Ltd. (SMM), the owner of the mine, with a portion of the results reported by MMAJ and SMM (1987) and Urashima et al. (1987). These studies and subsequent regional mapping and geophysical studies by SMM, studies on hydrothermal alteration by SMM and E. Izawa and geochemical studies of soil gas by A. Koga and S. Taguchi are incorporated in this paper. The objective of this paper is to review the studies previously published in Japanese, and to update the geological, geophysical and geochemical database for future reference in the exploration of epithermal gold veins in young volcanic terrain.
Discovery and development Location The Hishikari mine (32°00'N, 130°41'E) is located in the northeastern part of the Hokusatsu district, Kagoshima Prefecture, Kyushu, about 60 km north of Kagoshima city (Fig. 1 ). The area is immediately west of the Kakuto caldera, part of the Kagoshima Graben in which the active Kirishima volcano is located. The portal of the mine is at 265 m above sea level. The topography, ranging from 200 to 600 m in elevation, comprises densely timbered, hilly terrain separated by small valleys cultivated mainly with rice. Temperatures range
THE HISHIKARIGOLDDEPOSIT
3
0 I
i
~ 1
~ 2
."'"'l]3
L;,4 /-,
" 5
• 6
X +7
Fig. 1. Location and structural map of the Hokusatsu district. 1 = outcrops of the Shimanto Supergroup; 2 = Holocene volcanic rocks; 3 = caldera and basin, 0--Okuchi basin, K= Kakuto caldera, and A = Aira caldera; 4, Kagoshima graben (Tsuyuki, 1969); 5-- volcanic centers; 6 = gold deposits; 7--deep drill holes with elevation of the top of the Shimanto Supergroup (parenthesis indicates that the drill hole did not reach the basement), data from Aramaki (1968), Kubota (1986), MITI (1988) and Ikeda (pers. commun., 1988 and 1989). The area in the square is shown in Figure 3.
from a low of - 7 o C to a high of 35 ° C. Rainfall is rather high, averaging 2500 m m per annum.
History The first prospecting record in the Hishikari mine area dates back to 1750 (Kondoh, 1986). Since then, exploration and small-scale mining (limited to above 230 m elevation) has been intermittent. In 1903, near-surface exploitation began on three gold-bearing quartz-calcite-clay veins (Table 1 ). The veins are located 100 m directly above the top of the present Hishikari veins ( H o n k o veins), and are now interpreted to be an upper manifestation of mineralization at Hishikari. A crosscut was driven in 1933 about 180 m in length at an elevation of 230 m, using explosives and dewatering pumps. Ore was shipped to
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E. IZAWA ET AL.
TABLE
I
Near-surface veins of the old Hishikari-yamada mine, worked in the early 1900's (Fukuoka Bureau of InternationalTrade and Industry, 1959 ) Vein
Strike
Dip
Width
Grade
Remarks
(m) No. 1 No. 2 No. 3
N60°E N60°E N58°E
80°NW 80°NW 90 °
1.0 0.6 0.5-1.0
Au (g/t)
Ag (g/t)
30.3 18-32 21.0
26.2 5.0
"Moso" vein, main working Calcite-clay vein "Baka" vein, calcite-rich quartz vein
the Saganoseki copper smelter 170 km from the mine. The maximum grade of this ore was as high as 130 g/metric ton gold. In 1943 a plan of downward development was suspended owing to intensification of World War II. The crosscut of the 230 m level was re-opened in 1952, with about 40 m of drifting on the No. 3 vein; however, the results were discouraging (Nishizawa and Ibaraki, 1985 ). In 1973 the Taio Mining Co., a subsidiary of the Sumitomo Metal Mining Co., Ltd. (SMM), acquired the mineral rights.
Discovery Various geologists of the SMM group pointed out the possibility of concealed gold deposits underneath young volcanic rocks in the Hokusatsu district and recommended an exploration program using advanced techniques (Ikeda, 1952, 1968; Kobayashi et al., 1972; Nishizawa et al., 1973: all are unpublished company reports). However, no work was actually conducted until a systematic exploration program by the Metallic Minerals Exploration Promoting Agency of Japan, now the Metal Mining Agency of Japan (MMAJ), started in 1975. Between 1975 and 1978, regional geological mapping and reconnaissance geophysical surveys of the Hokusatsu district were carried out by MMAJ. One of the important conclusions from this survey was that most of the gold veins in the Hokusatsu district are hosted in propylitically altered andesites, which at that time were assumed to be "older andesites" of Miocene age and named the Hokusatsu Older Andesites (MITI, 1977, 1979). Later it was shown that the altered andesites actually have various ages. In 1975 and 1976, gravity surveys were made with 566 stations at about 1km intervals along roads, including 380 km ~ of the Hokusatsu district. Small gravity highs were identified over most of the known gold deposits of the district. Taking into account the regional geology, the anomaly was interpreted to principally reflect an uplifted mass of propylitized andesites and in part basement rocks, which are denser than the young volcanic rocks (MITI, 1976,
THE HISHIKARI GOLD DEPOSIT
5
25~
I00~
J
%
I0f
~
Legend 10
25-_I i
1 j
2
J
3
4
o 0
500 |
5
1000 m i
Fig. 2. Relationship between Bouguer gravity contours and resistivitydata. Localities of the old tunnel, as well as M M A J drillholes, are indicated. 1 =Bouguer gravity contour, reGal (MITI, 1977 );2 = Schlumberger resistivity,o h m - m (MITI, 1979 );3 = airborne E M anomalies, conductor extent, > 1.1 mhos ( M M A J , 1979 );4 = old Hishikari-yamada mine tunnel; 5 = M M A J drillholes.
1977). The gravity survey revealed a northeast-trending gravity high of 4 reGal amplitude in the Hishikari area (MITI, 1976). In 1978, electrical sounding (Schlumberger array) and heliborne electromagnetic (EM) methods were used over the Okuchi-Hishikari area. Both detected a low-resistivity zone over Hishikari which resembled that of the nearby Okuchi mine (MITI, 1979; MMAJ, 1979; Johnson and Fujita, 1985). In the Schlumberger resistivity section, a low-resistivity zone (3-7 ohm-m) at shallow depths and a high-resistivity zone ( > 100 ohm-m) at a depth of 200 m were interpreted as reflecting the hydrothermally altered Hokusatsu Older Andesites and an intrusive rock, respectively (MITI, 1979). In 1980, scout drilling was recommended to explore a deep target below the abandoned old tunnel, due to the gravity high anomaly overlapping low-resistivity anomalies from EM and Schlumberger data over the altered andesites
E. IZAWAET AL.
6 TABLE 2 Assay value for drill core samples of MMAJ (MITI, 1982) Drill hole
Interval (m)
Length (m)
Au (g/t)
Ag (g/t)
55MAHT-5
291.70-291.85
0.15
290.3
167.0
56MAHT-1
465.25-466.00 476.35-476.60
0.75 0.25
102.0 149.7
50.3 52.0
56MAHT-2
241.68-242.90 261.40-265.15 277.65-283.10 301.75-302.50
1.22 3.75 5.45 0.75
63.7 69.9 220.43 44.7
44.0 52.8 57.6 26.3
(Fig. 2 ). The drilling (55MAHT-5) started in late 1980; in February, 1981, a quartz vein was intersected at 291 m drilled depth which assayed 290.3 g/metric ton gold and 167 g/metric ton silver over 15 cm. The most interesting fact was that the quartz vein occurred in shale of the Shimanto Supergroup, which had not been considered as a common host rock for gold deposits in the Hokusatsu district. The next two holes, drilled from August to October, 1981, were located about 100 m east and 400 m west of the discovery hole, respectively, and also encountered high-grade gold veins (MITI, 1982). Table 2 shows the assay results of these three MMAJ drill holes.
Surface drilling program by SMM With MMAJ's consent, SMM commenced a follow-up drilling program in late 1981, to confirm the results of MMAJ scout drilling; these preliminary results indicated that gold-bearing quartz veins occurred over an area of 800 by 200 m, striking N70 ° to 80°E and dipping steeply north. Within a year SMM completed 18 holes on 8 panels totaling 6870 m. All the holes intersected gold mineralization, comparable with the MMAJ findings. The results indicated that gold veins were present over a minimum strike of 700 m, with a vertical range of 100 m. The reserves were then estimated to contain about 120 metric tons gold with an average grade of 80 g/metric ton gold between - 20 and + 130 m in elevation. SMM subsequently made a quick decision to conduct underground exploration, starting in January, 1983.
Development Hot groundwaters were a major obstacle to development. Surface drilling revealed that a volume of hot water with a temperature of 60-65 ° C was present within the vein system. The original static water level was 200 m above sea level when pumping started at the 100 m level in May, 1984. By the end of July, 1988, the hot water table had been lowered to 20 m above sea level. Under-
THE HISHIKARIGOLDDEPOSIT
'I
TABLE 3 Compositionof hot waters from the Hishikari mine (an average of waters from 8 drill holes on June 18, 1988)
T (°C)
pH
K (mg/1)
Na (mg/l)
Ca (mg/1)
Si02 (mg/1)
Cl (mg/1)
SO4 (mg/l)
HC03 (mg/l)
63.4
6.5
28
650
86
114
460
68
1,180
ground HQ size drill holes are used for dewatering; pumping stations are established at the 10 m level in the No. 2 inclined shaft, with hot water pumped up through the ventilation shaft. At present 9.5 m3/min of water ( including 1.2 m3/min of used mining water ) are pumped; 3 m3/min are delivered to the local hot spring spa at Yunoo, 4 km southwest of the mine. The remainder is cooled, treated and discharged to the river. A typical analysis of water is presented in Table 3. Tritium dating ( 1.3 + 0.6 to 1.5 + 0.3 Tritium Unit ) indicates that hot water in the current vein system is old, circulated meteoric water. Underground exploration commenced in January, 1983, with the driving of two parallel inclined shafts at an inclination of - 1 7 % (4.8 m wide by 3.8 m high), some 15-20 m apart. Pilot drilling was undertaken from the beginning of the development to determine the existence of water in fissures, cracks and voids. When a water-bearing zone is encountered, the zone is grouted with cement under pressure. The first crosscut intersected the Ryosen No. 2 vein at the 100 m level on July 13, 1985. Main levels have been developed at 100 m, 70 m and 40 m elevation. The No. 1 incline of 2.0 km length has reached - 50 m elevation. The total length of tunneling is 26 kin, including 10 km of drifting as of March, 1989. A trackless operation is used, with diesel-powered mobile machines. Although the mine is still at the exploration stage, it produces 350-400 metric tons per day of ore from drifting (and small-scale test mining); the total production from July, 1985 to December, 1988, has been 21.7 metric ton of gold and 14.3 metric ton of silver. The bulk of the ore is crushed and delivered to the S M M copper smelter at Niihama by truck and ship for use as silica flux. Ore reserves (at the end of 1988) are 1.4 million metric tons of 70.5 g/metric ton gold and 49.0 g/metric ton silver (98 metric tons of gold and 68 metric tons of silver contained) in the Honko ore zone. From September, 1987, to May, 1988, twenty seven drill holes totaling 11,477 m revealed a new vein system in the Yamada area, about 1 km southwest of the known main Hishikari area (the Honko area). The newly discovered ore reserves are estimated to be more than 2 million metric tons with average grade of 20-25 g/metric ton gold and 12-15 g/metric ton silver.
8
E. IZAWA ET AL.
GEOLOGY Principal rock units and structure
Regional geologic mapping and complementary age dating and geochemical ( 7
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THE HISHIKARI GOLD DEPOSIT
9
A
B His~ikori Mine
500I~.OOl--
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I00 I-
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LEGEND Alluvium and Kakuto Group Pyroclastic flows
I
Hishikari Upper Andesites Shishimano Dacites Hishikari Middle Andesites O
rr-
Kurozonsan Dacites
Dacitic pyroclastics Welded tuff Hypersthene-augite andesite Andesitic pyroclastics and volcanic conglomerate
Hishikari Lower Andesites Shimanto Supergroup
Acidic pumice flow and welded tuff Hypersthene-augite andesite Andesitic pyroclastics Biotite-hornblende dacite Dacitic pyroclastics Hypersthene-augite andesite Andesitic pyroclastics Hypersthene-augite bearing biotite-hornblende dacite
E
Cretaceous
I
m
Shale and sandstone
i
Quartz vein Fault (assumed)
Fig. 3. Geologic map of t h e Hishikari area a n d cross section along A-B. Square is area of Figures 17, 20, 22 a n d 23. Locations of samples with K-Ar ages are shown as closed circles.
studies on volcanic rocks in the mine area has continued with underground exploration and development. The petrographic features of volcanic rocks in the area are similar, often making it difficult to distinguish between units with different ages. The studies of K-Ar age dating and chemical compositions as well as paleomagnetic study on the volcanic rocks have provided an important basis for geologic mapping. Geologically, the mine area consists of the Shimanto Supergroup of Cretaceous age and volcanic rocks and alluvial deposits of Quaternary age (Fig. 3 ).
The Shimanto Supergroup The Shimanto Supergroup is not exposed in the vicinity of the mine, as it is generally present at an elevation less than - 4 0 0 m in the surrounding area (Fig. 1). Drilling and underground exploration, however, have revealed that
10
E. IZAWA ET AL.
the Shimanto Supergroup rises from 130 m to 0 m elevation (about 100-200 m below the surface) in the central part of the Hishikari deposit. It comprises shale, sandstone and their alternations, accompanied by a minor amount of tuffaceous shale and chert. Sandstone sometimes contains small fragments of shale, and lenticular fragments of sandstone often occur in shale. The inner structure of this unit is complex and it may be regarded a kind of slump deposit. Although not yet confirmed by fossil evidence, this unit is assigned to the Saiki Subgroup of the Morotsuka Group of Okumura et al. (1985) on the basis of the quartz-feldspar-rock fragment ratio (Ishihara et al., 1986 ). The Kawanabe Group of the Shimanto Belt in the Nansatsu district of southern Kyushu is middle to upper Cretaceous age (MITI, 1985), and is correlative with the Morotsuka Group. The strata of the Kawanabe Group and the Morotsuka Group were subjected to low-grade regional metamorphism (generally metamorphosed to the prehnite-pumpellyite facies). The shale and sandstone in the Hishikari mine consist mainly of quartz, albite, Fe-chlorite and sericite with minor calcite, pyrite and carbonaceous matter. This unit is unconformably overlain by the Hishikari Lower Andesites. Shale and sandstone up to 20 m below the unconformity are often reddish brown in color, owing to the presence of hematite. Abe et al. (1986) interpreted this hematite as due to weathering at the paleosurface prior to volcanism. About 60% of minable ore in the Honko area is hosted in sedimentary rocks of the Shimanto Supergroup. Hydrothermal alteration of this unit is seen only at the contact zone ( < 1 m wide) with quartz veins, where the rocks have a characteristic pale green color owing to chloritization and sericitization. Pyritization consisting of disseminated, euhedral pyrite along fractures is also common, and silicification is locally present, in particular near quartz veinlets.
The Quaternary system The Quaternary rocks in the mine consist of the Hishikari Lower Andesites, the Kurozonsan Dacites, the Hishikari Middle Andesites, the Shishimano Dacites and the Hishikari Upper Andesites, in chronological order of decreasing age. Representative chemical compositions are shown in Table 4. The western and southwestern part of the area is covered by younger pyroclastic flow deposits and alluvial deposits. These volcanic rocks are calc-alkaline and were subaerially deposited; their magnetic susceptibility shows typical magnetiteseries values (Table 4; see also Ishihara et al., 1986). The Hishikari Lower Andesites crop out near the mine and in the northern part of the area. Hypersthene-augite andesite lava flows predominate in the upper part of the sequence, and andesitic pyroclastic rocks with intercalated thin lava flows occur in the lower part; lacustrine sediments are locally present. The lavas contain phenocrysts of plagioclase (An45-6o), augite and hypersthene in order of abundance, which occur in a groundmass consisting of pale
THE HISHIKARI GOLD DEPOSIT
11
TABLE 4
Chemical composition of principal rock units Rock unit
Hishikari Lower Andesite
Sample no.
27645'
(wt.%) SiO2 Ti02 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P20s H20+ H20 Total
61.9 0.62 16.21 6.91 a 0.12 2.47 4.33 2.42 2.35 0.02 2.29 b
-
(ppm) S Cl Be Rb Sr Ba Zr V Cr Co Ni Cu Zn Pb As Sb Ag (ppb) Au Hg
99.64
I29102
62.75 0.65 16.27 6.49 a 0.11 2.46 5.59 3.13 2.23 0.11 0.80 0.32 100.91
Kurozonsan Dacite
Hishikari Middle Andesite
Shishimano Daeite
Hishikari Upper Andesite
40020'
24983'
24972'
27681'
71.1 0.43 14.25 3.22 a 0.16 0.62 2.61 3.55 3.19 0.16 0.77 b 101.06
80 70 <0.5 174 355 135 36 11 9 35 53 12 2 0.1 0.4
69.71 0.41 14.67 3.1Y
61.4 0.54 16.56 7.26 ~
0.09 0.62 2.63 3.85 2.98 0.10 0.65 0.17 99.03
0.06 2.98 5.97 3.13 2.20 0.54 0.82 b
67 < 10
225 470 26 25 3 4 10 43 10 3 0.4 <0.2
57 < 10
98.77
289 350 177 100 16 11 41 54 6 2 0.2 0.4
20 25 3 < 1 15 29 20 3 0.2 <0.2
< 1 20 372
70.2 0.30 15.18 1.93 ~ <0.01 0.32 2.04 3.70 3.81 0.05 1.23 b
<0.5 171 625
<0.5 121 261 620 170
< 1 50 706
101.46
90 340 1.5
75 335 260 140
16 30
x ( e m u / g × 1 0 -~)
K25252
I29082
69.68 0.35 15.54 1.78 0.33 0.05 0.34 2.15 3.38 3.34 0.04 1.46 0.46 98.93
80 120 155 261 590 210 38 < 10
8 20
60.3 0.56 15.53 7.21 a 0.13 3.16 6.51 2.66 2.09 0.07 0.84 b 99.06
<0.5 265 335 160 45 15 14 81 53 16 3 0.2 0.4 < 1 50
321
a = Total Fe as Fe2Oa, b = l o s s on ignition, ------not determined, Z = magnetic susceptibility. ' W < 10 ppm, B i < 2 ppm, M o < 1 p p m except for no. 40020 (5 p p m ) ; Au, As, Sb and Hg by NAA, and other elements by ICP at Chemex Lab. Ltd. 2XRF, except for major elements of I2908 (wet chemical analysis), at K y u s h u University. 27645 = N of mine; I2910 = E of mine; 40020 = N of Kurozonsan; K2525 -- W of Kurozonsan; 24983 = SW of Yoshimatsu; 24972 = E of mine; I2908 = S of mine; 27681 = E of Uono.
12
E. IZAWA E T AL.
brown glass, scattered plagioclase laths and granules of augite and hypersthene. The andesitic pyroclastic rocks range from fine tufts to volcanic breccias and are poorly sorted. Fragments are andesitic volcanic rocks, except for just above the unconformity, where fragments sometimes consist of shale and a small amount of sandstone. There are several thin layers ofpaleosol containing hematite in the sequence. They sometimes contain charcoal of wood trunks and carbonaceous material derived from plants. About 40% of ruinable ore in the Honko area occurs within this unit. In the vicinity of the mine the volcanic rocks have been argillized or altered to green colored rocks (due to formation of smectite or chlorite). The Kurozonsan Dacites overlie the Hishikari Lower Andesites in the northern part of the area. They are composed of hypersthene-augite-bearing biotitehornblende dacite lava flows, and a minor amount of welded tuff and pumice tuff at the base of the sequence. The texture of dacite is variable but typically is characterized by relatively abundant feldspar phenocrysts; flow banding is rare. Phenocrysts consist of plagioclase (An35_55), hornblende, hypersthene, augite and biotite. Under the microscope the groundmass consists of glass, and scattered plagioclase laths, hornblende and biotite. The Hishikari Middle Andesites are restricted in distribution to the southern and southeastern part of the mine area, where they appear to directly overlie the Hishikari Lower Andesites and are overlain by the Shishimano Dacites. They are composed mainly of hypersthene-augite andesite lava flows and their pyroclastic rocks. Phenocrysts are plagioclase (An4~_~o), augite, hypersthene and olivine. Groundmass consists of glass, plagioclase laths and granules of augite and hypersthene. The Shishimano Dacites crop out to the southeast of the mine, forming a remnant of volcanic topography. The unit consists of biotite-hornblende dacite lavas with their pyroclastic equivalent at its base. The dacite is typically porphyritic with phenocrysts of plagioclase (An3o_35), hornblende and biotite in a glassy groundmass. In some places it is aphyric and has flow banding. This unit and the Kurozonsan Dacites once were collectively named the Kurozonsan Rhyolites, and their emplacement was thought to be post mineralization (MITI, 1977, 1979). However, it has been recognized that gold-bearing quartz veinlets are present in the southwestern part of the unit where the rocks are argillized. In addition, more differentiated characteristics of mineralogy and rock chemistry, as well as younger K-Ar ages, clarified that the Shishimano Dacites should be separated from the Kurozonsan Dacites. The Hishikari Upper Andesites occur 2 km northeast of the mine, where they directly overlie the Hishikari Lower Andesites, and 4 km southeast of the mine,
THE HISHIKARI GOLD DEPOSIT
13
overlying the Shishimano Dacites. This unit consists of hypersthene-augite andesite lava flows and their pyroclastic rocks. Phenocrysts are plagioclase (An 6o-55), augite, hypersthene and olivine in a glassy groundmass.
Pyroclastic flow deposits predominate in the western part of the area, filling topographic depressions below 250 m elevation with thicknesses up to 70 m. They consist mainly of pumice flows erupted from the Aira caldera about 22,000 years ago (Kigoshi et al., 1972). In addition to this unit there are several older units of pyroclastic flow deposits such as the Kakuto pyroclastic flow deposit of 0.3 Ma (Miyachi, 1983, 1987).
Radiometric age dating K-Ar age determinations for the least altered volcanic rocks have been made on whole rock and feldspar. Most of the K-At dates were determined by Teledyne Isotopes and some were analyzed at the University of British Columbia. The newly obtained ages of 53 whole rocks and one mineral are presented in Table 5 with locations shown in Figure 3; results having errors of more than 20% have been discarded and are not shown. Newly obtained K-Ar ages for adularia-bearing ore, analyzed at the Okayama University of Sciences, are shown together with age data for composite ores (Nishizawa and Ibaraki, 1985) in Table 6. The volcanic rocks in the area have been correlated with the "Hokusatsu Older Andesites" of Miocene age and the "Hokusatsu Younger Andesites" of Pleistocene age (MITI, 1979). K-Ar analyses, however, show that most of them belong to the same volcanic unit of Pleistocene age. Andesites in the mine area are newly divided into three units as follows: the Hishikari Lower Andesites (0.95 + 0.09 to 1.78 _+0.15 Ma), the Hishikari Middle Andesites {0.78 + 0.08 to 0.79 _+0.05 Ma) and the Hishikari Upper Andesites (0.51 _+0.06 to 0.58 + 0.10 Ma). The "Kurozonsan Rhyolites" (MITI, 1979) were also divided into two units: the Kurozonsan Dacites (0.95 + 0.09 to 1.56 + 0.20 Ma) and the Shishimano Dacites {0.66_+ 0.04 to 1.1 ___0.14 Ma). K-Ar analyses on adularia-bearing ore clearly demonstrate a Pleistocene age of 0.84 + 0.07 to 1.01 _+0.08 Ma for gold mineralization at Hishikari (Table 6). Urashima and Ikeda (1987) reported a K-At age of 0.86 + 0.12 Ma for adulariabearing ore from the Hosen 1 vein. An exceptional age of 1.5 + 0.3 Ma reported by MITI (1983) was obtained for adularia from a vein located 400 m south of the major vein system. Ueno et al. (1987) studied paleomagnetism of volcanic rocks and veins in the Hishikari area, and argued that the normal polarity of magnetization of a hematite vein, which parallels an ore vein and seems to have formed during gold mineralization, might correspond to the Jaramillo normal subchron (0.92-0.97 Ma). These data indicate that the principal AuAg mineralization at Hishikari is closely related to the latest-stage volcanic
14
E. IZAWAET AL.
TABLE 5 K-Ar age data and location of analyzed rock samples Sample no.
K (wt.%)
4°At tad (nl/g)
Rad.4°Ar (atom.%)
Calculated age (Ma)
Latitude north
Longitude east
Remarks
34.4 35.5 25.9 58.7 29.2 21.6 34.9 51.1 34.7 35.0 34.2 28.9 23.9 36.1 28.0 34.6 38.8 42.4 23.5 39.9 41.3 46.8 37.4 46.2 44.4 59.5 29.4 25.8 31.9
1.43-+0.09 1.44-+0.10 1.07_+0.08 1.09_+0.05 1.78_+0.15 1.32_+0.18 1.44_+0.10 1.35_+0.07 1.26±0.06 1.24___0.08 1.18-+0.10 1.22-+0.10 1.15_+0.12 1.19_+0.09 1.00_+0.09 1.09_+0.07 1.39_+0.08 1.44_+0.07 1.15_+0.18 1.46_+0.08 1.62_+0.09 1.37_+0.07 1.31_+0.08 1.30_+0.1 1.60_+0.1 1.38_+0.07 1.43_+0.11 0.98_+0.10 0.95_+0.08
32°04'40 " 32°04'05 " 32o04'05 " 32°03'30" 32°03'30" 32°03'20" 32°03'00 " 32°02'55 " 32002'45 " 32°02'35 " 32°02'10" 32°02'10" 32002'00" 32°01'50" 32°01'50 " 32°01'40 " 32°01'35 " 32°01'20 " 32°01'10 " 32001'00 " 32°01'00" 32°00'50" 32000'45" 32000'45" 32000'40 " 32000'40 " 32°00'25 " 32°00'25 " 31°56'45"
130°43'50" 130°40'30 " 130°42'30 " 130°40'50 " 130042'05 " 130°41'40 " 130°42'50 " 130°43'55 " 130°43'30 " 130°44'05 " 130°40'55" 130041'40 " 130044'00" 130043'50" 130°42'50" 130°44'10" 130°42'40 " 130040'55 " 130°42'10 " 130042'20 " 130042'25 " 130°42'00 " 130°41'55" 130041'55" 130°41'00" 130°41'00 " 130°41'45 " 130041'50 " 130°40'25 "
WofMasaki E ofMasaki WofMasaki EofAoki IchiyamaRiver IchiyamaRiver Nishino SEofNishino S ofNishino NofHannyaji Kusumoto EofKusumoto Hannyaji SWofHannyaji NofUono Yamashita Uono Shinkawa NEofmine NEofmine NEofmine NEofmine NEofmine NEofmine Quarry in Maeda QuarryinMaeda Sofportal Eofportal Omure
18.7 25.2 17.9 15.9 32.5 27.9 45.9 55.0 28.2 28.2 33.6
1.04+__0.19 1.11_+0.13 1.01_+0.19 1.25_+0.21 1.15_+0.09 0.95_+0.09 1.11_+0.06 1.05_+0.05 1.56_+0.20 1.05_+0.09 1.16+0.21
32004'45 " 32°04'40 " 32004'40" 32°04'25" 32°04'20 " 32°04'15" 32°04'10 " 32°03'40" 32002'45 " 32002'40 " 32°00'25"
130°39'25 " 130041'50 " 130044'05 " 130°42'30" 130043'30" 130°39'15" 130°42'05 " 130°41'00" 130°43'40" 130°41'40 " 130°40'55"
N ofAoki NofKurozonsan NofMasaki NofKurozonsan WofMasaki NofAoki NofKurozonsan EofAoki SofNishino SWofKurozonsan Yamada
16.7 25.2 35.0
0.79±0.12 0.78±0.08 0.79_+0.05
32°00'20" 32°00'00" 31°58'20"
130°42'30 130°43'50" 130°41'30"
E of mine SofYoshimatsu Inabazaki
Hishikari Lower Andesites M-53 A-7 M-92 U-122 M-17 M-13 M-8 U-115 M-78 U-114 M-24 M-3 M-42 T-55 A-5 T-54 U-3 M-1 F-212 T-14 T-12 T-19 T-20 UBC-2 UBC-3 U-13 U-103 U-104 A-31
1.61 1.75 1.93 1.88 1.74 2.15 1.67 1.95 1.89 1.69 2.19 2.10 1.76 1.53 1.70 2.36 1.83 1.78 1.70 1.94 1.91 1.88 1.90 2.06 1.85 2.05 1.30 1.93 1.97
0.09 0.10 0.08 0.08 0.12 0.11 0.09 0.10 0.09 0.08 0.10 0.10 0.08 0.07 0.06 0.10 0.09 0.10 0.07 0.11 0.12 0.10 0.10 0.1087 0.1161 0.11 0.07 0.07 0.07
Kurozonsan Dacites M-217 M-220 F-7 M-70 M-22 M-215 A-10 U-120 M-204 M-5 M-91
3.35 3.02 4.08 2.47 3.80 3.25 2.56 2.50 1.82 2.43 3.32
0.14 0.13 0.16 0.12 0.17 0.12 0.11 0.10 0.11 0.10 0.15
Hishikari Middle Andesites A-16 U19 U-26
1.82 1.58 2.05
0.05 0.05 0.06
THE HISHIKARI GOLD DEPOSIT
15
TABLE 5 (continued) Sample no.
