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Ta Nang gold deposit in the black shales of Central Vietnam
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Ta Nang gold deposit in the black shales of Central Vietnam Tran Tuan Anh a, I.V. Gaskov b,c,*, Tran Trong Hoa a, A.S. Borisenko b,c, A.E. Izokh b,c, Pham Thi Dung a, Vu Hoang Li a, Nguyen Thi Mai a a
Institute of Geological Sciences, Vietnam Academy of Science and Technology, 84 Chua Lang, Dong Da, Hanoi, Vietnam b V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia c Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 4 July 2014; accepted 21 October 2014
Abstract The Ta Nang gold deposit is localized in Middle Jurassic black shales. The ore zone is a series of layer-by-layer crush zones and zones of hydrothermal rock alteration, ≤10 m in thickness and >2 km in length. It consists of quartz–sulfide veins, sulfidized black shales, and their hydrothermally altered varieties. Sulfide mineralization occurs as two assemblages: early pyrite–arsenopyrite and late chalcopyrite–sphalerite– galena. The pyrite–arsenopyrite assemblage is composed of different morphogenetic varieties. Coarse-crystalline arsenopyrite and pyrite aggregates and metacrystals of different orientations, 0.1 to 10 mm in size, are the most widespread. The chalcopyrite–sphalerite–galena assemblage is scarce. Along with the main ore minerals, it includes more rare minerals: pyrrhotite, lead sulfosalts (tsugaruite), and gold, which form a spatial assemblage with the main minerals or small inclusions in them. Gold occurs mainly as fine dissemination in cracks in pyrite, arsenopyrite, chalcopyrite, and quartz. Gold content in sulfidized carbonaceous shales is no more than tenths of ppm, averaging 0.38 ppm. This content in the quartz veins is considerably higher, averaging 3.92 ppm. Silver contents in the shales and quartz veins are similar and equal to 2.68 and 5.30 ppm, respectively. Also, the sulfidized rocks and veins have elevated contents of Fe, As, Pb, Zn, Cu, Cd, Ni, and Co; most of these elements (Fe, As, Pb, Zn, and Cu) make up their own sulfide minerals, and the others are trace elements. According to 39Ar/40Ar dating of sericite from the quartz–sulfide veins, their age is 129.3 ± 5.6 Ma, which is close to the age of the Cretaceous granite intrusions of the Deo Ca complex. These veins formed from moderately strong solutions (11.7–6.4 wt.% NaCl equiv) with the CH4 + N2 + CO2 gas phase at 340–130 °C. Judging from the S isotope composition (δ34S = 1.6–4.3‰), predominantly deep-seated endogenic sulfur participated in the formation of ore sulfide associations. Analysis of the distribution of gold shows that it was deposited together with sulfide minerals (galena, sphalerite, and chalcopyrite) at a later stage. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: gold; deposit; ore associations; granitoid magmatism; Central Vietnam
Introduction Endogenous gold deposits are widely spread in Central Vietnam and are the main sources of its extraction. The study and analysis of the occurrences of gold, found in this region, show their large variety and quite broad spreading in different ore clusters, such as Kham Duc, Bong-Mieu, Kong Chro, Dak To, and Ta Nang (Fig. 1). A multitude of existing classifications of the Au deposits, based on different criteria and approaches, beginning from the first half of the last century (Goryachev et al., 2010; Ivensen and Levin, 1975; Konstantinov et al., 1997; Lindgren, 1933; Nekrasov, 1991; Petrovskaya, 1973; Safonov, 1997; Spiridonov, 2010) have identified * Corresponding author. E-mail address: [email protected] ( I.V. Gaskov)
a great number of gold ore formations, from the most general to specific ones, which sometimes overlap and are difficult to interpret. That is why we will try to differentiate all identified gold ore objects of Central Vietnam on the basis of their mineral composition, geochemical specifics, the type of wallrock metasomatites, connection with magmatism, depth and formation temperatures. On the whole, the following gold ore and gold-containing formations can be identified in this region: gold-sulphide-quartz ones; low-sulphide goldquartz; gold-sulphide; gold-silver and gold-Cu-Mo (Au)-porphyry formations. Deposits of gold-sulphide-quartz formation, represented in all studied clusters, are the most widely developed in Central Vietnam. They are characterized by connection with granitoid magmatism, predominantly veined (Bai Go, Bai Can, Dak Ripen, Ta Nang, Kon Fam, etc.) and interlacing veined (Ta
1068-7971/$ - see front matter D 201 5, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2015.09.004
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Nung, Nui Kem, Bai Go) form of ore bodies; intensive development of berezite type of wallrock metasomatic rocks; moderately subsurface (2–5 km) conditions of formation. The differences in the mineral composition of ores and their geochemical specifics make it possible to distinguish several mineral types: gold-galena-sphalerite, gold-pyrrhotite-chalcopyrite, gold-pyrite, and gold-arsenopyrite. Mineralization of gold-quartz low-sulfide formation is more limitedly evidenced; its small occurences are found in ore cluster Kong Chro (T’pe, Kong Jyang), and sufficiently large quartz-vein formations—in the ore region Kham Duc. This type of mineralization is characterized by low content of sulfides (<5%, usually 1–2%), minimized manifestations of wallrock metasomatic rocks and relatively large (up to 0.6 mm) excretions of gold with high fineness. The mineralization of this type is formed at high temperatures (400– 300 ºC). Mineralization of gold-silver formation is found only in the southern part of Central Vietnam. Such type is characterized by the contidions of near-surface ore depositions, connection with volcanoplutonic complexes, and a peculiar low-temperature range of near-ore alterations, such as: argillization, silicification, adularization, propylitization, sericitization. The specifics of the mineral composition of ores is characterized by widespread development of chalcedonic, small-grained quartz, colloform structures in ores, low fineness of gold (typically 650–750‰), and low Au/Ag ratio—1 :10–1 :100. This type can be attributed to Trang Sim deposit and several ore occurences. Mineralization zones are found in Sa Thay area; they are related to gold-bearing Cu-Mo (Au)-porphyry type. This mineralization is not studied well yet, but it has promising prospects in the region, especially in the western part of it, and it needs specialized exploration works. The criteria for selection of this type of mineralization in the area of Sa Thai are the following: large-scale hydrothermal alteration zones and pyritization of rocks; development of sericitization and feldspathization in the metasomatic zones; presence of granite porphyry dikes; presence of chalcopyrite, molybdenite, and gold in ores. Prospectivity of Central Vietnam in relation to mineralization of gold in general and its particular ore formations is determined by the specifics of its geological setting. Extensive development of siliclastic series, enriched by carbonaceous matter and varying in age, multiple recurrence of mantle-crustal magmatism, and presence of a large ancient block (Kontum unit), repeatedly recycled by metamorphic and magmatic processes (P-T, J3-K1), demonstrate significant prospects of gold mineralization. The most interesting is the gold-sulphidequartz formation, which is represented in this region by various mineral types, such as: gold-pyrite-arsenopyrite (Ta Nang), gold-pyrite (Ho Gan), gold-pyrrhotite-chalcopyrite (Khe Rin, Bai Go, Dak Ripen, Dak Rong), gold-galenasphalerite (Kon Fam, Kong Jyang, Ta Nung, Bai Dat, Bai Go, Nui Kem). Mineralization of this formation forms mainly mineralized zones of crushing and foliation with dissemination
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Fig. 1. The scheme of gold mineralization development. 1, Kontum block of the Pre-Cambrian; 2, Late Paleozoic–Early Mesozoic orogenic belt Truong Son; 3, Dalat Late Mesozoic active margin; 4, splits; 5, margin of the study region; 6, age of gold mineralization. Map letterings, provinces of Central Vietnam; inset, location of the study region.
of gold-bearing sulphides in carbonaceous volcanic-siliclastic and siliclastic rocks. In many regions of the world deposits, related to “black shales” series of siliclastic complexes, are referred to as gold-ore “black shales” type of deposits. Among these type of deposits the most famous are: Sukhoi Log, Olympiada, Nezhdaninskoe, Natalkinskoe, Maiskoe, Sovetskoe in Russia; Muruntau, Kokpatas, Zarmitan, Daugyztau, Amantaitau in Uzbekistan; Bakyrchik in Kazakhstan; Chore in Tadzhikistan; Kumtore in Kyrgyzstan; Mazer Lod in the U.S.A.; Bendigo, Olympic Dam in Australia, many of which are characterized by unique gold reserves (Boyle, 1979; Buryak et al., 2002; Konstantinov et al., 2009; Narseev and Uvarov, 1997; Nekrasov, 1991; Novozhilov and Gavrilov, 1996; Safonov, 1997; Sorokin et al., 2013). One of the brightest representatives of black shales type in Central Vietnam is the gold-pyrite-arsenopyrite deposit Ta Nang, located in the ore cluster (bearing the same name)
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Fig. 2. Geological map of the Ta Nang ore deposit in Lam Dong province. 1, 2, Quaternary period: 1, Holocenic system, proluvial alluvial, sand-pebble and clay sediments; 2, Upper Pleistocene; alluvial sand and pebble sediments, gray and gray-brown silty clays; 3, Upper Cretaceous volcanic rocks of Don Dong formation (K2dd): lavas and tuffs of felsites, rhyolites and dacite-rhyolites; 4, felsite-porphyry dikes of Don Dong formation; 5, 6, Middle Jurassic (J2ln1–2) terrigenous sedimentations of La Nga formation: 5, top assise of La Nga formation (J2ln2), siltstones, mudstones and shales with interbedded sandstones, thickness ~ 1.5 km; 6, bottom pack of La Nga formation (J2ln1), siltstones and mudstones, thickness ~ 2 km; 7, Cretaceous intrusive formations of Deo Ca complex (Kdc): biotite granites, granosyenites and syenites; 8, gold-quartz-sulfide veins; 9, geological borders: conformable (a); nonconformable (b); 10, zones of crush: determined (a); assumed (b); 11, slope angle of rocks; 12, hornfelsed rocks in the contact area with granite bodies; 13, isolines of depths (m); 14, rivers and sikes; 15, the area of the Ta Nang deposit.
among Mesozoic black shales that form thick areas of development in this region.
