Stable isotope geochemistry of clay minerals from fossil and active hydrothermal systems, southwestern Hokkaido, Japan

Gexxhimica et Cosmochimica Acta. Vol. 59. No. 12, pp. 2545-2559, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/95 $9.50 + .oo

kizh

Pergamon

0016-7037( 95)00149-2

Stable isotope geochemistry of clay minerals from fossil and active hydrothermal systems, southwestern Hokkaido, Japan KATWMI

MARUMO,’

FREDJ. LONGSTAFFE’ and

OSAMU MATSUBAYA~

‘Mineral

Resources Division, Geological Survey of Japan, Hishashi l-l-3, Tsukuba, Ibaraki 305, Japan *Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada 3Mining College, Akita University, 1-I Tegatagakuen-cho, Akita 010, Japan (Received

October

14, 1992; accepted

in revisedform

March 22, 1995)

Abstract-Miocene submarine to Quatemary terrestrial volcanism in southwestern Hokkaido, Japan, is associated with hydrothermal clay alteration and mineralization, including Kuroko-type deposits at Kagenosawa (14.2 Ma, Cu > Zn, Pb > Au) and Minamishiraoi (12.5 Ma, Ba > Zn, Pb, Cu), vein-style mineralization at Date (5.2 Ma, Au-Ag-Cu-Pb-Zn) and Chitose (3.6 Ma, Au-Ag ), and geothermal activity at Noboribetsu ( I 1.8 Ma). The 6D and 6 ‘*O values of mica (sericite), mica-smectite, chlorite, chloritesmectite, nacrite, dickite, kaolinite, and smectite were used to deduce the type(s) of hydrothermal fluid at each locality. Calculated compositions for Minamishiraoi and Kagenosawa fluids suggest that seawater was dominant, but some mixing with magmatic water is also indicated, particularly for the polymetallic Kagenosawa deposit. Hydrothermal fluids at Date, Chitose, and the Nohoribetsu geothermal area were dominated by meteoric water. Minor involvement of magmatic water during mineralization at Date cannot he ruled out, but evolution of local meteoric water along an evaporation trend and/or an ‘*O-shift due to hydrothermal rock-meteoric water interaction also could have produced appropriate fluid compositions. The 6D and 6’aO values of modem hot-spring waters at Noboribetsu closely parallel fluid compositions calculated for the clay alteration at Date, Chitose, and Noboribetsu. Because relatively poor reproducibility was obtained for the 6D values of the swelling clays, additional tests were conducted. Stepwise heating showed that, for some smectitic clays, water evolved between 200 and 300°C had anomalously high SD values because of residual interlayer water. This error can be minimized by sufficiently long preheating (in vacua) at =200°C. In vacua TG patterns of other smectitic clays suggested gradual loss of hydroxyl-groups beginning near 200°C, rather than the more typical distinct separation between interlayer water at <2OO”C and hydroxyl-groups at >4OO“C. This behaviour constrains the maximum temperature that can he used for in vacua preheating. Furthermore, shifts to lower 6D values (by as much as 1%0) were obtained when this smectite was dispersed in low-D water for three weeks, perhaps indicating isotopic exchange. However, with appropriate care, bD values obtained by conventional procedures (including preheating to 5200°C) normally reproduced natural compositions of the smectitic clays with acceptable

accuracy and precision.

INTRODUCTION

thermal ore deposits. Since then, fundamental studies of hydrogen isotopic exchange in systems such as kaolinite-water and chlorite-water have included O’Neil and Kharaka ( 1976), Suzuoki and Epstein (1976), Liu and Epstein ( 1984), and Graham et al. ( 1987). Such minerals exhibit extremely slow hydrogen isotopic exchange with water at low temperatures. Accordingly, most experiments have heen conducted at temperatures much higher than typical for natural crystallization of these phases. Recently, Vennemann and O’Neil (1994) have reported lower temperature, hydrogen-isotope exchange experiments that use molecular hydrogen rather than water to infer ~c~sy-waar. In addition, using natural samples from the Ohnuma geothermal area of Japan, Marumo et al. ( 1980) calculated hydrogen aclsy_watcr values for hydrothermal kaolingroup minerals, sericite, and chlorite. Oxygen isotopic exchange between clay minerals and water occurs even more slowly than for hydrogen (e.g., Longstaffe, 1989), and makes experimental evaluation of oxygen-isotope oclay_warer even more difficult. Empirical estimates of oxygen isotopic ~c~ay-watcr, and their variation with temperature, have heen made for chlorite ( Wenner and Taylor, 197 1 ), kaolinite (Savin and Epstein, 1970; Lawrence and Taylor, 1971; Land and Dutton, 1978)) and illite (Eslinger and Savin, 1973; Yeh

Miocene submarine to Quatemary terrestrial volcanism occurred within a narrow region of southwestern Hokkaido, Japan, causing substantial hydrothermal rock-water interaction, and producing mineralization. The main objective of this paper is to use hydrogen- and oxygen-isotope compositions of secondary clay minerals formed in fossil and active hydrothermal systems at Minamishiraoi, Kagenosawa, Date, Chitose, and Nohorihetsu (Fig. 1) to deduce the hydrothermal fluid evolution that accompanied transition from marine to terrestrial volcanism. Such interpretations most commonly are based on data for kaolin-group minerals and mica (sericite) . Their stable isotope oclay_warcr values are moderately well known and their hydroxyl-groups generally believed to he least susceptible to alteration (Marumo, 1989). However, besides kaolin-group minerals and mica (sericite), the clay-alteration assemblage in Hokkaido also includes smectite, micasmectite, and chlorite-smectite (Table 1). The stable isotopic systematics of these latter phases are more poorly documented. Sheppard et al. ( 1969) were among the very first to demonstrate the usefulness of hydrogen- and oxygen-isotope clay geochemistry in determining the origin of fluids in hydro2545