K
4°Ar rad
(wt.%) (nl/g)
Rad.4°Ar
Calculated
age (Ma)
Latitude north
Longitude east
Remarks
(atom.%) 9.4 12.0 42.8 28.9 23.6 36.2
1.1 _+0.1 1.0 _+0.1 0.81_+0.04 0.66_+0.04 0.73_+0.08 0.73_+0.05
32o00'50 " 32000'30 " 32o00'25 " 31°59'55 " 31059'40 " 31059'20 "
130°42'50 " 130o42'35 " 130°43'50" 130°43'20" 130041'35 " 130°43'50 "
Eofmine Eofmine Wof¥oshimatsu SWofYoshimatsu N ofHirasawazu SofYoshimatsu
23.2 16.6 27.3 14.7 24.4
0.56_+0.06 0.58_+0.10 0.57_+0.05 0.55_+0.01 0.51_+0.06
32o01'40" 32°01'15" 31o59'25" 31°59'10" 31o58'55 "
130o43'22 " 130°43'15 " 130043'50 " 130043'50 " 130o43'25 "
Uono Uono-Yoshimatsu Sof¥oshimatsu SofYoshimatsu SofYoshimatsu
Shishimano Dacites UBC-4* 0.29 UBC-1 2.92 U-22 2.72 T-77 3.12 U-27 3.08 U-14 2.84
0.0126 0.1196 0.08 0.08 0.08 0.08
Hishikari UpperAndesites U-5 U-7 U-15 T-75 K-4
2.04 1.90 1.62 1.81 1.71
0.04 0.04 0.03 0.04 0.03
*Feldspar Analyses: Teledyne Isotopes, except for UBC-1,2,3 and 4 (University of British Columbia)~
TABLE 6 K-Ar age data of adularia bearing ores from the Hishikari deposit Sample no.
K (wt.%)
HA-1 HA-3 HA-19 HA-7 HA-18
4.30_+0.13 4.09+0.12 4.83_+0.15 5.13_+0.15 4.34-+0.13
UBC-A1 UBC-A2
12.58 12.39
4°At rad (nl/g)
Rad.4°Ar (Atom.%)
Calculated age ( M a )
Remarks
0.1590_+0.0187 0.1481_+0.0076 0.1859_+0.0116 0.1665___0.0123 0.1694_+0.0125
20.1 18.7 16.7 13.9 13.3
0.95_+0.12 0.93_+0.06 0.99_+0.07 0.84-+0.07 1.01_+0.08
100mL E40 RY-6W 100mL E40 RY-6W 100mL E40 RY-6E 25S 70mL W14 ZU-1W F W 70mL W14 Z U ° l W 80 S
0.4808 0.4688
34.8 41.0
0.98_+ 0.04 0.97_+ 0.04
composite sample a composite sample b
aAdularia from drill hole cores 0-1 ( 192.80 m, 214.98 m a n d 331.60 m ) a n d E l - 1 (258.90 m, 279.87 m a n d 324.75 m ) (Nishizawa a n d Ibaraki, 1985} bAdularia from drill hole cores E3-1 {197.55 m a n d 217.50 m ) a n d W2-2 (327.90 m, 328.00 m and 328.30 m ) (Nishizawa a n d Ibaraki, 1985). Analyses: Okayama University of Science, except for UBC-A1 a n d UBC-A2 (University of B r i t i s h Columbia).
activity of the Hishikari Lower Andesites, and overlaps with early-stage eruption of the Shishimano Dacites (Fig. 4). Geological structure The Hishikari which
coincides
mine lies close to the western with part
of the Kagoshima
margin graben
of the Kakuto (Fig. 1). The
caldera, Shimanto
16
E. IZAWA ET AL
6 Vore s
42I(n=7) c o
0
i 0
°6F c
I 2
I Dacites
"~-4I(n=17)~ ~'~ ® 2 "u
hi
rozonsQn
0 0
~s
1
FAndesites f: 6 ~ ( n = 3 7 ) Upper z
I
2
~
o
Hishikori Lower
,
0
1 K--Ar
age
2 , Mo
Fig. 4. Histogramsof K-Ar agesof volcanicrocksand ores fromthe Hishikari area.
Supergroup basement is present below - 1000 m elevation in the graben (2000 m beneath the western flanks of the Kirishima volcano) (Nakagawa et al., 1985). The Shimanto Supergroup is also very deep to the northwest in the Okuchi basin, as a drill hole to - 8 0 0 m elevation did not reach the basement (T. Ikeda, pets. commun., 1988) (Fig. 1 ). On the contrary, the Hishikari area is underlain by shallow basement (with the highest occurrence at 130 m elevation). A gravity survey by SMM showed a gravity high anomaly with a northeasttrending axis over the Hishikari deposit. The area (3 km by i kin) of the gravity high anomaly corresponds closely to the area of the basement high. The surface of the Shimanto Supergroup in the Honko area is similar in shape to an inverted keel, with a gentle plunge to the west-southwest (Fig. 5). Elevations of the top of the Shimanto Supergroup are - 1 0 0 m to - 2 0 0 m in the Yamada area, 1 km to the southwest. The top of the basement is a series of peaks and ridges which are separated by small valleys. This relief of the basement surface is similar to that of the present topography of the area. The contour map of the Shimanto Supergroup implies that this structure was the paleosurface prior to eruption of the Hishikari Lower Andesites. Veins coincide with the topographic high of the paleosurface (Fig. 5). The Shimanto Supergroup and the Hishikari Lower Andesites are in places fault bounded, as evidenced by slickensides on the contact. O n the district
FHE HISHIKARI GOLD DEPOSIT
~/180
w40
17
zc
o
/~-.--4,o
,o:_ :r6o
40
II I
-40
'-
,.._:..
.........
~___DA
/
//I,o
"°
~ _ ~ ,
/
•
.
0
Legend '"-J""
DA
A
B C
540 0
I00
200
300
400
500m
i......... i , i I I i I I I
Fig. 5. E l e v a t i o n o f the top of the Shimanto Supergroup based on drilling and underground development. A =elevation in meters, contour interval is 10 m. B---location of the Honko veins at 40 m elevation, abbreviations include: H O - - Hosen vein group; Z U = Zuisen vein group; R Y = R y o s e n vein group; and D A = Daisen vein group. C = projection of the inclined shafts. The base line of the mine grid is parallel to the general strike of the veins ( N 5 0 ° E ) .
scale, a block of the Shimanto Supergroup, bounded by faults on the north and south ends, may have been uplifted in the mine area. ORE DEPOSIT
Vein system Hishikari can be classified as an epithermal gold-silver-bearing quartz-adularia vein deposit. At present, development has focused on the Honko vein system. A separate vein system was recently discovered in the Yamada area 1 km southwest of the Honko deposit; it is hosted by the Hishikari Lower Andesites and consists of several major veins with numerous parallel veins. However, details of the Yamada area are poorly known, so only the Honko vein system is described here. The Honko vein system is composed of 3 major vein groups and numerous veinlets in an area 1100 m by 200 m (Table 7). They occur in both the Shi-
18
E. IZAWA ET AL.
AGE Shishima no Dacite
H ishikari Lower A n d e s ~ ~ - ' ~ . V ]OOML
,,,'~\ / x / \ / \ /~(
River
Portal ~ / \
/ \ / \ /\ / \ \ // \ /\ / \ \ / \ / \ / \
\ / \ / \ /
\
/
/ /\ ~/ ' / \ / ~
~ / \ / \ \ / \ /
" \ 1 \ 1 \ 1 \ 1 \ I \ 1 \ 1 \ 1 \ 1 \ 1 \ \ / \ / q / k / N / k / \ / \ / \ / \ / \ \ / \ / \ / \
. , , If,
\. / \ 1
l J
----
I I
Vein 0.8z,~1.01 Mo
I' 0.~5
~
\ • .
I\ I\~
rll~
:::
\ / \ / \ / \ / ' \/ \/ \ / \ , \ / \ / \/' \ / \ /\.
I \
~/////4"///,',x~
I
1!1 M o
\.
\,
\,
\'~
t.78 Ma
\ / \ 1 \ 1 \ ~ \ 1 \ 1 \ 1 , \/\,
"", Cretaceous.
Fig. 6. Schematic n o r t h w e s t - t r e n d i n g section across the Hishikari mine. T h i s cross section shows the central part of the H o n k o area, where most of the veins are hosted in sediments. Legend is the same as in Figure 3.
manto Supergroup and the Hishikari Lower Andesites, and generally strike N50 °E with an individual strike length of 300 to 400 m; the veins dip steeply north (70 ° to 90 ° ) and range from 1 to 3 m (maximum 8 m) in width. Known bonanza zones are located between 130 m and - 2 0 m elevations (Figs. 6, 7 and 8). The Ryosen vein group, which is composed of 5 unit veins (RY-1, 2, 3, 5 and 6), is located in the eastern part of the deposit. Veins are relatively narrow in width and have a moderate gold grade for veins at Hishikari. The Zuisen vein group consists of 2 unit veins, occurring on the hanging-wall side of the western part of the deposit. The Zuisen No. 1 vein is the widest (up to 8 m) and lowest in grade relative to the other veins. The Hosen vein group is composed of 4 unit veins (HO-1, 2, 3 and 5 ) and is located on the footwall side in the central to western part of the deposit. The Hosen veins (particularly Hosen No. 2) are well developed in the Shimanto Supergroup and form extremely rich bonanzas directly below the unconformity (Fig. 8). Only part of the Daisen No. 1 vein has been developed, and is not well known; it parallels the other vein groups on the hanging-wall side. Veins of the Honko area commonly show rhythmic crustiform banding, in general symmetrical from both walls; however, in wider veins, such as Zuisen
rile HISHIKARI GOLD DEPOSIT
19
W; OB
W~OB
E20B
E4 {)B
E~B
-~=f . . . . . . . . . . . . .
N20G
/-/
oo
100~ L ...........
"-
. . . . . . . . . . . . . .
. _ _ .D_A o - . - I.
.
.
.
.
N20G
Y
OG
70M >,/
N20G
5
_
/I
N20G
I
. . . . ........ .::_---:
O~
-
I
20MI
0 i
I O0 l
ZOOm i
Fig. 7. Distribution of veins a n d the S h i m a n t o Supergroup (hatched area) at the 100 m level, the 70 m level, the 40 m level a n d the - 20 m level. T h e dashed lines indicated assumed vein portions. Abbreviations are identified in the caption to Figure 5. T h e base line of the mine grid is parallel to the general strike of the veins (N50 °E).
No. 1, the banded structure is complex, showing repeated collapse and deposition, with drusy cavities common and as large as 3 m X 7 m X 5 m. The size of each vein and average Ag/Au ratio of the ore are shown in Table
20
E. IZAWA ET AL.
E :)B
E3OB
.k"
",.
E,~OB
~//
•,... _. . . . . . . . . - . , "i/
/
/
/
RY-2 ~o
W&QB
w3011
. ;o
W208
,o,o.
WlOO
.... ~----,',;-::
.....
On
EI OB , I,o0 .. M
"b=:-=..... ~'~.'i" '°
;---- _'°,°
,,
W; OB
OB
0;
~ B
E]
eavtt ?
/
~ ~ : ~ ' ~ - ~ ~.~ - ~
"~,.,~/_.~//..'x///'M~
~ 2 2- -
/-J
_ :-~/--'/~..~- . ,
~
,
I
OLIO
SO
lOOm,
Legend
l~7~s
---F
~c [ [o Fig. 8. Distribution of gold values of veins in longitudinal projections onto a vertical plane along the direction of N50°E. RY-2 = Ryosen No. 2 vein; ZU-1 =Zuisen No. i vein; A to D indicates high to low goldvalues,which are expressedby the product of the grade and the width of the veins. The ore correspondingto the highest gold value usually exceeds 100 g/t Au. The light dotted line, E, shows the unconformity between the Shimanto Supergroup and the Hishikari Lower Andesites. The heavy dashed line, F, shows the limits of ore grade (> 4 g/t Au) in the veins; the upper boundary coincides with a pinching out of the vein.
THE HISHIKARI GOLD DEPOSIT
21
TABLE 7
General feature of veins of the Hishikari deposit (Honko area) Vein
Strike
Dip
Length
Elevation
(m)
Top
Bottom
(m)
(m)
Average width
Ag/Au
(m)
(wt.)
ratio
Ryosen RY-1 RY-2 RY-3 RY-5 RY-6
N50°E N50°E N50°E N50 ° E N30°E
75°N 75°N 90 ° 80 ° N 80°N
200 200-300 250-300 150 + 300+
110 115 110 95 110
25 -50 -5 25 -50
0.55 0.94 0.38 0.58 1.69
2.72 0.82 1.18 2.68 0.59
N50°E N50 ° E N50 ° E N50°E
90 ° 80 ° N 80 ° N 75°N
280 450 80 350
95 90 85 90
-20 - 20 - 5 0
1.40 3.22 1.47 0.74
0.90 0.57 -
N50°E N50 ° E
80°N 80 °
450 350
90 80
-65 0
3.92
0.71 -
--
90 °
(300)
(115)
-
90 ° 90 ° 90 °
(250) {250) {150)
85 80 75
20 0 5
0.23 0.17 0.16
-
Hosen HO-1 HO-2 HO-3 HO-5
Zuisen ZU-1 ZU-2
Daisen DA-I
Undeveloped E F H
N50°E N55°E N55°E
Lengths and vertical ranges are confirmed values except for the figures in parentheses, which are assumed values.
7. The Ag/Au ratio varies considerably from > 1 in the upper levels to 0.5 in the deeper levels (Fig. 9). Vein minerals
Veins are composed mainly of quartz, adularia and clay minerals. More than 90% of the clay minerals are smectite, with minor amounts of sericite, chlorite and kaolinite. The principal metallic minerals are electrum, naumannite-aguilarite, pyrargyrite, chalcopyrite, pyrite and marcasite, with minor amounts of sphalerite, galena, stibnite, tetrahedrite, miargyrite, hessite (?), Ag-Au selenide, acanthite, Cd-sulfide (greenockite or hawleyite), and hematite (Urashima and Izawa, 1983). Lamellar quartz is common and, in places, there are minor amounts of carbonates (calcite and Mn-Fe-Ca carbonate), gypsum, truscottite (alkali-free and alkali-rich varieties), xonotlite, wairakite and lau-
22
E. IZAWAET AL.
----~Ag/Au mL
1.0 R~-6 ~ ¥ - 2
2.0 '
3.0 I ...........
100
70 40
~
"~"~
~
RY-I
10 -20 Fig. 9. Plot of Ag/Au weight ratio of ore vs. elevation. Abbreviations include: R Y = Ryosen vein group; H O = Hosen vein group; and Z U = Zuisen vein group.
montite (Urashima and Izawa, 1982; Izawa and Nakae, 1983; Izawa and Urashima, 1983). Major- and trace-element chemical analyses of bulk samples of ore are listed in Table 9. Adularia is the most widespread mineral with quartz. Major veins consist of 70% quartz and 30% adularia, and locally the latter is more abundant than the former. Adularia occurs as idiomorphic clustered crystals, intergrown with quartz, or fine-grained clayey aggregates of quartz and smectite or kaolinite. Chemically, A1203 and K20 show a positive correlation with Au and Ag, but SiO2 is negatively related to Au and Ag (Table 9). Veins containing abundant adularia hence tend to be richer in Au and Ag than quartz-rich veins. However, on a microscopic scale, Au and Ag minerals are sparse in monomineralic bands of adularia, but are abundant in aggregates of fine grained quartz, quartz-adularia, or quartz-clay minerals. Electrum generally tends to occur in gray or black bands and spots, and clayrich portions of veins. Several stages of electrum concentration are observed. Electrum is often associated with chalcopyrite or occurs as isolated grains in quartz. In extremely high-grade veins, electrum bands occurs between early adularia-quartz and later smectite-quartz bands. The grain size of electrum generally ranges from less than 1 pm to 25/zm, often around 10/~m, and rarely exceeds 100/~m. Electrum contains 66-81 wt.% gold (average 70 wt.% ); electrum of higher gold content tends to occur at higher elevation (Table 8). Ginguro (silver black) is composed mainly of chalcopyrite, electrum, naumannite, sphalerite, galena, pyrite and marcasite, with minor amounts of acanthite, aguilarite, tetrahedrite, greenockite (or hawleyite), Au-Ag selenide, hessite (?), pyrargyrite and miargyrite. Ginguro tends to occur at higher levels (RY-1, 3, 5; HO-1, 2; ZU-1 ) and on the outer side of the vein, and seems to be an earlier stage of mineralization. Table 10 lists the chemical compositions of
THE HISHIKARIGOLDDEPOSIT
23
TABLE 8
Chemical composition of electrum determined by microprobe analysis Elevation (m)
Au (wt.%)
Ag (wt.%)
Total (wt.%)
Ag (Atom.%)
Remarks
No. 12-162.1m
118
81.24 76.65 75.77 71.19 68.75
18.51 22.55 24.31 28.43 31.32
99.75 99.20 100.08 99.62 100.07
29.4 34.9 36.9 42.2 45.4
HLA
No. 12-162.2m
118
69.55 67.72
31.24 32.86
100.79 100.58
45.1 47.0
HLA
56MAHT-1-456.5m
80
73.06
26.70
99.76
40.0
SH
No. 13-215.0m
75
67.02
32.84
99.86
47.2
SH
No. 13-215.2m
75
74.47 71.23
25.47 28.53
100.44 99.76
38.9 42.2
SH
56MAHT-2-263.8m
58
69.58
30.27
99.85
44.3
SH
55MAHT-5-291.7m
51
67.84 66.82
32.70 32.89
100.54 99.71
46.8 47.3
SH
Sample no.
Drill core samples 1
Underground 2
RY-1
E18B
100
72.07 66.96
27.57 33.39
99.64 100.35
41.1 47.7
HLA
RY-2
E28B W
100
67.60
31.95
99.55
46.3
HLA
RY-2
E30B W
100
66.59
33.25
99.84
47.7
HLA
ZU-1
Wl4B E
70
75.45
25.21
100.66
37.9
SH
HO-1
E1B E
70
70.38
30.37
100.75
44.1
SH
HLA = hosted in the Hishikari Lower Andesites; SH = hosted in the Shimanto Supergroup. RY = Ryosen vein group; Z U - Z u i s e n vein group; and HO = Hosen vein group.
1Drill hole numbers and sampling depths. 2Urashima and Nedachi (1986).
silver minerals and other sulfide minerals, while the compositional variation of sphalerite is listed in Table 11. Compared with the upper part of the vein system, electrum and sulfides are less abundant and finer grained in the middle and deeper levels. Silver minerals increase in abundance with elevation, as shown by the increase in overall Ag/ Au ratio in the upper part of the veins (Fig. 9); in contrast, the Ag content of electrum tends to decrease slightly at the higher levels. Other mineralogical changes with elevation include an increase in the selenium content of nau-
(wt.%) Si02 TiO2 Al20s Fe203 FeO* MgO CaO Na20 K20 P205 S C02 H20+ H20Total
Sample no.
Vein: Location:
65.93 0.31 12.24 1.02 3.13 1.13 0.08 0.23 10.05 0.14 1.73 <0.1 1.17 0.12 97.38
RY-6 100mL E28.4B HK-19
76.55 0.09 9.06 0.38 1.79 0.42 0.12 0.32 7.79 <0.01 0.37 <0.1 0.62 0.14 97.76
RY- 1 70mL E16.1B-W HK-17 87.71 < 0.01 3.87 0.17 1.29 0.04 0.10 0.05 3.06 <0.01 0.22 0.35 0.87 0.12 97.87
RY-2 70mL E29.5B2 HK-01 84.51 < 0.01 6.24 0.43 1.11 0.13 0.14 0.05 4.88 0.02 0.36 <0.1 1.02 0.35 99.35
RY-2 70mL E27.3B-W HK-03
Chemical composition of ores from the Honko area of the Hishikari deposit
TABLE 9
76.92 < 0.01 10.08 0.20 0.63 0.09 0.06 0.35 9.43 <0.01 0.04 <0.1 0.55 0.15 98.62
HO- 1 70mL W0.3B°W HK-13 82.92 < 0.01 6.77 0.20 1.03 0.11 0.09 0.30 5.28 <0.01 0.18 <0.1 0.51 0.20 97.71
HO-2 70mL W4.0B-W HK-15
75.03 < 0.01 12.07 0.17 0.81 0.15 0.12 0.25 10.82 <0.01 0.14 0.17 0.61 0.26 100.62
HO-2 70mL W13B-E Hk-05
85.39 < 0.01 5.01 0.33 0.94 0.06 0.16 0.19 2.80 <0.01 0.09 <0.1 0.74 0.27 96.10
HO-3 70mL W14.8B-E HK-09
84.10 < 0.01 6.01 0.22 0.97 0.01 0.02 0.02 5.46 <0.01 0.04 0.17 0.92 0.34 98.30
ZU- 1 85mL W20.1B-W HK-07
90.50 < 0.01 1.86 0.16 1.26 0.03 0.07 0.34 1.00 <0.01 0.11 <0.1 0.72 0.14 96.31
ZU- 1 85mL Wl0.5-E HK-11
b~
3944.9 10559.4 9500 970 800 35.0 7 7 710 17 13 320 8.75 0.6 9 5.3 -
655.6 1378.2 800 105 71 3.3 8 5 420 100 57 142 0.45 0.1 9 5.0 0.11
38.9 31.7 490 77 42 0.6 11 10 90 32 14.8 1.0 0.10 6.8 1 2.1 0.32
164.2 94.3 265 37 16 0.6 7 2 75 270 81 2.0 2.75 0.7 4 5.3 1.90
132.6 166.2 12 25 52 0.4 5 1 51 9 70 2.0 0.50 0.1 11 6.1 0.16
745.2 1332.5 680 145 53 2.1 7 1 78 30 200 102 5.50 0.1 7 3.4 0.19
470.0 824.3 500 104 65 2.0 6 1 60 36 280 84 0.05 0.3 12 6.8 0.37
64.5 41.1 80 18 5 0.4 8 1 81 23 26 0.5 0.25 0.1 5 1.8 0.12
104.1 80.4 108 19 10 0.5 7 1 73 15 33 3.0 0.80 0.1 3 3.2 0.31
35.8 17.0 33 5 10 0.4 8 1 91 38 10.6 < 0.5 0.55 0.1 2 0.8 0.27
FeO* = FeO + Fe (sulfide) 1 ppm Sn and 5 ppm Ge were detected for all samples; In < 2 except for HK- 19 ( 8 ppm), Ba < 100 ppm, P t < 50 ppb, Pd < 10 ppb and Rh < 20 ppb. RY = Ryosen; HO = Hosen; ZU = Zuisen. HK-19=ginguro-bearing veinlet; HK-17=ginguro-bearing white quartz vein; HK-01 = w h i t e to gray banded quartz vein; HK-03 =irregularly banded quartz vein; HK- 13 = white banded quartz-adularia vein; HK- 15 = ginguro-bearing gray banded quartz vein; HK-05 = banded quartz-adularia vein; HK-09 = white quartz-clay vein; HK-07 = w h i t e compact quartz vein; HK-11 = w h i t e quartz vein with black bands.
(ppm) Au Ag Cu Pb Zn Cd Ni Co Mn As Sb Se Te Bi Ga Ti Hg
b~ O1
t~ *O o
o
r~
26
E. IZAWAET AL.
T A B L E 10 C h e m i c a l c o m p o s i t i o n of silver m i n e r a l s a n d o t h e r sulfide m i n e r a l s d e t e r m i n e d by microprobe a na l ys i s Sample no.*
Wt.% Ag
A t omi c Se/(Se+S) Cu
Zn
Fe
Pb
Sb
As
Se
S
Tot a l
Naumannite (-Aguilarite) No. 1 2 - 1 . 6 2 . 1 m No. 1 2- 162.2m No. 1 3 - 2 1 5 . 2 m 56MAHT-2-263.8m
78.33 0.52 0.13 78.54 1.39 1.16 0.20 81.16 78.49 1.39 0.90
-
21.88 15.14 16.61 12.48
1.15 102.01 0.88 2.91 99.34 0.68 3.44 101.21 0.66 3.77 97.03 0.57
Pyrargyrite No. 1 3 - 2 1 5 . 2 m
61.54 0.74
-
-
19.08 1.98
5.63
13.99 102.96
36.34
-
-
40.91
1.91
19.58
98.74
-
53.02 53.23
99.69 99.47
2.06 11.25
99.58
Miargyrite No. 13 - 215.0
-
Pyrite No. 13 - 215.2m 5 5 M A H T - 5 - 291.7m
-
-
-
-
46.64 45.72
0.03 0.52
Galena No. 12 - 162.1m
-
86.27
Stibnite M A H T - 1 - 465.3m
-
-
-
71.78
-
28.31
100.09
- - = not detected. *Drill hole n u m b e r s a n d s a m p l i n g depths.
T A B L E 11 C h e m i c a l c o m p o s i t i o n of s p h a l e r i t e d e t e r m i n e d b y m i c r o p r o b e a n a l y s i s S a m p l e no.*
Wt.% Zn
No. No. No. No.
12 - 162.1 m 12 - 162.2 m 3 - 203.2 m 13 - 215.0 m
No. 8 - 408.5 m
63.57 62.74 63.73 62.46 62.58 64.27
Fe
Cu
Cd
S
Total
0.40 0.50 2.97
0.35 0.46 -
3.54 3.42 2.31 2.90 2.92 -
32.85 33.15 33.03 33.17 33.07 33.38
99.96 99.30 99.06 99.28 99.52 100.62
Mole % FeS
Elevation and remarks
<0.02 <0.02 <0.02 0.70 0.88 5.1
118 118 85 75
m m m m
HLA HLA SH SH
11 m S H
H L A = h o s t e d in t h e H i s h i k a r i L o w e r A n d e s i t e s ; S H = h o s t e d i n t h e S h i m a n t o S u p e r g r o u p . -- = not detected. *D rill hole n u m b e r s a n d s a m p l i n g d e p t h s .
mannite
and
of analyses Stibnite,
a decrease
in the
iron
content
pyrite
and/or
of sphalerite,
though
the
number
is limited. fine-grained
pyrargyrite
occur
in cracks,
druses
and
THEHISHIKARIGOLDDEPOSIT
27
fractures in the vein at higher elevations, indicating their precipitation during the latest stage of mineralization.