Ta Nang ore region The Ta Nang ore region, located in the southern part of Lam Dong province in Central Vietnam 40 km south of Dalat City, includes the Ta Nang deposit, (bearing the same name as the ore region), and a number of small mineralization points. Terrigenous-sedimentary La Nga formations belonging to the Middle Jurassic age (J2ln1–2), Upper Cretaceous volcanic rocks of Don Duong formation (K2dd), and Cretaceous granitoid rocks of the Deo Ca complex (Kdc) are a part in the
geological structure of the ore region (Fig. 2). Siliciclastic sediments with total thickness of more than 3.5 km composes the main area of the region. Two bands can be distinguished in its structure: the lower one (J2ln1) with capacity of about 2 km, which is composed mainly of siltstones and mudstones with horizons of sandstones, and the upper one (J2ln2) with capacity of about 1.5 km, represented mostly by sandstones and siltstones together with argillites and their schistose analogues. Rocks are crushed into folds, broken by a series of faults of a northeastern and northwestern direction. At the place of contact with granitic intrusions they are metamorphosed and transformed into hornfels. In the north of the area these deposits with stratigraphic unconformity are overlain by volcanic rocks of the Dong Dong formation (K2dd). Stratified
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Fig. 3. The scheme of geological structure of the Ta Nang deposit. 1, 2, terrigenous sedimentations of the upper assise of La Nga formation (J2ln2): 1, gray and dark-gray siltstones and shales; 2, gray and dark-gray sandstones. 3, ore zone: brecciated hydrothermally altered sulfided sandstones and shales; gold-quartz-sulfide veins; 4, zones of crush; 5, mining.
buildups of this formation are represented by lavas, tuffs and ingnibrites of rhyolitic and rhyolite-dacite composition, and subvolcanic dikes are comprised of rhyolite porphyries. Intrusive rocks within the field occur in its western part and are presented by small bodies of the Deo Ca complex belonging to the Cretaceous (Kdc). These rocks break through the Middle Jurassic (J2ln1-2) terrigenous-sedimentary blankets of the La Nga formation with development of hornfelsification zones. In some places they are closed by Upper Cretaceous volcanogenic rocks with rhyolite and rhyolite-dacite composition of the Dong Dong formation (Nguen Khoa Son, 2011). Two phases are identified as a part of the intrusive bodies: early one, composed of biotite granites, and late one, represented by biotite-hornblende granites, biotite granosyenites and syenites. Mineral assemblage of these rocks includes plagioclase, K-feldspar, quartz, biotite, hornblende, sphene, orthite, apatite, zircon and magnetite. The rocks belong to calc-alkaline varieties with different ratios of K2O/Na2O. Among these rocks high-K, high sodium, and K-Na differences are distinguished (Tran Trong Hoa et al., 2005). Ta Nang deposit The Ta Nang deposit is located in the high mountainous part of the region in the valley of the Siu Ngang River and is found on the dislocated terrigenous-sedimentary rocks of the upper member of the La Nga formation (Fig. 2). Gray and
dark-gray sandstones, siltstones, and mudstones, dating back to Jurassic period are a part of geological setting of the deposit; they are often schistose and rich in carbonaceous substance. The rocks have a northeast strike (strike azimuth 75°) and a southeast fall at an angle of 60°–65°. The ore zone is represented by a series of sublayer zones of crushing and of hydrothermally altering rocks stretching over 2 km (Fig. 3). Gold ore mineralization is closely associated with sulfided quartz, quartz-carbonaceous veins, with sulfided schistose host rocks, and with their hydrothermally altered differences in minor quantity (Fig. 4). Visible sulphide mineralization is represented mainly by arsenopyrite and pyrite impregnation. Thickness of the largest zone of mineralization reaches up to 10 m, and its length—up to several hundred meters. Besides that, thin zones of crushing (up to 1–2 m) with ore mineralization are widespread, stretching tens of meters. Change of host carbonaceous rocks has a local character and is often manifested on their contact areas of quartz veins in the form of rims up to 2–3 cm, as well as along specific cracks in the host rocks, which are sometimes traced by small crystals of arsenopyrite. It is expressed in the clarification (decarbonization) of carbonaceous shales, appearance of newly formed sericite that replaces feldspars in sandstones, and minor amounts of chlorite, carbonates (ankerite, calcite) and small disseminations of anthraxolite. In general, mineral composition of these metasomatic rocks can be represented as following: sericite—60%, quartz—30%, carbonates—up to 5%, pyrite,
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Fig. 4. Inner construction of ore zone: gold-quartz-sulfide veins in the beetled and hydrothermally altered carbonaceous shales (a photography in the gallery).
arsenopyrite—5%, which corresponds to the overall composition of beresites. Variously oriented quartz-sulfide veins are located in the central parts of zones of crushing with thickness from a few centimeters to 0.5 meters. Small admixture of sericite is found in the composition of quartz veins. Sulphide mineralization in the host rocks and quartz veins is similar in composition and represented by two mineral assemblages: early pyrite-arsenopyrite and late chalcopyrite-sphalerite-galena, attributed by N.V. Petrovskaya (1973) to their own stable mineral assemblages of gold deposits. Pyrite-arsenopyrite assemblage forms different morphogenetic disseminations (Fig. 5, I–III). The most widespread are large-crystalline aggregates and differently oriented metacrystals of arsenopyrite and pyrite with the size ranging from 0.1 mm to 10 mm. Pyrite crystals in the polished sections have the shape of the fragments of cubic structures (plates, diamonds, triangles), and arsenopyrite is mostly represented by short prysmatic crystals. In addition, arsenopyrite forms fine-grained aggregates (~ 0.1 mm), cementing fragments of large crystals of pyrite and arsenopyrite, and it is released in the form of disseminations among carbonaceous shales, while pyrite composes thin globular formations (<0.1 mm) in carbonaceous shales (Fig. 5, III). In the mixed pyrite-arsenopyrite aggregates later development of arsenopyrite is noted, which often replaces pyrite. Chalcopyrite-sphalerite-galena assemblage generally has a more limited development. The main minerals of this assem-
blage dimensionally are closely associated with other rare minerals, such as: pyrrhotite, sulfosalts of lead (tsugaruite), which sometimes also form small inclusions in the major minerals (sphalerite and chalcopyrite). Galena is the most widespread among the minerals of this assemblage. It often assembles fine-crystalline aggregates with sphalerite and also forms small veins the size of 0.1 × 15 mm (Fig. 5, IV); it is represented in the form of rounded inclusions in pyrite and formless disseminations around pyrite the size of up to 1.5 mm. Entrapped metacrystals of pyrite and arsenopyrite are quite often found inside the dissemination of galena. Sphalerite has a more limited distribution; it forms small disseminations (up to 1–2 mm) and joint aggregates with galena and develops inside the cracks in pyrite and arsenopyrite. Almost all forms of sphalerite contain nonuniform emulsive dispersion of chalcopyrite. Chalcopyrite is found in small amounts in the field, and in addition to the emulsive impregnation in sphalerite it forms small (0.01–0.1 mm) shapeless inclusions in arsenopyrite and pyrite as well as dissemination in quartz veins the size of up to 0.5 mm. All noted rare minerals form individual microscopic grains in veined quartz or in the main ore minerals and can be recorded mainly with a microscope and a scanning electronic microscope (Fig. 6). Gold is the main industrial component of the deposit; it mainly forms a thin dispersion, invisible to the naked eye. We have identified small (0.01–0.05 mm) spots of gold in pyrite, arsenopyrite, chalcopyrite, and quartz in the polished sections
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Fig. 5. Types of ore mineralization in the Ta Nang deposit: I, coarse-grained arsenopyrite dissemination in a quartz vein; II, coarse-grained cubic habitus dissemination of pyrite in quartz vein; III, small impregnated disseminations of pyrite and arsenopyrite in black shales; IV, pockety-veined segregations of chalcopyrite-sphaleritegalena mineral assemblage. Full size.
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Fig. 6. Dissemination of ore minerals and rare minerals under the electronic microscope. Pyr, Pyrite; Apyr, arsenopyrite; Chp, copper pyrite; Gl, galena; Sf, sphalerite; Sm, smaitite; Ts, tsugaruite; Q, quartz.
(Fig. 7). Using atomic absorption analysis (Institute of Geology and Mineralogy SB RAS) and ICP MS method in the LTD laboratory (Ontario, Canada) the content of gold and silver in the host sulfided rocks (carbonaceous shales), in
quartz-sulphide veins and in the main ore minerals—pyrite and arsenopyrite was examined. Results of analyses of these two methods are almost identical. Au content in the shale does not exceed 0.n ppm with an average of 0.38 ppm, and in quartz
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Fig. 7. Dissemination of fine-grained gold in quartz (a) and in chalcopyrite in association with galena (b). The compositions of mineral phases are shown in the diagrams. 1, gold; 2, chalcopyrite.