K. Marumo, F. J. Longstaffe, and 0. Matsubaya

2546

. Kuroko- t ype deposit *Au-Ag-Cu-Pb-Zn

vein type

deposit

o Au-Ag vein type deposit oDrillhole elte for geothermal exploration . Kuroko-type d Active

deposit

qCHITOSE

volcano E144’

E140’

N42

Fro. 1. Location of ore deposits and geothermal area investigated in this study. The distribution of active volcanoes is also shown.

and Savin, 1977). In addition, Savin and Lee (1988) have described semiempirical methods to calculate oxygen-isotope ~~~~~~~~~~~ based on the structure and chemical composition of the clay mineral. Relatively few systematic stable isotopic studies have been performed on smectite, mica-smectite, or chlorite-smectite, despite their abundance in hydrothermal environments. Isotope exchange studies generally are not possible because these minerals convert to other phases (e.g., sericite and chlorite) at temperatures well below those needed for experiments. Interlayer water also complicates the system because the extent of hydrogen isotopic exchange among hydroxyl-groups, interlayer water, and bulk water is not well understood. Residual

interlayer water also can be retained by the clay, depending on procedures used to dehydrate smectitic clays prior to isotopic analysis. With these uncertainties in mind, a subsiduary objective of this paper is to relate isotopic results for smectite, mica-smectite, and chlorite-smectite to those for kaolinite and mica (sericite) from the mineralized deposits in southwestern Hokkaido, and provide at least empirical evidence for the isotopic behaviour of these swelling clays in a natural hydrothermal system. Analytically, special attention was directed towards the dehydration behaviour of each swelling clay, including possible variability in the temperature at which interlayer water is removed completely, and the sensitivity of 83 values

Table 1. (kc-deposit charactistics Host Rock and Aee of

Des&t

h4iidiiti0~

Andcsitic or dacitic lavas and

pymclastic rocks; 12.5M.6Ma

Mineral Assemble in Alteration Zone

Fluid-InclusionTemperatures

*93M/S (mica layers %O%),

5230 to 250°C for qusrtz veinlet

nacrite,hick&e &d 4c/S in footwall-alterationzone beneath btite-bearing silicified breccia; the peripheralpart of the.zone containskaolin& and smectite

in barite-bearingsilicitied breccia; 180 to 19OoCfor suhaleritein the footwall-alteration’xone; cl 4ooC for barite in the peripheraJpart of the footwall-alterationzone

And&tic or dacitic lavas and pymclastic rocks; 14.2M.7Ma

Mica and MIS

Kuroko-type(Cu>Zn. Pb>Au)

5250 to 280°C for quartz veinlet in the Cu-bearing siliceom ore

l%Z Au-Ag-Cu-PbZn vein type

And&tic or dacitic volcanic ycch4;i quartz porphm;

I M mica, M/S, US and smectite in earlier stage;dickite. nacrite and sudoitein later stage

5220 to 260% for quartz veinlet

Chitose Au-Ag vein type

Andesitic lava and quartz porphyry; 3.6&0.6Ma

1M and 2Ml micas and kaolinite

a200 to 25OoCfor Om level quartz veins; 270 to 3lOoC for -420m level quartz veins

Noboribetsugeothermalarea

D&e tuffsand and&tic lavas; 1.8Ma to present

Kaolinite, dickite.,nacrite. MIS. US, smectis and moxdenite(at <3OOmdepth); Ib and IIb chlorites. 1M and 2M, micas. laumontiteand wabakite (at >3OOmdepth)

7180 to 230°C for quartz veins

Kagenosawa

.

.

which coexist with 1M mica and M/S (at c5OOm depth); 230 10 28OT for quartzand calciteveins

which coexist with lb and Ilb chlorites;320 to 38OoCfor quartz veins which coexistwith 2M 1

mica

‘After MARUMO (1985) and MARUMO and SAWAl (1986); %fter MARUMO et al. (1985); %nica-smectite;Chlorite-smectite; sYONEDA (personalcommunication);6YAJIMA (1979); ‘New Energy and Industrial Technology DevelopmentOrganizationof Japan (1991)

Isotope composition of hydrothermal clay in Japan

obtained for ‘ ‘hydroxyl-group" tional procedures) interlayer water.