Homogenization temperatures of fluid inclusions Homogenization temperatures of 232 fluid inclusions in quartz, adularia and calcite were measured on 42 samples from underground and drill holes. These inclusions are two-phase and liquid-dominated. The results are summarized in Figure 10. Fluid inclusions in quartz are very poorly developed and are less than 10/zm in size. No temperature correction for the pressure of formation was made. Unusually high temperatures were obtained for several inclusions in quartz samples from four locations. This may be due to two-phase trapping (and would indicate boiling) at about 250°C between - 2 0 m and 90 m elevation. These unusually high values are excluded from Figure 10. The mean temperature for five inclusions from two adularia samples is 241 ° C. Homogenization temperatures for quartz from veins hosted in the basement Calcite
160 ± 48"C (n=41)
0
Quartz
in andesites)
197 +- 35"C
Quartz
in basement)
213¢23"C (n =129)
o .2 ul 20D 15-
~
IO-
E z
o
5]
Adularia
r - - I 7"C 241+-
(n:51
0
O0 ,
1
,
150 Homogenization
J 200 temperature,
ml-~ J p 250 "C
Fig. 10. Histograms of homogenization temperatures of fluid inclusions from the Hishikari deposit.
28
E. IZAWA ET AL.
sediments averaged 213 ° C, with most values ranging from 195 to 230 ° C. Homogenization temperatures for quartz in the Hishikari Lower Andesites are scattered over a wide range with an average fo 197°C. Calcite also shows scattered values of temperatures from 233°C to 91°C, averaging 160°C and with no definite peak temperature. These data indicate that the temperature of ascending fluid may have been as high as 250 ° C at the greatest depth of mineralization; however, the representative temperature of gold deposition was 210°C in basement at about 25 m elevation, and less than 200 °C in volcanic rocks at about 100 m elevation. Deposition of barren quartz and calcite continued during waning stages of the Hishikari hydrothermal system to temperatures as low as 90 ° C. GEOCHEMICALCHARACTERISTICS
Hydrothermal alteration More than 200 samples collected from surface and about 300 samples from 22 drill holes were examined by means of X-ray diffraction. A number of igneous and hydrothermal minerals such as silica polymorphs, feldspar, clay minerals, zeolites, carbonates and sulfides were identified. The least altered andesitic rocks consist mainly of plagioclase, cristobalite a n d / o r tridymite. Quartz and potassium feldspar are also major constituents of the Shishimano Dacites.
Definition of alteration zones On the basis of the mineral assemblage, hydrothermally altered volcanic rocks can be grouped into four mineralogical zones as shown in Figure 11. The deepest and innermost chlorite-sericite zone (IV) is defined by the common presence of chlorite with local sericite. The typical mineral assemblage is quartz, chlorite, adularia, calcite, and plagioclase, with minor interstratified chlorite/ smectite or sericite/smectite. The interstratified clay mineral zone (III) is characterized by the presence of interstratified chlorite/smectite a n d / o r sericite/smectite. Quartz, adularia, calcite, smectite and laumontite are commonly present. The presence of quartz with smectite a n d / o r kaolin minerals, and the absence of chlorite, sericite and interstratified clay minerals define the quartzsmectite zone (II). Cristobalite a n d / o r tridymite occur with smectite in the cristobalite-smectite zone (I). Although smectite is the essential alteration mineral in both the quartz-smectite and the cristobalite-smectite zones, kaolinite or halloysite are locally predominant (the kaolin mineral subzone). Hydrothermal alteration of basement sediments is developed only in narrow halos to the veins. The alteration is represented by the chlorite-sericite assem-
]'HE HISHIKARI GOLD DEPOSIT
Least ~Itered
29
I Cr-Sm Ka
II
Qz-Sm Sm
Ka
III
IV
Int. Clay
Ch-Se
)lagioclase :r, Tr tuartz <-feldspar (aol in min. ~mectite
:/s, M/S :hlorite 5ericite :pt, Stl 4d, Eps ~m, Ac :alcite
;ypsum }yrite
Fig. 11. Alteration zoning by mineral assemblage. Cr-Sm=cristobalite-smectite zone; QzSm = quartz-smectite zone;Int. Clay = interstratified clay mineral zone; Ch-Se = chlorite-sericite zone; Ka=kaolin subzone; Sm=smectite subzone. Cr=cristobalite; Tr=tridymite; Kaolin min. = kaolinite and halloysite; C/S = interstratified chlorite/smectite; M/S = interstratified sericite/smectite; Cpt = clinoptilolite; Stl= stilbite; Md = mordenite; Eps = epistilbite; Lm = laumontite; A c = analcite. blage, though it is often difficult to distinguish the assemblage from the regional metamorphic chlorite-sericite. Distribution of alteration zones
The vertical and spatial distribution of alteration zones is shown in Figures 12 and 13, which on the whole indicate a zonal arrangement of alteration. Chlorite-sericite alteration (zone IV) is the principal alteration directly associated with high-grade gold mineralization. Interstratified clay mineral (zone III) and quartz-smectite (zone II) alteration envelopes the mineralized center and forms a nearly horizontal layer of intense argillation located 50 to 100 m above the Hishikari vein system. These alteration zones are surrounded by the cristobalite-smectite zone (zone I) and least-altered rocks. There is a slight downward drape to the alteration zones, reflecting distance from the veins (Fig. 12). In addition to the above-mentioned alteration, sporadic alunite-quartz (or alunite-cristobalite) alteration is present at higher elevations (350-400 m elevation) in the eastern part of the Hishikari area. Geochemical anomalies
C h e m i c a l c h a n g e s d u e to a l t e r a t i o n
Studies of chemical variation related to gold mineralization have not been
30
E. IZAWA ET AL.
N6 )G
N" ~G
N; QQ
0G
S20G
540G
S60G
f
400ML
E20B 300Mr
L
L r
L ~
200MI
IOOML
0ML
-100ML
OG
NZOG
0G
S20G
S40G
S60G
400ML
W80B 300ML
L
200ML
v v v
v
vI IO(~L
OML
-IOOML
Fig. 12. Vertical alteration zoning on the E20B and the W80B cross sections along the direction of N40 °W {mine grid is in Fig. 5 and locations of the sections are in Fig. 13). I=cristobalitesmectite zone; H= quartz-smectite zone; III= interstratified clay mineral zone; IV=chlorite-sericite zone. Legend is same as in Figure 3 except for andesitic pyroclastic rocks of the Hishikari Lower Andesites which are shown by the area without pattern. completed yet, t h o u g h analyses of major and minor elements for the leastaltered and altered volcanic rocks, soil and plants in the Hishikari area were listed by M M A J (1988). Variation in major-element concentrations of volcanic rocks from the surface are generally small except for N a 2 0 values, which are remarkably depleted in the highly argillized rocks over the H o n k o vein system. The analytical data ( M M A J , 1988) also show background values for several
]~HE HISHIKARI G O L D DEPOSIT
0
0.5
I
3]
1kin ~\
=
I
\
25
\
,
25 ~
--,..
oo
).
Yamada~>,,.~, @
o
,;-
¢.J
/
o
"~.
,
o
,~
o~
o
..
o
o
N
,S ," I
/,"
oc
f
~
{
•
Q
c
'
\
•
"\
o
} .
I
"-.J
*
Fig. 13. Areal alteration zoning. I=cristobalite-smectite zone; //--quartz-smectite zone; I I I = interstratified clay mineral zone. The western side of the dashed line is covered by young
pyroclastic flow deposits. Small circles show sample locations; open circles denote the least altered rocks and solid circles refer to altered rocks. Light lines or dashed lines show the surface projection of the veins. Area of low resistivity {less than 25 ohm-m), determined by CSAMT (Kawasaki et al., 1986), is outlined by the heavy line. Mine grid, 0G ( N50 ° E) and 0B (N40W), and the locations of cross sections (E20B and WSOB in Fig. 12) are shown.
trace elements in Quaternary volcanic rocks of the Hishikari area. Most of the volcanic rocks contain 2-5 p p m As, 10-40 ppb Hg and < 1-1 ppb Au. In a few cases, argillized surface rocks directly above the Honko veins contain up to 15 ppm As, 1500 ppb Hg and 31 ppb Au. It is interesting to note that leaves of some plants like Callicarpa moles contain rather high gold (10-41 ppb on a dry basis) in the area over the vein system (laterally up to 400 m from the surface projection of the Honko veins). The gold content of sandstone and shale of the Shimanto Supergroup ranges from < 1 to 3 ppb for the least-altered rocks and ranges up to 64 ppb for altered rocks (Ishihara et al., 1986). Variations of JlsO in the Hishikari area Oxygen isotopic compositions of ore and whole rock were analyzed at the University of Queensland, Australia, with the results listed in Table 12. A1-
32
E. IZAWAET AL.
TABLE 12 Oxygen isotope data for whole rocks and ores from the Hishikari mine area (Takaoka, 1987) Sample no.
Location
Rock and ore
Ores (quartz RY-2 HO-1 QV
and adularia) 70mL E drive 18 + 1.0m 70mL E drive 40+2.3m DW-20 71.5m
Ryosen No.2 vein Hosen No.1 vein Quartz vein
+ 6.8 + 8.3 + 8.8
In Hishikari Lower Andesites In Shimanto Supergroup In Shimanto Supergroup
Volcanic rocks (least altered) U-13 Quarry in Yamada T-12 NE of mine T-20 NE of mine G- 1 NW of Yoshimatsu
Pyroxene Pyroxene Pyroxene Pyroxene
+ 8.5 +9.6 + 9.1 + 9.2
Hishikari Lower Andesites Hishikari Lower Andesites Hishikari Lower Andesites Hishikari Lower Andesites
Volcanic rocks (altered) HL-1 56MAHT-1 38.5m HL-2 56MAHT-1 100.0m HL-3 56MAHT-1 157.3m HL-4 56MAHT-1 200.0m HL-5 56MAHT-1 248.0m HL-6 56MAHT-1 300.0m HL-7 56MAHT-1 320.0m HL-8 56MAHT-1 359.7m HL-9 56MAHT-1 390.3m TN-1 No.1 Incline TN-2 No.1 Incline TN-3 No.1 Incline TN-4 No.1 Incline TN-5 No.1 Incline TN-6 No.1 Incline TN-7 No.1 Incline TN-8 No.1 Incline TN-9 No.1 Incline TN-10 DV-2 6.0m TN-11 DV-2 40.2m TN-12 DV-2 79.1m TN-13 DV-2 120.2m TN-14 DV-2 160.0m TN-15 DV-2 201.4m TN-16 DV-2 239.6m TN-17 DV-2 282.2m TN-18 DV-2 294.5m TN-19 DV-2 320.0m TN-20 DV-2 355.1m
Lapilli tuff Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccm Tuff breccia Tuff breccia Andesite Tuff breccia Volcanic breccia Tuff breccia Lapilli tuff LapiUi tuff Andesite Tuff breccia Lapillituff Tuff breccia Tuff breccia Lapillituff
+8.1 + 8.8 + 10.0 + 5.6 + 6.8 + 7.4 + 3.3 + 6.2 + 4.1 +5.5 +4.0 +6.0 +4.8 +4.9 +6.3 +5.5 +6.8 +4.7 + 6.6 + 6.9 + 6.4 + 6.5 +4.0 +4.3 +6.6 +5.5 +7.2 + 5.0 +6.3
qz-Kf-sm-ch/sm-lm-cal-py qz-sm-ch/sm-lm-py Pale green (arg) qz-Kf-sm-ch/sm-lm-py sm-ser/sm-ch/sm qz-Kf-sm-ch/sm-lm-py qz-Kf-sm-ch/sm-cal qz-piag-sm-ch/sm qz-Kf-ch-ser Greenish white (arg, py) Pale green (arg, py) Pale green (arg, py) Pale green (arg, cal) Pale green (arg, py) Pale green (arg, py) Light greenish gray (arg) Green (silic, py) Dark green (propylitic) Greenish brown (arg, py) Greenish brown (arg, py) Light green ( arg, py ) Pale green (arg, py) Light gray (silic) Gray (chloritized) Pale green (arg, py) Light green (chloritized) Light green (py) Bluish green Greenish white (arg)
Shimanto Supergroup SH-1 DW-20 21.4 m SH-2 DW-20 49.0m SH-3 DW-20 70.3m SH-4 56MAHT-1 398.3m
Shale Shale Shale Shale
+12.3 +10.5 +14.4 +11.8
oz-se-ch
andesite andesite andesite andesite
J180 (%0)
Remarks
THE HISHIKARI GOLD DEPOSIT
33
T A B L E 12 (continued) Sample no.
Location
Rock and ore
$lsO (Too)
Sandstone > shale Shale Shale Sandstone > shale Shale Sandstone=shale Shale
+ 12.5 + 11.4 + 12.6 - 11.0 + 12.2 +10.0 + 12.3
Remarks
Shimanto Supergroup SH-5 SH-6 SH-7 SH-8 SH-9 SH-10 SH-11
56MAHT-1 56MAHT-1 56MAHT-1 56MAHT-1 56MAHT-1 56MAHT-1 56MAHT-1
432.6m 461.2m 511.4m 550.0m 600.0m 654.0m 701.0m
qz-plag-se-ch-py qz-plag-se-ch qz-plag-se-ch
Arg = argillic alteration; silie = silicification; qz = quartz; plag = plagioclase; K f = K-feldspar; s m - smectite; s e / s m - i n t e r s t r a t i f i e d sericite/smectite; se=sericite; c h / s m = i n t e r s t r a t i f i e d chlorite/smectite; ch = chlorite; lm -- laumontite; cal -- calcite; p y - - pyrite.
though the number of data are limited, $1so values of quartz veins range from + 8.8 to + 6.8%o, similar to those of quartz veins from the nearby Kushikino gold deposit (Matsuhisa et al., 1985 ). The temperatures of mineralization were typically around 210 °C in the basement and less than 200 oC in the Hishikari Lower Andesites on the basis of fluid-inclusion data (as discussed above). Based on the isotopic composition and formation temperature of the quartz, the 81s0 values of the hydrothermal fluids responsible for gold mineralization may be calculated from the equation for quartz-water isotopic fractionation given by Matsuhisa et al. (1979). Calculated 81so values for the fluid range between - 2 and -5%o, much heavier than that of local meteoric waters (-7.2%o; Matsubaya et al., 1975); this indicates extensive isotopic shifting by waterrock interaction. The whole-rock 8 ' s o values of andesitic rocks in the Hishikari area range from +3.3 to +10.0%o. The 81so values of the least-altered volcanic rocks from the Hishikari mine area range from + 8.5 to + 9.6%o, slightly higher than those of intermediate lavas (e.g., + 6 to +8%o; Taylor, 1968). A small amount of smectite was observed under the microscope in three of these four andesites. The rocks in the intensely argillized zone (the upper part of the interstratified clay mineral zone) covering the zone of high-grade veins also have higher 81so values (+8.1 to +10.0%o). These oxygen isotope results suggest that lowtemperature hydrothermal alteration has increased the whole-rock $ aso values (due to the increased fractionation factor at lower temperatures). In contrast, relatively low 8~so values (ranging from + 7.4 to + 3.3%o, averaging + 5.7%o ) were measured for altered volcanic rocks from the lower part of the interstratified clay zone and from the chlorite-sericite zone. The lower 81sO values of altered rocks are attributed to interaction of the rocks with hydrothermal waters; these were most likely local meteoric waters that were heated and isotopically shifted at greater depths. In particular, the lowest ei'so
34
E. IZAWAET AL.
values were ibr samples located close to (or over) quartz veins, indicating the highest temperatures a n d / o r the least amount of shallow water-rock interaction water-rock interaction (i.e. a higher permeability). Most Cretaceous rocks of the Shimanto Supergroup in southwest Kagoshima Prefecture have been metamorphosed at least to prehnite-pumpellyite facies (MITI, 1985), and hydrothermal alteration of sandstone and shale of the Hishikari deposit is generally not conspicuous. The sedimentary rocks have a narrow range of ~lso values of + 10.0 to + 14.4%o; a low g 1 8 0 anomaly was not observed, indicating very little interaction (due to low permeabilities) with hydrothermal fluids.
Volatile components in soil gas Since the middle of the 1970s, the geochemistry of volatile components such as Hg, CO2 and radon (Rn and T n ) in soil gas has become one of the most useful exploration tools in active geothermal fields. This is because these volatile components can migrate from deep geothermal fluids to the surface through fractures such as faults and veins (e.g., Koga and Noda, 1974, 1975; Stoker and Kruger, 1975; Whitehead, 1981 ). The Hishikari gold veins are accompanied by hot spring water at a temperature of about 65 ° C and with high CO2 contents, up to 500 mg/kg. Therefore, the geochemical prospecting technique using soil gas was applied to reveal the relationship between the distribution of the volatiles at the surface and the known gold vein systems, and also to estimate the extension of the veins. Sampling: Sampling points were arranged to cross the projection of the vein system to the surface, with some additional points near the center of the vein system. The sampling was initiated by making a hole 60 cm deep and 4 cm in diameter with an iron pipe. Radon and CO2 were analyzed in situ. All work carried out in the field takes approximately 20 minutes per sample point. Rn and Tn: After extracting the iron pipe, a portable radon detector (alpharay scintillation counter; Radon Detector RD200, EDA Instruments Inc., Canada) was positioned in the hole (Fig. 14). Soil gas was then introduced to the fluorescent cell and counted for three successive one minute intervals. Although a r a d o n / t h o r o n ratio ( R n / T n ) was obtained from the three successive measurements, only the total counts of the three measurements are discussed in this study. C02: After measuring Rn, CO2 was measured using a Kitagawa Precision Gas Detector (Komyo Rikagaku Kogyo Ltd., Japan). The detector carries out a dry analysis based on the principles of chemical reaction and physical absorption. After introducing 100 ml of soil gas into the detector tube from the bottom of the hole, the concentration of CO2 was determined by reading the length of reagent reacted in the tube with the CO2 of the soil gas. Hg: A 10-cm-long pure gold wire with a diameter of 1 m m was left for three
THE HISHIKARI GOLD DEPOSIT
35
~ RStopper.~ ubber ~ P
GasCell
~ee D lco ,rJI
Gas Fig. 14. Samplinghole and equipmentsetup for the radon detector.
days to capture Hg gas in the soil gas. A soil sample from the bottom of the hole was also collected for determination of Hg in the soil. The amount of Hg adsorbed on the gold wire and that of the soil sample was determined in the laboratory by flameless atomic absorption spectrometry. The amount that can be detected by our instrument is 0.1 × 10-gg of Hg, such that the detection limit is 0.1 ppb if 1 g of soil is used.
Inferred fracture zones: The distribution of radon, COe and Hg in soil gas, and Hg in soil, are shown in Figure 15. The values above the background concentration of each volatile component for the area are divided into three groups. Relatively high concentrations of each volatile gas correspond to the subsurface position of the known Honko vein system. In addition, high-volatile concentrations are also located in the eastern and western areas on the extension of the veins, and also to the north and south. In particular, all of the components are anomalously high and widely distributed in the west. Radon has a half life of 3.82 days for R n 222 and 54.5 seconds for T n (Rn22°), respectively. Therefore, high radon concentrations indicate the existence of highly permeable zones which permit a short period of migration from the deep Rn source; alternatively, the source is close to the surface. The CO2 concentrations in hot spring waters from the Hishikari mine are high; therefore, CO2 also appears to be a good indicator of fracture distribution. However, radon and C02 may be affected by groundwater due to their relatively high solubilities. Since Hg is very volatile, and its vapor pressure is sensitive to slight temperature variations, Hg in soil gas effectively indicates the location of permeable fractures connected with relatively high-temperature fluid. Although high concentration of Hg in soil is evidence for hydrothermal activity, if Hg in the corresponding soil gas is low, sealing of the fracture is indicated.
36
E. IZAWAET AL. w~o
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Fig. 15. Geochemical maps showing the distribution of radon (Rn + T n ) , CO2 and Hg in soil gas, and Hg in soil. The vein pattern was projected to the surface from 70 mL and 40 mL. The base line of the mine grid is parallel to the general strike of the veins (N50°E). Rn: the numbers with solid circles are total radon counts of three successive one-minute intervals. C02: the numbers with solid circles show volume percent of C02. -,
THE HISHIKARI GOLD DEPOSIT
wso
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38
E. IZAWAET AL.
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Fig. 16. Fracture zones inferred by geochemical prospecting using volatile components. Hatched areas are fracture zones determined from multiple anomalies of volatiles, and dotted areas by a single gas anomaly. The base line of the mine grid is parallel to the general strike of the veins (N50°E).
The hatched areas in Figure 16 show the location of fracture zones inferred from anomalously high concentrations of three or four volatile components. The deduced fracture zones agree with the known Honko vein system. The results also indicate that the Honko zone probably branches off to the westsouthwest and southwest at its western end. The wide shaded zone to the west indicates that fractures are common in this area. In fact, the recently discovered Yamada vein system is likely to be an extension of the southwest branch. GEOPHYSICAL EXPLORATION
In the Hishikari area it was shown during the early exploration stage that gravity and resistivity anomalies have a close relation to the mineralized zone (Fig. 2). The magnetic data from the airborne survey, however, showed no definite anomalies over the Hishikari ore zone (Johnson and Fujita, 1985) because the area is covered by volcanic rocks that are little altered and have relatively high magnetic susceptibilities.
THE HISHIKARI GOLD DEPOSIT
39
During the period 1982 to 1987 a comprehensive suite of geophysical exploration techniques was applied over the Hishikari vein system by SMM to clarify the extension of buried mineralization and to correlate the anomalies detected by various methods, and also to assess their applicability for similar mineral deposits. Structural models obtained from gravity, Schlumberger, controlled source audio frequency magnetoteUuric (CSAMT) and magnetotelluric (MT) data, and the distribution of hydrothermal alteration (conducting zone) indicated by CSAMT, transient electromagnetic (TEM) and induced polarization (IP) data are discussed below, and are compared with known geology.
Subsurface structure obtained from various geophysical surveys Gravity survey The reconnaissance gravity survey by MMAJ revealed a 4-reGal gravity high over and slightly northeast of the present Hishikari veins. The most probable source of this gravity high is the very shallow basement, which rises to 130 m elevation at its shallowest level (about 100 m below the surface). The gravity map by MMAJ (MITI, 1977) thus reflects the general feature of the subsurface structure at Hishikari, and was useful in targeting exploration. A further detailed gravity survey was carried out by SMM over and around the Hishikari area in order to elucidate the correlation between the gravity high and the mineralized zone, and to locate other small gravity highs. Preliminary results were described by Kawasaki et al. (1986). Using two La Coste & Romberg Model G gravimeters, 1230 observations were made at 200-300-m intervals along all the roads and paths over an area 18 km east-west by 24 km north-south. A part of the Bouguer anomaly map near the Hishikari mine is shown in Figure 17. The most conspicuous feature is the previously mentioned gravity high centered directly over the Hishikari deposit. This "Hishikari gravity high", bounded by the 12.5-mGal contour, is elongate southwestward and shows steep peripheral gradients toward the northwest and particularly toward the southeast. Except for this feature, the area is characterized by a relatively fiat gravity field, with contours of 0.5-mGal intervals generally trending east northeast-west southwest. The gravity low present in the northwest corner of Figure 17 is the southeast extension of the low anomaly of the Okuchi basin. The Shimanto Supergroup comprises sandstone and shale which have the highest density among rocks in the area (average wet density of drill core: 2.62 g/cm3). The overlying Hishikari Lower Andesites consist of abundant tuff breccias and a small amount of lava flows and have lower densities (average: 2.37 g/cm 3) than the basement rocks. Among the younger volcanic rocks fresh andesites have a comparable density with that of the basement rocks, and pyroclastic flow deposits have an average density of 2.25 g/cm ~. The density
40
E. IZAWAET AL.
Fig. 17. Bouguer anomaly map. Contour interval is 0.5 reGal for reduction density = 2.40 g / c m 3. Open circles show locations of gravity stations. The dotted area indicates the mineralized zone of the Hishikari deposit. The area is shown in Figure 3. Lines A-A', B-B' and C-C' are the locations of the sections shown in Figures 18 and 19.
of the entire volcanic pile thus averages 2.40 g/cm3; a density contrast of 0.20.25 g/cm ~ was used in the depth-to-basement analysis. The Hishikari gravity high infers local doming of the basement (the Shimanto Supergroup); numerical modelling by a three-dimensional analysis indicates that the vein system occurs where the basement rises to an elevation above 100 m (Kawasaki et al., 1986). This doming was confirmed by drilling and underground development. The elevation of the top of the Shimanto Supergroup outside of the Hishikari area, obtained from the gravity distribution, is generally - 5 0 0 m to below - 1 0 0 0 m, agreeing with known depths. It is concluded that the gravity distribution in the area primarily reflects the basem e n t topography.
THE HISHIKARI GOLD DEPOSIT
41
Resistivity structure The Schlumberger soundings were conducted over the area on two lines 9 km long running in a northeast direction at 1450-m spacing in order to analyze the subsurface structure (Yokoyama et al., 1987). A maximum electrode spacing of AB/2 = 2000 m was used, and readings were taken every 250 m; these were modified to AB/2 = 4000 m and 125-m intervals over the mineralized zone in the central part of the traverses. Figure 18 shows the results of the sections analyzed along line-A and line-B of Figure 17. In spite of the two lines being 1450 m apart, there is a close correlation between the two analyzed sections, demonstrating the continuation of geologic structure along the northeast direction. The areas can be basically interpreted as having a three-layer structure of high-low-high resistivity. The upper layer, with high values of 100-1000 m ohm-m, corresponds to the unaltered volcanic rocks. This layer is almost absent directly over the Hishikari veins, as seen around Station 18 of line-A, with the thickness increasing toward the northwest and southeast. The second layer generally has a low resistivity of 10-20 ohm-m, and can be interpreted as due to hydrothermally altered volcanic rocks consisting mainly of the Hishikari Lower Andesites and the lower part of the Shishimano Dacites. Extremely low values (2-8 ohm-m) observed around the vein system are due to intense hydrothermal alteration. There are relatively high-resistivity layers beneath the first layer away from the mineralized zone, in particular to the southeast. These probably represent a weakening in the intensity of alteration. The third layer of relatively high resistivity {80-150 ohm-m) corresponds to the Shimanto Supergroup. The top of the third layer is shallow over the mineralized zone, e.g., 130 m below the surface on line-A and 150 m on line-B. The depths increase gradually toward both ends of the lines to about 800 m below the surface at the northwestern ends. Toward the southeast the depths increase abruptly between stations 20 and 21, suggesting a fault-like structure, which is also inferred by topographic features running northeast to southwest. Another step down structure is seen further to the southeast, also corresponding to a topographic lineation which is interpreted as a fault {Fig. 3). In situ electrical logging shows that resistivity values of andesites and tuff breccia range from 125 to 250 ohm-m at shallow levels and decrease to about 20 ohm-m in the hydrothermally altered zone. Resistivity values of the basement range from 100 to 150 ohm-m for shales and from 200 to 300 ohm-m for sandstone. Although these values are not representative of the resistivity of rocks over the whole area, they are consistent with resistivity values obtained from the Schlumberger soundings. As stated above, the depth to the top of the third layer relates to that of the Shimanto Supergroup. This shows the usefulness of the Schlumberger sound-
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THE HISHIKARI GOLD DEPOSIT
43
ing to understand the subsurface structure in the Hishikari mine area, for which two-dimensional modeling of multi-layers is primarily applicable. In Figure 18 the profiles of the top of the basement obtained from threedimensional gravity analysis are superimposed on the Schlumberger cross sections. The profiles are similar to those of Schlumberger soundings; in particular, the two depth estimates coincide in the central part of line-B. There is, however, a large discrepancy to the southeast. The depth to the top of the basement obtained by the gravity survey might be too shallow as shown, because gravity data were analyzed on the assumption of the two-layer structure, neglecting the presence of the high-resistivity intermediate layer with presumably high density to the southeast. A high-resistivity zone ( > 500 ohm-m) is suggested at depths of more than 1000 m below the surface. Although the electrode spacing (AB/2) is too short to delineate the complete shape of this high-resistivity zone, the Schlumberger section provides the possibility for the presence of a shallow intrusive body or a thermally metamorphosed zone related to the intrusive rock. The C S A M T (controlled source audio-frequency magneto-telluric) survey is a rapid and relatively inexpensive method to determine the resistivity distribution at shallow to medium depths over an area, since only the receiver is moved from station to station once the current sources are set up. A Zonge GDP 12 system was used. The depth of penetration of the CSAMT survey is estimated to be about 800 m in this area, on the basis of the skin depth calculation and signal-to-noise ratio. The depth to the top of the resistivity basement obtained from one-dimensional analysis is in general similar to those of Schlumberger soundings. However, the lateral continuation of the resistivity layers is often not obvious, probably owing to static anomalies related to the presence of a thick layer with low resistivity near the surface. Thus the estimated depth to the basement did not agree with the actual depth in the central part of the Honko ore zone. A resistivity structure cross section (Fig. 19) along an approximately n o r t h south line in the western part of the survey area shows basically a three-layer structure of high-low-high resistivity, similar to that in the Honko ore zone described by Kawasaki et al. {1986). In Figure 19, the depth to the basement is about 400 m in the central part and becomes deeper (about 800 m) towards the north and south; in particular, it steepens abruptly to the south, as observed in the Honko ore zone. An M T (magnetotelluric) survey was carried out using a Beicip M T 5C system in order to know the large-scale resistivity structure at depth and to evaluate its applicability to gold exploration (Kawasaki et al., 1986). Since the survey area is densely inhabited and there are several high-voltage power lines, two sets of five-component data acquisition systems (reference and mobile
44
E. IZAWAET AL.
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THE HISHIKARI GOLD DEPOSIT
45
systems) were employed to increase the reliability of computed M T parameters. Thirty observation stations were scattered over the whole area as uniformly as possible. CSAMT measurements were also made at seven of the sites for comparison, and to obtain shallow information. The distribution of apparent resistivity at low frequencies of 0.01-0.1 Hz reflects the deep subsurface structure, which can not be obtained by other electrical and electromagnetic surveys. Figure 20 shows the distribution of apparent resistivity obtained by the transverse electric (TE) mode at 0.03 Hz. A wide-spread low-resistivity zone is present at depth to the southeast of the mineralized zone. A resistivity cross section (Fig. 21 ) was obtained by Bostic inversion using apparent resistivities at 0.03 to 200 Hz and apparent phase curves for nine measured points. This analysis reveals an overall feature of the vertical distribution of resistivity in this area. There are marked low-resistivity zones (a few ohm-m) at shallow levels which coincide with the zone of veins, and at deep levels (more than 10 km below the surface) that might correspond to a partially molten magma.