veins its concentrations are much higher and are equal to 3.92 ppm on average (Table 1). Shales and veins have close silver content, which is on average, respectively, 2.68 and 5.30 ppm. As it is demonstrated in many publications (Ashley et al., 2000; Gaskov et al., 2006; Genkin, 1998; Genkin et al., 2002; Kovalev et al., 2011; Starova and Bocharov, 1977; Tyukova and Voroshin, 2007; Vilor et al., 2014; Volkov et al., 2006; Vos et al., 2005), arsenopyrite-pyrite paragenesis in gold-sulphide deposits, localized in black shale series, is the most common. However, gold is concentrated predominantly in arsenopyrite. According to these researches, the most gold-bearing is early thin needle-shaped arsenopyrite, which is nonstoichiometric in its chemical composition and is characterized by the deficit of As with the ratio of S/As 1.16–1.21. As it has been demonstrated above, large prismatical arsenopyrite is the most widely developed in the Ta Nang deposit; it forms large crystals (1.5 cm) and nests in quartz veins and sulfided carbonaceous rocks. Fine-grained aggregates of arsenopyrite are also characterized by short-prismatic
habitus and are found in very small amounts. Sometimes they form a thin layered dissemination in carbonaceous shales and develop in the cracks of macrocrystalline arsenopyrite. The ratio of S/As does not exceed 1.12. Pyrite mainly forms metacrystals and diffuse dissemination in altered rocks and in quartz veins, and rarely—thin fines of globular pyrite in carbonaceous shales. Analysis of monofractions of arsenopyrite and pyrite disseminations demonstrated similar concentrations of Au and Ag in them (Table 2), which generally correlate with their content in the samples, from which they are extracted. The ratio of Au/Ag in the analyzed samples and minerals is characterized by wide variations (Tables 1, 2). Minimum range of changes in this ratio is found in the sulfided carbonaceous shales (0.06–0.44), and maximum one—in arsenopyrite (0.42–8.77). Microscopic studies show that gold in these minerals, as well as a in quartz veins, is developing in the cracks and is connected with its later deposition. The composition of gold, studied mainly in native alluvial formations, developed in
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Table 1. Concentrations of Au, Ag, and trace elements in the ores of the Ta Nang deposit Sample
Rock, ores
Au
Ag
Au/Ag
ppm
Cu
Cd
Mo
Mn
Sr
Pb
Ni
Zn
Ba
Bi
Co
ppm
As
Fe
%
T05-H12/1
Black sulphidized shales 0.49
2.46
0.20
55.8
0.3
3.5
637
259
339
37.9
93.6
133
5.25
13
1.43
5.5
T05-H13/1
0.37
4.17
0.09
72.6
0.2
1.7
517
144
55.9
64.9
624
117
1.39
29
1.19
5.9
T05-H13/2
0.28
4.32
0.06
74.9
0.4
6.7
729
174
73.4
59.2
163
309
0.95
16
0.03
5.9
T05-H13/4
0.57
1.28
0.44
14.2
3.9
0.6
314
68.2
511
7.5
776
56
0.78
2
0.62
1.6
T05-H15/2
0.17
1.21
0.14
5.1
<0.1
0.1
61
5.6
7.8
2.2
5.6
59
0.1
<1
0.18
0.7
Average
0.38
2.68
0.14
44.5
1.2
3.1
451.6 130.2 197.4 34.3
332.44
134.8 1.6
15
0.70
3.9
0.80
2.87
0.28
48.5
2.3
1.1
228
118
105
2.38
<1
1.89
4.2
T05-H13/5
9.63
6.78
1.42
505
1.8
0.5
140
14.1
1670 50.8
352
77
1.94
17
2.55
6.8
T05-H13/6
3.98
3.16
1.25
30.9
< 0.1 0.5
185
5.8
1280 4.2
29.8
9
0.8
1
8.68
8.6
T05-H14/1
4.20
5.17
0.81
146
< 0.1 12.6
42
20.2
162
54.4
30.8
19
1.49
34
7.48
12
T05-H14/2
0.94
1.1
0.85
32.5
< 0.1 1.4
171
12.1
46.1
47.3
35.9
82
0.66
16
2.71
7.3
T05-H15/1
6.96
20.3
0.34
149
60.3
0.3
50
4.7
320
9.4
10800
49
0.27
3
0.10
7.1
T05-H15/2
1.46
2.44
0.60
28.1
8.5
0.6
48
1.9
485
3.7
1580
3
0.38
3
0.12
1.1
T05-H16/2
3.39
0.6
5.65
7.2
< 0.1 1.9
55
1.9
30
7.2
3
4
0.41
6
> 10 9.4
Average
3.92
5.30
0.74
118.4 18.23 2.4
114.9 11.5
639.1 24.3
1618.7
43.5
1.04
11.4
3.36
7.1
5.05
8.14
0.62
40.4
181
3370 6.9
936
144
1.75
5
1.40
2.8
T05-H12/2
T05-H17/2
Quartz-sulphide veins
Concentrate
7.6
alluvial deposits of the Sue Ngang River, varies widely (Table 3). All analyzed samples form a row in which two discrete groups can be identified: 1—with fineness of 527.3– 802.0‰ and 2—941.3–996.7‰; the first group corresponds to endogenous gold, and the second, most probably, to gold, modified in exogenous conditions. The main impurity of endogenous gold is Ag—up 46.05%, and in some samples up to 0.28% of Hg is found. Amount of Ag falls sharply in the modified gold. Analysis of gold-bearing samples of ore to determine the presence of such elements as Ag, Cu, Cd, Mn, Pb, Ni, Zn, As, Ba, Be, Bi, Br, Ca, Co, Cr, Cs, Fe, Hf, Ga, Ge, In, Li, Mg, Na, Nb, Rb, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Tl, U, V, W, Y, Zr, K, including such REE as La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Tb, Ho, Hg, Er, conducted using ISP MS at the LTD
0.7
31.2
1120 17.3
14.7
laboratory (Ontario, Canada), has found their low concentrations, close to Clarke values. Fe, As, Pb, Zn, Cu, Cd, Ni, Co are found in high concentrations in the sulfided rocks and veins, most of which (Fe, As, Pb, Zn, Cu) form their own sulfide minerals, and others are a part of their composition as trace elements (Table 1). However, the level of concentration
Table 3. Content of free gold in the Ta Nang deposit Sample
Number Cu of grains
Au
Hg
Ag
Total
DL-3067-K
5
0
52.73
0.02
46.05
98.80
DL-3067-C
3
0
53.41
0.04
45.97
99.41
DL-3138 DK
1
0
58.34
0
41.59
99.93
DL-3078-C
4
0
59.71
0.28
38.99
98.98
DL-3138 CC
1
0
59.78
0
40.23
100.01
Table 2. Gold content in pyrite and arsenopyrite from the Ta Nang deposit
DL-3138 CK
1
0
60.96
0
39.35
100.31
Sample
DL-3138 DC
1
0
62.66
0
37.02
99.68
DL-3138 BC
1
0
64.50
0
35.34
99.84
Mineral
Au
Ag
Au/Ag
T05-H12/1
Pyrite
5.6
6.9
0.81
T05-H12/2
Arsenopyrite
7.6
18.0
0.42
T05-H13/1
Pyrite
1.0
5.9
0.16
T05-H13/1
Arsenopyrite
6.2
12.0
0.51
T05-H13/5
Pyrite
17.0
12.0
T05-H13/5
Arsenopyrite
10.0
T05-H13/6
Pyrite
T05-H13/6
DL-3137 BC
3
0
65.96
0
31.88
97.84
DL-3138 AC
1
0
72.52
0
27.38
99.9
DL-3137 CC
1
0
80.20
0
19.71
99.91
DL-3070-K
1
0
93.41
0.17
5.11
98.7
1.42
DL-3070
1
0
93.43
0.21
5.21
98.85
3.5
2.86
DL-3137 AC
3
0
96.39
0
2.97
99.36
4.7
5.2
0.90
DL-3138 AK
1
0
96.99
0.02
2.22
99.24
Arsenopyrite
11.0
2.9
3.80
DL-3137 BK
2
0
98.88
0.05
0.78
99.71
T05-H14/1
Pyrite
1.3
1.1
1.18
DL-3137 CK
1
0
99.23
0
0
99.23
T05-H15/2-1
Arsenopyrite
5.0
0.57
8.77
DL-3138 BK
1
0
99.67
0.02
0.49
100.19
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Tran Tuan Anh et al. / Russian Geology and Geophysics 56 (2015) 1414–1427
Table 4. Correlation of elements in the gold ore mineralization of the Ta Nang deposit Au
Ag
Cu
Cd
Mo
Mn
Pb
Au
1.00
Ag
0.63
1.00
Cu
0.77
0.37
1.00
Cd
0.43
0.91
0.12
Mo
–0.07
–0.06
0.06
–0.22
1.00
Mn
–0.48
–0.19
–0.11
–0.28
0.19
1.00
Pb
0.49
0.22
0.24
–0.01
–0.29
–0.15
1.00
Ni
–0.04
–0.05
0.42
–0.28
0.56
0.54
–0.27
Ni
Zn
As
Ba
Bi
Co
Fe
1.00
1.00
Zn
0.41
0.90
0.13
0.95
–0.22
–0.25
–0.06
–0.24
1.00
As
0.26
–0.24
–0.03
–0.29
0.30
–0.34
–0.03
–0.06
–0.30
Ba
–0.28
0.03
0.02
–0.14
0.20
0.81
0.08
0.50
–0.14
–0.49
1.00
Bi
–0.07
–0.10
0.16
–0.24
0.18
0.53
0.20
0.32
–0.25
–0.09
0.30
1.00
1.00
Co
0.04
–0.03
0.35
–0.27
0.71
0.28
–0.26
0.90
–0.23
0.15
0.20
0.22
1.00
Fe
0.39
0.17
0.28
0.02
0.51
–0.09
–0.20
0.45
0.03
0.74
–0.16
0.06
0.56
1.00
Note. In the sampling with n = 14, significant correlation coefficient (r) > 0.54.
Table 5. Composition of ore minerals from the Ta Nang deposit Test No.
Number of analyses
Fe
Cu
Zn
Pb
S
As
Sb
Bi
Ag
Cd
Mn
Co
Ni
Total
1
33.24
–
–
0
20.60
42.79
0
–
–
–
–
0.02
0.60
97.21
2
34.03
–
–
0.01
20.30
43.97
0
–
–
–
–
0.03
0.02
98.36
3
34.04
–
–
0
20.73
43.10
0.03
–
–
–
–
0.01
0.03
97.93
Arsenopyrite T05-H13/1
4
34.25
–
–
0
20.94
42.76
0
–
–
–
–
0.03
0.26
98.25
T05-H13/2
1
33.95
–
–
0
20.23
43.93
0
–
–
–
–
0.10
0.05
98.26
2
33.95
–
–
0.04
21.14
42.85
0
–
–
–
–
0.05
0.03
98.06
T05-H13/4
1
35.16
–
–
0
21.69
42.53
0
–
–
–
–
0.05
0.01
99.43
T05-H13/5
1
34.76
–
–
0
20.86
43.21
0.02
–
–
–
–
0.04
0
98.89
2
34.95
–
–
0
21.33
42.93
0.01
–
–
–
–
0.05
0.01
99.27
3
35.32
–
–
0.01
21.85
42.59
0
–
–
–
–
0.05
0.01
99.82
1
34.12
–
–
0.02
20.40
43.36
0.03
–
–
–
–
0.05
0.03
98.01
2
34.63
–
–
0.05
21.38
42.56
0
–
–
–
–
0.03
0.02
98.67
3
34.87
–
–
0
21.33
43.41
0
–
–
–
–
0.04
0
99.65
1
45.45
–
–
0.04
50.81
0
–
0.02
0
–
–
0.04
0.05
96.41
2
45.24
–
–
0.13
51.89
0
–
0.03
0
–
–
0.05
0
97.34
T05-H13/6
Pyrite T05-H13/4
3
45.98
–
–
0.08
52.65
0
–
0.02
0
–
–
0.06
0.04
98.83
T05-H13/5
1
46.84
–
–
0.05
53.28
0
–
0.00
0
–
–
0.05
0
99.22
T05-H13/6
1
45.92
–
–
0
52.88
0
–
0.01
0
–
–
0.04
0
98.84
2
45.25
–
–
0
52.84
0
–
0.01
0
–
–
0.04
0
98.14
T05-H15/2
3
45.18
–
–
0.02
51.66
0
–
0
0
–
–
0.03
0.05
99.39
1
45.94
–
–
0
52.92
0
–
0.03
0
–
–
0.05
0
98.94
2
46.03
–
–
0.01
52.06
0
–
0
0
–
–
0.03
0
98.13
(continued on next page)
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Tran Tuan Anh et al. / Russian Geology and Geophysics 56 (2015) 1414–1427
Table 5 (continued) Test No.