hydrogen (using convento retention of, or isotopic exchange with,

ANALYTICAL

PROCEDURES

Samples of the alteration haloes were collected from drill cores and outcrops at the Minamishiraoi deposit and the Noboribetsu geothermal area. Sampling of the other deposits was restricted to poorly exposed outcrops. Clay-size fractions (<2 pm) were obtained by gently crushing samples, followed by suspension and dispersion in distilled water for one week (except where otherwise indicated; see below). These fractions were further purified until only the desired clay phase was detectable by X-ray diffraction (XRD). Most XRD an&es were performed on randomly oriented, <2 pm material, usine a Rieaku diffractometer (20 mA and 35 kV), Cuba radiation, 1” diLerge;ce, and scatter slits,‘and a scanning rate of 2”28/min. The type. of mica-smectite was determined by XRD using ethylene glycolsolvated, oriented smears on glass slides, following Watanabe ( 1981). The Fe*+ content of chlorite was estimated using the XRU intensities of the 0.47, 0.7, and 1.4 nm basal diffractions, obtained using ‘/6”divergence and scatter slits (Oinuma et al., 1972). Grainby-grain chemical analyses of smectite, chlorite-smectite, and chlorite were obtained at 100 kV accelerating voltage using a Philips CMlZlSTEM transmission electron microscope (ATEM) equipped with EDS (EDAX 9900). More than twenty grains were analyzed for each sample to obtain average (Fe*’ + Fe’+)/(Fe*’ + Fe3+ + Mg*+) ratios. Standard wet chemical methods were used to verify the ATEM results (Table 2). The difference between (Fe*+ + Fe”+)/ (Fe*’ + Fe3+ + Mg*+) ratios obtained using each method was minimal (e.g., <0.025 for chlorite). Thermogravimetric analyses (TG) of smectite, mica-smectite, and chlorite-smectite were performed at 1 atmosphere total pressure and under vacuum, using a Rigaku TASlOO instrument. About 30 mg samples were heated to 1ooo”C at 2O”CYmin.Because interlayer and surface-sorbed water are lost rapidly under vacuum at room temperature, samples were maintained under vacuum for about 1 h before heating to stabilize the TG curves. The hydrogen- and oxygen-isotope data for the clays am reported in the normal C-notation relative to the V-SMOW standard (Tables 3.4.5). Hydrogen-isotope analyses were obtained using a MAT 250 mass spectrometer. Oxygen-isotope values were obtained using a VG-SIRA mass spectrometer. An oxygen isotope, COI-H20 fractionation factor of 1.0412 at 25°C was used to calibrate the massspectrometer reference gas. Water was evolved from clay minerals by heating under vacuum at temperatures up to 1000°C (after Bigeleisen et al., 1952; Godfrey, 1962). Hydrogen produced during this process was moxidized immediately to H20 by reaction with CuO at 500600°C. Uranium maintained at 800°C was then used to reduce all water to hydrogen. Clay hydrogen-isotope analyses obtained using the standard protocol (removal of water released up to 200°C prior to collection of water evolved between 200 and 1000°C; Savin and Epstein, 1970) are listed in Table 5. Reproducibility for kaolin-group minerals analyzed using the standard protocol averaged < 41X0. Additional 6D measurements were made to test the sensitivity of swelling clays to preparation procedures (see below). Oxygen was extracted from the clay minerals using the BrFs method of Clayton and Mayeda ( 1963). and quantitatively converted to CO2 over red-hot graphite. Prior to reaction with BrF, , the samples were heated under vacuum for 20 h at 200°C. All samples were analyzed at least twice; triplicate analyses were performed on three samples. The standard deviation calculated from the pooled residual variance of renlicate 6 “0 values is ?0.16%0. A 6 ‘*O value of +9.66 2 0.13%0 was’ obtained for several aliquots of silica standard NBS28 analyzed at the same time as the clay minerals. ANALYTICAL

HYDROGEN ISOTOPE VARIABILITY SWELLING CLAYS

OF

Prior to using the 6D values of hydroxyl-groups from Hokkaido swelling clays to elucidate the evolution of hydrother-

2547

mal fluids, possible effects due to interlayer water were investigated. First, representative samples of smectite, micasmectite, and chlorite-smectite were subjected to stepwise heating in vacua to measure the 6D values of water evolved at <25”C, 25-lOO”C, IOO-2OO”C, 200-300°C. 300-4OO”C, and 400- 1000°C (Table 3). Most samples were maintained at the top of the temperature range for 1 h; evolution of water at each step was monitored using a vacuum gauge, and generally was completed in 20-50 minutes. The volume of hydrogen produced at each step (wt% water in Table 3) was measured using a Hg manometer. Second, the preheating period was varied to evaluate the possible consequences for the quantity and m values of evolved water produced using the standard protocol (Table 4). Third, to evaluate whether the 6D values of swelling clays could be affected by dispersion in distilled water during clay separation, separate aliquots of smectite and mica-smectite samples were allowed to stand in this water for one and three weeks, respectively (Table 4). Mineralogy Samples of dioctahedral smectite were analyzed from the Minamishiraoi and Date deposits and the Noboribetsu geothermal area. ATEM data are shown in Fig. 2a-c; the wet chemical analysis available for one sample (#56-S-82.5 m, Table 2) is consistent with the ATEM results. The relative abundances of A1203, Fez03, and MgO of most smectite samples lie within the montmorillonite field, but some samples from the Date deposit (#83092105, Fig. 2b) have compositions typical of beidellite, whereas sample #57-l-44 m (Minamishiraoi deposit) is richer in Fe and Mg (i.e., nontronitic, Fig. 2a). Mica-smectite from the Minamishiraoi deposit contains 60 to >80% mica (sericite) layers and has a Reichweite value > 1; K20 values (4.7-7.7 wt%, Table 2) are consistent with the percentage of mica layers determined using XRD. Micasmectite from the Date deposit (#83092007 and #83092206) contains >80% mica layers and has Reichweite values > 1. Dioctahedral chlorite-smectite from the Minamishiraoi and Date deposits, and the Noboribetsu geothermal area exhibits a sharp XRD diffraction at about 3.0 nm; it has been identified as Fe-poor tosudite (Fe0 + Fez09 < 0.57 wt%; ATRM data) (i.e., regularly interstratified aluminous chlorite and montmorillonite) . TG-DTA Dehydration Patterns Thermogravimetric (TG) and the differential thermal analysis (DTA) curves for smectite (sample #82102805) from the Noboribetsu geothermal area are shown in Fig. 3, produced in air, together with the TG curve obtained under vacuum. At 1 atmosphere, the smectite showed 9.928% loss in weight up to 250°C and 0.972% weight loss between 250 and 400°C. The in vacua weight loss at ~250°C diminished to 1.8278, indicating that most interlayer water was lost prior to the TG determination. In vacua TG curves for smectite samples from the Date and Minamishiraoi deposits are shown in Fig. 4. Loss of interlayer water and hydroxyl-groups was distinctly separated for the Date smectite (sample #83092105) but was much more grad-

K. Marumo, F. J. Longstaffe, and 0. Matsubaya

2548

MINAMISHIRAOI

MO

MO

Smectite i\ A’203

Fe24

(4

0

Y

”

”

A1203

“c-8.