Delineation of near-surface alteration zones Resistivity distribution from high-frequency sounding data The resistivity structure of this area consists basically of three layers, highlow-high. The intermediate layer of low resistivity, which is located in the vicinity of mineralized zones, can be attributed to the hydrothermally altered rocks. It is thus important for delineation of mineralized zones to know the depth and the areal distribution of the low-resistivity layer. Kawasaki et al. (1986) reported the results of CSAMT and M T surveys. The distribution of apparent resistivity obtained by CSAMT at 128 Hz represents a depth of 200-300 m from the surface. A low-resistivity zone of less than 25 ohm-m is present in the Hishikari area, trending northeast (Fig. 13). In the western part of the low-resistivity zone, about 1 km southwest of the Honko area, the Yamada vein system was recently discovered. The distribution of apparent resistivity obtained by M T at higher frequencies (10-100 Hz) coincides well with that at similar frequencies of the CSAMT survey. The transient EM survey was carried out in the western part of the Hishikari area, and a distribution of low-resistivity ( < 5 ohm-m) anomalies similar to the results of CSAMT was obtained (Kawasaki et al., 1987). The low-resistivity zones at the shallow depth overlap the Hishikari gravity high, indicating that mineralization took place in the area of uplifted basement and was accompanied by intense argillic alteration at shallow levels.
Results of the IP survey As described above, the low-resistivity zone extends northeast to southwest, enveloping the gold-bearing quartz veins. In underground, wall rocks of quartz
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THE HISHIKARI GOLD DEPOSIT
47
veins are often impregnated with pyrite and thus IP anomalies may discriminate mineralized zones. Using a Scintrex IPR-11 system, a time-domain IP survey was conducted. A primary voltage Vp, a secondary voltage Vs, chargeability ( Vd Vp) of ten slices (Mo-Mg) after the current-off were observed. A Dipole-Dipole array, with basic electrode spacing (a= 50 m) and electrode separation (n= 1-6 and n= 27 ) was used. Twenty two lines of 42 line-km total were measured at every 50 m with a 300-800-m line interval.
Detailed apparent resistivity distribution of the shallow level was obtained by the IP survey, and the expanding Eltran configuration provided information of the deeper portion by increasing electrode separation (n). Figure 22 shows the distribution of apparent resistivity for n - 4, indicating a relatively shallow
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Fig. 22. Apparent D C resistivity contour m a p using dipole-dipole array with a = 50 m and n = 4 (in ohm-m).
48
E. IZAWA ET AL.
T "\
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Fig. 23. Apparent ehargeability contour map using dipole-dil~le array with a = 50 m and n = 4 (in mV/V). Chargeability of window M8 (1050-1410 ms) is shown. depth (around 125 m from the surface). A contour of 10 ohm-m surrounds the low-resistivity area which contains the ore zones. The area is elongate toward the southwest, as noted for other electrical and electromagnetic surveys. To the northeast, the low-resistivity zone terminates abruptly at line R, where elevations increase and less altered volcanic rocks cover the area. T h u s the eastern extension of the altered zone is not clear. The low-resistivity zone extends towards the southwest, though the width is narrow at line G, where elevations become higher. The gravity map (Fig. 17) shows a change in the pattern of contours across Sendai River, indicating a structural change coincident with the river. This implies that the southwestern end of the low-resistivity zone exists around line Q.
I P anomalies of this system are expressed by the chargeability in m V / V (decay voltage divided by primary voltage and duration). Figure 23 is a plan
rilE HISHIKARIGOLDDEPOSIT
49
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30ohm m
tO0
200
100
_J0
~
50
I00
--1
150
1 ~
2C0 m
3ohm-m
Fig. 24. IP pseudosection along a part of line C in Figure 23. Upper section, apparent resistivity in ohm-m; middle section, apparent chargeability of duration time 1050-1410 ms (in mV/V); lower section, resistivity structure derived from two-dimensional simulation analysis using finiteelement method, with the Honko vein system shown by cross hatching.
of the chargeability of the 9th slice (Ms) obtained for an electrode separation of n = 4. Although amplitudes of IP responses in this area are weak compared with those in areas of sulfide deposits, the trend of high chargeability is present over the Honko ore zone toward the southwest, in particular around E-, F- and
50
E. IZAWAET AL.
G-lines. The recently discovered Yamada ore zone exists near the high-chargeability area around E- and F-lines. The pseudosection of C-line (Fig. 24) shows the apparent resistivity, chargeability and resistivity structure obtained by the two-dimensional analysis. The Honko ore zone, which exists directly below point 15, is associated with the "pants legs" pattern, the typical downward flaring shaped IP anomaly, and shows low resistivity from the surface. The results of the IP survey suggest that the method is effective in prospecting for epithermal gold quartz veins. The high-chargeability zone within the low-resistivity area could be attributed to the presence of pyrite impregnation in the argillic alteration zone, and correlates well with the presence of quartz veins.
DISCUSSION AND CONCLUSIONS
Quaternary magmatism and gold mineralization Magmatism has migrated eastward in southern Kyushu from middle Miocene to the present (Izawa and Urashima, 1987). Andesitic volcanism in the Hishikari area, which comprises part of this magmatism, began in the early Quaternary and continued for more than one million years. During the waning stages of volcanism, felsic magma probably intruded to a shallow level, with a portion of the magma extruding as the Shishimano Dacites. The result of the MT survey suggests the presence of a partially molten magma at deep levels (Fig. 21 ), with a shallow intrusive body indicated by the Schlumberger sounding (Fig. 18 ). The magmatic intrusion-induced uplifting of the basement block caused high-angle fracturing of the basement sediments and overlying volcanic rocks. The associated heat anomaly generated the Hishikari hydrothermal system one million years ago. The extremely high-grade gold deposit in the Honko area at Hishikari formed near the unconformity between sedimentary rocks of the Shimanto Supergroup and andesitic volcanic rocks of Quaternary age (Fig. 8). Although the mechanism of gold deposition is not fully understood, mineralization was strongly affected by the presence of this unconformity (Ishihara et al., 1986; Ishihara and Morishita, 1987; Izawa, 1988). The presence of bladed quartz and large amounts of adularia, as well as evidence from fluid inclusions, suggest that boiling was common during mineralization of Hishikari. In several active geothermal fields, bladed calcite is observed in zones of boiling (Browne and Ellis, 1970; Keith and Muffler, 1978). Urashiriia (1956) described platy calcite filled with granular quartz and adularia aggregates in quartz veins of the Konomai epithermal gold veins in Hokkaido; he concluded that lamellar quartz was formed by the dissolution of the platy calcite. Thus the common presence of lamellar quartz in the Hishikari
THE HISHIKARI GOLD DEPOSIT
51
Honko veins indicates that bladed calcite was once deposited during boiling and subsequently dissolved during cooling, leaving the lamellar quartz. The occurrence of sericite in the wall rock adjacent to veins, in contrast to the dominance of adularia in the veins, is similar to that noted at many other epithermal deposits (Buchanan, 1981). This shift in K-silicate mineralogy supports the above suggestion that boiling was common in the veins, as gas loss favors adularia over sericite. Homogenization temperatures of fluid inclusions are plotted against elevation for Hishikari (Fig. 25). At the nearby active geothermal field of Kirishima, fluid inclusion temperatures show a wide range at each depth (Taguchi and Hayashi, 1983); the maximum temperatures at each depth are usually on or near the boiling-point curve of water, with fluid-inclusion temperatures rangElevation
Paleowater table
m
Paleodepth, m
,
-
,
0-
4O0 o
30O
aCI
1 O0
__~resentsu~ce ~o
200
200 . . . . .
o
® °~o
oo--ooo o: :: °.oo
o
o
0
oo
o
o o
o
300
~
o
,~
o
°
oo
•
o
!i__oi:i i °° * ~
°o
ooo
o o I
I
l
100
150
200
I~
o
500
,/ 250
Temperature,~C
Fig. 25. Fluid-inclusion temperatures versus elevation. Boiling-point curve for solution of 2 wt.% NaC1. Solid circles denote data for adularia at an elevation of 80 m.
52
E. IZAWA ET AL.
ing down to the present (subboiling) measured temperatures, thus recording cooling of that system. This relation between fluid-inclusion temperatures and the boiling-point curve may be applied to the Hishikari hydrothermal system. Assuming an average temperature of 243 ° C for adularia at 80 m elevation, the hydrothermal water table would have been at least 470 m in elevation if constrained by boiling relations for fluids of 2 wt.% NaC1. This implies that the depth of erosion ranges up to 240 m over the Honko vein system. The range of fluid-inclusion temperatures records cooling below boiling point temperatures, as at Kirishima. The cluster of fluid-inclusion temperatures around 200 ° C may reflect temperatures of gold mineralization. Visible electrum bands are often present between early adularia-quartz and the later smectite-quartz bands in the veins, and have extremely high gold values. This mineralogical change observed over a narrow band in the veins, and the distribution of fluid-inclusion temperatures, indicate that hydrothermal waters cooled from about 250 ° C to less than 200 ° C at the site of gold deposition around the unconformity. Since such rapid cooling can not be attributed to simple boiling, mixing with cooler groundwater is a possibility (Izawa, 1988). Mixing of ascending high-temperature fluids with low-temperature groundwaters, resulting in rapid cooling and oversaturation of silica relative to quartz solubility, is also supported by several lines of evidence. Fine aggregates of adularia and kaolinite occasionally occur in the quartz veins. Normally, adularia and kaolinite are separated by the sericite stability field. However, adularia-kaolinite can be a stable assemblage if the silica content of waters is near cristobalite-saturation over the temperature range of 150 to 200 ° C, i.e. supersaturated with respect to quartz (Izawa, 1985: calculation using thermodynamic data of Helgeson, 1969). Although the equilibrium relation is not conclusive on the basis of different thermodynamic data (for example, Helgeson et al., 1978), the coexistence of adularia and kaolinite without sericite may indicate a high silica concentration. Truscottite is a minor mineral in the Honko veins, as in other high-grade gold deposits. On the basis of synthetic experiments (Harker, 1964), it was suggested that truscottite forms from waters supersaturated with respect to quartz. Therefore, the presence of truscottite and the apparent equilibrium between adularia and kaolinite indicate that the ascending fluids were cooling very rapidly (due to rapid upflow and boiling as well as subsequent mixing with ground waters), such that quartz deposition could not maintain the silica concentration at quartz saturation. The presence of a large volume of low-temperature water may also be deduced from oxygen isotopic data of altered rocks. There are significantly large variations (up to 6%o ) in the oxygen isotopic values of altered volcanic rocks between 100 and 200 m elevation, with the heaviest values for the shallowest samples. Using the fractionation between water and quartz (Matsuhisa et al.,
T H E HISHIKARI GOLD DEPOSIT
53
1979), this variation would imply a temperature differential as large as 80 oC, which can not be explained by simple cooling of ascending fluids along the boiling-point curve (Fig. 25). Thus the shallow, relatively heavy oxygen isotopic compositions support the presence of a marginal water related to the argillic alteration. This water would likely have been steam-heated to temperatures of 100-150 oC, as noted in active geothermal systems (Hedenquist and Browne, 1989). In addition, whole-rock oxygen isotopic variations as large as 3%o over a few tens of meters laterally suggest a variable degree of water-rock interaction, as the lowest oxygen isotope compositions appear most closely related to fractures. This is consistent with large volumes of fluid flow in veins in contrast to the adjacent wall rock. The volcanic rocks contain magnetite and hematite, and the basement sedimentary rocks near the unconformity are hematite-bearing, the latter probably due to weathering at the sediment paleosurface. This indicates relatively oxidizing conditions as compared with the deeper basement environment, where pyrite and pyrrhotite occur with carbonaceous matter. The iron content of sphalerite shows a marked decrease (from 5 to < 0.02 mole % FeS ) with about a 100-m increase in elevation from 10 m. This decrease in FeS content of the sphalerite reflects an increase in the redox state of the fluid at higher elevations (probably due to mixing with steam-heated groundwater that equilibrated along the relatively oxidized unconformity). Deeply circulating waters, oxygen-shifted from water-rock interaction at depth, ascended along fractures; given the mineralogy of the system, gold would have been transported as a sulfide complex. The complicated structure of veins implies repeated fracturing (a sudden drop in pressure) and sealing. Therefore, gold deposition occurred following H2S loss due to boiling, as well as after rapid cooling and oxidation caused by mixing of the ascending fluids with cooler, oxidized (and probably steam-heated) groundwaters at the permeable zone around the unconformity. In the sedimentary rocks, the pH was buffered at close to neutral by the coexistence of potassium feldspar and sericite. The nearneutral pH is also indicated by the presence of calcite, laumontite and smectite in the high level alteration zone. This mechanism of gold deposition related to the unconformity will be clarified through a comparative study of the somewhat lower-grade gold mineralization hosted largely in the volcanic rocks of the Yamada area when mine development reaches that area.
Significance to exploration Gravity and resistivity anomalies are important in target selection in the Hokusatsu volcanic region of Kyushu. Subsurface structures can be recognized by a gravity survey using several observation points per kin2;upliftedbasement blocks represented by gravity highs are favorable to mineralization, probably because they focused fluid flow as discussed above. Following a gravity survey,
54
E. IZAWAET AL.
Schlumberger vertical soundings are useful to obtain the depth to the top of the basement and to know the approximate resistivity structure. Resistivity surveys and an IP survey can delineate the areal distribution of the subsurface hydrothermal alteration related to mineralization. Geochemical exploration using Hg, CO2 and radon in soil gas is effective in tracing the buried fracture zones. ACKNOWLEDGMENTS
We are grateful to the management of the Sumitomo Metal Mining Co. Ltd. for permission to publish this paper. Special thanks go to J.W. Hedenquist and R.W. Henley for their encouragement and helpful comments. E.I. also wishes to acknowledge the help of Y. Nakae with chemical analyses, Y. Nagae with XRD and XRF studies, and J. Nishioka and M. Saito with fluid-inclusion studies. REFERENCES Abe, I., Suzuki, H., Isogami, A. and Goto, T., 1986. Geology and development of the Hishikari mine. Min. Geol., 36:117-130 ( in Japanese, with English abstract). Aramaki, S., 1968. Geology of the Kakuto basin, southern Kyushu and the earthquake swarm from February, 1968. Bull. Earthquake Res. Inst., 46:1325-1343 (in Japanese with English abstract}. Browne, P.R.L. and Ellis, A.J., 1970. The Ohaki-Broadlands hydrothermal area, New Zealand: mineralogy and related geochemistry. Am. J. Sci., 269: 97-131. Buchanan, L.J., 1981. Precious metal deposits associated with volcanic environments in the southwest. In: W.R. Dickinson and W.D. Payne (Editors), Relations of Tectonics to Ore Deposits in the Southern Cordillera. Ariz. Geol. Soc. Dig., 14: 237-267. Fukuoka Bureau of International Trade and Industry, 1959. Mining Industry in Kyushu. Mine Safety Department, Fukuoka Bureau of International Trade and Industry, Fukuoka, 372 pp. (in Japanese }. Harker, R.I., 1964. Dehydration series in the system CaSiO3-SiO2-H20. J. Am. Ceram. Soc., 47: 521-529. Hedenquist, J.W. and Browne, P.R.L., 1989. The evolution of the Waiotapu geothermal system, New Zealand, based on the chemical and isotopic composition of its fluids, minerals and rocks. Geochim. Cosmochim. Acta, 53: 2235-2257. Helgeson, H.C., 1969. Thermodynamics of hydrothermal systems at elevated temperatures and pressures. Am. J. Sci., 267: 729-804. Helgeson, H.C., Delany, J.M., Nesbitt, H.W. and Bird, D.K., 1978. Summary and critique of the thermodynamic properties of rock forming minerals. Am. J. Sci., 278-A: 1-229. Ishihara, S. and Morishita, Y., 1987. Gold deposits in the Hokusatsu district, especially the Hishikari deposit: structural controls and genetic model. Geol. News, 389:7-20 (in Japanese ). Ishihara, S., Sakamaki, Y., Sasaki, A., Teraoka, Y. and Terashima, S., 1986. Role of the basement in the genesis of the Hishikari gold-quartz vein deposit, southern Kyushu, Japan. Min. Geol., 36: 495-509. Izawa, E., 1985. Alteration zoning and clay minerals in epithermal gold-silver deposits - examination of geothermal model. In: Y. Urashima {Editor), Gold-Silver Ores in Japan, no. 3. Min. Metall. Inst. Jpn., Tokyo, pp. 133-154 (in Japanese). Izawa, E., 1988. Science of exploration for gold. Kagaku (Science), 58:15-23 (in Japanese). Izawa, E. and Nakae, Y., 1983. Xonotlite, truscottite and gyrolite from gold veins in Kyushu.
THEHISHIKARIGOLDDEPOSIT
55
Abstr. Mineral. Soc. Japan, 1983 Annu. Meet., Kobe, p. 14 (in Japanese). Izawa, E. and Urashima, Y., 1983. Hydrothermal alterationon drillcores from the Hishikari gold deposit (abstract).Min. Geol., 33:50 (in Japanese). Izawa, E. and Urashima, Y., 1987. Geologic and tectonic setting of the epithermal gold and geothermal areas in Kyushu. In: Y. Urashima (Editor), Gold Deposits and Geothermal Fields in Kyushu. Soc. Min. Geol. Jpn., Guidebook, 2: 1-12. Johnson, I.M. and Fujita, M., 1985. The Hishikari gold deposit: an airborne E M discovery. C I M Bull.,78(no. 876): 61-66. Kawasaki, K., Okada, K. and Kubota, R., 1986. Geophysical surveys in the Hishikari mine area. Min. Geol., 36:131-147 (in Japanese, with English abstract). Kawasaki, K., Okada, K. and Matsuda, Y., 1987. T E M sounding survey in the Hishikari mine area. Proc. 77th Soc. Exploration Geophysicists of Japan Conference, 1987, Kyoto, pp. 198201 (in Japanese). Keith, T.E.C. and Muffler, L.J.P., 1978. Minerals produced during cooling and hydrothermal alteration of ash flow tufffrom Yellowstone drillhole Y-5. J. Volcanol. Geotherm. Res., 3: 373402. Kigoshi, K., Fukuoka, T. and Yokoyama, S., 1972. 14C age of Tsumaya pyroclastic flow, Aira caldera, southern Kyushu, Japan. J. Volcanol. Soc. Jpn., Second series, 17:1-8 (in Japanese, with English abstract). Koga, A. and Noda, T., 1974. Volatiles as geochemical indicators for geothermal prospecting. Oita Prefecture Hot Spring Reports, 25:48-52 (in Japanese). Koga, A. and Noda, T., 1975. Geochemical prospecting in vapor-dominated fields for geothermal exploration. Proc. of 2nd U.N. Symposium on the Development and Utilization of Geothermal Resources, vol. 3, San Francisco, CA, pp. 761-766. Kondoh, K., 1986. Exploration and development of Hishikari gold mine. Min. Geol., 36:1-9 (in Japanese ). Kubota, Y., 1986. Geological and geotectonic setting of gold-silver mineralization in the Hokusatsu district, southern Kyushu, Japan. Min. Geol., 36:459-474 (in Japanese, with English abstract). Matsubaya, O., Ueda, A., Kusakabe, M., Matsuhisa, Y., Sakai, H. and Sasaki, A., 1975. An isotopic study of the volcanoes and the hot springs in Satsuma Iwo-jima and some areas in Kyushu. Bull. Geol. Surv. Jpn., 26:375-392 (in Japanese, with English abstract). Matsuhisa, Y., Goldsmith, J.R. and Clayton, R.N., 1979. Oxygen isotopic fractionation in the system quartz-albite-anorthite-water. Geochim. Cosmochim. Acta, 43: 1131-1140. Matsuhisa, Y., Morishita, Y. and Sato, T., 1985. Oxygen and carbon isotope variations in goldbearing hydrothermal veins in the Kushikino mining area, southern Kyushu, Japan, Econ. Geol., 80: 283-293. MITI, 1976. Report of Basic Geological Survey of Gold Mine Area: Hokusatsu Area, 1975 fiscal year. Ministry of International Trade and Industry, Tokyo, 21 pp. (in Japanese). MITI, 1977. Report of Basic Geological Survey of Gold Mine Area: Hokusatsu Area, 1976 fiscal year. Ministry of International Trade and Industry, Tokyo, 47 pp. (in Japanese ). MITI, 1979. Report of Regional Geological Survey: Hokusatsu-Kushikino Area, 1978 fiscal year. Ministry of International Trade and Industry, Tokyo, 92 pp. (in Japanese). MITI, 1982. Report of Regional Geological Survey: Hokusatsu-Kushikino Area, 1981 fiscal year. Ministry of International Trade and Industry, Tokyo, 81 pp. (in Japanese). MITI, 1983. Report of Regional Geological Survey: Hokusatsu-Kushikino Area, 1982 fiscal year. Ministry of International Trade and Industry, Tokyo, 65 pp. (in Japanese). MITI, 1985. Report of Regional Geological Survey: Nansatsu Area, 1984 fiscal year. Ministry of International Trade and Industry, Tokyo, 180 pp. (in Japanese). MITI, 1988. Report of Regional Geological Structure Survey: Hokusatsu-Kushikino Area, 1987 fiscal year. Ministry of International Trade and Industry, Tokyo, 69 pp. (in Japanese). Miyachi, M., 1983. Contribution of zircon fission-track ages to stratigraphic studies in southern Kyushu, Japan. J. Jpn. Assoc. Mineral. Petrol. Econ. Geol., 78: 170-181. Miyachi, M., 1987. Correlations of the pyroclastic flow deposits in southern Kyushu, Japan. Re-
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ports Earth Science, College General Education, Kyushu Univ., 25:9-38 (in Japanese, with English abstract). M M A J , 1979. Report of Development and Survey of Exploration Techniques for Mineral Resources, 1978 fiscalyear: (Airborne Electromagnetic Survey in the Hokusatsu Area). Metal Mining Agency of Japan, Tokyo, 122 pp. (in Japanese). MMAJ, 1988. Report of Survey for Rare Metal Resources, 1987 fiscal year: (Development of Exploration Techniques of Floating Trace Metals). Metal Mining Agency of Japan, Tokyo, 195 pp. (in Japanese). MMAJ and SMM, 1987. Discovery and development of Hishikari mine. Mining Geol., 37: 227236 (in Japanese, with English abstract). Nakagawa, S., Kuriyama, T. and Sakaguchi, K., 1985. Geothermal system in the west Kirishima geothermal area, Kagoshima Prefecture. J. Geotherm. Res. Soc. Jpn., 7:329-343 (in Japanese, with English abstract). Nishizawa, N. and Ibaraki, 1985. Exploration of the Hishikari mine. In: Y. Urashima (Editor), Gold-Silver Ores in Japan, no. 3. Min. Metall. Inst. Jpn., Tokyo, pp. 1-17 (in Japanese). Okumura, K., Teraoka, Y. and Sugiyama, Y., 1985. Geology of the Kamae district: Quadrangle series, scale 1:50,000. Geol. Surv. Jpn., 58 pp. (in Japanese, with English abstract). Stoker, A.K. and Kruger, P., 1975. Radon in geothermal reservoirs. Proc. of 2nd U. N. Symposium on the Development and Utilization of Geothermal Resources, vol. 3, San Francisco, CA, pp. 1797-1804. Taguchi, S. and Hayashi, M., 1983. Past and present subsurface thermal structures of the Kirishima geothermal area, Japan. Geotherm. Res. Council Trans., 7: 199-203. Takaoka, H., 1987. Final report on the case study of oxygen isotopic exploration (Hishikari and Urushi-Ora areas). Tech. Rep. Mines Resour. Div. SMM, 4:10-17 (in Japanese). Taylor, H.P., Jr., 1968. The oxygen isotope geochemistry of igneous rocks. Contrib. Mineral. Petrol., 19: 1-71. Tsuyuki, T., 1969. Geological study of hot springs in Kyushu, Japan (5): some hot springs in the Kagoshima graben, with special reference to thermal water reservoir. Mem. Fac. Sci., Kagoshima Univ. (Geol. and Biol. ), 2:85-101 (in Japanese, with English abstract). Ueno, H., Nedachi, M., Urashima, Y., Ibaragi, K. and Suzuki, R., 1987. Paleomagnetism of the Hishikari gold deposits; preliminary report. Rock Magnetism and Paleogeophysics, Japanese National Committee for the Dynamics and. Evolution of the Lithosphere Project, 14: 31-35. Urashima, Y., 1956. "Bosa" quartz veins, especially the fine-grained quartz aggregates, of the Konomai mine in Hokkaido, Japan. J. Fac. Sci., Hokkaido Univ., Ser. IV, 9: 371-380. Urashima, Y., Ibaraki, K. and Suzuki, R., 1987. The Hishikari gold-silver deposit. In: Y. Urashima (Editor), Gold Deposits and Geothermal Fields in Kyushu. Soc. Min. Geol. Jpn., Guidebook, 2: 27-38. Urashima, Y. and Ikeda, T., 1987. K-Ar ages for adularia from the Fuke, Okuchi, Hishikari, Kuronita, and Hanakago gold-silver deposits, Kagoshima Prefecture, Japan. Min. Geol., 37: 205213 (in Japanese, with English abstract ). Urashima, Y. and Izawa, E., 1982. Hydrothermal alteration observed in exploration drill cores of the Hishikari gold deposit. Autumn Meeting of Mining Metal. Inst. Japan, 1982, Proc., G: 1316 (in Japanese). Urashima, Y. and Izawa, E., 1983. Ores from drill-cores of the Hishikari gold deposit (abstract). Min. Geol., 33:50 (in Japanese). Urashima, Y. and Nedachi, M., 1986. Electrum from the Hishikari ore deposit (abstract). 1986 Annual Meeting Japanese Assoc. Mineral. Petrol. Econ. Geol., Mineral. Soc. Japan and Soc. Min. Geol. Japan, p. 43 (in Japanese). Whitehead, N.E., 1981. A test of radon ground measurements as a geothermal prospecting tool in New Zealand. N. Z. J. Sci., 24: 59-64. Yokoyama, T., Takagi, M. and Takahashi, N., 1987. Subsurface structure of the Hishikari gold mine - comparison of Schlumberger and other geophysical surveys. Proc. 77th Soc. Exploration Geophysicists of Japan Conference, 1987, Kyoto, pp. 194-197 (in Japanese ).