Number of analyses
Fe
Cu
Zn
Pb
S
As
Sb
Bi
Ag
Cd
Mn
Co
Ni
Total
T05-H15/2
3
45.84
–
–
0.05
52.32
0
–
0
0
–
–
0.05
0.02
98.27
4
47.24
–
–
0
52.98
0
–
0
0.03
–
–
0.05
0
100.39
5
45.77
–
–
0.03
52.85
0
–
0
0.01
–
–
0.03
0
98.68
6
46.44
–
–
0.06
52.36
0
–
0
0.04
–
–
0.05
0.02
98.99
1
43.66
–
–
0
52.52
0.05
–
0
0
–
–
0
1.97
98.2
2
45.8
–
–
0
53.01
0.27
–
0
0
–
–
0
0.47
99.73
1
45.18
–
–
0.02
51.66
2.44
–
0
0
–
–
0.03
0.05
99.39
1
59.99
–
–
–
38.7
0.13
–
–
–
–
–
–
–
98.83
2
60.78
–
–
–
38.69
0.16
–
–
–
–
–
–
–
99.63
1
28.96
34.17
1.41
35.26
–
–
–
0.17
0.02
–
–
–
99.99
2
28.93
34.06
1.58
35.26
–
–
–
0.01
0
–
–
–
99.84
T05-H13/4
T05-H12/3 Pyrrhotite T05-H13/2
Chalcopyrite T05-H13/1
Galena T05-H13/5
T05-H13/6
T05-H15/2
1
85.91
13.65
0.07
0.03
0.21
–
–
0
99.97
2
86.24
13.16
0.01
0.1
0
–
–
0
99.53
3
86.20
13.18
0.06
0.01
0
–
–
0
99.45
4
87.08
13.28
0.06
0.07
0
–
–
0
100.57
1
87.13
13.3
0.1
0
0
–
–
0
100.53
2
87.63
13.14
0.03
0
0
–
–
0.01
100.81
3
86.24
13.14
0
0
0
–
–
0.04
99.42
4
86.74
13.04
0.04
0
0
–
–
0.02
99.84
5
87.29
13.08
0.03
0
0
–
–
0.01
100.41
1
87.47
13.03
0.07
0.1
0
–
–
0
100.67
Sphalerite T05-H13/1
T05-H15/2
1
4.7
0.06
62.39
–
33.72
–
–
–
0.02
0.44
0.01
–
–
101.33
2
6.66
0.93
58.88
–
34.00
–
–
–
0.03
0.42
0.01
–
–
100.93
3
4.76
0.71
58.36
–
33.96
–
–
–
0.02
0.43
0.00
–
–
98.24
4
4.44
0.18
61.46
–
34.15
–
–
–
0.01
0.43
0.02
–
–
100.7
5
4.95
0.05
60.65
–
33.98
–
–
–
0.01
0.40
0.02
–
–
100.15
1
4.54
0.02
61.26
–
33.49
–
–
–
0.01
0.38
0
–
–
99.78
2
3.46
0.02
63.27
–
33.74
–
–
–
0.02
0.37
0.02
–
–
100.95
0.26
–
–
73.75
15.48
10.9
–
–
–
–
–
–
–
100.39
Tsugaruite T05-H13/4
Note: Dash, Below the detection limit (<0.01%).
of these elements in the sulfided carbonaceous shales is significantly lower compared with quartz-sulfide veins, which is, most probably, connected with smaller amounts of sulfides therein. Additionally, increased contents of Mn, Ba and Sr are found in the shales (Table 1). Also, some samples with high Zn, Pb, Cu contents tend to have higher Cd, and in samples, where Fe and As are predominant, concentrations of Co, Ni, Mo are increasing. Calculation of the pair correlation coefficients shows direct positive correlation of Au with Ag and Cu, and Ag with Zn and Cd; also, positive direct correlation
of Co with Fe and Ni; Fe with As; Mn with Ba is found (Table 4). Analysis of the content of the main ore minerals, conducted at the Analytical Center of the Institute of Geology and Mineralogy SB RAS (Russia) using microanalyzer Camebax micro, shows their following features. In the field arsenopyrite is characterized by stable chemical composition with close values of atomic ratio of S/As, varying in the range of 1.08–1.12. Among the impurity elements, found in arsenopyrite, there is up to 0.6% of Ni and in some samples—Sb
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Tran Tuan Anh et al. / Russian Geology and Geophysics 56 (2015) 1414–1427
and Pb at the level of sensitivity of X-ray spectral microanalysis (Table 5). Pyrite corresponds to the stoichiometry in its composition. Out of the impurity elements some of its samples also have an increased Ni content up to 1.97%, As up to 2.44%, and small amounts of Co, Bi, and Ag. Minerals, belonging to late assemblage, have a wider range of trace elements. Sphalerite permanently contains few percentages of iron admixture, and Cu and Cd—0.n%. In addition it notes the following Ag and Mn (Table 5). Besides that, it has traces of Ag and Mn (Table 5). Galena contains increased admixture of Ag and Bi (0.1%), and chalcopyrite—increased content of Zn and a small admixture of Ag and Cd. Only separate microscopic inclusions of tsugaruite have been revealed in sulfosalts, in which Pb as a basic cation was found (Table 5).
Genesis of the Ta Nang deposit As it has already been noted, the Ta Nang deposit is localized among clastic terrigenous sedimentations, belonging to the Middle Jurassic Period (J2ln1–2), enriched with carbonaceous material (La Nga formation). It is found in the sublayer area of crushing and hydrothermal alteration and is represented by the development of quartz-sulfide veins and sulphide mineralization in the host rocks. Determining the time of formation of sericite from quartz-sulfide veins using the method of Ar39/Ar40 dating indicates that the age of formation of these veins along with sulfide mineralization was 129.3 ± 5.6 Ma. This indicates that formation of ores of the Ta Nang deposit is close in time with the development the Cretaceous granitic intrusive formations of the Deo Ca complex (145– 99.6 Ma), which break through the Upper Jurassic deposits (145 Ma) of the La Nga formation and are covered by volcanic rocks of the Don Dong formation, belonging to the Upper Cretaceous (99.6 Ma) (Vu Nhu Hung et al., 2003). Comparison of the spectrogram graphs of rare earth elements, stand-
ardized on the basis of chondrite (Boynton, 1984) and drawn for quartz-sulfide veins and sulfided shales of the Ta Nang deposit, with the spectra of rare earth elements of the granites of the Deo Ca complex and volcanites of the Dong Dong formation (K2dd), shows similar configurations of the charts of ore formations and granites and their significant difference with the volcanite graphs. The spectrograms of ore formations and granites are noted by enrichment with light lanthanoides and manifestation of minor europium minimum (Fig. 8), which can probably be indirect evidence of the connection between ore-forming hydrothermal solutions with these granites. The relationship of ore minerals and the specifics of development of their different morphological features show that in the formation of the Ta Nang deposit two stages can be identified: early quartz-pyrite-arsenopyrite and late chalcopyrite-sphalerite-galena. Mineral assemblages of these stages are related to hydrothermal ore formations with similar isotopic composition of sulfur of their sulfides, varying within (δ34S 1.6–4.3‰) (Table 6), which indicates that there is a single deep endogenous source of sulfur. Slight weighting of the isotopic composition of sulfur in comparison with meteorite analogue can be explained by partial assimilation of the weighted rocky sulfur during the formation of fine crystalline, mainly framboidal mineralization of pyrite and arsenopyrite in carbonaceous shales during their diagenetic transformation. Later on, during the formation of mineralization zone in the areas of crushing and foliation this dispersed mineralization was partially mobilized and redeposited in the ore zones along with endogenous sulfide mineralization. The possibility of such redistribution is indicated by a redeposition of carbonaceous material of host rocks with formation of patches and dispersions of anthraxolite (amorphous conductive carbon) and graphite in altered rocks and quartz-sulfide veins. Isotopic composition of carbon in these structures shows that the deposition of graphite and anthraxolite was due to redeposition of slightly metamorphosed carbonaceous substances of host rocks (Table 7). Physical-chemical conditions of formation of quartz-sulfide veins, according to the study of fluid inclusions in quartz, are characterized by a narrow temperature range— 305–245 °C. They were formed out of moderately concen-
Table 6. Isotopic composition of sulfur in the main ore minerals of the Ta Nang deposit
Fig. 8. Distribution of rare earth elements in quartz-sulfide veins: (1) black sulphidized shales; (2) Ta Nang deposit, granites of Deo Ca complex (3) and volcanites of Don Dong formation (4).
Sample
Mineral
δ34S
DL-3067
Arsenopyrite
1.6
DL-3068
Arsenopyrite
2.0
DL-3074-5
Arsenopyrite
4.3
DL-3074-9
Arsenopyrite
4.1
DL-3076-5
Arsenopyrite
4.0
DL-3075-9
Pyrite
3.5
DL-3076-9
Sphalerite
3.5
DL-3076-a
Galena
3.9
DL-3079a-9
Galena
1.7
DL-3077
Galena
2.3
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Tran Tuan Anh et al. / Russian Geology and Geophysics 56 (2015) 1414–1427
Table 7. Carbon isotopic composition of carbonaceous material from the ore gold deposits (%) Deposit
δ13C
Carbonaceous material
Ta Nang
–19.7
Anthraxolite
Ta Nang
–18.5
DL-3061
Carbon shale
Ta Nang
–18.0
DL-3080
Graphite
Ta Nang
–21.5
Sample
Mineral
DL-3067 D 1-3067
Table 8. Results of thermobarogeochemical studies of fluid inclusions in quartz from the Ta Nang deposits Tice melt, °C
ΣC, wt.% NaCl equiv
Sample
Thom, °C
Teutec, °C
DL-3079
305–275
–50 /–49
–8 / –6
11.7–9.2
DL-3070
250–245
–22
–5 / –4
7.8–6.4
trated solutions (11.7–6.4 wt.% NaCl equiv) with the methane-nitrogen-carbon dioxide gas phase (CO2 + N2 + CH4) (Table 8). Gold has a clear correlation with the sulphides, according to the analysis of its distribution and its dissemination forms. It develops as thin spots in the cracks of disseminations of pyrite, arsenopyrite, quartz, and is located in the form of spots or is intergrown with the minerals of late assemblage (galena, sphalerite, chalcopyrite), which indicates that it was formed at a later stage together with these sulfide minerals. Thus, studies in the Ta Nang deposit indicate that it relates to endogenous hydrothermal formations of later associtation, the formation of which occurred in the Cretaceous (129.3 ± 5.6 Ma) and is associated with the invasion of granitoid intrusions of the Deo Ca complex. The process of ore mineralization occured in two stages: early quartz-pyritearsenopyrite and late chalcopyrite-galena-sphalerite, which is prospective for gold. Gold is characterized by wide variations in fineness (527.3–802.0‰) and forms small and thin spots in quartz and sulphides. Black shale series of La Nga formation due to its sand-shale composition and enrichment with carbonaceous matter was a favorable environment for the deposition of ore components of postmagmatic hydrothermal solutions. Therefore, these strata in the areas of development of granitoid magmatism of the Deo Ca complex may be prospective for the detection of such deposits. This work was supported by the project “Complex and Rational Use of Mineral Resources for Economic and Social Development of Thai Nguyen region, TN3/T05” and 105.06.76.09 (NAFOSTED).