”

0

Y

90

”

0

”

”

”

Fe303

MO

DATE Smectite

ual for the Minamishiraoi smectite (sample #57-l-44 m). The Noboribetsu smectite (82102805, Fig. 3) exhibits intermediate behaviour, but is most similar to the Date smectite. The weight loss during in vacua TG analysis between 250 and 1000°C is much higher for the Minamishiraoi smectite (6.916%) than the Noboribetsu or Date samples (4.7115.043%) (Table 4). Such behaviour may be induced by rapid heating (20”Umin) during TG analysis since, during stepwise heating, the amount of water evolved between 200 and 1000°C (5.11 wt%) is comparable to the Date (5.04 wt%) and Noboribetsu smectites (4.33 wt%) (Table 4). In vacua TG curves for Minamishiraoi mica-smectite (60% mica layers, sample #57-6-55 m) and tosudite (sample #57l-90 m) are also illustrated in Fig. 4. Dehydration of interlayer water and hydroxyl-groups was very clearly separated for the mica-smectite at C2OO”C. Separation was not quite so effective for the tosudite; nevertheless,
04

NOBORIBETSU

WI0

Smectite

(4

/“YVV~VV”~~ A1203

90

Fe203

FIG. 2. ATEM data for smectite. Total Fe was calculated as FezOB. (a) Minamishiraoi deposit: Cl sa. 56-5-70.5 m; 0 sa. 56-5-82.5 m; V sa. 57-l-44 m; (b) Date deposit: Cl sa. 83092105; V sa. 83092221; 0 sa. 83092223; (c) Noboribetsu geothermal area: 0 sa. 82102805; 0 sa. 82102806, V sa. 82102809.

The quantity of water evolved, and its 6D value, obtained from in vacua, stepwise heating of the smectite, mica-smecthe, and tosudite samples described above are illustrated in Fig. 5a and b, respectively, and summarized in Table 3. All three smectite samples exhibit similar behaviour. The 6D values of water collected at room temperature are negative (-77 to -71%0), whereas those collected at 25-100°C and lOO200°C are positive (+3 to +9%0, +24 to +2%0, and +29 to +14%0, respectively, for Minamishiraoi, Date, and Noboribetsu smectite). This pattern shows that interlayer water dehydrated in vacua at room temperature was depleted of deuterium relative to residual interlayer water extracted at higher temperatures. That most of the interlayer water has lower 6D values, and the remaining small amount, released at higher temperatures has higher 6D values than the distilled water, is consistent with a Rayleigh-type distillation process during which deuterium was concentrated in the residual interlayer water. We can calculate the weighted average 6D value (-57 + 5%0) of the interlayer water from the data given in Table 3, assuming that all of it was captured in the fractions released at 25°C and 25- 100°C; the average value is slightly higher (-54 -C 4560) if the fraction collected at lOO-200°C is also included. These values are higher than that of the distilled water (-68 t 5%0) used to produce these clay separates. Because interlayer water undergoes very rapid, in situ, isotopic exchange with atmospheric water vapour (Savin, 1967), interpretation of the former’s stable isotope compositions is very difficult. However, if the 6D values for interlayer water obtained in this study retain an isotopic signature derived from the distilled water, an ointi~y~w~~~_~t~ws~r of 1.0118- 1.0150 can be calculated. Such values are consistent with the prediction of Lawrence and Taviani (1988) for this fractionation factor, but entirely opposite in sign to that measured by France-Lanord and Sheppard ( 1992). This significant discrepancy in the apparent nature of the interlayer water-bulk water fractionation factor remains to be investigated further.

Isotope composition

2549

of hydrothermal clay in Japan

Table 2. Chemistry of clay minerals 56-5-82.5m smectite 51.10 0.53 24.13 1.27

SiO2 Tie A2oj Fe203 z FE Na20 K20 2%;

5%655m ‘M/S 52.65 1.12 26.30 0.16

0.40

IB2- 1478m IIb chlorite 26.78 0.88 17.70 3.98

56-2-60m US 47.58 3E

IB3-329.5m lb chlorite 31.71 0.10 17.90 4.21

0.01 0.51

0.02

0.01

15.76 0.51

20.21 0.75

2.26 1.84 0.05 0.15 14.23 4.16

0.95 2.52 0.05 4.74 4.89

0.48 0.35 FE

22.21 0.20 0.28 0.07 10.63 0.76

12.23 0.31 0.53 0.73 10.39

4:82

9Et 100.24 10::: Total 1Mica-smectite with 60% mica layers; ~icaknu&e

For the Date and Noboribetsu smectites, 6D values of water dehydrated between 200 and 300°C remain significantly higher than those of water evolved at 300-400°C and 4001000°C. This behaviour suggests that minor quantities of Drich residual interlayer water, as well as water evolved from hydroxyl-groups, may have contributed to the 200-300°C aliquot. On one hand, such contamination, as calculated using the data in Table 3, could have increased the cumulative 6D value of water evolved from 200- 1000°C by up to 1.5-3.5%0. On the other hand, however, the cumulative 6D values calculated for both samples using data only for 300-400°C and 400- 1000°C aliquots in Table 3 ( -64, -68%0) are virtually identical to the results obtained using the conventional (200-

99.76

lOkE

with >80% mica layers

1000°C) protocol (-64, -67%0) (Table 4). Likewise, for both samples, the amount of water released using the conventional protocol is very similar to that obtained during the stepwise heating and the TG analysis (Table 4). For these two samples, at least, the possible effects of residual interlayer water (200-300°C) seem to be too small to be uniquely discerned within the typical precision of the hydrogen-isotope method. For the Minamishiraoi smectite, the 6D value for the 200300°C aliquot is similar to higher temperature fractions. The cumulative SD value (-27%0) calculated for 200-1ooO”C (Table 3) is even lower than that obtained using the conventional protocol ( -24%0) (Table 4), despite the longer total preheating time in the latter analysis. Little evidence exists

NOBORIBETSU Smectite,

82102805

TG in 1 atmosphere

Jib%

MINAMISHIRAOI Mica-Smectite

# 4.793 % t ----__----__--

MINAMISHIRAO, /

*r--,

Chlorite-Smectite 6.737%

200

400

800

800 %

Temperature

I

400

800

800 C

Temperature FIG. 3. Comparison of TG curves for the Noboribetsu smectite produced under 1 atmosphere pressure and in vacua. The DTA curve produced under 1 atmosphere pressure is also shown.