1
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
The Hishikari gold deposit: high-grade epithermal veins in Quaternary volcanics of southern Kyushu, Japan EIJI IZAWA1, YUKITOSHI URASHIMA2, KENZO IBARAKI3, RYOICHI SUZUKI3, TAKE0 YOKOYAMA3, KIYOSHI KAWASAKI3, AKITO KOGA4 and SACHIHIR0 TAGUCHI5
1Department of Mining, Kyushu University, Higashiku, Fukuoka 812, Japan 2Department of Environmental Science, Kagoshima University, Japan 3Sumitomo Metal Mining Co. Ltd., Japan 4Geothermal Research Centre, Kyushu University, Fukuoka, Japan ~Department of Earth Science, Fukuoka University, Fukuoka, Japan (Received March 31, 1989; revised and accepted July 13, 1989)
ABSTRACT Izawa, E., Urashima Y., Ibaraki, K., Suzuki, R., Yokoyama, T., Kawasaki, K., Koga, A. and Taguchi, S., 1990. The Hishikari gold deposit: high-grade epithermal veins in Quaternary volcanics of southern Kyushu, Japan. In: J.W. Hedenquist, N.C. White and G. Siddeley (Editors), Epithermal Gold Mineralization of the Circum-Pacific: Geology, Geochemistry, Origin and Exploration, II. J. Geochem. Explor., 36: 1-56. The Hishikari epithermal gold-silver deposit is located in northeastern Kagoshima Prefecture, Kyushu, Japan. Geological and geophysical surveys played important roles in the discovery, made in 1981. Subsequent mine development has proved Hishikari to be one of the major gold deposits in the western Pacific. The production from July, 1985, to December, 1988, totaled 21.7 metric tons of gold and 14.3 metric tons of silver. Ore reserves are estimated to be 1.4 million metric tons at an average grade of 70 g/metric ton gold (98 metric tons of contained gold) in the Honko ore zone and approximately 2 million metric tons at 20 to 25 g/metric ton gold in the newly discovered Vamada ore zone. The geology of the mine area is composed of basement sediments of the Cretaceous Shimanto Supergroup, and volcanic rocks of Quaternary age. The deposit is of the quartz-adularia vein type, with associated electrum, naumannite-aguilarite, pyrargyrite and smectite. K-Ar age dating of adularia bearing ore indicates a Pleistocene age of 0.84 _+0.07 to 1.01 _+0.08 Ma for gold mineralization. Fluid-inclusionstudies indicate that the representative temperature of gold deposition was 210 ° C in the basement and less than 200 ° C in the overlying volcanic rocks. J~s0 values of quartz veins range from + 8.8 to + 6.8%o. Chlorite and sericite alteration is directly associated with high-grade gold mineralization. Alteration zones of interstratified clay minerals and quartz-smectite envelope the mineralized center and form a near-horizontal layer of intense argillization, located 50-100 m above the Hishikari vein system. These alteration zones are surrounded by a zone of cristobalite-smectite and essentially unaltered rocks. Subsurface structures were recognized by a detailed gravity survey, with uplifted basement blocks
0375-6742/90/$03.50
© 1990 Elsevier Science Publishers B.V.
2
E. IZAWAET AL.
represented by gravity highs. Schlumberger vertical soundings are useful in estimating the depth to the basement and to identify the approximate resistivity structure, while resistivity and IP surveys define the areal distribution of the hydrothermal alteration related to mineralization. Geochemical exploration using Hg, C02 and radon in soil gas has been effective in tracing fracture zones. The extremely high-grade gold mineralization is focused near the unconformity between basement sediments and overlying volcanic rocks of the Honko area. The high grades of gold may be explained as a combination of two processes. As the higher-temperature fluids ascended, boiling resulted in gold deposition; further mineralization was favored by subsequent mixing of the deep fluids with steam-heated groundwaters near the unconformity, causing rapid cooling and oxidation.
INTRODUCTION
The recent development of the Hishikari mine following the discovery of gold veins in 1981 has proven Hishikari to be one of the major gold deposits in the Western Pacific. Geologic and geophysical surveys played important roles in the discovery of the rich veins beneath an old, small mining site (MITI, 1982). Initial mineralogical and fluid-inclusion studies using core samples were made by Urashima and Izawa (1982). There was a subsequent detailed description of the veins by Abe et al. (1986), and the geophysical surveys were discussed by Kawasaki et al. (1986); these studies were conducted during the early state of mine development. The regional geology has been newly studied by the Sumitomo Metal Mining Co., Ltd. (SMM), the owner of the mine, with a portion of the results reported by MMAJ and SMM (1987) and Urashima et al. (1987). These studies and subsequent regional mapping and geophysical studies by SMM, studies on hydrothermal alteration by SMM and E. Izawa and geochemical studies of soil gas by A. Koga and S. Taguchi are incorporated in this paper. The objective of this paper is to review the studies previously published in Japanese, and to update the geological, geophysical and geochemical database for future reference in the exploration of epithermal gold veins in young volcanic terrain.
Discovery and development Location The Hishikari mine (32°00'N, 130°41'E) is located in the northeastern part of the Hokusatsu district, Kagoshima Prefecture, Kyushu, about 60 km north of Kagoshima city (Fig. 1 ). The area is immediately west of the Kakuto caldera, part of the Kagoshima Graben in which the active Kirishima volcano is located. The portal of the mine is at 265 m above sea level. The topography, ranging from 200 to 600 m in elevation, comprises densely timbered, hilly terrain separated by small valleys cultivated mainly with rice. Temperatures range
THE HISHIKARIGOLDDEPOSIT
3
0 I
i
~ 1
~ 2
."'"'l]3
L;,4 /-,
" 5
• 6
X +7
Fig. 1. Location and structural map of the Hokusatsu district. 1 = outcrops of the Shimanto Supergroup; 2 = Holocene volcanic rocks; 3 = caldera and basin, 0--Okuchi basin, K= Kakuto caldera, and A = Aira caldera; 4, Kagoshima graben (Tsuyuki, 1969); 5-- volcanic centers; 6 = gold deposits; 7--deep drill holes with elevation of the top of the Shimanto Supergroup (parenthesis indicates that the drill hole did not reach the basement), data from Aramaki (1968), Kubota (1986), MITI (1988) and Ikeda (pers. commun., 1988 and 1989). The area in the square is shown in Figure 3.
from a low of - 7 o C to a high of 35 ° C. Rainfall is rather high, averaging 2500 m m per annum.
History The first prospecting record in the Hishikari mine area dates back to 1750 (Kondoh, 1986). Since then, exploration and small-scale mining (limited to above 230 m elevation) has been intermittent. In 1903, near-surface exploitation began on three gold-bearing quartz-calcite-clay veins (Table 1 ). The veins are located 100 m directly above the top of the present Hishikari veins ( H o n k o veins), and are now interpreted to be an upper manifestation of mineralization at Hishikari. A crosscut was driven in 1933 about 180 m in length at an elevation of 230 m, using explosives and dewatering pumps. Ore was shipped to
4
E. IZAWA ET AL.
TABLE
I
Near-surface veins of the old Hishikari-yamada mine, worked in the early 1900's (Fukuoka Bureau of InternationalTrade and Industry, 1959 ) Vein
Strike
Dip
Width
Grade
Remarks
(m) No. 1 No. 2 No. 3
N60°E N60°E N58°E
80°NW 80°NW 90 °
1.0 0.6 0.5-1.0
Au (g/t)
Ag (g/t)
30.3 18-32 21.0
26.2 5.0
"Moso" vein, main working Calcite-clay vein "Baka" vein, calcite-rich quartz vein
the Saganoseki copper smelter 170 km from the mine. The maximum grade of this ore was as high as 130 g/metric ton gold. In 1943 a plan of downward development was suspended owing to intensification of World War II. The crosscut of the 230 m level was re-opened in 1952, with about 40 m of drifting on the No. 3 vein; however, the results were discouraging (Nishizawa and Ibaraki, 1985 ). In 1973 the Taio Mining Co., a subsidiary of the Sumitomo Metal Mining Co., Ltd. (SMM), acquired the mineral rights.
Discovery Various geologists of the SMM group pointed out the possibility of concealed gold deposits underneath young volcanic rocks in the Hokusatsu district and recommended an exploration program using advanced techniques (Ikeda, 1952, 1968; Kobayashi et al., 1972; Nishizawa et al., 1973: all are unpublished company reports). However, no work was actually conducted until a systematic exploration program by the Metallic Minerals Exploration Promoting Agency of Japan, now the Metal Mining Agency of Japan (MMAJ), started in 1975. Between 1975 and 1978, regional geological mapping and reconnaissance geophysical surveys of the Hokusatsu district were carried out by MMAJ. One of the important conclusions from this survey was that most of the gold veins in the Hokusatsu district are hosted in propylitically altered andesites, which at that time were assumed to be "older andesites" of Miocene age and named the Hokusatsu Older Andesites (MITI, 1977, 1979). Later it was shown that the altered andesites actually have various ages. In 1975 and 1976, gravity surveys were made with 566 stations at about 1km intervals along roads, including 380 km ~ of the Hokusatsu district. Small gravity highs were identified over most of the known gold deposits of the district. Taking into account the regional geology, the anomaly was interpreted to principally reflect an uplifted mass of propylitized andesites and in part basement rocks, which are denser than the young volcanic rocks (MITI, 1976,
THE HISHIKARI GOLD DEPOSIT
5
25~
I00~
J
%
I0f
~
Legend 10
25-_I i
1 j
2
J
3
4
o 0
500 |
5
1000 m i
Fig. 2. Relationship between Bouguer gravity contours and resistivitydata. Localities of the old tunnel, as well as M M A J drillholes, are indicated. 1 =Bouguer gravity contour, reGal (MITI, 1977 );2 = Schlumberger resistivity,o h m - m (MITI, 1979 );3 = airborne E M anomalies, conductor extent, > 1.1 mhos ( M M A J , 1979 );4 = old Hishikari-yamada mine tunnel; 5 = M M A J drillholes.
1977). The gravity survey revealed a northeast-trending gravity high of 4 reGal amplitude in the Hishikari area (MITI, 1976). In 1978, electrical sounding (Schlumberger array) and heliborne electromagnetic (EM) methods were used over the Okuchi-Hishikari area. Both detected a low-resistivity zone over Hishikari which resembled that of the nearby Okuchi mine (MITI, 1979; MMAJ, 1979; Johnson and Fujita, 1985). In the Schlumberger resistivity section, a low-resistivity zone (3-7 ohm-m) at shallow depths and a high-resistivity zone ( > 100 ohm-m) at a depth of 200 m were interpreted as reflecting the hydrothermally altered Hokusatsu Older Andesites and an intrusive rock, respectively (MITI, 1979). In 1980, scout drilling was recommended to explore a deep target below the abandoned old tunnel, due to the gravity high anomaly overlapping low-resistivity anomalies from EM and Schlumberger data over the altered andesites
E. IZAWAET AL.
6 TABLE 2 Assay value for drill core samples of MMAJ (MITI, 1982) Drill hole
Interval (m)
Length (m)
Au (g/t)
Ag (g/t)
55MAHT-5
291.70-291.85
0.15
290.3
167.0
56MAHT-1
465.25-466.00 476.35-476.60
0.75 0.25
102.0 149.7
50.3 52.0
56MAHT-2
241.68-242.90 261.40-265.15 277.65-283.10 301.75-302.50
1.22 3.75 5.45 0.75
63.7 69.9 220.43 44.7
44.0 52.8 57.6 26.3
(Fig. 2 ). The drilling (55MAHT-5) started in late 1980; in February, 1981, a quartz vein was intersected at 291 m drilled depth which assayed 290.3 g/metric ton gold and 167 g/metric ton silver over 15 cm. The most interesting fact was that the quartz vein occurred in shale of the Shimanto Supergroup, which had not been considered as a common host rock for gold deposits in the Hokusatsu district. The next two holes, drilled from August to October, 1981, were located about 100 m east and 400 m west of the discovery hole, respectively, and also encountered high-grade gold veins (MITI, 1982). Table 2 shows the assay results of these three MMAJ drill holes.
Surface drilling program by SMM With MMAJ's consent, SMM commenced a follow-up drilling program in late 1981, to confirm the results of MMAJ scout drilling; these preliminary results indicated that gold-bearing quartz veins occurred over an area of 800 by 200 m, striking N70 ° to 80°E and dipping steeply north. Within a year SMM completed 18 holes on 8 panels totaling 6870 m. All the holes intersected gold mineralization, comparable with the MMAJ findings. The results indicated that gold veins were present over a minimum strike of 700 m, with a vertical range of 100 m. The reserves were then estimated to contain about 120 metric tons gold with an average grade of 80 g/metric ton gold between - 20 and + 130 m in elevation. SMM subsequently made a quick decision to conduct underground exploration, starting in January, 1983.
Development Hot groundwaters were a major obstacle to development. Surface drilling revealed that a volume of hot water with a temperature of 60-65 ° C was present within the vein system. The original static water level was 200 m above sea level when pumping started at the 100 m level in May, 1984. By the end of July, 1988, the hot water table had been lowered to 20 m above sea level. Under-
THE HISHIKARIGOLDDEPOSIT
'I
TABLE 3 Compositionof hot waters from the Hishikari mine (an average of waters from 8 drill holes on June 18, 1988)
T (°C)
pH
K (mg/1)
Na (mg/l)
Ca (mg/1)
Si02 (mg/1)
Cl (mg/1)
SO4 (mg/l)
HC03 (mg/l)
63.4
6.5
28
650
86
114
460
68
1,180
ground HQ size drill holes are used for dewatering; pumping stations are established at the 10 m level in the No. 2 inclined shaft, with hot water pumped up through the ventilation shaft. At present 9.5 m3/min of water ( including 1.2 m3/min of used mining water ) are pumped; 3 m3/min are delivered to the local hot spring spa at Yunoo, 4 km southwest of the mine. The remainder is cooled, treated and discharged to the river. A typical analysis of water is presented in Table 3. Tritium dating ( 1.3 + 0.6 to 1.5 + 0.3 Tritium Unit ) indicates that hot water in the current vein system is old, circulated meteoric water. Underground exploration commenced in January, 1983, with the driving of two parallel inclined shafts at an inclination of - 1 7 % (4.8 m wide by 3.8 m high), some 15-20 m apart. Pilot drilling was undertaken from the beginning of the development to determine the existence of water in fissures, cracks and voids. When a water-bearing zone is encountered, the zone is grouted with cement under pressure. The first crosscut intersected the Ryosen No. 2 vein at the 100 m level on July 13, 1985. Main levels have been developed at 100 m, 70 m and 40 m elevation. The No. 1 incline of 2.0 km length has reached - 50 m elevation. The total length of tunneling is 26 kin, including 10 km of drifting as of March, 1989. A trackless operation is used, with diesel-powered mobile machines. Although the mine is still at the exploration stage, it produces 350-400 metric tons per day of ore from drifting (and small-scale test mining); the total production from July, 1985 to December, 1988, has been 21.7 metric ton of gold and 14.3 metric ton of silver. The bulk of the ore is crushed and delivered to the S M M copper smelter at Niihama by truck and ship for use as silica flux. Ore reserves (at the end of 1988) are 1.4 million metric tons of 70.5 g/metric ton gold and 49.0 g/metric ton silver (98 metric tons of gold and 68 metric tons of silver contained) in the Honko ore zone. From September, 1987, to May, 1988, twenty seven drill holes totaling 11,477 m revealed a new vein system in the Yamada area, about 1 km southwest of the known main Hishikari area (the Honko area). The newly discovered ore reserves are estimated to be more than 2 million metric tons with average grade of 20-25 g/metric ton gold and 12-15 g/metric ton silver.
8
E. IZAWA ET AL.
GEOLOGY Principal rock units and structure
Regional geologic mapping and complementary age dating and geochemical ( 7
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THE HISHIKARI GOLD DEPOSIT
9
A
B His~ikori Mine
500I~.OOl--
_lsoo
I00 I-
0;
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o
lpoo
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LEGEND Alluvium and Kakuto Group Pyroclastic flows
I
Hishikari Upper Andesites Shishimano Dacites Hishikari Middle Andesites O
rr-
Kurozonsan Dacites
Dacitic pyroclastics Welded tuff Hypersthene-augite andesite Andesitic pyroclastics and volcanic conglomerate
Hishikari Lower Andesites Shimanto Supergroup
Acidic pumice flow and welded tuff Hypersthene-augite andesite Andesitic pyroclastics Biotite-hornblende dacite Dacitic pyroclastics Hypersthene-augite andesite Andesitic pyroclastics Hypersthene-augite bearing biotite-hornblende dacite
E
Cretaceous
I
m
Shale and sandstone
i
Quartz vein Fault (assumed)
Fig. 3. Geologic map of t h e Hishikari area a n d cross section along A-B. Square is area of Figures 17, 20, 22 a n d 23. Locations of samples with K-Ar ages are shown as closed circles.
studies on volcanic rocks in the mine area has continued with underground exploration and development. The petrographic features of volcanic rocks in the area are similar, often making it difficult to distinguish between units with different ages. The studies of K-Ar age dating and chemical compositions as well as paleomagnetic study on the volcanic rocks have provided an important basis for geologic mapping. Geologically, the mine area consists of the Shimanto Supergroup of Cretaceous age and volcanic rocks and alluvial deposits of Quaternary age (Fig. 3 ).
The Shimanto Supergroup The Shimanto Supergroup is not exposed in the vicinity of the mine, as it is generally present at an elevation less than - 4 0 0 m in the surrounding area (Fig. 1). Drilling and underground exploration, however, have revealed that
10
E. IZAWA ET AL.
the Shimanto Supergroup rises from 130 m to 0 m elevation (about 100-200 m below the surface) in the central part of the Hishikari deposit. It comprises shale, sandstone and their alternations, accompanied by a minor amount of tuffaceous shale and chert. Sandstone sometimes contains small fragments of shale, and lenticular fragments of sandstone often occur in shale. The inner structure of this unit is complex and it may be regarded a kind of slump deposit. Although not yet confirmed by fossil evidence, this unit is assigned to the Saiki Subgroup of the Morotsuka Group of Okumura et al. (1985) on the basis of the quartz-feldspar-rock fragment ratio (Ishihara et al., 1986 ). The Kawanabe Group of the Shimanto Belt in the Nansatsu district of southern Kyushu is middle to upper Cretaceous age (MITI, 1985), and is correlative with the Morotsuka Group. The strata of the Kawanabe Group and the Morotsuka Group were subjected to low-grade regional metamorphism (generally metamorphosed to the prehnite-pumpellyite facies). The shale and sandstone in the Hishikari mine consist mainly of quartz, albite, Fe-chlorite and sericite with minor calcite, pyrite and carbonaceous matter. This unit is unconformably overlain by the Hishikari Lower Andesites. Shale and sandstone up to 20 m below the unconformity are often reddish brown in color, owing to the presence of hematite. Abe et al. (1986) interpreted this hematite as due to weathering at the paleosurface prior to volcanism. About 60% of minable ore in the Honko area is hosted in sedimentary rocks of the Shimanto Supergroup. Hydrothermal alteration of this unit is seen only at the contact zone ( < 1 m wide) with quartz veins, where the rocks have a characteristic pale green color owing to chloritization and sericitization. Pyritization consisting of disseminated, euhedral pyrite along fractures is also common, and silicification is locally present, in particular near quartz veinlets.
The Quaternary system The Quaternary rocks in the mine consist of the Hishikari Lower Andesites, the Kurozonsan Dacites, the Hishikari Middle Andesites, the Shishimano Dacites and the Hishikari Upper Andesites, in chronological order of decreasing age. Representative chemical compositions are shown in Table 4. The western and southwestern part of the area is covered by younger pyroclastic flow deposits and alluvial deposits. These volcanic rocks are calc-alkaline and were subaerially deposited; their magnetic susceptibility shows typical magnetiteseries values (Table 4; see also Ishihara et al., 1986). The Hishikari Lower Andesites crop out near the mine and in the northern part of the area. Hypersthene-augite andesite lava flows predominate in the upper part of the sequence, and andesitic pyroclastic rocks with intercalated thin lava flows occur in the lower part; lacustrine sediments are locally present. The lavas contain phenocrysts of plagioclase (An45-6o), augite and hypersthene in order of abundance, which occur in a groundmass consisting of pale
THE HISHIKARI GOLD DEPOSIT
11
TABLE 4
Chemical composition of principal rock units Rock unit
Hishikari Lower Andesite
Sample no.
27645'
(wt.%) SiO2 Ti02 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P20s H20+ H20 Total
61.9 0.62 16.21 6.91 a 0.12 2.47 4.33 2.42 2.35 0.02 2.29 b
-
(ppm) S Cl Be Rb Sr Ba Zr V Cr Co Ni Cu Zn Pb As Sb Ag (ppb) Au Hg
99.64
I29102
62.75 0.65 16.27 6.49 a 0.11 2.46 5.59 3.13 2.23 0.11 0.80 0.32 100.91
Kurozonsan Dacite
Hishikari Middle Andesite
Shishimano Daeite
Hishikari Upper Andesite
40020'
24983'
24972'
27681'
71.1 0.43 14.25 3.22 a 0.16 0.62 2.61 3.55 3.19 0.16 0.77 b 101.06
80 70 <0.5 174 355 135 36 11 9 35 53 12 2 0.1 0.4
69.71 0.41 14.67 3.1Y
61.4 0.54 16.56 7.26 ~
0.09 0.62 2.63 3.85 2.98 0.10 0.65 0.17 99.03
0.06 2.98 5.97 3.13 2.20 0.54 0.82 b
67 < 10
225 470 26 25 3 4 10 43 10 3 0.4 <0.2
57 < 10
98.77
289 350 177 100 16 11 41 54 6 2 0.2 0.4
20 25 3 < 1 15 29 20 3 0.2 <0.2
< 1 20 372
70.2 0.30 15.18 1.93 ~ <0.01 0.32 2.04 3.70 3.81 0.05 1.23 b
<0.5 171 625
<0.5 121 261 620 170
< 1 50 706
101.46
90 340 1.5
75 335 260 140
16 30
x ( e m u / g × 1 0 -~)
K25252
I29082
69.68 0.35 15.54 1.78 0.33 0.05 0.34 2.15 3.38 3.34 0.04 1.46 0.46 98.93
80 120 155 261 590 210 38 < 10
8 20
60.3 0.56 15.53 7.21 a 0.13 3.16 6.51 2.66 2.09 0.07 0.84 b 99.06
<0.5 265 335 160 45 15 14 81 53 16 3 0.2 0.4 < 1 50
321
a = Total Fe as Fe2Oa, b = l o s s on ignition, ------not determined, Z = magnetic susceptibility. ' W < 10 ppm, B i < 2 ppm, M o < 1 p p m except for no. 40020 (5 p p m ) ; Au, As, Sb and Hg by NAA, and other elements by ICP at Chemex Lab. Ltd. 2XRF, except for major elements of I2908 (wet chemical analysis), at K y u s h u University. 27645 = N of mine; I2910 = E of mine; 40020 = N of Kurozonsan; K2525 -- W of Kurozonsan; 24983 = SW of Yoshimatsu; 24972 = E of mine; I2908 = S of mine; 27681 = E of Uono.
12
E. IZAWA E T AL.
brown glass, scattered plagioclase laths and granules of augite and hypersthene. The andesitic pyroclastic rocks range from fine tufts to volcanic breccias and are poorly sorted. Fragments are andesitic volcanic rocks, except for just above the unconformity, where fragments sometimes consist of shale and a small amount of sandstone. There are several thin layers ofpaleosol containing hematite in the sequence. They sometimes contain charcoal of wood trunks and carbonaceous material derived from plants. About 40% of ruinable ore in the Honko area occurs within this unit. In the vicinity of the mine the volcanic rocks have been argillized or altered to green colored rocks (due to formation of smectite or chlorite). The Kurozonsan Dacites overlie the Hishikari Lower Andesites in the northern part of the area. They are composed of hypersthene-augite-bearing biotitehornblende dacite lava flows, and a minor amount of welded tuff and pumice tuff at the base of the sequence. The texture of dacite is variable but typically is characterized by relatively abundant feldspar phenocrysts; flow banding is rare. Phenocrysts consist of plagioclase (An35_55), hornblende, hypersthene, augite and biotite. Under the microscope the groundmass consists of glass, and scattered plagioclase laths, hornblende and biotite. The Hishikari Middle Andesites are restricted in distribution to the southern and southeastern part of the mine area, where they appear to directly overlie the Hishikari Lower Andesites and are overlain by the Shishimano Dacites. They are composed mainly of hypersthene-augite andesite lava flows and their pyroclastic rocks. Phenocrysts are plagioclase (An4~_~o), augite, hypersthene and olivine. Groundmass consists of glass, plagioclase laths and granules of augite and hypersthene. The Shishimano Dacites crop out to the southeast of the mine, forming a remnant of volcanic topography. The unit consists of biotite-hornblende dacite lavas with their pyroclastic equivalent at its base. The dacite is typically porphyritic with phenocrysts of plagioclase (An3o_35), hornblende and biotite in a glassy groundmass. In some places it is aphyric and has flow banding. This unit and the Kurozonsan Dacites once were collectively named the Kurozonsan Rhyolites, and their emplacement was thought to be post mineralization (MITI, 1977, 1979). However, it has been recognized that gold-bearing quartz veinlets are present in the southwestern part of the unit where the rocks are argillized. In addition, more differentiated characteristics of mineralogy and rock chemistry, as well as younger K-Ar ages, clarified that the Shishimano Dacites should be separated from the Kurozonsan Dacites. The Hishikari Upper Andesites occur 2 km northeast of the mine, where they directly overlie the Hishikari Lower Andesites, and 4 km southeast of the mine,
THE HISHIKARI GOLD DEPOSIT
13
overlying the Shishimano Dacites. This unit consists of hypersthene-augite andesite lava flows and their pyroclastic rocks. Phenocrysts are plagioclase (An 6o-55), augite, hypersthene and olivine in a glassy groundmass.
Pyroclastic flow deposits predominate in the western part of the area, filling topographic depressions below 250 m elevation with thicknesses up to 70 m. They consist mainly of pumice flows erupted from the Aira caldera about 22,000 years ago (Kigoshi et al., 1972). In addition to this unit there are several older units of pyroclastic flow deposits such as the Kakuto pyroclastic flow deposit of 0.3 Ma (Miyachi, 1983, 1987).