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Ta Nang gold deposit in the black shales of Central Vietnam Tran Tuan Anh a, I.V. Gaskov b,c,*, Tran Trong Hoa a, A.S. Borisenko b,c, A.E. Izokh b,c, Pham Thi Dung a, Vu Hoang Li a, Nguyen Thi Mai a a
Institute of Geological Sciences, Vietnam Academy of Science and Technology, 84 Chua Lang, Dong Da, Hanoi, Vietnam b V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia c Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 4 July 2014; accepted 21 October 2014
Abstract The Ta Nang gold deposit is localized in Middle Jurassic black shales. The ore zone is a series of layer-by-layer crush zones and zones of hydrothermal rock alteration, ≤10 m in thickness and >2 km in length. It consists of quartz–sulfide veins, sulfidized black shales, and their hydrothermally altered varieties. Sulfide mineralization occurs as two assemblages: early pyrite–arsenopyrite and late chalcopyrite–sphalerite– galena. The pyrite–arsenopyrite assemblage is composed of different morphogenetic varieties. Coarse-crystalline arsenopyrite and pyrite aggregates and metacrystals of different orientations, 0.1 to 10 mm in size, are the most widespread. The chalcopyrite–sphalerite–galena assemblage is scarce. Along with the main ore minerals, it includes more rare minerals: pyrrhotite, lead sulfosalts (tsugaruite), and gold, which form a spatial assemblage with the main minerals or small inclusions in them. Gold occurs mainly as fine dissemination in cracks in pyrite, arsenopyrite, chalcopyrite, and quartz. Gold content in sulfidized carbonaceous shales is no more than tenths of ppm, averaging 0.38 ppm. This content in the quartz veins is considerably higher, averaging 3.92 ppm. Silver contents in the shales and quartz veins are similar and equal to 2.68 and 5.30 ppm, respectively. Also, the sulfidized rocks and veins have elevated contents of Fe, As, Pb, Zn, Cu, Cd, Ni, and Co; most of these elements (Fe, As, Pb, Zn, and Cu) make up their own sulfide minerals, and the others are trace elements. According to 39Ar/40Ar dating of sericite from the quartz–sulfide veins, their age is 129.3 ± 5.6 Ma, which is close to the age of the Cretaceous granite intrusions of the Deo Ca complex. These veins formed from moderately strong solutions (11.7–6.4 wt.% NaCl equiv) with the CH4 + N2 + CO2 gas phase at 340–130 °C. Judging from the S isotope composition (δ34S = 1.6–4.3‰), predominantly deep-seated endogenic sulfur participated in the formation of ore sulfide associations. Analysis of the distribution of gold shows that it was deposited together with sulfide minerals (galena, sphalerite, and chalcopyrite) at a later stage. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: gold; deposit; ore associations; granitoid magmatism; Central Vietnam
Introduction Endogenous gold deposits are widely spread in Central Vietnam and are the main sources of its extraction. The study and analysis of the occurrences of gold, found in this region, show their large variety and quite broad spreading in different ore clusters, such as Kham Duc, Bong-Mieu, Kong Chro, Dak To, and Ta Nang (Fig. 1). A multitude of existing classifications of the Au deposits, based on different criteria and approaches, beginning from the first half of the last century (Goryachev et al., 2010; Ivensen and Levin, 1975; Konstantinov et al., 1997; Lindgren, 1933; Nekrasov, 1991; Petrovskaya, 1973; Safonov, 1997; Spiridonov, 2010) have identified * Corresponding author. E-mail address: [email protected] ( I.V. Gaskov)
a great number of gold ore formations, from the most general to specific ones, which sometimes overlap and are difficult to interpret. That is why we will try to differentiate all identified gold ore objects of Central Vietnam on the basis of their mineral composition, geochemical specifics, the type of wallrock metasomatites, connection with magmatism, depth and formation temperatures. On the whole, the following gold ore and gold-containing formations can be identified in this region: gold-sulphide-quartz ones; low-sulphide goldquartz; gold-sulphide; gold-silver and gold-Cu-Mo (Au)-porphyry formations. Deposits of gold-sulphide-quartz formation, represented in all studied clusters, are the most widely developed in Central Vietnam. They are characterized by connection with granitoid magmatism, predominantly veined (Bai Go, Bai Can, Dak Ripen, Ta Nang, Kon Fam, etc.) and interlacing veined (Ta
1068-7971/$ - see front matter D 201 5, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2015.09.004
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Nung, Nui Kem, Bai Go) form of ore bodies; intensive development of berezite type of wallrock metasomatic rocks; moderately subsurface (2–5 km) conditions of formation. The differences in the mineral composition of ores and their geochemical specifics make it possible to distinguish several mineral types: gold-galena-sphalerite, gold-pyrrhotite-chalcopyrite, gold-pyrite, and gold-arsenopyrite. Mineralization of gold-quartz low-sulfide formation is more limitedly evidenced; its small occurences are found in ore cluster Kong Chro (T’pe, Kong Jyang), and sufficiently large quartz-vein formations—in the ore region Kham Duc. This type of mineralization is characterized by low content of sulfides (<5%, usually 1–2%), minimized manifestations of wallrock metasomatic rocks and relatively large (up to 0.6 mm) excretions of gold with high fineness. The mineralization of this type is formed at high temperatures (400– 300 ºC). Mineralization of gold-silver formation is found only in the southern part of Central Vietnam. Such type is characterized by the contidions of near-surface ore depositions, connection with volcanoplutonic complexes, and a peculiar low-temperature range of near-ore alterations, such as: argillization, silicification, adularization, propylitization, sericitization. The specifics of the mineral composition of ores is characterized by widespread development of chalcedonic, small-grained quartz, colloform structures in ores, low fineness of gold (typically 650–750‰), and low Au/Ag ratio—1 :10–1 :100. This type can be attributed to Trang Sim deposit and several ore occurences. Mineralization zones are found in Sa Thay area; they are related to gold-bearing Cu-Mo (Au)-porphyry type. This mineralization is not studied well yet, but it has promising prospects in the region, especially in the western part of it, and it needs specialized exploration works. The criteria for selection of this type of mineralization in the area of Sa Thai are the following: large-scale hydrothermal alteration zones and pyritization of rocks; development of sericitization and feldspathization in the metasomatic zones; presence of granite porphyry dikes; presence of chalcopyrite, molybdenite, and gold in ores. Prospectivity of Central Vietnam in relation to mineralization of gold in general and its particular ore formations is determined by the specifics of its geological setting. Extensive development of siliclastic series, enriched by carbonaceous matter and varying in age, multiple recurrence of mantle-crustal magmatism, and presence of a large ancient block (Kontum unit), repeatedly recycled by metamorphic and magmatic processes (P-T, J3-K1), demonstrate significant prospects of gold mineralization. The most interesting is the gold-sulphidequartz formation, which is represented in this region by various mineral types, such as: gold-pyrite-arsenopyrite (Ta Nang), gold-pyrite (Ho Gan), gold-pyrrhotite-chalcopyrite (Khe Rin, Bai Go, Dak Ripen, Dak Rong), gold-galenasphalerite (Kon Fam, Kong Jyang, Ta Nung, Bai Dat, Bai Go, Nui Kem). Mineralization of this formation forms mainly mineralized zones of crushing and foliation with dissemination
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Fig. 1. The scheme of gold mineralization development. 1, Kontum block of the Pre-Cambrian; 2, Late Paleozoic–Early Mesozoic orogenic belt Truong Son; 3, Dalat Late Mesozoic active margin; 4, splits; 5, margin of the study region; 6, age of gold mineralization. Map letterings, provinces of Central Vietnam; inset, location of the study region.
of gold-bearing sulphides in carbonaceous volcanic-siliclastic and siliclastic rocks. In many regions of the world deposits, related to “black shales” series of siliclastic complexes, are referred to as gold-ore “black shales” type of deposits. Among these type of deposits the most famous are: Sukhoi Log, Olympiada, Nezhdaninskoe, Natalkinskoe, Maiskoe, Sovetskoe in Russia; Muruntau, Kokpatas, Zarmitan, Daugyztau, Amantaitau in Uzbekistan; Bakyrchik in Kazakhstan; Chore in Tadzhikistan; Kumtore in Kyrgyzstan; Mazer Lod in the U.S.A.; Bendigo, Olympic Dam in Australia, many of which are characterized by unique gold reserves (Boyle, 1979; Buryak et al., 2002; Konstantinov et al., 2009; Narseev and Uvarov, 1997; Nekrasov, 1991; Novozhilov and Gavrilov, 1996; Safonov, 1997; Sorokin et al., 2013). One of the brightest representatives of black shales type in Central Vietnam is the gold-pyrite-arsenopyrite deposit Ta Nang, located in the ore cluster (bearing the same name)
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Fig. 2. Geological map of the Ta Nang ore deposit in Lam Dong province. 1, 2, Quaternary period: 1, Holocenic system, proluvial alluvial, sand-pebble and clay sediments; 2, Upper Pleistocene; alluvial sand and pebble sediments, gray and gray-brown silty clays; 3, Upper Cretaceous volcanic rocks of Don Dong formation (K2dd): lavas and tuffs of felsites, rhyolites and dacite-rhyolites; 4, felsite-porphyry dikes of Don Dong formation; 5, 6, Middle Jurassic (J2ln1–2) terrigenous sedimentations of La Nga formation: 5, top assise of La Nga formation (J2ln2), siltstones, mudstones and shales with interbedded sandstones, thickness ~ 1.5 km; 6, bottom pack of La Nga formation (J2ln1), siltstones and mudstones, thickness ~ 2 km; 7, Cretaceous intrusive formations of Deo Ca complex (Kdc): biotite granites, granosyenites and syenites; 8, gold-quartz-sulfide veins; 9, geological borders: conformable (a); nonconformable (b); 10, zones of crush: determined (a); assumed (b); 11, slope angle of rocks; 12, hornfelsed rocks in the contact area with granite bodies; 13, isolines of depths (m); 14, rivers and sikes; 15, the area of the Ta Nang deposit.
among Mesozoic black shales that form thick areas of development in this region.