FIG. 4. In vacua TG curves for samples of Date smectite, and Minamishiraoi smectite, mica-smectite and chlorite-smectite.

K. Marumo, F. J. Longstaffe, and 0. Matsubaya

2550

(4 12

0

*

smectite

(57-l -44m)

---O---

smectite

(63092105)

----O----

smectite

(62102605)

--4--

mica-smectite

---B---

chlorite-smectite

200

(57-6-55m)

600

400 Temperature

(b)

-30

-

-40

-

-50

-

-60

-

(57-l-QOm)

1000

600

“C

*

smectite

(57-l

--O---

smectite

(63092105)

----O----

smectite

(62102605)

_-*-_

mica-smectite

___m--

chlorite-smectite

--------------

.-.-..

-44m)

(57-6-55m) (57-l-QOm)

__*---

____._.,___-_~_-_.“~_____

,...a-...,..., 0

200

:

____*-----

. 600

400

Temperature

0

..--0

____----

600

.

.

. 1000

“C

FIG. 5. Stepwise heating for Minamishiraoi (sa. 57-l-44 m), Date (sa. 83092105) and Noboribetsu (sa. 82102805) smectites, Minamishiraoi mica-smectite (sa. 57-6-55 m) and Minamishiraoi chlorite-smectite (sa. 57-l-90 m). (a) Wt% water evolved during each temperature interval (plotted at maximum of each interval). (b) 6D values of water evolved during each temperature- interval (plotted at maximum of each interval).

for retention of D-rich, residual interlayer water above 200°C. Yet the quantity of water evolved between 200-300°C was the largest of all samples examined (0.59 wt%), suggesting that this smectite released hydroxyl-groups at lower temperatures than the other samples examined. The lack of sharp distinction between release of interlayer water and water evolved from hydroxyl-groups was illustrated earlier for this sample by the TG curve (Fig. 4, Table 4). Together, these results illustrate the difficulty in defining a unique dehydration temperature (and time) for separation of water released from interlayer vs. hydroxyl sites in smectites. The variability in retention temperatures for hydroxyl-groups (and the implied

variability in clay crystallinity) also pose further problems for isotopic analysis, including variable susceptibility to isotopic exchange (see below ) . Interlayer water removed in vacua from mica-smectite and chlorite-smectite (tosudite) at room temperature also was depleted of deuterium relative to remaining interlayer water extracted at 25 100°C and lOO-200°C (Fig. 5a, b, Table 3). No isotopic evidence for retention of interlayer water above 200°C was observed, consistent with the very close similarity between the wt% water determined from TG analysis (2501000°C) and stepwise heating from 200-1000°C (Table 4). Very similar 6D values (-25 vs. -27%0, Table 4) were ob-

Isotope composition

of hydrothermal clay in Japan

2551

Table 3. 6D values of waterevolved from swelling clays’ during 2ste.pwiseheating undervacuum Sample

3Bulk 25“C 25-1OOoC sD%owt.% 8DD960wt.% 8D%o

Smectite 457-l-44m -27 10.77 -73 583092105 -63 10.12 -77 682102805 -65 6.46 -71 Mica-smectite 457-6-55m -31 3.52 -74 Chlorite-smectite 457-l-VOm -27 3.99 -70

loo-2OOY 200-ulooc 300-4txFC wt.% 6D4%0 wt.% 8D960 wt% SDL

400-1OOOY wt.% 8DV&

1.87 +3 2.29 +24 1.41 +29

0.98 i9 0.31 +29 0.42 +14

0.59 -25 0.32 -39 0.46 -36

0.81 -26 0.83 -74 0.76 -64

3.71 -27 3.89 -62 3.11 -69

0.65 +21

0.23

-7

0.25 -28

0.85 -45

3.78 -28

1.51

0.49

-21

0.39 -39

0.87 -34

7.70 -26

-6

lone-weekseparation procedute; 2temperatummaintainedat maximum of each range for about 6Omin 3caIculatedbulk 3D value, 2OG1000oC. 4Minamishiraoideposit; 5Date deposit; 6Noboribetsu&posit Preheating and Settling Times, and Hydrogen-Isotope Reproducibility

tained for the tosudite using stepwise heating and the conventional protocol. Agreement for the mica-smectite was poorer ( -25 vs. -3 1%0), in large part because of one peculiarly low 6D value (-45%0) obtained for the 300~400°C step (Table 3). Insufficient sample remained to repeat this measurement.

Using the conventional protocol (one week separation in distilled water, in vacua preheating to 200°C; collection of

Table 4. Reproduci~lityof ewlved watercontentsand6D valuesof clay minerals tWt.% Water *Pm-Heating from Xi Time (mm) G?5ooc 250-looooc Kaolin(Nabadbetsu) 82102808 (kaobnite)

Wt.% Water aD %o 2O&lOOPC SMOW

-91

:: 35

13.60 13.44 13.66 13.91 13.81 13.83

:z 30 60

4.72 4.70 4.65 4.67

-85 -83 -85 -90

40 60 46

82102814 (kaolin&e) 82102817 (dickite+ naciite) D3-3OOmL(1M + 2Ml mica) D342OmL (1M + 2Ml mica)

Chkwite-Smectite (Mnamishimoi) 1.753 8.737 57-l-VOm(-50% chloritelayers): one week separation 83 one week separation(stcpwirc heati@ 6O@laPc + 60@2oooc Mica-Smectite(M~iraoi) 562xiOnlnl~0%f): one week separation 57-8-44&m ~~~o~rs) one weekseparation 57-6-55m (-&% mica layers): 1.316 4.793 threeweek separation one week separation one week separation(stepwise hestina) 60~1ax Smectite 57-144m (Minantishiraoi): th& week separation