Radiometric age dating K-Ar age determinations for the least altered volcanic rocks have been made on whole rock and feldspar. Most of the K-At dates were determined by Teledyne Isotopes and some were analyzed at the University of British Columbia. The newly obtained ages of 53 whole rocks and one mineral are presented in Table 5 with locations shown in Figure 3; results having errors of more than 20% have been discarded and are not shown. Newly obtained K-Ar ages for adularia-bearing ore, analyzed at the Okayama University of Sciences, are shown together with age data for composite ores (Nishizawa and Ibaraki, 1985) in Table 6. The volcanic rocks in the area have been correlated with the "Hokusatsu Older Andesites" of Miocene age and the "Hokusatsu Younger Andesites" of Pleistocene age (MITI, 1979). K-Ar analyses, however, show that most of them belong to the same volcanic unit of Pleistocene age. Andesites in the mine area are newly divided into three units as follows: the Hishikari Lower Andesites (0.95 + 0.09 to 1.78 _+0.15 Ma), the Hishikari Middle Andesites {0.78 + 0.08 to 0.79 _+0.05 Ma) and the Hishikari Upper Andesites (0.51 _+0.06 to 0.58 + 0.10 Ma). The "Kurozonsan Rhyolites" (MITI, 1979) were also divided into two units: the Kurozonsan Dacites (0.95 + 0.09 to 1.56 + 0.20 Ma) and the Shishimano Dacites {0.66_+ 0.04 to 1.1 ___0.14 Ma). K-Ar analyses on adularia-bearing ore clearly demonstrate a Pleistocene age of 0.84 + 0.07 to 1.01 _+0.08 Ma for gold mineralization at Hishikari (Table 6). Urashima and Ikeda (1987) reported a K-At age of 0.86 + 0.12 Ma for adulariabearing ore from the Hosen 1 vein. An exceptional age of 1.5 + 0.3 Ma reported by MITI (1983) was obtained for adularia from a vein located 400 m south of the major vein system. Ueno et al. (1987) studied paleomagnetism of volcanic rocks and veins in the Hishikari area, and argued that the normal polarity of magnetization of a hematite vein, which parallels an ore vein and seems to have formed during gold mineralization, might correspond to the Jaramillo normal subchron (0.92-0.97 Ma). These data indicate that the principal AuAg mineralization at Hishikari is closely related to the latest-stage volcanic
14
E. IZAWAET AL.
TABLE 5 K-Ar age data and location of analyzed rock samples Sample no.
K (wt.%)
4°At tad (nl/g)
Rad.4°Ar (atom.%)
Calculated age (Ma)
Latitude north
Longitude east
Remarks
34.4 35.5 25.9 58.7 29.2 21.6 34.9 51.1 34.7 35.0 34.2 28.9 23.9 36.1 28.0 34.6 38.8 42.4 23.5 39.9 41.3 46.8 37.4 46.2 44.4 59.5 29.4 25.8 31.9
1.43-+0.09 1.44-+0.10 1.07_+0.08 1.09_+0.05 1.78_+0.15 1.32_+0.18 1.44_+0.10 1.35_+0.07 1.26±0.06 1.24___0.08 1.18-+0.10 1.22-+0.10 1.15_+0.12 1.19_+0.09 1.00_+0.09 1.09_+0.07 1.39_+0.08 1.44_+0.07 1.15_+0.18 1.46_+0.08 1.62_+0.09 1.37_+0.07 1.31_+0.08 1.30_+0.1 1.60_+0.1 1.38_+0.07 1.43_+0.11 0.98_+0.10 0.95_+0.08
32°04'40 " 32°04'05 " 32o04'05 " 32°03'30" 32°03'30" 32°03'20" 32°03'00 " 32°02'55 " 32002'45 " 32°02'35 " 32°02'10" 32°02'10" 32002'00" 32°01'50" 32°01'50 " 32°01'40 " 32°01'35 " 32°01'20 " 32°01'10 " 32001'00 " 32°01'00" 32°00'50" 32000'45" 32000'45" 32000'40 " 32000'40 " 32°00'25 " 32°00'25 " 31°56'45"
130°43'50" 130°40'30 " 130°42'30 " 130°40'50 " 130042'05 " 130°41'40 " 130°42'50 " 130°43'55 " 130°43'30 " 130°44'05 " 130°40'55" 130041'40 " 130044'00" 130043'50" 130°42'50" 130°44'10" 130°42'40 " 130040'55 " 130°42'10 " 130042'20 " 130042'25 " 130°42'00 " 130°41'55" 130041'55" 130°41'00" 130°41'00 " 130°41'45 " 130041'50 " 130°40'25 "
WofMasaki E ofMasaki WofMasaki EofAoki IchiyamaRiver IchiyamaRiver Nishino SEofNishino S ofNishino NofHannyaji Kusumoto EofKusumoto Hannyaji SWofHannyaji NofUono Yamashita Uono Shinkawa NEofmine NEofmine NEofmine NEofmine NEofmine NEofmine Quarry in Maeda QuarryinMaeda Sofportal Eofportal Omure
18.7 25.2 17.9 15.9 32.5 27.9 45.9 55.0 28.2 28.2 33.6
1.04+__0.19 1.11_+0.13 1.01_+0.19 1.25_+0.21 1.15_+0.09 0.95_+0.09 1.11_+0.06 1.05_+0.05 1.56_+0.20 1.05_+0.09 1.16+0.21
32004'45 " 32°04'40 " 32004'40" 32°04'25" 32°04'20 " 32°04'15" 32°04'10 " 32°03'40" 32002'45 " 32002'40 " 32°00'25"
130°39'25 " 130041'50 " 130044'05 " 130°42'30" 130043'30" 130°39'15" 130°42'05 " 130°41'00" 130°43'40" 130°41'40 " 130°40'55"
N ofAoki NofKurozonsan NofMasaki NofKurozonsan WofMasaki NofAoki NofKurozonsan EofAoki SofNishino SWofKurozonsan Yamada
16.7 25.2 35.0
0.79±0.12 0.78±0.08 0.79_+0.05
32°00'20" 32°00'00" 31°58'20"
130°42'30 130°43'50" 130°41'30"
E of mine SofYoshimatsu Inabazaki
Hishikari Lower Andesites M-53 A-7 M-92 U-122 M-17 M-13 M-8 U-115 M-78 U-114 M-24 M-3 M-42 T-55 A-5 T-54 U-3 M-1 F-212 T-14 T-12 T-19 T-20 UBC-2 UBC-3 U-13 U-103 U-104 A-31
1.61 1.75 1.93 1.88 1.74 2.15 1.67 1.95 1.89 1.69 2.19 2.10 1.76 1.53 1.70 2.36 1.83 1.78 1.70 1.94 1.91 1.88 1.90 2.06 1.85 2.05 1.30 1.93 1.97
0.09 0.10 0.08 0.08 0.12 0.11 0.09 0.10 0.09 0.08 0.10 0.10 0.08 0.07 0.06 0.10 0.09 0.10 0.07 0.11 0.12 0.10 0.10 0.1087 0.1161 0.11 0.07 0.07 0.07
Kurozonsan Dacites M-217 M-220 F-7 M-70 M-22 M-215 A-10 U-120 M-204 M-5 M-91
3.35 3.02 4.08 2.47 3.80 3.25 2.56 2.50 1.82 2.43 3.32
0.14 0.13 0.16 0.12 0.17 0.12 0.11 0.10 0.11 0.10 0.15
Hishikari Middle Andesites A-16 U19 U-26
1.82 1.58 2.05
0.05 0.05 0.06
THE HISHIKARI GOLD DEPOSIT
15
TABLE 5 (continued) Sample no.
K
4°Ar rad
(wt.%) (nl/g)
Rad.4°Ar
Calculated
age (Ma)
Latitude north
Longitude east
Remarks
(atom.%) 9.4 12.0 42.8 28.9 23.6 36.2
1.1 _+0.1 1.0 _+0.1 0.81_+0.04 0.66_+0.04 0.73_+0.08 0.73_+0.05
32o00'50 " 32000'30 " 32o00'25 " 31°59'55 " 31059'40 " 31059'20 "
130°42'50 " 130o42'35 " 130°43'50" 130°43'20" 130041'35 " 130°43'50 "
Eofmine Eofmine Wof¥oshimatsu SWofYoshimatsu N ofHirasawazu SofYoshimatsu
23.2 16.6 27.3 14.7 24.4
0.56_+0.06 0.58_+0.10 0.57_+0.05 0.55_+0.01 0.51_+0.06
32o01'40" 32°01'15" 31o59'25" 31°59'10" 31o58'55 "
130o43'22 " 130°43'15 " 130043'50 " 130043'50 " 130o43'25 "
Uono Uono-Yoshimatsu Sof¥oshimatsu SofYoshimatsu SofYoshimatsu
Shishimano Dacites UBC-4* 0.29 UBC-1 2.92 U-22 2.72 T-77 3.12 U-27 3.08 U-14 2.84
0.0126 0.1196 0.08 0.08 0.08 0.08
Hishikari UpperAndesites U-5 U-7 U-15 T-75 K-4
2.04 1.90 1.62 1.81 1.71
0.04 0.04 0.03 0.04 0.03
*Feldspar Analyses: Teledyne Isotopes, except for UBC-1,2,3 and 4 (University of British Columbia)~
TABLE 6 K-Ar age data of adularia bearing ores from the Hishikari deposit Sample no.
K (wt.%)
HA-1 HA-3 HA-19 HA-7 HA-18
4.30_+0.13 4.09+0.12 4.83_+0.15 5.13_+0.15 4.34-+0.13
UBC-A1 UBC-A2
12.58 12.39
4°At rad (nl/g)
Rad.4°Ar (Atom.%)
Calculated age ( M a )
Remarks
0.1590_+0.0187 0.1481_+0.0076 0.1859_+0.0116 0.1665___0.0123 0.1694_+0.0125
20.1 18.7 16.7 13.9 13.3
0.95_+0.12 0.93_+0.06 0.99_+0.07 0.84-+0.07 1.01_+0.08
100mL E40 RY-6W 100mL E40 RY-6W 100mL E40 RY-6E 25S 70mL W14 ZU-1W F W 70mL W14 Z U ° l W 80 S
0.4808 0.4688
34.8 41.0
0.98_+ 0.04 0.97_+ 0.04
composite sample a composite sample b
aAdularia from drill hole cores 0-1 ( 192.80 m, 214.98 m a n d 331.60 m ) a n d E l - 1 (258.90 m, 279.87 m a n d 324.75 m ) (Nishizawa a n d Ibaraki, 1985} bAdularia from drill hole cores E3-1 {197.55 m a n d 217.50 m ) a n d W2-2 (327.90 m, 328.00 m and 328.30 m ) (Nishizawa a n d Ibaraki, 1985). Analyses: Okayama University of Science, except for UBC-A1 a n d UBC-A2 (University of B r i t i s h Columbia).
activity of the Hishikari Lower Andesites, and overlaps with early-stage eruption of the Shishimano Dacites (Fig. 4). Geological structure The Hishikari which
coincides
mine lies close to the western with part
of the Kagoshima
margin graben
of the Kakuto (Fig. 1). The
caldera, Shimanto
16
E. IZAWA ET AL
6 Vore s
42I(n=7) c o
0
i 0
°6F c
I 2
I Dacites
"~-4I(n=17)~ ~'~ ® 2 "u
hi
rozonsQn
0 0
~s
1
FAndesites f: 6 ~ ( n = 3 7 ) Upper z
I
2
~
o
Hishikori Lower
,
0
1 K--Ar
age
2 , Mo
Fig. 4. Histogramsof K-Ar agesof volcanicrocksand ores fromthe Hishikari area.
Supergroup basement is present below - 1000 m elevation in the graben (2000 m beneath the western flanks of the Kirishima volcano) (Nakagawa et al., 1985). The Shimanto Supergroup is also very deep to the northwest in the Okuchi basin, as a drill hole to - 8 0 0 m elevation did not reach the basement (T. Ikeda, pets. commun., 1988) (Fig. 1 ). On the contrary, the Hishikari area is underlain by shallow basement (with the highest occurrence at 130 m elevation). A gravity survey by SMM showed a gravity high anomaly with a northeasttrending axis over the Hishikari deposit. The area (3 km by i kin) of the gravity high anomaly corresponds closely to the area of the basement high. The surface of the Shimanto Supergroup in the Honko area is similar in shape to an inverted keel, with a gentle plunge to the west-southwest (Fig. 5). Elevations of the top of the Shimanto Supergroup are - 1 0 0 m to - 2 0 0 m in the Yamada area, 1 km to the southwest. The top of the basement is a series of peaks and ridges which are separated by small valleys. This relief of the basement surface is similar to that of the present topography of the area. The contour map of the Shimanto Supergroup implies that this structure was the paleosurface prior to eruption of the Hishikari Lower Andesites. Veins coincide with the topographic high of the paleosurface (Fig. 5). The Shimanto Supergroup and the Hishikari Lower Andesites are in places fault bounded, as evidenced by slickensides on the contact. O n the district
FHE HISHIKARI GOLD DEPOSIT
~/180
w40
17
zc
o
/~-.--4,o
,o:_ :r6o
40
II I
-40
'-
,.._:..
.........
~___DA
/
//I,o
"°
~ _ ~ ,
/
•
.
0
Legend '"-J""
DA
A
B C
540 0
I00
200
300
400
500m
i......... i , i I I i I I I
Fig. 5. E l e v a t i o n o f the top of the Shimanto Supergroup based on drilling and underground development. A =elevation in meters, contour interval is 10 m. B---location of the Honko veins at 40 m elevation, abbreviations include: H O - - Hosen vein group; Z U = Zuisen vein group; R Y = R y o s e n vein group; and D A = Daisen vein group. C = projection of the inclined shafts. The base line of the mine grid is parallel to the general strike of the veins ( N 5 0 ° E ) .
scale, a block of the Shimanto Supergroup, bounded by faults on the north and south ends, may have been uplifted in the mine area. ORE DEPOSIT
Vein system Hishikari can be classified as an epithermal gold-silver-bearing quartz-adularia vein deposit. At present, development has focused on the Honko vein system. A separate vein system was recently discovered in the Yamada area 1 km southwest of the Honko deposit; it is hosted by the Hishikari Lower Andesites and consists of several major veins with numerous parallel veins. However, details of the Yamada area are poorly known, so only the Honko vein system is described here. The Honko vein system is composed of 3 major vein groups and numerous veinlets in an area 1100 m by 200 m (Table 7). They occur in both the Shi-
18
E. IZAWA ET AL.
AGE Shishima no Dacite
H ishikari Lower A n d e s ~ ~ - ' ~ . V ]OOML
,,,'~\ / x / \ / \ /~(
River
Portal ~ / \
/ \ / \ /\ / \ \ // \ /\ / \ \ / \ / \ / \
\ / \ / \ /
\
/
/ /\ ~/ ' / \ / ~
~ / \ / \ \ / \ /
" \ 1 \ 1 \ 1 \ 1 \ I \ 1 \ 1 \ 1 \ 1 \ 1 \ \ / \ / q / k / N / k / \ / \ / \ / \ / \ \ / \ / \ / \
. , , If,
\. / \ 1
l J
----
I I
Vein 0.8z,~1.01 Mo
I' 0.~5
~
\ • .
I\ I\~
rll~
:::
\ / \ / \ / \ / ' \/ \/ \ / \ , \ / \ / \/' \ / \ /\.
I \
~/////4"///,',x~
I
1!1 M o
\.
\,
\,
\'~
t.78 Ma
\ / \ 1 \ 1 \ ~ \ 1 \ 1 \ 1 , \/\,
"", Cretaceous.
Fig. 6. Schematic n o r t h w e s t - t r e n d i n g section across the Hishikari mine. T h i s cross section shows the central part of the H o n k o area, where most of the veins are hosted in sediments. Legend is the same as in Figure 3.
manto Supergroup and the Hishikari Lower Andesites, and generally strike N50 °E with an individual strike length of 300 to 400 m; the veins dip steeply north (70 ° to 90 ° ) and range from 1 to 3 m (maximum 8 m) in width. Known bonanza zones are located between 130 m and - 2 0 m elevations (Figs. 6, 7 and 8). The Ryosen vein group, which is composed of 5 unit veins (RY-1, 2, 3, 5 and 6), is located in the eastern part of the deposit. Veins are relatively narrow in width and have a moderate gold grade for veins at Hishikari. The Zuisen vein group consists of 2 unit veins, occurring on the hanging-wall side of the western part of the deposit. The Zuisen No. 1 vein is the widest (up to 8 m) and lowest in grade relative to the other veins. The Hosen vein group is composed of 4 unit veins (HO-1, 2, 3 and 5 ) and is located on the footwall side in the central to western part of the deposit. The Hosen veins (particularly Hosen No. 2) are well developed in the Shimanto Supergroup and form extremely rich bonanzas directly below the unconformity (Fig. 8). Only part of the Daisen No. 1 vein has been developed, and is not well known; it parallels the other vein groups on the hanging-wall side. Veins of the Honko area commonly show rhythmic crustiform banding, in general symmetrical from both walls; however, in wider veins, such as Zuisen
rile HISHIKARI GOLD DEPOSIT
19
W; OB
W~OB
E20B
E4 {)B
E~B
-~=f . . . . . . . . . . . . .
N20G
/-/
oo
100~ L ...........
"-
. . . . . . . . . . . . . .
. _ _ .D_A o - . - I.
.
.
.
.
N20G
Y
OG
70M >,/
N20G
5
_
/I
N20G
I
. . . . ........ .::_---:
O~
-
I
20MI
0 i
I O0 l
ZOOm i
Fig. 7. Distribution of veins a n d the S h i m a n t o Supergroup (hatched area) at the 100 m level, the 70 m level, the 40 m level a n d the - 20 m level. T h e dashed lines indicated assumed vein portions. Abbreviations are identified in the caption to Figure 5. T h e base line of the mine grid is parallel to the general strike of the veins (N50 °E).
No. 1, the banded structure is complex, showing repeated collapse and deposition, with drusy cavities common and as large as 3 m X 7 m X 5 m. The size of each vein and average Ag/Au ratio of the ore are shown in Table
20
E. IZAWA ET AL.
E :)B
E3OB
.k"
",.
E,~OB
~//
•,... _. . . . . . . . . - . , "i/
/
/
/
RY-2 ~o
W&QB
w3011
. ;o
W208
,o,o.
WlOO
.... ~----,',;-::
.....
On
EI OB , I,o0 .. M
"b=:-=..... ~'~.'i" '°
;---- _'°,°
,,
W; OB
OB
0;
~ B
E]
eavtt ?
/
~ ~ : ~ ' ~ - ~ ~.~ - ~
"~,.,~/_.~//..'x///'M~
~ 2 2- -
/-J
_ :-~/--'/~..~- . ,
~
,
I
OLIO
SO
lOOm,
Legend
l~7~s
---F
~c [ [o Fig. 8. Distribution of gold values of veins in longitudinal projections onto a vertical plane along the direction of N50°E. RY-2 = Ryosen No. 2 vein; ZU-1 =Zuisen No. i vein; A to D indicates high to low goldvalues,which are expressedby the product of the grade and the width of the veins. The ore correspondingto the highest gold value usually exceeds 100 g/t Au. The light dotted line, E, shows the unconformity between the Shimanto Supergroup and the Hishikari Lower Andesites. The heavy dashed line, F, shows the limits of ore grade (> 4 g/t Au) in the veins; the upper boundary coincides with a pinching out of the vein.
THE HISHIKARI GOLD DEPOSIT
21
TABLE 7
General feature of veins of the Hishikari deposit (Honko area) Vein
Strike
Dip
Length
Elevation
(m)
Top
Bottom
(m)
(m)
Average width
Ag/Au
(m)
(wt.)
ratio
Ryosen RY-1 RY-2 RY-3 RY-5 RY-6
N50°E N50°E N50°E N50 ° E N30°E
75°N 75°N 90 ° 80 ° N 80°N
200 200-300 250-300 150 + 300+
110 115 110 95 110
25 -50 -5 25 -50
0.55 0.94 0.38 0.58 1.69
2.72 0.82 1.18 2.68 0.59
N50°E N50 ° E N50 ° E N50°E
90 ° 80 ° N 80 ° N 75°N
280 450 80 350
95 90 85 90
-20 - 20 - 5 0
1.40 3.22 1.47 0.74
0.90 0.57 -
N50°E N50 ° E
80°N 80 °
450 350
90 80
-65 0
3.92
0.71 -
--
90 °
(300)
(115)
-
90 ° 90 ° 90 °
(250) {250) {150)
85 80 75
20 0 5
0.23 0.17 0.16
-
Hosen HO-1 HO-2 HO-3 HO-5
Zuisen ZU-1 ZU-2
Daisen DA-I
Undeveloped E F H
N50°E N55°E N55°E
Lengths and vertical ranges are confirmed values except for the figures in parentheses, which are assumed values.
7. The Ag/Au ratio varies considerably from > 1 in the upper levels to 0.5 in the deeper levels (Fig. 9). Vein minerals
Veins are composed mainly of quartz, adularia and clay minerals. More than 90% of the clay minerals are smectite, with minor amounts of sericite, chlorite and kaolinite. The principal metallic minerals are electrum, naumannite-aguilarite, pyrargyrite, chalcopyrite, pyrite and marcasite, with minor amounts of sphalerite, galena, stibnite, tetrahedrite, miargyrite, hessite (?), Ag-Au selenide, acanthite, Cd-sulfide (greenockite or hawleyite), and hematite (Urashima and Izawa, 1983). Lamellar quartz is common and, in places, there are minor amounts of carbonates (calcite and Mn-Fe-Ca carbonate), gypsum, truscottite (alkali-free and alkali-rich varieties), xonotlite, wairakite and lau-
22
E. IZAWAET AL.
----~Ag/Au mL
1.0 R~-6 ~ ¥ - 2
2.0 '
3.0 I ...........
100
70 40
~
"~"~
~
RY-I
10 -20 Fig. 9. Plot of Ag/Au weight ratio of ore vs. elevation. Abbreviations include: R Y = Ryosen vein group; H O = Hosen vein group; and Z U = Zuisen vein group.
montite (Urashima and Izawa, 1982; Izawa and Nakae, 1983; Izawa and Urashima, 1983). Major- and trace-element chemical analyses of bulk samples of ore are listed in Table 9. Adularia is the most widespread mineral with quartz. Major veins consist of 70% quartz and 30% adularia, and locally the latter is more abundant than the former. Adularia occurs as idiomorphic clustered crystals, intergrown with quartz, or fine-grained clayey aggregates of quartz and smectite or kaolinite. Chemically, A1203 and K20 show a positive correlation with Au and Ag, but SiO2 is negatively related to Au and Ag (Table 9). Veins containing abundant adularia hence tend to be richer in Au and Ag than quartz-rich veins. However, on a microscopic scale, Au and Ag minerals are sparse in monomineralic bands of adularia, but are abundant in aggregates of fine grained quartz, quartz-adularia, or quartz-clay minerals. Electrum generally tends to occur in gray or black bands and spots, and clayrich portions of veins. Several stages of electrum concentration are observed. Electrum is often associated with chalcopyrite or occurs as isolated grains in quartz. In extremely high-grade veins, electrum bands occurs between early adularia-quartz and later smectite-quartz bands. The grain size of electrum generally ranges from less than 1 pm to 25/zm, often around 10/~m, and rarely exceeds 100/~m. Electrum contains 66-81 wt.% gold (average 70 wt.% ); electrum of higher gold content tends to occur at higher elevation (Table 8). Ginguro (silver black) is composed mainly of chalcopyrite, electrum, naumannite, sphalerite, galena, pyrite and marcasite, with minor amounts of acanthite, aguilarite, tetrahedrite, greenockite (or hawleyite), Au-Ag selenide, hessite (?), pyrargyrite and miargyrite. Ginguro tends to occur at higher levels (RY-1, 3, 5; HO-1, 2; ZU-1 ) and on the outer side of the vein, and seems to be an earlier stage of mineralization. Table 10 lists the chemical compositions of
THE HISHIKARIGOLDDEPOSIT
23
TABLE 8
Chemical composition of electrum determined by microprobe analysis Elevation (m)
Au (wt.%)
Ag (wt.%)
Total (wt.%)
Ag (Atom.%)
Remarks
No. 12-162.1m
118
81.24 76.65 75.77 71.19 68.75
18.51 22.55 24.31 28.43 31.32
99.75 99.20 100.08 99.62 100.07
29.4 34.9 36.9 42.2 45.4
HLA
No. 12-162.2m
118
69.55 67.72
31.24 32.86
100.79 100.58
45.1 47.0
HLA
56MAHT-1-456.5m
80
73.06
26.70
99.76
40.0
SH
No. 13-215.0m
75
67.02
32.84
99.86
47.2
SH
No. 13-215.2m
75
74.47 71.23
25.47 28.53
100.44 99.76
38.9 42.2
SH
56MAHT-2-263.8m
58
69.58
30.27
99.85
44.3
SH
55MAHT-5-291.7m
51
67.84 66.82
32.70 32.89
100.54 99.71
46.8 47.3
SH
Sample no.
Drill core samples 1
Underground 2
RY-1
E18B
100
72.07 66.96
27.57 33.39
99.64 100.35
41.1 47.7
HLA
RY-2
E28B W
100
67.60
31.95
99.55
46.3
HLA
RY-2
E30B W
100
66.59
33.25
99.84
47.7
HLA
ZU-1
Wl4B E
70
75.45
25.21
100.66
37.9
SH
HO-1
E1B E
70
70.38
30.37
100.75
44.1
SH
HLA = hosted in the Hishikari Lower Andesites; SH = hosted in the Shimanto Supergroup. RY = Ryosen vein group; Z U - Z u i s e n vein group; and HO = Hosen vein group.
1Drill hole numbers and sampling depths. 2Urashima and Nedachi (1986).
silver minerals and other sulfide minerals, while the compositional variation of sphalerite is listed in Table 11. Compared with the upper part of the vein system, electrum and sulfides are less abundant and finer grained in the middle and deeper levels. Silver minerals increase in abundance with elevation, as shown by the increase in overall Ag/ Au ratio in the upper part of the veins (Fig. 9); in contrast, the Ag content of electrum tends to decrease slightly at the higher levels. Other mineralogical changes with elevation include an increase in the selenium content of nau-
(wt.%) Si02 TiO2 Al20s Fe203 FeO* MgO CaO Na20 K20 P205 S C02 H20+ H20Total
Sample no.
Vein: Location:
65.93 0.31 12.24 1.02 3.13 1.13 0.08 0.23 10.05 0.14 1.73 <0.1 1.17 0.12 97.38
RY-6 100mL E28.4B HK-19
76.55 0.09 9.06 0.38 1.79 0.42 0.12 0.32 7.79 <0.01 0.37 <0.1 0.62 0.14 97.76
RY- 1 70mL E16.1B-W HK-17 87.71 < 0.01 3.87 0.17 1.29 0.04 0.10 0.05 3.06 <0.01 0.22 0.35 0.87 0.12 97.87
RY-2 70mL E29.5B2 HK-01 84.51 < 0.01 6.24 0.43 1.11 0.13 0.14 0.05 4.88 0.02 0.36 <0.1 1.02 0.35 99.35
RY-2 70mL E27.3B-W HK-03
Chemical composition of ores from the Honko area of the Hishikari deposit
TABLE 9
76.92 < 0.01 10.08 0.20 0.63 0.09 0.06 0.35 9.43 <0.01 0.04 <0.1 0.55 0.15 98.62
HO- 1 70mL W0.3B°W HK-13 82.92 < 0.01 6.77 0.20 1.03 0.11 0.09 0.30 5.28 <0.01 0.18 <0.1 0.51 0.20 97.71
HO-2 70mL W4.0B-W HK-15
75.03 < 0.01 12.07 0.17 0.81 0.15 0.12 0.25 10.82 <0.01 0.14 0.17 0.61 0.26 100.62
HO-2 70mL W13B-E Hk-05
85.39 < 0.01 5.01 0.33 0.94 0.06 0.16 0.19 2.80 <0.01 0.09 <0.1 0.74 0.27 96.10
HO-3 70mL W14.8B-E HK-09
84.10 < 0.01 6.01 0.22 0.97 0.01 0.02 0.02 5.46 <0.01 0.04 0.17 0.92 0.34 98.30
ZU- 1 85mL W20.1B-W HK-07
90.50 < 0.01 1.86 0.16 1.26 0.03 0.07 0.34 1.00 <0.01 0.11 <0.1 0.72 0.14 96.31
ZU- 1 85mL Wl0.5-E HK-11
b~
3944.9 10559.4 9500 970 800 35.0 7 7 710 17 13 320 8.75 0.6 9 5.3 -
655.6 1378.2 800 105 71 3.3 8 5 420 100 57 142 0.45 0.1 9 5.0 0.11
38.9 31.7 490 77 42 0.6 11 10 90 32 14.8 1.0 0.10 6.8 1 2.1 0.32
164.2 94.3 265 37 16 0.6 7 2 75 270 81 2.0 2.75 0.7 4 5.3 1.90
132.6 166.2 12 25 52 0.4 5 1 51 9 70 2.0 0.50 0.1 11 6.1 0.16
745.2 1332.5 680 145 53 2.1 7 1 78 30 200 102 5.50 0.1 7 3.4 0.19
470.0 824.3 500 104 65 2.0 6 1 60 36 280 84 0.05 0.3 12 6.8 0.37
64.5 41.1 80 18 5 0.4 8 1 81 23 26 0.5 0.25 0.1 5 1.8 0.12
104.1 80.4 108 19 10 0.5 7 1 73 15 33 3.0 0.80 0.1 3 3.2 0.31
35.8 17.0 33 5 10 0.4 8 1 91 38 10.6 < 0.5 0.55 0.1 2 0.8 0.27
FeO* = FeO + Fe (sulfide) 1 ppm Sn and 5 ppm Ge were detected for all samples; In < 2 except for HK- 19 ( 8 ppm), Ba < 100 ppm, P t < 50 ppb, Pd < 10 ppb and Rh < 20 ppb. RY = Ryosen; HO = Hosen; ZU = Zuisen. HK-19=ginguro-bearing veinlet; HK-17=ginguro-bearing white quartz vein; HK-01 = w h i t e to gray banded quartz vein; HK-03 =irregularly banded quartz vein; HK- 13 = white banded quartz-adularia vein; HK- 15 = ginguro-bearing gray banded quartz vein; HK-05 = banded quartz-adularia vein; HK-09 = white quartz-clay vein; HK-07 = w h i t e compact quartz vein; HK-11 = w h i t e quartz vein with black bands.