Ta Nang ore region The Ta Nang ore region, located in the southern part of Lam Dong province in Central Vietnam 40 km south of Dalat City, includes the Ta Nang deposit, (bearing the same name as the ore region), and a number of small mineralization points. Terrigenous-sedimentary La Nga formations belonging to the Middle Jurassic age (J2ln1–2), Upper Cretaceous volcanic rocks of Don Duong formation (K2dd), and Cretaceous granitoid rocks of the Deo Ca complex (Kdc) are a part in the
geological structure of the ore region (Fig. 2). Siliciclastic sediments with total thickness of more than 3.5 km composes the main area of the region. Two bands can be distinguished in its structure: the lower one (J2ln1) with capacity of about 2 km, which is composed mainly of siltstones and mudstones with horizons of sandstones, and the upper one (J2ln2) with capacity of about 1.5 km, represented mostly by sandstones and siltstones together with argillites and their schistose analogues. Rocks are crushed into folds, broken by a series of faults of a northeastern and northwestern direction. At the place of contact with granitic intrusions they are metamorphosed and transformed into hornfels. In the north of the area these deposits with stratigraphic unconformity are overlain by volcanic rocks of the Dong Dong formation (K2dd). Stratified
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Fig. 3. The scheme of geological structure of the Ta Nang deposit. 1, 2, terrigenous sedimentations of the upper assise of La Nga formation (J2ln2): 1, gray and dark-gray siltstones and shales; 2, gray and dark-gray sandstones. 3, ore zone: brecciated hydrothermally altered sulfided sandstones and shales; gold-quartz-sulfide veins; 4, zones of crush; 5, mining.
buildups of this formation are represented by lavas, tuffs and ingnibrites of rhyolitic and rhyolite-dacite composition, and subvolcanic dikes are comprised of rhyolite porphyries. Intrusive rocks within the field occur in its western part and are presented by small bodies of the Deo Ca complex belonging to the Cretaceous (Kdc). These rocks break through the Middle Jurassic (J2ln1-2) terrigenous-sedimentary blankets of the La Nga formation with development of hornfelsification zones. In some places they are closed by Upper Cretaceous volcanogenic rocks with rhyolite and rhyolite-dacite composition of the Dong Dong formation (Nguen Khoa Son, 2011). Two phases are identified as a part of the intrusive bodies: early one, composed of biotite granites, and late one, represented by biotite-hornblende granites, biotite granosyenites and syenites. Mineral assemblage of these rocks includes plagioclase, K-feldspar, quartz, biotite, hornblende, sphene, orthite, apatite, zircon and magnetite. The rocks belong to calc-alkaline varieties with different ratios of K2O/Na2O. Among these rocks high-K, high sodium, and K-Na differences are distinguished (Tran Trong Hoa et al., 2005). Ta Nang deposit The Ta Nang deposit is located in the high mountainous part of the region in the valley of the Siu Ngang River and is found on the dislocated terrigenous-sedimentary rocks of the upper member of the La Nga formation (Fig. 2). Gray and
dark-gray sandstones, siltstones, and mudstones, dating back to Jurassic period are a part of geological setting of the deposit; they are often schistose and rich in carbonaceous substance. The rocks have a northeast strike (strike azimuth 75°) and a southeast fall at an angle of 60°–65°. The ore zone is represented by a series of sublayer zones of crushing and of hydrothermally altering rocks stretching over 2 km (Fig. 3). Gold ore mineralization is closely associated with sulfided quartz, quartz-carbonaceous veins, with sulfided schistose host rocks, and with their hydrothermally altered differences in minor quantity (Fig. 4). Visible sulphide mineralization is represented mainly by arsenopyrite and pyrite impregnation. Thickness of the largest zone of mineralization reaches up to 10 m, and its length—up to several hundred meters. Besides that, thin zones of crushing (up to 1–2 m) with ore mineralization are widespread, stretching tens of meters. Change of host carbonaceous rocks has a local character and is often manifested on their contact areas of quartz veins in the form of rims up to 2–3 cm, as well as along specific cracks in the host rocks, which are sometimes traced by small crystals of arsenopyrite. It is expressed in the clarification (decarbonization) of carbonaceous shales, appearance of newly formed sericite that replaces feldspars in sandstones, and minor amounts of chlorite, carbonates (ankerite, calcite) and small disseminations of anthraxolite. In general, mineral composition of these metasomatic rocks can be represented as following: sericite—60%, quartz—30%, carbonates—up to 5%, pyrite,
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Fig. 4. Inner construction of ore zone: gold-quartz-sulfide veins in the beetled and hydrothermally altered carbonaceous shales (a photography in the gallery).
arsenopyrite—5%, which corresponds to the overall composition of beresites. Variously oriented quartz-sulfide veins are located in the central parts of zones of crushing with thickness from a few centimeters to 0.5 meters. Small admixture of sericite is found in the composition of quartz veins. Sulphide mineralization in the host rocks and quartz veins is similar in composition and represented by two mineral assemblages: early pyrite-arsenopyrite and late chalcopyrite-sphalerite-galena, attributed by N.V. Petrovskaya (1973) to their own stable mineral assemblages of gold deposits. Pyrite-arsenopyrite assemblage forms different morphogenetic disseminations (Fig. 5, I–III). The most widespread are large-crystalline aggregates and differently oriented metacrystals of arsenopyrite and pyrite with the size ranging from 0.1 mm to 10 mm. Pyrite crystals in the polished sections have the shape of the fragments of cubic structures (plates, diamonds, triangles), and arsenopyrite is mostly represented by short prysmatic crystals. In addition, arsenopyrite forms fine-grained aggregates (~ 0.1 mm), cementing fragments of large crystals of pyrite and arsenopyrite, and it is released in the form of disseminations among carbonaceous shales, while pyrite composes thin globular formations (<0.1 mm) in carbonaceous shales (Fig. 5, III). In the mixed pyrite-arsenopyrite aggregates later development of arsenopyrite is noted, which often replaces pyrite. Chalcopyrite-sphalerite-galena assemblage generally has a more limited development. The main minerals of this assem-
blage dimensionally are closely associated with other rare minerals, such as: pyrrhotite, sulfosalts of lead (tsugaruite), which sometimes also form small inclusions in the major minerals (sphalerite and chalcopyrite). Galena is the most widespread among the minerals of this assemblage. It often assembles fine-crystalline aggregates with sphalerite and also forms small veins the size of 0.1 × 15 mm (Fig. 5, IV); it is represented in the form of rounded inclusions in pyrite and formless disseminations around pyrite the size of up to 1.5 mm. Entrapped metacrystals of pyrite and arsenopyrite are quite often found inside the dissemination of galena. Sphalerite has a more limited distribution; it forms small disseminations (up to 1–2 mm) and joint aggregates with galena and develops inside the cracks in pyrite and arsenopyrite. Almost all forms of sphalerite contain nonuniform emulsive dispersion of chalcopyrite. Chalcopyrite is found in small amounts in the field, and in addition to the emulsive impregnation in sphalerite it forms small (0.01–0.1 mm) shapeless inclusions in arsenopyrite and pyrite as well as dissemination in quartz veins the size of up to 0.5 mm. All noted rare minerals form individual microscopic grains in veined quartz or in the main ore minerals and can be recorded mainly with a microscope and a scanning electronic microscope (Fig. 6). Gold is the main industrial component of the deposit; it mainly forms a thin dispersion, invisible to the naked eye. We have identified small (0.01–0.05 mm) spots of gold in pyrite, arsenopyrite, chalcopyrite, and quartz in the polished sections
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Fig. 5. Types of ore mineralization in the Ta Nang deposit: I, coarse-grained arsenopyrite dissemination in a quartz vein; II, coarse-grained cubic habitus dissemination of pyrite in quartz vein; III, small impregnated disseminations of pyrite and arsenopyrite in black shales; IV, pockety-veined segregations of chalcopyrite-sphaleritegalena mineral assemblage. Full size.
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Fig. 6. Dissemination of ore minerals and rare minerals under the electronic microscope. Pyr, Pyrite; Apyr, arsenopyrite; Chp, copper pyrite; Gl, galena; Sf, sphalerite; Sm, smaitite; Ts, tsugaruite; Q, quartz.
(Fig. 7). Using atomic absorption analysis (Institute of Geology and Mineralogy SB RAS) and ICP MS method in the LTD laboratory (Ontario, Canada) the content of gold and silver in the host sulfided rocks (carbonaceous shales), in
quartz-sulphide veins and in the main ore minerals—pyrite and arsenopyrite was examined. Results of analyses of these two methods are almost identical. Au content in the shale does not exceed 0.n ppm with an average of 0.38 ppm, and in quartz
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Fig. 7. Dissemination of fine-grained gold in quartz (a) and in chalcopyrite in association with galena (b). The compositions of mineral phases are shown in the diagrams. 1, gold; 2, chalcopyrite.