3.064

one week separation one week separation(stcpwi~huting) 565-70.5m (Mhiamishiraoi): threeweek separation one.week sepamtion 83092105 (Date): 2.799 thmeweek separation one week separation one week separation(rtepwiac~) 82102805 (Nobotibetsu): tbreeweeksepamtion ’ one wed sepanuion

oneweekseparation (~tcpwirc heuiag)

-25 -27

10s 85 50

4.35 4.52 4.52

-32 -27 -25

120 105 67

4.37 4.70 4.43

-38 -42 -28

4.48 4.% 4.88

-33

f3 + 6002aYc

6.916

87 140 185 63@1C@‘C!+ 59@2CKPC

5.043

1: -86 -87 -87

1::

6.47 5.15 4.41 5.11

:: -24 -27

4.88 4.79 5.04

-62 -64 -63

4.35 4.39 4.33

-65 -67 -65

60 170 12

60 60 6O~laFc + 60@2oooc 4.711

z

6OOla-PC + 60@2OOY!

tin vacno beatingrateof 2OY!/min;2in vacnu,2oooC unlessotherwiselisted

2552

K. Marumo, F. J. Longstaffe, and 0. Matsubaya

water released from 200-lOOO”C), the wt% water and 6D values of kaolin-group minerals were highly reproducible (5 20.12 wt%, < ?0.5%0), and showed no significant variation for preheating times that ranged from 35 to 60 min (Table 4). Similar behaviour was observed for fine-grained mica (sericite) ( 20.01 wt%, 5 ?2.5%0; Table 4) _ Because of the potential involvement of interlayer water, poorer reproducibility was anticipated for chlorite-smectite, mica-smectite, and smectite. To minimize this difficulty, longer preheating times were used (50-170 min, Table 4). For the chlorite-smectite, reproducibility of both water content and 6D values remained excellent (20.02 wt%, 2 1x0, Table 4), even though stepwise heating was used to obtain one of the results. Two sets of data were obtained for the mica-smectite and smectite samples, for one-week and three-week separations in distilled water (6D = -68 + 5%0), respectively. As discussed earlier, reproducibility of water content and SD values for mica-smectite using the one-week procedure was 50.04 wt% and +3%0; lower water contents and 6D values were correlated with longer preheating times. Reproducibility for “oneweek” smectites was similar (t0.03-0.35 wt%, +0.51.5%0), although no consistent pattern was discerned among preheating time, water content, and m values (Table 4). For the “three-week” mica-smectites and smectites, reproducibility of water contents and 6D values was poorer, 20.09 to 20.66 wt% and ?2-3%0, respectively. Here again, in almost every case, longer preheating times resulted in lower wt% water evolved and lower 6D values (Table 4). Such behaviour can be explained by more complete removal of residual, Drich interlayer water. The most surprising observation was the substantial difference in 6D values obtained for some smectite and mica-smectite samples exposed to distilled water for three vs. one weeks (Tables 4, 5; e.g., samples 56-5-70.5 m, 57-l-44 m, 57-g-44.5 m, and 82102809). For smectite samples in which a clear separation between interlayer and hydroxyl water removal was observed in TG patterns, the differences in 6D values are small, and not significant (Fig. 3, sample 82102805: -65%0 vs. -66 2 1%0; Fig. 4, sample 83092105: -62%0 vs. -63.5 +- 0.5%0). Only slightly lower 6D values were obtained for three week vs. one week samples of mica-smectite 57-6-55 m (-33X0 vs. -28 2 3%0) and 56-2-60 m (-29.5 t 2.5%0 vs. -25%0), consistent with reasonable separation for loss of interlayer and hydroxyl water on the TG curve (Fig. 4). The biggest difference between three week and one week separates was obtained for smectite 57-l-44 m: -44 2 3%0 vs. -25.5 + 1.5%0.Notably, the TG pattern of this sample indicates progressive loss of hydroxyl-groups over a continuum that merges with release of interlayer water (Fig. 4). Large differences also were obtained for smectite 56-5-70.5 m (-31 +- 3700 vs. -14%0) and mica-smectite 57-8-44.5 m (-40 + 2%0vs. -28%0), but TG curves were not obtained for these samples. From the existing data, the cause of this variation cannot be demonstrated unequivocally. Because of the similarity in water contents between three and one week preparations of a given sample (Table 4), the differences in 6D values likely do not arise from preferential retention of interlayer water. We speculate that more poorly crystallized samples (i.e.,

those lacking clear thermal separation for release of interlayer water vs. hydroxyl-groups) began to exchange hydrogen isotopes with the low-D distilled water incorporated into the interlayer during the longer period of suspension. Despite the difficulties described above, collectively, these tests demonstrate that the hydrogen-isotope compositions obtained for Hokkaido swelling clays using conventional analytical approaches normally are sufficiently reliable for geological applications, provided that preheating times are sufficiently long. But some caution must be exercised. Retention of minor quantities of residual interlayer water will lead to 6D values that are slightly too high. More worrisome is the possibility that certain smectitic clays (varieties that begin to release hydroxyl-groups at lower, rather than higher, temperatures) are susceptible to hydrogen isotopic exchange at low temperatures, not just in the laboratory, but also in nature. Whether and how this process occurs, and whether oxygenisotopes also participate, needs future investigation. Accordingly, to minimize potential errors, only results for samples prepared by one-week separation procedure are used in the discussion that follows. HYDROTHERMAL FLUID EVOLUTION, SOUTHWESTERN HOKKAIDO General Geological Background