(ppm) Au Ag Cu Pb Zn Cd Ni Co Mn As Sb Se Te Bi Ga Ti Hg
b~ O1
t~ *O o
o
r~
26
E. IZAWAET AL.
T A B L E 10 C h e m i c a l c o m p o s i t i o n of silver m i n e r a l s a n d o t h e r sulfide m i n e r a l s d e t e r m i n e d by microprobe a na l ys i s Sample no.*
Wt.% Ag
A t omi c Se/(Se+S) Cu
Zn
Fe
Pb
Sb
As
Se
S
Tot a l
Naumannite (-Aguilarite) No. 1 2 - 1 . 6 2 . 1 m No. 1 2- 162.2m No. 1 3 - 2 1 5 . 2 m 56MAHT-2-263.8m
78.33 0.52 0.13 78.54 1.39 1.16 0.20 81.16 78.49 1.39 0.90
-
21.88 15.14 16.61 12.48
1.15 102.01 0.88 2.91 99.34 0.68 3.44 101.21 0.66 3.77 97.03 0.57
Pyrargyrite No. 1 3 - 2 1 5 . 2 m
61.54 0.74
-
-
19.08 1.98
5.63
13.99 102.96
36.34
-
-
40.91
1.91
19.58
98.74
-
53.02 53.23
99.69 99.47
2.06 11.25
99.58
Miargyrite No. 13 - 215.0
-
Pyrite No. 13 - 215.2m 5 5 M A H T - 5 - 291.7m
-
-
-
-
46.64 45.72
0.03 0.52
Galena No. 12 - 162.1m
-
86.27
Stibnite M A H T - 1 - 465.3m
-
-
-
71.78
-
28.31
100.09
- - = not detected. *Drill hole n u m b e r s a n d s a m p l i n g depths.
T A B L E 11 C h e m i c a l c o m p o s i t i o n of s p h a l e r i t e d e t e r m i n e d b y m i c r o p r o b e a n a l y s i s S a m p l e no.*
Wt.% Zn
No. No. No. No.
12 - 162.1 m 12 - 162.2 m 3 - 203.2 m 13 - 215.0 m
No. 8 - 408.5 m
63.57 62.74 63.73 62.46 62.58 64.27
Fe
Cu
Cd
S
Total
0.40 0.50 2.97
0.35 0.46 -
3.54 3.42 2.31 2.90 2.92 -
32.85 33.15 33.03 33.17 33.07 33.38
99.96 99.30 99.06 99.28 99.52 100.62
Mole % FeS
Elevation and remarks
<0.02 <0.02 <0.02 0.70 0.88 5.1
118 118 85 75
m m m m
HLA HLA SH SH
11 m S H
H L A = h o s t e d in t h e H i s h i k a r i L o w e r A n d e s i t e s ; S H = h o s t e d i n t h e S h i m a n t o S u p e r g r o u p . -- = not detected. *D rill hole n u m b e r s a n d s a m p l i n g d e p t h s .
mannite
and
of analyses Stibnite,
a decrease
in the
iron
content
pyrite
and/or
of sphalerite,
though
the
number
is limited. fine-grained
pyrargyrite
occur
in cracks,
druses
and
THEHISHIKARIGOLDDEPOSIT
27
fractures in the vein at higher elevations, indicating their precipitation during the latest stage of mineralization.
Homogenization temperatures of fluid inclusions Homogenization temperatures of 232 fluid inclusions in quartz, adularia and calcite were measured on 42 samples from underground and drill holes. These inclusions are two-phase and liquid-dominated. The results are summarized in Figure 10. Fluid inclusions in quartz are very poorly developed and are less than 10/zm in size. No temperature correction for the pressure of formation was made. Unusually high temperatures were obtained for several inclusions in quartz samples from four locations. This may be due to two-phase trapping (and would indicate boiling) at about 250°C between - 2 0 m and 90 m elevation. These unusually high values are excluded from Figure 10. The mean temperature for five inclusions from two adularia samples is 241 ° C. Homogenization temperatures for quartz from veins hosted in the basement Calcite
160 ± 48"C (n=41)
0
Quartz
in andesites)
197 +- 35"C
Quartz
in basement)
213¢23"C (n =129)
o .2 ul 20D 15-
~
IO-
E z
o
5]
Adularia
r - - I 7"C 241+-
(n:51
0
O0 ,
1
,
150 Homogenization
J 200 temperature,
ml-~ J p 250 "C
Fig. 10. Histograms of homogenization temperatures of fluid inclusions from the Hishikari deposit.
28
E. IZAWA ET AL.
sediments averaged 213 ° C, with most values ranging from 195 to 230 ° C. Homogenization temperatures for quartz in the Hishikari Lower Andesites are scattered over a wide range with an average fo 197°C. Calcite also shows scattered values of temperatures from 233°C to 91°C, averaging 160°C and with no definite peak temperature. These data indicate that the temperature of ascending fluid may have been as high as 250 ° C at the greatest depth of mineralization; however, the representative temperature of gold deposition was 210°C in basement at about 25 m elevation, and less than 200 °C in volcanic rocks at about 100 m elevation. Deposition of barren quartz and calcite continued during waning stages of the Hishikari hydrothermal system to temperatures as low as 90 ° C. GEOCHEMICALCHARACTERISTICS
Hydrothermal alteration More than 200 samples collected from surface and about 300 samples from 22 drill holes were examined by means of X-ray diffraction. A number of igneous and hydrothermal minerals such as silica polymorphs, feldspar, clay minerals, zeolites, carbonates and sulfides were identified. The least altered andesitic rocks consist mainly of plagioclase, cristobalite a n d / o r tridymite. Quartz and potassium feldspar are also major constituents of the Shishimano Dacites.
Definition of alteration zones On the basis of the mineral assemblage, hydrothermally altered volcanic rocks can be grouped into four mineralogical zones as shown in Figure 11. The deepest and innermost chlorite-sericite zone (IV) is defined by the common presence of chlorite with local sericite. The typical mineral assemblage is quartz, chlorite, adularia, calcite, and plagioclase, with minor interstratified chlorite/ smectite or sericite/smectite. The interstratified clay mineral zone (III) is characterized by the presence of interstratified chlorite/smectite a n d / o r sericite/smectite. Quartz, adularia, calcite, smectite and laumontite are commonly present. The presence of quartz with smectite a n d / o r kaolin minerals, and the absence of chlorite, sericite and interstratified clay minerals define the quartzsmectite zone (II). Cristobalite a n d / o r tridymite occur with smectite in the cristobalite-smectite zone (I). Although smectite is the essential alteration mineral in both the quartz-smectite and the cristobalite-smectite zones, kaolinite or halloysite are locally predominant (the kaolin mineral subzone). Hydrothermal alteration of basement sediments is developed only in narrow halos to the veins. The alteration is represented by the chlorite-sericite assem-
]'HE HISHIKARI GOLD DEPOSIT
Least ~Itered
29
I Cr-Sm Ka
II
Qz-Sm Sm
Ka
III
IV
Int. Clay
Ch-Se
)lagioclase :r, Tr tuartz <-feldspar (aol in min. ~mectite
:/s, M/S :hlorite 5ericite :pt, Stl 4d, Eps ~m, Ac :alcite
;ypsum }yrite
Fig. 11. Alteration zoning by mineral assemblage. Cr-Sm=cristobalite-smectite zone; QzSm = quartz-smectite zone;Int. Clay = interstratified clay mineral zone; Ch-Se = chlorite-sericite zone; Ka=kaolin subzone; Sm=smectite subzone. Cr=cristobalite; Tr=tridymite; Kaolin min. = kaolinite and halloysite; C/S = interstratified chlorite/smectite; M/S = interstratified sericite/smectite; Cpt = clinoptilolite; Stl= stilbite; Md = mordenite; Eps = epistilbite; Lm = laumontite; A c = analcite. blage, though it is often difficult to distinguish the assemblage from the regional metamorphic chlorite-sericite. Distribution of alteration zones
The vertical and spatial distribution of alteration zones is shown in Figures 12 and 13, which on the whole indicate a zonal arrangement of alteration. Chlorite-sericite alteration (zone IV) is the principal alteration directly associated with high-grade gold mineralization. Interstratified clay mineral (zone III) and quartz-smectite (zone II) alteration envelopes the mineralized center and forms a nearly horizontal layer of intense argillation located 50 to 100 m above the Hishikari vein system. These alteration zones are surrounded by the cristobalite-smectite zone (zone I) and least-altered rocks. There is a slight downward drape to the alteration zones, reflecting distance from the veins (Fig. 12). In addition to the above-mentioned alteration, sporadic alunite-quartz (or alunite-cristobalite) alteration is present at higher elevations (350-400 m elevation) in the eastern part of the Hishikari area. Geochemical anomalies
C h e m i c a l c h a n g e s d u e to a l t e r a t i o n
Studies of chemical variation related to gold mineralization have not been
30
E. IZAWA ET AL.
N6 )G
N" ~G
N; QQ
0G
S20G
540G
S60G
f
400ML
E20B 300Mr
L
L r
L ~
200MI
IOOML
0ML
-100ML
OG
NZOG
0G
S20G
S40G
S60G
400ML
W80B 300ML
L
200ML
v v v
v
vI IO(~L
OML
-IOOML
Fig. 12. Vertical alteration zoning on the E20B and the W80B cross sections along the direction of N40 °W {mine grid is in Fig. 5 and locations of the sections are in Fig. 13). I=cristobalitesmectite zone; H= quartz-smectite zone; III= interstratified clay mineral zone; IV=chlorite-sericite zone. Legend is same as in Figure 3 except for andesitic pyroclastic rocks of the Hishikari Lower Andesites which are shown by the area without pattern. completed yet, t h o u g h analyses of major and minor elements for the leastaltered and altered volcanic rocks, soil and plants in the Hishikari area were listed by M M A J (1988). Variation in major-element concentrations of volcanic rocks from the surface are generally small except for N a 2 0 values, which are remarkably depleted in the highly argillized rocks over the H o n k o vein system. The analytical data ( M M A J , 1988) also show background values for several
]~HE HISHIKARI G O L D DEPOSIT
0
0.5
I
3]
1kin ~\
=
I
\
25
\
,
25 ~
--,..
oo
).
Yamada~>,,.~, @
o
,;-
¢.J
/
o
"~.
,
o
,~
o~
o
..
o
o
N
,S ," I
/,"
oc
f
~
{
•
Q
c
'
\
•
"\
o
} .
I
"-.J
*
Fig. 13. Areal alteration zoning. I=cristobalite-smectite zone; //--quartz-smectite zone; I I I = interstratified clay mineral zone. The western side of the dashed line is covered by young
pyroclastic flow deposits. Small circles show sample locations; open circles denote the least altered rocks and solid circles refer to altered rocks. Light lines or dashed lines show the surface projection of the veins. Area of low resistivity {less than 25 ohm-m), determined by CSAMT (Kawasaki et al., 1986), is outlined by the heavy line. Mine grid, 0G ( N50 ° E) and 0B (N40W), and the locations of cross sections (E20B and WSOB in Fig. 12) are shown.
trace elements in Quaternary volcanic rocks of the Hishikari area. Most of the volcanic rocks contain 2-5 p p m As, 10-40 ppb Hg and < 1-1 ppb Au. In a few cases, argillized surface rocks directly above the Honko veins contain up to 15 ppm As, 1500 ppb Hg and 31 ppb Au. It is interesting to note that leaves of some plants like Callicarpa moles contain rather high gold (10-41 ppb on a dry basis) in the area over the vein system (laterally up to 400 m from the surface projection of the Honko veins). The gold content of sandstone and shale of the Shimanto Supergroup ranges from < 1 to 3 ppb for the least-altered rocks and ranges up to 64 ppb for altered rocks (Ishihara et al., 1986). Variations of JlsO in the Hishikari area Oxygen isotopic compositions of ore and whole rock were analyzed at the University of Queensland, Australia, with the results listed in Table 12. A1-
32
E. IZAWAET AL.
TABLE 12 Oxygen isotope data for whole rocks and ores from the Hishikari mine area (Takaoka, 1987) Sample no.
Location
Rock and ore
Ores (quartz RY-2 HO-1 QV
and adularia) 70mL E drive 18 + 1.0m 70mL E drive 40+2.3m DW-20 71.5m
Ryosen No.2 vein Hosen No.1 vein Quartz vein
+ 6.8 + 8.3 + 8.8
In Hishikari Lower Andesites In Shimanto Supergroup In Shimanto Supergroup
Volcanic rocks (least altered) U-13 Quarry in Yamada T-12 NE of mine T-20 NE of mine G- 1 NW of Yoshimatsu
Pyroxene Pyroxene Pyroxene Pyroxene
+ 8.5 +9.6 + 9.1 + 9.2
Hishikari Lower Andesites Hishikari Lower Andesites Hishikari Lower Andesites Hishikari Lower Andesites
Volcanic rocks (altered) HL-1 56MAHT-1 38.5m HL-2 56MAHT-1 100.0m HL-3 56MAHT-1 157.3m HL-4 56MAHT-1 200.0m HL-5 56MAHT-1 248.0m HL-6 56MAHT-1 300.0m HL-7 56MAHT-1 320.0m HL-8 56MAHT-1 359.7m HL-9 56MAHT-1 390.3m TN-1 No.1 Incline TN-2 No.1 Incline TN-3 No.1 Incline TN-4 No.1 Incline TN-5 No.1 Incline TN-6 No.1 Incline TN-7 No.1 Incline TN-8 No.1 Incline TN-9 No.1 Incline TN-10 DV-2 6.0m TN-11 DV-2 40.2m TN-12 DV-2 79.1m TN-13 DV-2 120.2m TN-14 DV-2 160.0m TN-15 DV-2 201.4m TN-16 DV-2 239.6m TN-17 DV-2 282.2m TN-18 DV-2 294.5m TN-19 DV-2 320.0m TN-20 DV-2 355.1m
Lapilli tuff Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccia Tuff breccm Tuff breccia Tuff breccia Andesite Tuff breccia Volcanic breccia Tuff breccia Lapilli tuff LapiUi tuff Andesite Tuff breccia Lapillituff Tuff breccia Tuff breccia Lapillituff
+8.1 + 8.8 + 10.0 + 5.6 + 6.8 + 7.4 + 3.3 + 6.2 + 4.1 +5.5 +4.0 +6.0 +4.8 +4.9 +6.3 +5.5 +6.8 +4.7 + 6.6 + 6.9 + 6.4 + 6.5 +4.0 +4.3 +6.6 +5.5 +7.2 + 5.0 +6.3
qz-Kf-sm-ch/sm-lm-cal-py qz-sm-ch/sm-lm-py Pale green (arg) qz-Kf-sm-ch/sm-lm-py sm-ser/sm-ch/sm qz-Kf-sm-ch/sm-lm-py qz-Kf-sm-ch/sm-cal qz-piag-sm-ch/sm qz-Kf-ch-ser Greenish white (arg, py) Pale green (arg, py) Pale green (arg, py) Pale green (arg, cal) Pale green (arg, py) Pale green (arg, py) Light greenish gray (arg) Green (silic, py) Dark green (propylitic) Greenish brown (arg, py) Greenish brown (arg, py) Light green ( arg, py ) Pale green (arg, py) Light gray (silic) Gray (chloritized) Pale green (arg, py) Light green (chloritized) Light green (py) Bluish green Greenish white (arg)
Shimanto Supergroup SH-1 DW-20 21.4 m SH-2 DW-20 49.0m SH-3 DW-20 70.3m SH-4 56MAHT-1 398.3m
Shale Shale Shale Shale
+12.3 +10.5 +14.4 +11.8
oz-se-ch
andesite andesite andesite andesite
J180 (%0)
Remarks
THE HISHIKARI GOLD DEPOSIT
33
T A B L E 12 (continued) Sample no.
Location
Rock and ore
$lsO (Too)
Sandstone > shale Shale Shale Sandstone > shale Shale Sandstone=shale Shale
+ 12.5 + 11.4 + 12.6 - 11.0 + 12.2 +10.0 + 12.3
Remarks
Shimanto Supergroup SH-5 SH-6 SH-7 SH-8 SH-9 SH-10 SH-11
56MAHT-1 56MAHT-1 56MAHT-1 56MAHT-1 56MAHT-1 56MAHT-1 56MAHT-1
432.6m 461.2m 511.4m 550.0m 600.0m 654.0m 701.0m
qz-plag-se-ch-py qz-plag-se-ch qz-plag-se-ch
Arg = argillic alteration; silie = silicification; qz = quartz; plag = plagioclase; K f = K-feldspar; s m - smectite; s e / s m - i n t e r s t r a t i f i e d sericite/smectite; se=sericite; c h / s m = i n t e r s t r a t i f i e d chlorite/smectite; ch = chlorite; lm -- laumontite; cal -- calcite; p y - - pyrite.
though the number of data are limited, $1so values of quartz veins range from + 8.8 to + 6.8%o, similar to those of quartz veins from the nearby Kushikino gold deposit (Matsuhisa et al., 1985 ). The temperatures of mineralization were typically around 210 °C in the basement and less than 200 oC in the Hishikari Lower Andesites on the basis of fluid-inclusion data (as discussed above). Based on the isotopic composition and formation temperature of the quartz, the 81s0 values of the hydrothermal fluids responsible for gold mineralization may be calculated from the equation for quartz-water isotopic fractionation given by Matsuhisa et al. (1979). Calculated 81so values for the fluid range between - 2 and -5%o, much heavier than that of local meteoric waters (-7.2%o; Matsubaya et al., 1975); this indicates extensive isotopic shifting by waterrock interaction. The whole-rock 8 ' s o values of andesitic rocks in the Hishikari area range from +3.3 to +10.0%o. The 81so values of the least-altered volcanic rocks from the Hishikari mine area range from + 8.5 to + 9.6%o, slightly higher than those of intermediate lavas (e.g., + 6 to +8%o; Taylor, 1968). A small amount of smectite was observed under the microscope in three of these four andesites. The rocks in the intensely argillized zone (the upper part of the interstratified clay mineral zone) covering the zone of high-grade veins also have higher 81so values (+8.1 to +10.0%o). These oxygen isotope results suggest that lowtemperature hydrothermal alteration has increased the whole-rock $ aso values (due to the increased fractionation factor at lower temperatures). In contrast, relatively low 8~so values (ranging from + 7.4 to + 3.3%o, averaging + 5.7%o ) were measured for altered volcanic rocks from the lower part of the interstratified clay zone and from the chlorite-sericite zone. The lower 81sO values of altered rocks are attributed to interaction of the rocks with hydrothermal waters; these were most likely local meteoric waters that were heated and isotopically shifted at greater depths. In particular, the lowest ei'so
34
E. IZAWAET AL.
values were ibr samples located close to (or over) quartz veins, indicating the highest temperatures a n d / o r the least amount of shallow water-rock interaction water-rock interaction (i.e. a higher permeability). Most Cretaceous rocks of the Shimanto Supergroup in southwest Kagoshima Prefecture have been metamorphosed at least to prehnite-pumpellyite facies (MITI, 1985), and hydrothermal alteration of sandstone and shale of the Hishikari deposit is generally not conspicuous. The sedimentary rocks have a narrow range of ~lso values of + 10.0 to + 14.4%o; a low g 1 8 0 anomaly was not observed, indicating very little interaction (due to low permeabilities) with hydrothermal fluids.
Volatile components in soil gas Since the middle of the 1970s, the geochemistry of volatile components such as Hg, CO2 and radon (Rn and T n ) in soil gas has become one of the most useful exploration tools in active geothermal fields. This is because these volatile components can migrate from deep geothermal fluids to the surface through fractures such as faults and veins (e.g., Koga and Noda, 1974, 1975; Stoker and Kruger, 1975; Whitehead, 1981 ). The Hishikari gold veins are accompanied by hot spring water at a temperature of about 65 ° C and with high CO2 contents, up to 500 mg/kg. Therefore, the geochemical prospecting technique using soil gas was applied to reveal the relationship between the distribution of the volatiles at the surface and the known gold vein systems, and also to estimate the extension of the veins. Sampling: Sampling points were arranged to cross the projection of the vein system to the surface, with some additional points near the center of the vein system. The sampling was initiated by making a hole 60 cm deep and 4 cm in diameter with an iron pipe. Radon and CO2 were analyzed in situ. All work carried out in the field takes approximately 20 minutes per sample point. Rn and Tn: After extracting the iron pipe, a portable radon detector (alpharay scintillation counter; Radon Detector RD200, EDA Instruments Inc., Canada) was positioned in the hole (Fig. 14). Soil gas was then introduced to the fluorescent cell and counted for three successive one minute intervals. Although a r a d o n / t h o r o n ratio ( R n / T n ) was obtained from the three successive measurements, only the total counts of the three measurements are discussed in this study. C02: After measuring Rn, CO2 was measured using a Kitagawa Precision Gas Detector (Komyo Rikagaku Kogyo Ltd., Japan). The detector carries out a dry analysis based on the principles of chemical reaction and physical absorption. After introducing 100 ml of soil gas into the detector tube from the bottom of the hole, the concentration of CO2 was determined by reading the length of reagent reacted in the tube with the CO2 of the soil gas. Hg: A 10-cm-long pure gold wire with a diameter of 1 m m was left for three
THE HISHIKARI GOLD DEPOSIT
35
~ RStopper.~ ubber ~ P
GasCell
~ee D lco ,rJI
Gas Fig. 14. Samplinghole and equipmentsetup for the radon detector.
days to capture Hg gas in the soil gas. A soil sample from the bottom of the hole was also collected for determination of Hg in the soil. The amount of Hg adsorbed on the gold wire and that of the soil sample was determined in the laboratory by flameless atomic absorption spectrometry. The amount that can be detected by our instrument is 0.1 × 10-gg of Hg, such that the detection limit is 0.1 ppb if 1 g of soil is used.
Inferred fracture zones: The distribution of radon, COe and Hg in soil gas, and Hg in soil, are shown in Figure 15. The values above the background concentration of each volatile component for the area are divided into three groups. Relatively high concentrations of each volatile gas correspond to the subsurface position of the known Honko vein system. In addition, high-volatile concentrations are also located in the eastern and western areas on the extension of the veins, and also to the north and south. In particular, all of the components are anomalously high and widely distributed in the west. Radon has a half life of 3.82 days for R n 222 and 54.5 seconds for T n (Rn22°), respectively. Therefore, high radon concentrations indicate the existence of highly permeable zones which permit a short period of migration from the deep Rn source; alternatively, the source is close to the surface. The CO2 concentrations in hot spring waters from the Hishikari mine are high; therefore, CO2 also appears to be a good indicator of fracture distribution. However, radon and C02 may be affected by groundwater due to their relatively high solubilities. Since Hg is very volatile, and its vapor pressure is sensitive to slight temperature variations, Hg in soil gas effectively indicates the location of permeable fractures connected with relatively high-temperature fluid. Although high concentration of Hg in soil is evidence for hydrothermal activity, if Hg in the corresponding soil gas is low, sealing of the fracture is indicated.
36
E. IZAWAET AL. w~o
w,:o
e,:o
6
•
/
06:6~
• •
O•
~:2 0,77 •
~s3 0178
:239
0 2 8°
~.s Oz~
zu-~ "
.................
~
•
0343
~150
02s4
•
03o2
N40
O.
D*-
..... 0~3 " 2"2~. . . . . . . . . . • • • .Y-G ~,~27o zu-2 • ~_%/,6~. ~ w..~
• -
o _,o
?,43
o.. .......
02S2 0867
05
•
. . ~ : ~ . ~ . o -o,,.
~ ~
•
0216
•
wn I
din*
Ws~ 03e?
o
0-
~2~ "~-" O~s2
• •
•
•
0336
Rn+Tn (counts/3min)
S20-
O~
3so<__O 250<_• < 350
0194 0178
150_< • < 250 50_< • < 150 e < 50
Radon
~'"
Vein 7OraL Vein Z*OmL-
--,
i
n
6
i
w~o
I
w,~o
O~so
I
E,:O
6
•
O 0 70
•
• 0s0
O080
I
400m
260
/
•
•
••
03.3 •
N40
•
017o • o 7o Oo 6s •070
0453
Oo Oo~
zu, " -
0o7o "
......................... oq~~7~ - - Oo 60---0,__-].....
•070 •
~343
zu-2
.-~':~-3~,.
-
0200
•
•
•
•
O. ."Y=6 . . ' i . / .
2.~0
@no
0o~5 •
0050
0 2 oo 03~
T a ",ooo
0o90 •
HO-~
o
•
•
•
•
•
•
520-
COz (%)
toosO 0.50< • <1.00
. .~-'" Vein 70mL Vein 40m
0.30<_ • <0.50 0.10<_ • <0.30
C02
• <0.10 i
J
i
i
i
I
6
2(30
i
J
400m i
Fig. 15. Geochemical maps showing the distribution of radon (Rn + T n ) , CO2 and Hg in soil gas, and Hg in soil. The vein pattern was projected to the surface from 70 mL and 40 mL. The base line of the mine grid is parallel to the general strike of the veins (N50°E). Rn: the numbers with solid circles are total radon counts of three successive one-minute intervals. C02: the numbers with solid circles show volume percent of C02. -,
THE HISHIKARI GOLD DEPOSIT
wso
37
w~o
002
6
E~O
Oo?
0o, Oo.5
O03
Oo6 Oos
O,, O23,
O 0 ~.
O o ,;
O1.1
~.~ O0s
............. ,-.
.....
O0 ~.
O21
~-~q~o
o~.,
~
Or4
~,~!;t ......
Oo9
z+~.
,+,
""
&
"i,9
. . . . .
N 40
~?_
. . . . .
O0.3
oo~
9.o.9
,~,~,,, _
~
'
"::;:'-:+
~
RY'~"
0o5
;
O01
O0.5
..........