veins its concentrations are much higher and are equal to 3.92 ppm on average (Table 1). Shales and veins have close silver content, which is on average, respectively, 2.68 and 5.30 ppm. As it is demonstrated in many publications (Ashley et al., 2000; Gaskov et al., 2006; Genkin, 1998; Genkin et al., 2002; Kovalev et al., 2011; Starova and Bocharov, 1977; Tyukova and Voroshin, 2007; Vilor et al., 2014; Volkov et al., 2006; Vos et al., 2005), arsenopyrite-pyrite paragenesis in gold-sulphide deposits, localized in black shale series, is the most common. However, gold is concentrated predominantly in arsenopyrite. According to these researches, the most gold-bearing is early thin needle-shaped arsenopyrite, which is nonstoichiometric in its chemical composition and is characterized by the deficit of As with the ratio of S/As 1.16–1.21. As it has been demonstrated above, large prismatical arsenopyrite is the most widely developed in the Ta Nang deposit; it forms large crystals (1.5 cm) and nests in quartz veins and sulfided carbonaceous rocks. Fine-grained aggregates of arsenopyrite are also characterized by short-prismatic
habitus and are found in very small amounts. Sometimes they form a thin layered dissemination in carbonaceous shales and develop in the cracks of macrocrystalline arsenopyrite. The ratio of S/As does not exceed 1.12. Pyrite mainly forms metacrystals and diffuse dissemination in altered rocks and in quartz veins, and rarely—thin fines of globular pyrite in carbonaceous shales. Analysis of monofractions of arsenopyrite and pyrite disseminations demonstrated similar concentrations of Au and Ag in them (Table 2), which generally correlate with their content in the samples, from which they are extracted. The ratio of Au/Ag in the analyzed samples and minerals is characterized by wide variations (Tables 1, 2). Minimum range of changes in this ratio is found in the sulfided carbonaceous shales (0.06–0.44), and maximum one—in arsenopyrite (0.42–8.77). Microscopic studies show that gold in these minerals, as well as a in quartz veins, is developing in the cracks and is connected with its later deposition. The composition of gold, studied mainly in native alluvial formations, developed in
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Table 1. Concentrations of Au, Ag, and trace elements in the ores of the Ta Nang deposit Sample
Rock, ores
Au
Ag
Au/Ag
ppm
Cu
Cd
Mo
Mn
Sr
Pb
Ni
Zn
Ba
Bi
Co
ppm
As
Fe
%
T05-H12/1
Black sulphidized shales 0.49
2.46
0.20
55.8
0.3
3.5
637
259
339
37.9
93.6
133
5.25
13
1.43
5.5
T05-H13/1
0.37
4.17
0.09
72.6
0.2
1.7
517
144
55.9
64.9
624
117
1.39
29
1.19
5.9
T05-H13/2
0.28
4.32
0.06
74.9
0.4
6.7
729
174
73.4
59.2
163
309
0.95
16
0.03
5.9
T05-H13/4
0.57
1.28
0.44
14.2
3.9
0.6
314
68.2
511
7.5
776
56
0.78
2
0.62
1.6
T05-H15/2
0.17
1.21
0.14
5.1
<0.1
0.1
61
5.6
7.8
2.2
5.6
59
0.1
<1
0.18
0.7
Average
0.38
2.68
0.14
44.5
1.2
3.1
451.6 130.2 197.4 34.3
332.44
134.8 1.6
15
0.70
3.9
0.80
2.87
0.28
48.5
2.3
1.1
228
118
105
2.38
<1
1.89
4.2
T05-H13/5
9.63
6.78
1.42
505
1.8
0.5
140
14.1
1670 50.8
352
77
1.94
17
2.55
6.8
T05-H13/6
3.98
3.16
1.25
30.9
< 0.1 0.5
185
5.8
1280 4.2
29.8
9
0.8
1
8.68
8.6
T05-H14/1
4.20
5.17
0.81
146
< 0.1 12.6
42
20.2
162
54.4
30.8
19
1.49
34
7.48
12
T05-H14/2
0.94
1.1
0.85
32.5
< 0.1 1.4
171
12.1
46.1
47.3
35.9
82
0.66
16
2.71
7.3
T05-H15/1
6.96
20.3
0.34
149
60.3
0.3
50
4.7
320
9.4
10800
49
0.27
3
0.10
7.1
T05-H15/2
1.46
2.44
0.60
28.1
8.5
0.6
48
1.9
485
3.7
1580
3
0.38
3
0.12
1.1
T05-H16/2
3.39
0.6
5.65
7.2
< 0.1 1.9
55
1.9
30
7.2
3
4
0.41
6
> 10 9.4
Average
3.92
5.30
0.74
118.4 18.23 2.4
114.9 11.5
639.1 24.3
1618.7
43.5
1.04
11.4
3.36
7.1
5.05
8.14
0.62
40.4
181
3370 6.9
936
144
1.75
5
1.40
2.8
T05-H12/2
T05-H17/2
Quartz-sulphide veins
Concentrate
7.6
alluvial deposits of the Sue Ngang River, varies widely (Table 3). All analyzed samples form a row in which two discrete groups can be identified: 1—with fineness of 527.3– 802.0‰ and 2—941.3–996.7‰; the first group corresponds to endogenous gold, and the second, most probably, to gold, modified in exogenous conditions. The main impurity of endogenous gold is Ag—up 46.05%, and in some samples up to 0.28% of Hg is found. Amount of Ag falls sharply in the modified gold. Analysis of gold-bearing samples of ore to determine the presence of such elements as Ag, Cu, Cd, Mn, Pb, Ni, Zn, As, Ba, Be, Bi, Br, Ca, Co, Cr, Cs, Fe, Hf, Ga, Ge, In, Li, Mg, Na, Nb, Rb, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Tl, U, V, W, Y, Zr, K, including such REE as La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Tb, Ho, Hg, Er, conducted using ISP MS at the LTD
0.7
31.2
1120 17.3
14.7
laboratory (Ontario, Canada), has found their low concentrations, close to Clarke values. Fe, As, Pb, Zn, Cu, Cd, Ni, Co are found in high concentrations in the sulfided rocks and veins, most of which (Fe, As, Pb, Zn, Cu) form their own sulfide minerals, and others are a part of their composition as trace elements (Table 1). However, the level of concentration
Table 3. Content of free gold in the Ta Nang deposit Sample
Number Cu of grains
Au
Hg
Ag
Total
DL-3067-K
5
0
52.73
0.02
46.05
98.80
DL-3067-C
3
0
53.41
0.04
45.97
99.41
DL-3138 DK
1
0
58.34
0
41.59
99.93
DL-3078-C
4
0
59.71
0.28
38.99
98.98
DL-3138 CC
1
0
59.78
0
40.23
100.01
Table 2. Gold content in pyrite and arsenopyrite from the Ta Nang deposit
DL-3138 CK
1
0
60.96
0
39.35
100.31
Sample
DL-3138 DC
1
0
62.66
0
37.02
99.68
DL-3138 BC
1
0
64.50
0
35.34
99.84
Mineral
Au
Ag
Au/Ag
T05-H12/1
Pyrite
5.6
6.9
0.81
T05-H12/2
Arsenopyrite
7.6
18.0
0.42
T05-H13/1
Pyrite
1.0
5.9
0.16
T05-H13/1
Arsenopyrite
6.2
12.0
0.51
T05-H13/5
Pyrite
17.0
12.0
T05-H13/5
Arsenopyrite
10.0
T05-H13/6
Pyrite
T05-H13/6
DL-3137 BC
3
0
65.96
0
31.88
97.84
DL-3138 AC
1
0
72.52
0
27.38
99.9
DL-3137 CC
1
0
80.20
0
19.71
99.91
DL-3070-K
1
0
93.41
0.17
5.11
98.7
1.42
DL-3070
1
0
93.43
0.21
5.21
98.85
3.5
2.86
DL-3137 AC
3
0
96.39
0
2.97
99.36
4.7
5.2
0.90
DL-3138 AK
1
0
96.99
0.02
2.22
99.24
Arsenopyrite
11.0
2.9
3.80
DL-3137 BK
2
0
98.88
0.05
0.78
99.71
T05-H14/1
Pyrite
1.3
1.1
1.18
DL-3137 CK
1
0
99.23
0
0
99.23
T05-H15/2-1
Arsenopyrite
5.0
0.57
8.77
DL-3138 BK
1
0
99.67
0.02
0.49
100.19
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Tran Tuan Anh et al. / Russian Geology and Geophysics 56 (2015) 1414–1427
Table 4. Correlation of elements in the gold ore mineralization of the Ta Nang deposit Au
Ag
Cu
Cd
Mo
Mn
Pb
Au
1.00
Ag
0.63
1.00
Cu
0.77
0.37
1.00
Cd
0.43
0.91
0.12
Mo
–0.07
–0.06
0.06
–0.22
1.00
Mn
–0.48
–0.19
–0.11
–0.28
0.19
1.00
Pb
0.49
0.22
0.24
–0.01
–0.29
–0.15
1.00
Ni
–0.04
–0.05
0.42
–0.28
0.56
0.54
–0.27
Ni
Zn
As
Ba
Bi
Co
Fe
1.00
1.00
Zn
0.41
0.90
0.13
0.95
–0.22
–0.25
–0.06
–0.24
1.00
As
0.26
–0.24
–0.03
–0.29
0.30
–0.34
–0.03
–0.06
–0.30
Ba
–0.28
0.03
0.02
–0.14
0.20
0.81
0.08
0.50
–0.14
–0.49
1.00
Bi
–0.07
–0.10
0.16
–0.24
0.18
0.53
0.20
0.32
–0.25
–0.09
0.30
1.00
1.00
Co
0.04
–0.03
0.35
–0.27
0.71
0.28
–0.26
0.90
–0.23
0.15
0.20
0.22
1.00
Fe
0.39
0.17
0.28
0.02
0.51
–0.09
–0.20
0.45
0.03
0.74
–0.16
0.06
0.56
1.00
Note. In the sampling with n = 14, significant correlation coefficient (r) > 0.54.
Table 5. Composition of ore minerals from the Ta Nang deposit Test No.
Number of analyses
Fe
Cu
Zn
Pb
S
As
Sb
Bi
Ag
Cd
Mn
Co
Ni
Total
1
33.24
–
–
0
20.60
42.79
0
–
–
–
–
0.02
0.60
97.21
2
34.03
–
–
0.01
20.30
43.97
0
–
–
–
–
0.03
0.02
98.36
3
34.04
–
–
0
20.73
43.10
0.03
–
–
–
–
0.01
0.03
97.93
Arsenopyrite T05-H13/1
4
34.25
–
–
0
20.94
42.76
0
–
–
–
–
0.03
0.26
98.25
T05-H13/2
1
33.95
–
–
0
20.23
43.93
0
–
–
–
–
0.10
0.05
98.26
2
33.95
–
–
0.04
21.14
42.85
0
–
–
–
–
0.05
0.03
98.06
T05-H13/4
1
35.16
–
–
0
21.69
42.53
0
–
–
–
–
0.05
0.01
99.43
T05-H13/5
1
34.76
–
–
0
20.86
43.21
0.02
–
–
–
–
0.04
0
98.89
2
34.95
–
–
0
21.33
42.93
0.01
–
–
–
–
0.05
0.01
99.27
3
35.32
–
–
0.01
21.85
42.59
0
–
–
–
–
0.05
0.01
99.82
1
34.12
–
–
0.02
20.40
43.36
0.03
–
–
–
–
0.05
0.03
98.01
2
34.63
–
–
0.05
21.38
42.56
0
–
–
–
–
0.03
0.02
98.67
3
34.87
–
–
0
21.33
43.41
0
–
–
–
–
0.04
0
99.65
1
45.45
–
–
0.04
50.81
0
–
0.02
0
–
–
0.04
0.05
96.41
2
45.24
–
–
0.13
51.89
0
–
0.03
0
–
–
0.05
0
97.34
T05-H13/6
Pyrite T05-H13/4
3
45.98
–
–
0.08
52.65
0
–
0.02
0
–
–
0.06
0.04
98.83
T05-H13/5
1
46.84
–
–
0.05
53.28
0
–
0.00
0
–
–
0.05
0
99.22
T05-H13/6
1
45.92
–
–
0
52.88
0
–
0.01
0
–
–
0.04
0
98.84
2
45.25
–
–
0
52.84
0
–
0.01
0
–
–
0.04
0
98.14
T05-H15/2
3
45.18
–
–
0.02
51.66
0
–
0
0
–
–
0.03
0.05
99.39
1
45.94
–
–
0
52.92
0
–
0.03
0
–
–
0.05
0
98.94
2
46.03
–
–
0.01
52.06
0
–
0
0
–
–
0.03
0
98.13
(continued on next page)
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Tran Tuan Anh et al. / Russian Geology and Geophysics 56 (2015) 1414–1427
Table 5 (continued) Test No.