In southwestern Hokkaido, Kuroko-type deposits, which are associated with submarine volcanism, occur in close spatial association with Quatemary terrestrial volcanoes and an active geothermal system. The Kuroko-type Minamishiraoi deposit, for example, is located only 10 km from the Noboribetsu active geothermal area (Fig. 1). These deposits and the geothermal system occur in small sedimentary basins (20-80 km long, lo-20 km wide; Yajima, 1979) underlain by phyllites, slates, and quartz diorite of probable Paleozoic to Early Mesozoic age. They are filled with sediments, and andesitic to dacitic lavas and pyroclastics of Tertiary age plus tuffs, marine and nonmarine sediments, and andesite lavas of Quatemary age (Yahata, 1989). Volcanism accompanied Kuroko-type mineralization (Minamishiraoi and Kagenosawa deposits) in the Middle Miocene, and Au-Ag-Cu-Pb-Zn vein type (Date deposit) and Au-Ag vein type mineralization (Chitose deposit) in the Late Miocene to Pliocene. Holocene volcanism provides the heat for the active Noboribetsu hydrothermal system. Differences in mineralization among the deposits reflect changes in the hydrothermal fluid which, in turn, can be related to the tectonic evolution of southwestern Hokkaido. Until the Middle Miocene, sediments were deposited in a marine environment, and the hydrothermal systems responsible for Kuroko-type mineralization should have been dominated by seawater. Uplift in the Late Miocene and Pliocene would have facilitated introduction of meteoric water. Such changes should be apparent in the isotopic compositions of hydrothermal clay alteration about each deposit. The nature of this clay alteration at each locality is summarized in Table 1. Hydrogen- and Oxygen-Isotope

Systematics

The hydrogen and oxygen isotopic compositions of clay minerals and associated hydrothermal fluids for each deposit

Isotope composition of hydrothermal clay in Japan are illustrated in Fig. 6, and listed in Table 5. The 6D values of hydrothermal fluids were estimated using the clay-water relationships for kaolinite, mica (sericite), mica-smectite, and chlorite from Marumo et al. ( 1980), and smectite from Yeh ( 1980). Using the hydrogen-isotope, kaolinite-water equation of Lambert and Epstein ( 1980) would lower the 6D values of water reported here by about 11%0,but would not change the general sense of the arguments. The clay-water relationships used to obtain 6”O values for the hydrothermal fluids were taken from Savin and Lee ( 1988) for smectite, mica-smectite, and mica (sericite) (after Yeh and Savin, 1977; Eslinger and Savin; 1973), Cole ( 1985) for chlorite, and Savin and Lee ( 1988) for kaolinite (after Kulla and Anderson, 1978). Using the oxygen-isotope, kaolinite-water equation of Land and Dutton ( 1978) would lower the 6 “‘0 values of water reported here by about 2%0, but again would not affect the general sense of our conclusions. Fluid inclusion temperatures available for quartz and sphalerite intimately associated with the specific clay alteration zones (Table 1) were used in the calculations. Minamishiraoi

deposit (Kuroko-type)

The Minamishiraoi deposit consists of an upper, massive, stratiform bat-he orebody, a lower barite-beating, silicified breccia, and a clay-rich, footwall alteration zone (Table 1). The deposit is only weakly mineralized. The 6D (-38%0) and 6”O ( +7.5%0) values of mica-smectite (sample 57-8-81 m) located just beneath the bat-he-bearing, silicified breccia correspond to hydrothermal fluid values of 6D = - 11%0 and 6 “0 = +2.6 2 0.4%0 (box 1 in Fig. 6), assuming ctystallization at 240 2 10°C. Further into the footwall alteration zone, 6D and S”O values for mica-smectite samples (56-2-60 m, 56-8-99 m, 57-261 m, 57-8-44.5 m; mica layers > 80%) range from -28 to -23%0 and +7.9 to +9.2%0, respectively. Hydrothetmal fluid compositions of 6D = -1 to +3%0 and 6”O = +0.4 to +2.2%0 (box 2 in Fig. 6) result, assuming crystallization at the same temperature as coexisting sphalerite (185 + 5°C). Similar fluid compositions can be calculated for crystallization of nacrite at these temperatures (6D = - 11 to - 1O%O; 6’sO = +0.6 to + 1.2%0; box 3 in Fig. 6). Dickite and kaolinite have 6D values that are virtually identical, but 6”O values average 0.8 and 2%0 higher than nacrite. These compositions suggest that dickite and kaolinite formed from very similar fluids as nacrite, but as temperatures declined from -185°C to -150°C. Such behaviour is consistent with the footwall alteration pattern (Table 1 ), and the change in kaolin polytype. The 6D and 6”O values of (aluminous) chloritesmectite (-34 to -24%0, +7.7 to +8.1%0) are similar to those of kaolin-group minerals, especially dickite and nacrite, consistent with the close association of these phases in the footwall alteration. These clays apparently fractionate hydrogenand oxygen-isotopes in a very similar manner at these temperatures. The 6D values of smectite samples from the surface (-3 1 to -20%0) vs. those from drillholes ( -27 to - 14%0) are very similar, suggesting that postcrystallization hydrogen-isotope exchange with local meteoric water (SD = -76 to -50%0) has not been a serious problem. The smectite 6D values also

2553

are very similar to those of mica-smectite (-28 to -23%0) and kaolin-group minerals ( -28 to -24%0), all of which occur together in peripheral portions of the footwall alteration zone (Table 1). Smectite 6”O values, however, are much higher (+ 13.4 to +20.2%0), suggesting its crystallization at lower temperatures. Temperatures of 115- 135°C can be calculated for the lowest smectite 6”O values, assuming the same fluid compositions as for mica-smectite and kaolin-group minerals. This result is in excellent agreement with barite fluid-inclusion temperatures from the periphery of the alteration zone ( < 14O”C, Table 1). Furthermore, using these temperatures, and the average 6D value of the smectite (-23%0), fluid 6D values of 0 to +2.5%0 can be calculated (box 4 in Fig. 6), in good agreement with values obtained for the mica-smectite from the footwall alteration (box 2 in Fig. 6). The one micasmectite sample (57-6-55) with similar 6 “0 ( + 12.0%0) and 6D (-25%0) values, and a high proportion of smectite layers (Table 5). likely formed from the same fluid, probably at a slightly higher temperature ( 120- 150°C). Finally, temperatures of 55-70°C can be calculated for the smectite sample with the highest 6”O value, assuming formation from the same fluids in a cooling system. The isotopic compositions calculated for the hydrothermal fluids all plot between seawater and typical Kuroko fluids (Fig. 6, Hokuroku Kuroko ore field, Urabe, 1987). Fluid compositions for clay alteration developed at lower temperatures are most similar to seawater. Kagenosawa

deposit ( Kuroko-type)