01,t
zu,
O,.o
o 01.4
10.3
o0.+ 11.0
/
J'
O0? •02 002
003
0o6
-
103 O1.6
O03
•0 2
• o4 %.i
520
002 •o6
Hg in soil air (lOgg/3days)
15mO
O0.3
~ " " Vein 70mL Vein 40rnL
1.1<-0 <15
0.7_< • <1.1 0.3<_ • <0.7
Hg in gas
• <0.3 i
L
i
WSO
i
I
W40
6
260
i
i
6
~6om i
E40
O168
/
0127 02 033
025 05O
~246
017
051
0365
In, 1122
e;n
03O •
................
026;
079
......
..........
~O,o,
•
oA-;
°" . . . . . . . . . .
•152
O2=+ +
ZU-I
331 , .~.+++~r
2%1 ,
~.
e++
~i.+ ...........
•
2
zu-z ,~Os8
..... ~
N40
079
OS90 O~72
O234
=
~.150
RY-6
~
~
• ......
- +-
•99 0183
-"~+'
a ~ '~°
121B
0356
!-123 0158 060
•;3s
~-~.o-, 8
o
•
• 0233
096 08~
• 520
11o7
Hg in soil(ppb) 041o • 155
275~0 200~ • 125 <_• 50_< • • =
< 275 <200 <125 < 50
....--Vein 7OraL J Vein 40mL-
Hg in soil i
i
i
; i
i
26o
~6om i
Hg in soil gas: the numbers with the solid circles show the weight of Hg (in 10 -9 g), which was adsorbed on a gold wire over 3 days. Open circles indicate points which were not measured. Hg in soil: the numbers with the solid circles are Hg contents in ppb.
38
E. IZAWAET AL.
wBo : ~
w~,o
E~0
6
'~i:;~
/ •
"///////~/J//~
";~::::Z<"//b"
N40
~{i~,i::i ~i
.............
ii:i
.
520 -
~
....... 2(~0
i
i
i
i
i
I
i
Vein
70mL
Vein
40rnL"
i i.OOm
i
Fig. 16. Fracture zones inferred by geochemical prospecting using volatile components. Hatched areas are fracture zones determined from multiple anomalies of volatiles, and dotted areas by a single gas anomaly. The base line of the mine grid is parallel to the general strike of the veins (N50°E).
The hatched areas in Figure 16 show the location of fracture zones inferred from anomalously high concentrations of three or four volatile components. The deduced fracture zones agree with the known Honko vein system. The results also indicate that the Honko zone probably branches off to the westsouthwest and southwest at its western end. The wide shaded zone to the west indicates that fractures are common in this area. In fact, the recently discovered Yamada vein system is likely to be an extension of the southwest branch. GEOPHYSICAL EXPLORATION
In the Hishikari area it was shown during the early exploration stage that gravity and resistivity anomalies have a close relation to the mineralized zone (Fig. 2). The magnetic data from the airborne survey, however, showed no definite anomalies over the Hishikari ore zone (Johnson and Fujita, 1985) because the area is covered by volcanic rocks that are little altered and have relatively high magnetic susceptibilities.
THE HISHIKARI GOLD DEPOSIT
39
During the period 1982 to 1987 a comprehensive suite of geophysical exploration techniques was applied over the Hishikari vein system by SMM to clarify the extension of buried mineralization and to correlate the anomalies detected by various methods, and also to assess their applicability for similar mineral deposits. Structural models obtained from gravity, Schlumberger, controlled source audio frequency magnetoteUuric (CSAMT) and magnetotelluric (MT) data, and the distribution of hydrothermal alteration (conducting zone) indicated by CSAMT, transient electromagnetic (TEM) and induced polarization (IP) data are discussed below, and are compared with known geology.
Subsurface structure obtained from various geophysical surveys Gravity survey The reconnaissance gravity survey by MMAJ revealed a 4-reGal gravity high over and slightly northeast of the present Hishikari veins. The most probable source of this gravity high is the very shallow basement, which rises to 130 m elevation at its shallowest level (about 100 m below the surface). The gravity map by MMAJ (MITI, 1977) thus reflects the general feature of the subsurface structure at Hishikari, and was useful in targeting exploration. A further detailed gravity survey was carried out by SMM over and around the Hishikari area in order to elucidate the correlation between the gravity high and the mineralized zone, and to locate other small gravity highs. Preliminary results were described by Kawasaki et al. (1986). Using two La Coste & Romberg Model G gravimeters, 1230 observations were made at 200-300-m intervals along all the roads and paths over an area 18 km east-west by 24 km north-south. A part of the Bouguer anomaly map near the Hishikari mine is shown in Figure 17. The most conspicuous feature is the previously mentioned gravity high centered directly over the Hishikari deposit. This "Hishikari gravity high", bounded by the 12.5-mGal contour, is elongate southwestward and shows steep peripheral gradients toward the northwest and particularly toward the southeast. Except for this feature, the area is characterized by a relatively fiat gravity field, with contours of 0.5-mGal intervals generally trending east northeast-west southwest. The gravity low present in the northwest corner of Figure 17 is the southeast extension of the low anomaly of the Okuchi basin. The Shimanto Supergroup comprises sandstone and shale which have the highest density among rocks in the area (average wet density of drill core: 2.62 g/cm3). The overlying Hishikari Lower Andesites consist of abundant tuff breccias and a small amount of lava flows and have lower densities (average: 2.37 g/cm 3) than the basement rocks. Among the younger volcanic rocks fresh andesites have a comparable density with that of the basement rocks, and pyroclastic flow deposits have an average density of 2.25 g/cm ~. The density
40
E. IZAWAET AL.
Fig. 17. Bouguer anomaly map. Contour interval is 0.5 reGal for reduction density = 2.40 g / c m 3. Open circles show locations of gravity stations. The dotted area indicates the mineralized zone of the Hishikari deposit. The area is shown in Figure 3. Lines A-A', B-B' and C-C' are the locations of the sections shown in Figures 18 and 19.
of the entire volcanic pile thus averages 2.40 g/cm3; a density contrast of 0.20.25 g/cm ~ was used in the depth-to-basement analysis. The Hishikari gravity high infers local doming of the basement (the Shimanto Supergroup); numerical modelling by a three-dimensional analysis indicates that the vein system occurs where the basement rises to an elevation above 100 m (Kawasaki et al., 1986). This doming was confirmed by drilling and underground development. The elevation of the top of the Shimanto Supergroup outside of the Hishikari area, obtained from the gravity distribution, is generally - 5 0 0 m to below - 1 0 0 0 m, agreeing with known depths. It is concluded that the gravity distribution in the area primarily reflects the basem e n t topography.
THE HISHIKARI GOLD DEPOSIT
41
Resistivity structure The Schlumberger soundings were conducted over the area on two lines 9 km long running in a northeast direction at 1450-m spacing in order to analyze the subsurface structure (Yokoyama et al., 1987). A maximum electrode spacing of AB/2 = 2000 m was used, and readings were taken every 250 m; these were modified to AB/2 = 4000 m and 125-m intervals over the mineralized zone in the central part of the traverses. Figure 18 shows the results of the sections analyzed along line-A and line-B of Figure 17. In spite of the two lines being 1450 m apart, there is a close correlation between the two analyzed sections, demonstrating the continuation of geologic structure along the northeast direction. The areas can be basically interpreted as having a three-layer structure of high-low-high resistivity. The upper layer, with high values of 100-1000 m ohm-m, corresponds to the unaltered volcanic rocks. This layer is almost absent directly over the Hishikari veins, as seen around Station 18 of line-A, with the thickness increasing toward the northwest and southeast. The second layer generally has a low resistivity of 10-20 ohm-m, and can be interpreted as due to hydrothermally altered volcanic rocks consisting mainly of the Hishikari Lower Andesites and the lower part of the Shishimano Dacites. Extremely low values (2-8 ohm-m) observed around the vein system are due to intense hydrothermal alteration. There are relatively high-resistivity layers beneath the first layer away from the mineralized zone, in particular to the southeast. These probably represent a weakening in the intensity of alteration. The third layer of relatively high resistivity {80-150 ohm-m) corresponds to the Shimanto Supergroup. The top of the third layer is shallow over the mineralized zone, e.g., 130 m below the surface on line-A and 150 m on line-B. The depths increase gradually toward both ends of the lines to about 800 m below the surface at the northwestern ends. Toward the southeast the depths increase abruptly between stations 20 and 21, suggesting a fault-like structure, which is also inferred by topographic features running northeast to southwest. Another step down structure is seen further to the southeast, also corresponding to a topographic lineation which is interpreted as a fault {Fig. 3). In situ electrical logging shows that resistivity values of andesites and tuff breccia range from 125 to 250 ohm-m at shallow levels and decrease to about 20 ohm-m in the hydrothermally altered zone. Resistivity values of the basement range from 100 to 150 ohm-m for shales and from 200 to 300 ohm-m for sandstone. Although these values are not representative of the resistivity of rocks over the whole area, they are consistent with resistivity values obtained from the Schlumberger soundings. As stated above, the depth to the top of the third layer relates to that of the Shimanto Supergroup. This shows the usefulness of the Schlumberger sound-
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THE HISHIKARI GOLD DEPOSIT
43
ing to understand the subsurface structure in the Hishikari mine area, for which two-dimensional modeling of multi-layers is primarily applicable. In Figure 18 the profiles of the top of the basement obtained from threedimensional gravity analysis are superimposed on the Schlumberger cross sections. The profiles are similar to those of Schlumberger soundings; in particular, the two depth estimates coincide in the central part of line-B. There is, however, a large discrepancy to the southeast. The depth to the top of the basement obtained by the gravity survey might be too shallow as shown, because gravity data were analyzed on the assumption of the two-layer structure, neglecting the presence of the high-resistivity intermediate layer with presumably high density to the southeast. A high-resistivity zone ( > 500 ohm-m) is suggested at depths of more than 1000 m below the surface. Although the electrode spacing (AB/2) is too short to delineate the complete shape of this high-resistivity zone, the Schlumberger section provides the possibility for the presence of a shallow intrusive body or a thermally metamorphosed zone related to the intrusive rock. The C S A M T (controlled source audio-frequency magneto-telluric) survey is a rapid and relatively inexpensive method to determine the resistivity distribution at shallow to medium depths over an area, since only the receiver is moved from station to station once the current sources are set up. A Zonge GDP 12 system was used. The depth of penetration of the CSAMT survey is estimated to be about 800 m in this area, on the basis of the skin depth calculation and signal-to-noise ratio. The depth to the top of the resistivity basement obtained from one-dimensional analysis is in general similar to those of Schlumberger soundings. However, the lateral continuation of the resistivity layers is often not obvious, probably owing to static anomalies related to the presence of a thick layer with low resistivity near the surface. Thus the estimated depth to the basement did not agree with the actual depth in the central part of the Honko ore zone. A resistivity structure cross section (Fig. 19) along an approximately n o r t h south line in the western part of the survey area shows basically a three-layer structure of high-low-high resistivity, similar to that in the Honko ore zone described by Kawasaki et al. {1986). In Figure 19, the depth to the basement is about 400 m in the central part and becomes deeper (about 800 m) towards the north and south; in particular, it steepens abruptly to the south, as observed in the Honko ore zone. An M T (magnetotelluric) survey was carried out using a Beicip M T 5C system in order to know the large-scale resistivity structure at depth and to evaluate its applicability to gold exploration (Kawasaki et al., 1986). Since the survey area is densely inhabited and there are several high-voltage power lines, two sets of five-component data acquisition systems (reference and mobile
44
E. IZAWAET AL.
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THE HISHIKARI GOLD DEPOSIT
45
systems) were employed to increase the reliability of computed M T parameters. Thirty observation stations were scattered over the whole area as uniformly as possible. CSAMT measurements were also made at seven of the sites for comparison, and to obtain shallow information. The distribution of apparent resistivity at low frequencies of 0.01-0.1 Hz reflects the deep subsurface structure, which can not be obtained by other electrical and electromagnetic surveys. Figure 20 shows the distribution of apparent resistivity obtained by the transverse electric (TE) mode at 0.03 Hz. A wide-spread low-resistivity zone is present at depth to the southeast of the mineralized zone. A resistivity cross section (Fig. 21 ) was obtained by Bostic inversion using apparent resistivities at 0.03 to 200 Hz and apparent phase curves for nine measured points. This analysis reveals an overall feature of the vertical distribution of resistivity in this area. There are marked low-resistivity zones (a few ohm-m) at shallow levels which coincide with the zone of veins, and at deep levels (more than 10 km below the surface) that might correspond to a partially molten magma.
Delineation of near-surface alteration zones Resistivity distribution from high-frequency sounding data The resistivity structure of this area consists basically of three layers, highlow-high. The intermediate layer of low resistivity, which is located in the vicinity of mineralized zones, can be attributed to the hydrothermally altered rocks. It is thus important for delineation of mineralized zones to know the depth and the areal distribution of the low-resistivity layer. Kawasaki et al. (1986) reported the results of CSAMT and M T surveys. The distribution of apparent resistivity obtained by CSAMT at 128 Hz represents a depth of 200-300 m from the surface. A low-resistivity zone of less than 25 ohm-m is present in the Hishikari area, trending northeast (Fig. 13). In the western part of the low-resistivity zone, about 1 km southwest of the Honko area, the Yamada vein system was recently discovered. The distribution of apparent resistivity obtained by M T at higher frequencies (10-100 Hz) coincides well with that at similar frequencies of the CSAMT survey. The transient EM survey was carried out in the western part of the Hishikari area, and a distribution of low-resistivity ( < 5 ohm-m) anomalies similar to the results of CSAMT was obtained (Kawasaki et al., 1987). The low-resistivity zones at the shallow depth overlap the Hishikari gravity high, indicating that mineralization took place in the area of uplifted basement and was accompanied by intense argillic alteration at shallow levels.
Results of the IP survey As described above, the low-resistivity zone extends northeast to southwest, enveloping the gold-bearing quartz veins. In underground, wall rocks of quartz
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THE HISHIKARI GOLD DEPOSIT
47
veins are often impregnated with pyrite and thus IP anomalies may discriminate mineralized zones. Using a Scintrex IPR-11 system, a time-domain IP survey was conducted. A primary voltage Vp, a secondary voltage Vs, chargeability ( Vd Vp) of ten slices (Mo-Mg) after the current-off were observed. A Dipole-Dipole array, with basic electrode spacing (a= 50 m) and electrode separation (n= 1-6 and n= 27 ) was used. Twenty two lines of 42 line-km total were measured at every 50 m with a 300-800-m line interval.
Detailed apparent resistivity distribution of the shallow level was obtained by the IP survey, and the expanding Eltran configuration provided information of the deeper portion by increasing electrode separation (n). Figure 22 shows the distribution of apparent resistivity for n - 4, indicating a relatively shallow
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Fig. 22. Apparent D C resistivity contour m a p using dipole-dipole array with a = 50 m and n = 4 (in ohm-m).
48
E. IZAWA ET AL.
T "\
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Fig. 23. Apparent ehargeability contour map using dipole-dil~le array with a = 50 m and n = 4 (in mV/V). Chargeability of window M8 (1050-1410 ms) is shown. depth (around 125 m from the surface). A contour of 10 ohm-m surrounds the low-resistivity area which contains the ore zones. The area is elongate toward the southwest, as noted for other electrical and electromagnetic surveys. To the northeast, the low-resistivity zone terminates abruptly at line R, where elevations increase and less altered volcanic rocks cover the area. T h u s the eastern extension of the altered zone is not clear. The low-resistivity zone extends towards the southwest, though the width is narrow at line G, where elevations become higher. The gravity map (Fig. 17) shows a change in the pattern of contours across Sendai River, indicating a structural change coincident with the river. This implies that the southwestern end of the low-resistivity zone exists around line Q.
I P anomalies of this system are expressed by the chargeability in m V / V (decay voltage divided by primary voltage and duration). Figure 23 is a plan
rilE HISHIKARIGOLDDEPOSIT
49
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of the chargeability of the 9th slice (Ms) obtained for an electrode separation of n = 4. Although amplitudes of IP responses in this area are weak compared with those in areas of sulfide deposits, the trend of high chargeability is present over the Honko ore zone toward the southwest, in particular around E-, F- and
50
E. IZAWAET AL.
G-lines. The recently discovered Yamada ore zone exists near the high-chargeability area around E- and F-lines. The pseudosection of C-line (Fig. 24) shows the apparent resistivity, chargeability and resistivity structure obtained by the two-dimensional analysis. The Honko ore zone, which exists directly below point 15, is associated with the "pants legs" pattern, the typical downward flaring shaped IP anomaly, and shows low resistivity from the surface. The results of the IP survey suggest that the method is effective in prospecting for epithermal gold quartz veins. The high-chargeability zone within the low-resistivity area could be attributed to the presence of pyrite impregnation in the argillic alteration zone, and correlates well with the presence of quartz veins.
DISCUSSION AND CONCLUSIONS
Quaternary magmatism and gold mineralization Magmatism has migrated eastward in southern Kyushu from middle Miocene to the present (Izawa and Urashima, 1987). Andesitic volcanism in the Hishikari area, which comprises part of this magmatism, began in the early Quaternary and continued for more than one million years. During the waning stages of volcanism, felsic magma probably intruded to a shallow level, with a portion of the magma extruding as the Shishimano Dacites. The result of the MT survey suggests the presence of a partially molten magma at deep levels (Fig. 21 ), with a shallow intrusive body indicated by the Schlumberger sounding (Fig. 18 ). The magmatic intrusion-induced uplifting of the basement block caused high-angle fracturing of the basement sediments and overlying volcanic rocks. The associated heat anomaly generated the Hishikari hydrothermal system one million years ago. The extremely high-grade gold deposit in the Honko area at Hishikari formed near the unconformity between sedimentary rocks of the Shimanto Supergroup and andesitic volcanic rocks of Quaternary age (Fig. 8). Although the mechanism of gold deposition is not fully understood, mineralization was strongly affected by the presence of this unconformity (Ishihara et al., 1986; Ishihara and Morishita, 1987; Izawa, 1988). The presence of bladed quartz and large amounts of adularia, as well as evidence from fluid inclusions, suggest that boiling was common during mineralization of Hishikari. In several active geothermal fields, bladed calcite is observed in zones of boiling (Browne and Ellis, 1970; Keith and Muffler, 1978). Urashiriia (1956) described platy calcite filled with granular quartz and adularia aggregates in quartz veins of the Konomai epithermal gold veins in Hokkaido; he concluded that lamellar quartz was formed by the dissolution of the platy calcite. Thus the common presence of lamellar quartz in the Hishikari
THE HISHIKARI GOLD DEPOSIT
51
Honko veins indicates that bladed calcite was once deposited during boiling and subsequently dissolved during cooling, leaving the lamellar quartz. The occurrence of sericite in the wall rock adjacent to veins, in contrast to the dominance of adularia in the veins, is similar to that noted at many other epithermal deposits (Buchanan, 1981). This shift in K-silicate mineralogy supports the above suggestion that boiling was common in the veins, as gas loss favors adularia over sericite. Homogenization temperatures of fluid inclusions are plotted against elevation for Hishikari (Fig. 25). At the nearby active geothermal field of Kirishima, fluid inclusion temperatures show a wide range at each depth (Taguchi and Hayashi, 1983); the maximum temperatures at each depth are usually on or near the boiling-point curve of water, with fluid-inclusion temperatures rangElevation
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Fig. 25. Fluid-inclusion temperatures versus elevation. Boiling-point curve for solution of 2 wt.% NaC1. Solid circles denote data for adularia at an elevation of 80 m.
52
E. IZAWA ET AL.
ing down to the present (subboiling) measured temperatures, thus recording cooling of that system. This relation between fluid-inclusion temperatures and the boiling-point curve may be applied to the Hishikari hydrothermal system. Assuming an average temperature of 243 ° C for adularia at 80 m elevation, the hydrothermal water table would have been at least 470 m in elevation if constrained by boiling relations for fluids of 2 wt.% NaC1. This implies that the depth of erosion ranges up to 240 m over the Honko vein system. The range of fluid-inclusion temperatures records cooling below boiling point temperatures, as at Kirishima. The cluster of fluid-inclusion temperatures around 200 ° C may reflect temperatures of gold mineralization. Visible electrum bands are often present between early adularia-quartz and the later smectite-quartz bands in the veins, and have extremely high gold values. This mineralogical change observed over a narrow band in the veins, and the distribution of fluid-inclusion temperatures, indicate that hydrothermal waters cooled from about 250 ° C to less than 200 ° C at the site of gold deposition around the unconformity. Since such rapid cooling can not be attributed to simple boiling, mixing with cooler groundwater is a possibility (Izawa, 1988). Mixing of ascending high-temperature fluids with low-temperature groundwaters, resulting in rapid cooling and oversaturation of silica relative to quartz solubility, is also supported by several lines of evidence. Fine aggregates of adularia and kaolinite occasionally occur in the quartz veins. Normally, adularia and kaolinite are separated by the sericite stability field. However, adularia-kaolinite can be a stable assemblage if the silica content of waters is near cristobalite-saturation over the temperature range of 150 to 200 ° C, i.e. supersaturated with respect to quartz (Izawa, 1985: calculation using thermodynamic data of Helgeson, 1969). Although the equilibrium relation is not conclusive on the basis of different thermodynamic data (for example, Helgeson et al., 1978), the coexistence of adularia and kaolinite without sericite may indicate a high silica concentration. Truscottite is a minor mineral in the Honko veins, as in other high-grade gold deposits. On the basis of synthetic experiments (Harker, 1964), it was suggested that truscottite forms from waters supersaturated with respect to quartz. Therefore, the presence of truscottite and the apparent equilibrium between adularia and kaolinite indicate that the ascending fluids were cooling very rapidly (due to rapid upflow and boiling as well as subsequent mixing with ground waters), such that quartz deposition could not maintain the silica concentration at quartz saturation. The presence of a large volume of low-temperature water may also be deduced from oxygen isotopic data of altered rocks. There are significantly large variations (up to 6%o ) in the oxygen isotopic values of altered volcanic rocks between 100 and 200 m elevation, with the heaviest values for the shallowest samples. Using the fractionation between water and quartz (Matsuhisa et al.,
T H E HISHIKARI GOLD DEPOSIT
53
1979), this variation would imply a temperature differential as large as 80 oC, which can not be explained by simple cooling of ascending fluids along the boiling-point curve (Fig. 25). Thus the shallow, relatively heavy oxygen isotopic compositions support the presence of a marginal water related to the argillic alteration. This water would likely have been steam-heated to temperatures of 100-150 oC, as noted in active geothermal systems (Hedenquist and Browne, 1989). In addition, whole-rock oxygen isotopic variations as large as 3%o over a few tens of meters laterally suggest a variable degree of water-rock interaction, as the lowest oxygen isotope compositions appear most closely related to fractures. This is consistent with large volumes of fluid flow in veins in contrast to the adjacent wall rock. The volcanic rocks contain magnetite and hematite, and the basement sedimentary rocks near the unconformity are hematite-bearing, the latter probably due to weathering at the sediment paleosurface. This indicates relatively oxidizing conditions as compared with the deeper basement environment, where pyrite and pyrrhotite occur with carbonaceous matter. The iron content of sphalerite shows a marked decrease (from 5 to < 0.02 mole % FeS ) with about a 100-m increase in elevation from 10 m. This decrease in FeS content of the sphalerite reflects an increase in the redox state of the fluid at higher elevations (probably due to mixing with steam-heated groundwater that equilibrated along the relatively oxidized unconformity). Deeply circulating waters, oxygen-shifted from water-rock interaction at depth, ascended along fractures; given the mineralogy of the system, gold would have been transported as a sulfide complex. The complicated structure of veins implies repeated fracturing (a sudden drop in pressure) and sealing. Therefore, gold deposition occurred following H2S loss due to boiling, as well as after rapid cooling and oxidation caused by mixing of the ascending fluids with cooler, oxidized (and probably steam-heated) groundwaters at the permeable zone around the unconformity. In the sedimentary rocks, the pH was buffered at close to neutral by the coexistence of potassium feldspar and sericite. The nearneutral pH is also indicated by the presence of calcite, laumontite and smectite in the high level alteration zone. This mechanism of gold deposition related to the unconformity will be clarified through a comparative study of the somewhat lower-grade gold mineralization hosted largely in the volcanic rocks of the Yamada area when mine development reaches that area.
Significance to exploration Gravity and resistivity anomalies are important in target selection in the Hokusatsu volcanic region of Kyushu. Subsurface structures can be recognized by a gravity survey using several observation points per kin2;upliftedbasement blocks represented by gravity highs are favorable to mineralization, probably because they focused fluid flow as discussed above. Following a gravity survey,
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Schlumberger vertical soundings are useful to obtain the depth to the top of the basement and to know the approximate resistivity structure. Resistivity surveys and an IP survey can delineate the areal distribution of the subsurface hydrothermal alteration related to mineralization. Geochemical exploration using Hg, CO2 and radon in soil gas is effective in tracing the buried fracture zones. ACKNOWLEDGMENTS
We are grateful to the management of the Sumitomo Metal Mining Co. Ltd. for permission to publish this paper. Special thanks go to J.W. Hedenquist and R.W. Henley for their encouragement and helpful comments. E.I. also wishes to acknowledge the help of Y. Nakae with chemical analyses, Y. Nagae with XRD and XRF studies, and J. Nishioka and M. Saito with fluid-inclusion studies. REFERENCES Abe, I., Suzuki, H., Isogami, A. and Goto, T., 1986. Geology and development of the Hishikari mine. Min. Geol., 36:117-130 ( in Japanese, with English abstract). Aramaki, S., 1968. Geology of the Kakuto basin, southern Kyushu and the earthquake swarm from February, 1968. Bull. Earthquake Res. Inst., 46:1325-1343 (in Japanese with English abstract}. Browne, P.R.L. and Ellis, A.J., 1970. The Ohaki-Broadlands hydrothermal area, New Zealand: mineralogy and related geochemistry. Am. J. Sci., 269: 97-131. Buchanan, L.J., 1981. Precious metal deposits associated with volcanic environments in the southwest. In: W.R. Dickinson and W.D. Payne (Editors), Relations of Tectonics to Ore Deposits in the Southern Cordillera. Ariz. Geol. Soc. Dig., 14: 237-267. Fukuoka Bureau of International Trade and Industry, 1959. Mining Industry in Kyushu. Mine Safety Department, Fukuoka Bureau of International Trade and Industry, Fukuoka, 372 pp. (in Japanese }. Harker, R.I., 1964. Dehydration series in the system CaSiO3-SiO2-H20. J. Am. Ceram. Soc., 47: 521-529. Hedenquist, J.W. and Browne, P.R.L., 1989. The evolution of the Waiotapu geothermal system, New Zealand, based on the chemical and isotopic composition of its fluids, minerals and rocks. Geochim. Cosmochim. Acta, 53: 2235-2257. Helgeson, H.C., 1969. Thermodynamics of hydrothermal systems at elevated temperatures and pressures. Am. J. Sci., 267: 729-804. Helgeson, H.C., Delany, J.M., Nesbitt, H.W. and Bird, D.K., 1978. Summary and critique of the thermodynamic properties of rock forming minerals. Am. J. Sci., 278-A: 1-229. Ishihara, S. and Morishita, Y., 1987. Gold deposits in the Hokusatsu district, especially the Hishikari deposit: structural controls and genetic model. Geol. News, 389:7-20 (in Japanese ). Ishihara, S., Sakamaki, Y., Sasaki, A., Teraoka, Y. and Terashima, S., 1986. Role of the basement in the genesis of the Hishikari gold-quartz vein deposit, southern Kyushu, Japan. Min. Geol., 36: 495-509. Izawa, E., 1985. Alteration zoning and clay minerals in epithermal gold-silver deposits - examination of geothermal model. In: Y. Urashima {Editor), Gold-Silver Ores in Japan, no. 3. Min. Metall. Inst. Jpn., Tokyo, pp. 133-154 (in Japanese). Izawa, E., 1988. Science of exploration for gold. Kagaku (Science), 58:15-23 (in Japanese). Izawa, E. and Nakae, Y., 1983. Xonotlite, truscottite and gyrolite from gold veins in Kyushu.
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