Number of analyses
Fe
Cu
Zn
Pb
S
As
Sb
Bi
Ag
Cd
Mn
Co
Ni
Total
T05-H15/2
3
45.84
–
–
0.05
52.32
0
–
0
0
–
–
0.05
0.02
98.27
4
47.24
–
–
0
52.98
0
–
0
0.03
–
–
0.05
0
100.39
5
45.77
–
–
0.03
52.85
0
–
0
0.01
–
–
0.03
0
98.68
6
46.44
–
–
0.06
52.36
0
–
0
0.04
–
–
0.05
0.02
98.99
1
43.66
–
–
0
52.52
0.05
–
0
0
–
–
0
1.97
98.2
2
45.8
–
–
0
53.01
0.27
–
0
0
–
–
0
0.47
99.73
1
45.18
–
–
0.02
51.66
2.44
–
0
0
–
–
0.03
0.05
99.39
1
59.99
–
–
–
38.7
0.13
–
–
–
–
–
–
–
98.83
2
60.78
–
–
–
38.69
0.16
–
–
–
–
–
–
–
99.63
1
28.96
34.17
1.41
35.26
–
–
–
0.17
0.02
–
–
–
99.99
2
28.93
34.06
1.58
35.26
–
–
–
0.01
0
–
–
–
99.84
T05-H13/4
T05-H12/3 Pyrrhotite T05-H13/2
Chalcopyrite T05-H13/1
Galena T05-H13/5
T05-H13/6
T05-H15/2
1
85.91
13.65
0.07
0.03
0.21
–
–
0
99.97
2
86.24
13.16
0.01
0.1
0
–
–
0
99.53
3
86.20
13.18
0.06
0.01
0
–
–
0
99.45
4
87.08
13.28
0.06
0.07
0
–
–
0
100.57
1
87.13
13.3
0.1
0
0
–
–
0
100.53
2
87.63
13.14
0.03
0
0
–
–
0.01
100.81
3
86.24
13.14
0
0
0
–
–
0.04
99.42
4
86.74
13.04
0.04
0
0
–
–
0.02
99.84
5
87.29
13.08
0.03
0
0
–
–
0.01
100.41
1
87.47
13.03
0.07
0.1
0
–
–
0
100.67
Sphalerite T05-H13/1
T05-H15/2
1
4.7
0.06
62.39
–
33.72
–
–
–
0.02
0.44
0.01
–
–
101.33
2
6.66
0.93
58.88
–
34.00
–
–
–
0.03
0.42
0.01
–
–
100.93
3
4.76
0.71
58.36
–
33.96
–
–
–
0.02
0.43
0.00
–
–
98.24
4
4.44
0.18
61.46
–
34.15
–
–
–
0.01
0.43
0.02
–
–
100.7
5
4.95
0.05
60.65
–
33.98
–
–
–
0.01
0.40
0.02
–
–
100.15
1
4.54
0.02
61.26
–
33.49
–
–
–
0.01
0.38
0
–
–
99.78
2
3.46
0.02
63.27
–
33.74
–
–
–
0.02
0.37
0.02
–
–
100.95
0.26
–
–
73.75
15.48
10.9
–
–
–
–
–
–
–
100.39
Tsugaruite T05-H13/4
Note: Dash, Below the detection limit (<0.01%).
of these elements in the sulfided carbonaceous shales is significantly lower compared with quartz-sulfide veins, which is, most probably, connected with smaller amounts of sulfides therein. Additionally, increased contents of Mn, Ba and Sr are found in the shales (Table 1). Also, some samples with high Zn, Pb, Cu contents tend to have higher Cd, and in samples, where Fe and As are predominant, concentrations of Co, Ni, Mo are increasing. Calculation of the pair correlation coefficients shows direct positive correlation of Au with Ag and Cu, and Ag with Zn and Cd; also, positive direct correlation
of Co with Fe and Ni; Fe with As; Mn with Ba is found (Table 4). Analysis of the content of the main ore minerals, conducted at the Analytical Center of the Institute of Geology and Mineralogy SB RAS (Russia) using microanalyzer Camebax micro, shows their following features. In the field arsenopyrite is characterized by stable chemical composition with close values of atomic ratio of S/As, varying in the range of 1.08–1.12. Among the impurity elements, found in arsenopyrite, there is up to 0.6% of Ni and in some samples—Sb
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Tran Tuan Anh et al. / Russian Geology and Geophysics 56 (2015) 1414–1427
and Pb at the level of sensitivity of X-ray spectral microanalysis (Table 5). Pyrite corresponds to the stoichiometry in its composition. Out of the impurity elements some of its samples also have an increased Ni content up to 1.97%, As up to 2.44%, and small amounts of Co, Bi, and Ag. Minerals, belonging to late assemblage, have a wider range of trace elements. Sphalerite permanently contains few percentages of iron admixture, and Cu and Cd—0.n%. In addition it notes the following Ag and Mn (Table 5). Besides that, it has traces of Ag and Mn (Table 5). Galena contains increased admixture of Ag and Bi (0.1%), and chalcopyrite—increased content of Zn and a small admixture of Ag and Cd. Only separate microscopic inclusions of tsugaruite have been revealed in sulfosalts, in which Pb as a basic cation was found (Table 5).
Genesis of the Ta Nang deposit As it has already been noted, the Ta Nang deposit is localized among clastic terrigenous sedimentations, belonging to the Middle Jurassic Period (J2ln1–2), enriched with carbonaceous material (La Nga formation). It is found in the sublayer area of crushing and hydrothermal alteration and is represented by the development of quartz-sulfide veins and sulphide mineralization in the host rocks. Determining the time of formation of sericite from quartz-sulfide veins using the method of Ar39/Ar40 dating indicates that the age of formation of these veins along with sulfide mineralization was 129.3 ± 5.6 Ma. This indicates that formation of ores of the Ta Nang deposit is close in time with the development the Cretaceous granitic intrusive formations of the Deo Ca complex (145– 99.6 Ma), which break through the Upper Jurassic deposits (145 Ma) of the La Nga formation and are covered by volcanic rocks of the Don Dong formation, belonging to the Upper Cretaceous (99.6 Ma) (Vu Nhu Hung et al., 2003). Comparison of the spectrogram graphs of rare earth elements, stand-
ardized on the basis of chondrite (Boynton, 1984) and drawn for quartz-sulfide veins and sulfided shales of the Ta Nang deposit, with the spectra of rare earth elements of the granites of the Deo Ca complex and volcanites of the Dong Dong formation (K2dd), shows similar configurations of the charts of ore formations and granites and their significant difference with the volcanite graphs. The spectrograms of ore formations and granites are noted by enrichment with light lanthanoides and manifestation of minor europium minimum (Fig. 8), which can probably be indirect evidence of the connection between ore-forming hydrothermal solutions with these granites. The relationship of ore minerals and the specifics of development of their different morphological features show that in the formation of the Ta Nang deposit two stages can be identified: early quartz-pyrite-arsenopyrite and late chalcopyrite-sphalerite-galena. Mineral assemblages of these stages are related to hydrothermal ore formations with similar isotopic composition of sulfur of their sulfides, varying within (δ34S 1.6–4.3‰) (Table 6), which indicates that there is a single deep endogenous source of sulfur. Slight weighting of the isotopic composition of sulfur in comparison with meteorite analogue can be explained by partial assimilation of the weighted rocky sulfur during the formation of fine crystalline, mainly framboidal mineralization of pyrite and arsenopyrite in carbonaceous shales during their diagenetic transformation. Later on, during the formation of mineralization zone in the areas of crushing and foliation this dispersed mineralization was partially mobilized and redeposited in the ore zones along with endogenous sulfide mineralization. The possibility of such redistribution is indicated by a redeposition of carbonaceous material of host rocks with formation of patches and dispersions of anthraxolite (amorphous conductive carbon) and graphite in altered rocks and quartz-sulfide veins. Isotopic composition of carbon in these structures shows that the deposition of graphite and anthraxolite was due to redeposition of slightly metamorphosed carbonaceous substances of host rocks (Table 7). Physical-chemical conditions of formation of quartz-sulfide veins, according to the study of fluid inclusions in quartz, are characterized by a narrow temperature range— 305–245 °C. They were formed out of moderately concen-
Table 6. Isotopic composition of sulfur in the main ore minerals of the Ta Nang deposit
Fig. 8. Distribution of rare earth elements in quartz-sulfide veins: (1) black sulphidized shales; (2) Ta Nang deposit, granites of Deo Ca complex (3) and volcanites of Don Dong formation (4).
Sample
Mineral
δ34S
DL-3067
Arsenopyrite
1.6
DL-3068
Arsenopyrite
2.0
DL-3074-5
Arsenopyrite
4.3
DL-3074-9
Arsenopyrite
4.1
DL-3076-5
Arsenopyrite
4.0
DL-3075-9
Pyrite
3.5
DL-3076-9
Sphalerite
3.5
DL-3076-a
Galena
3.9
DL-3079a-9
Galena
1.7
DL-3077
Galena
2.3
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Tran Tuan Anh et al. / Russian Geology and Geophysics 56 (2015) 1414–1427
Table 7. Carbon isotopic composition of carbonaceous material from the ore gold deposits (%) Deposit
δ13C
Carbonaceous material
Ta Nang
–19.7
Anthraxolite
Ta Nang
–18.5
DL-3061
Carbon shale
Ta Nang
–18.0
DL-3080
Graphite
Ta Nang
–21.5
Sample
Mineral
DL-3067 D 1-3067
Table 8. Results of thermobarogeochemical studies of fluid inclusions in quartz from the Ta Nang deposits Tice melt, °C
ΣC, wt.% NaCl equiv
Sample
Thom, °C
Teutec, °C
DL-3079
305–275
–50 /–49
–8 / –6
11.7–9.2
DL-3070
250–245
–22
–5 / –4
7.8–6.4
trated solutions (11.7–6.4 wt.% NaCl equiv) with the methane-nitrogen-carbon dioxide gas phase (CO2 + N2 + CH4) (Table 8). Gold has a clear correlation with the sulphides, according to the analysis of its distribution and its dissemination forms. It develops as thin spots in the cracks of disseminations of pyrite, arsenopyrite, quartz, and is located in the form of spots or is intergrown with the minerals of late assemblage (galena, sphalerite, chalcopyrite), which indicates that it was formed at a later stage together with these sulfide minerals. Thus, studies in the Ta Nang deposit indicate that it relates to endogenous hydrothermal formations of later associtation, the formation of which occurred in the Cretaceous (129.3 ± 5.6 Ma) and is associated with the invasion of granitoid intrusions of the Deo Ca complex. The process of ore mineralization occured in two stages: early quartz-pyritearsenopyrite and late chalcopyrite-galena-sphalerite, which is prospective for gold. Gold is characterized by wide variations in fineness (527.3–802.0‰) and forms small and thin spots in quartz and sulphides. Black shale series of La Nga formation due to its sand-shale composition and enrichment with carbonaceous matter was a favorable environment for the deposition of ore components of postmagmatic hydrothermal solutions. Therefore, these strata in the areas of development of granitoid magmatism of the Deo Ca complex may be prospective for the detection of such deposits. This work was supported by the project “Complex and Rational Use of Mineral Resources for Economic and Social Development of Thai Nguyen region, TN3/T05” and 105.06.76.09 (NAFOSTED).
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