The Kagenosawa deposit is composed of siliceous, Cu-Aurich, stockwork ore, similar to Kuroko deposits in the Hokuroku ore field. Most of the black (Pb-Zn-rich) ore overlying the stockwork has been eroded. Mica (sericite) and micasmectite comprise the principal clay alteration. For temperatures of 250-280°C (Table 1). the 6D (-47 to -4l%b0) and 6 “0 ( +6.1 to +6.%0) values of the 1 M micas in stockwork ore correspond to hydrothermal fluids with SD values = -20 to - 14%. and 6 “0 values = + 1.6 to +3.3%0 (box 5 in Fig. 6). Such values lie within the range previously determined for Kuroko hydrothermal fluids. Date Au-Ag-Cu-Pb-Zn

deposit (vein-&e)

The Date deposit consists of numerous, Cu-Pb-Zn sulfidebearing quartz veins hosted by Middle Miocene andesitic to dacitic volcanic breccia and quartz porphyry. The quartz veins are enveloped by an alteration halo that is 1 km long and 500 m wide, with inner portions containing mica (sericite) and outer portions, chlorite- smectite, mica- smectite , and smectite. These phyllosilicates have been replaced by dickite, nacrite and/or sudoite (Al-rich chlorite) in the most intensively altered zones. The SD (-81 to -63%0) and 6”O (+3.3 to +4.1%0) values of 1 M mica (sericite) , mica-smectite, and chlorite-smectite (tosudite) from the Date deposit are very similar (and hence probably of common origin), and significantly lower than values for the same phases in the Kuroko-type deposits (Table 5). Fluid-inclusion data (220-26(X) are available only for

K. Marumo, F. J. Longstaffe. and 0. Matsubaya

2554 10 0 -10 -20 -30 -40 3 z

-50

s

-60 -70 -80 -90 -100 -110 -10

-5

0

5

10

15

20

FOG.6. 6D vs. 6’*0 values for clay minerals and associated hydrothermal fluids from southwestern Hokkaido. Only the data for one-week separates (see Table 5 and text) have been plotted or used in the calculations of hydrothermal fluid compositions. The meteoric water line of Craig (1961). the 6D and 6’sO values for local meteoric water in southwestern Hokkaido (open diamonds), and hot-spring water from the Noboribetsu geothermal area (half-filled diamonds) (N.E.I.T.D.O., Japan, 1991) are also shown. The fields for Kuroko ore-fluids (Urabe, 1987). felsic magmatic water (Taylor, 1992) and andesitic water (Giggenbach, 1992) are also indicated. Open circles = mica-smectite; circles with cross = mica (sericite); open triangles = kaolin-group minerals; half-filled squares = chlorite-smectite; open squares = smectite; * = chlorite. Numbered boxes correspond to calculated hydrothermal fluid compositions: Minamishiraoi deposit (Kuroko-type): Box 1 = mica-smectite just below silicified breccia; box 2 = mica-smectite in footwall alteration zone; box 3 = nacrite in the footwall alteration zone; box 4 = smectite in the footwall alteration zone. Kagenosawa deposit (Kuroko-type): box 5 = mica. Date Au-Ag-Cu-Pb-Zn deposit (vein-type): box 6 = mica; box 7 = smectite. Chitose Au-Ag deposit (vein-type): box 8 = mica. Noboribetsu geothermal area: box 9 = mica; box 10 = kaolin-group minerals; box 11 = smectite; box 12 = chlorite.

quartz veins associated with the mica (Table 1). Hydrothermal fluids in equilibrium with the mica have 6D and 6”‘O values of -54 to -36%0 and -2.3 to -0.2%0, respectively (box 6 in Fig. 6). Hot-spring waters from the nearby Noboribetsu geothermal area attain such compositions; hence, hydrothermal fluids of such origin could account for the Date “mica” field. Alternatively, the “mica” field lies on a putative mixing line between local meteoric water and fields proposed for “andesite water” or “felsic magmatic water” by Giggenbach (1992) and Taylor ( 1992) (Fig. 6). Therefore, possible involvement of magmatic water in formation of this polymetallic, vein-type mineralization cannot be ruled out. The &D values of smectite samples range from -68 to -57%0, overlapping the range of the mica, mica-smectite, and chlorite-smectite. A direct temperature estimate is not available for smectite crystallization; the one smectite 6”O value of +9. I%0 suggests formation at lower temperatures than micaceous phases. Assuming crystallization at 150°C. SD and 6’*0 values of -56 to -47%0 and -1.1%0, respectively, can be. calculated for the hydrothermal fluid (box 7 in Fig. 6), virtually identical to those obtained for the mica. The 6D values of dickite and nacrite (-91 to -78%0) crystallized after mica and smectite require water with lower 6D values (-76 to -63%0at 150°C; -70 to -57 at 100°C). Local meteoric water has the appropriate composition (-76 to

-50960, Fig. 6). Hence, the clay mineralogy, paragenesis, and isotopic data for the Date deposit are consistent with a hydrothermal fluid evolution from hotter, evolved brines of meteoric origin (*magma& fluids) to cooler, relatively unmodified, local meteoric waters.

Chitose Au-Ag deposit (vein-type) The Chitose deposit contains more than forty gold-bearing quartz veins hosted by Middle to Late Miocene andesitic lava and quartz porphyry. The veins are associated with secondary kaolinite near the surface and 1 M + 2 M, mica (sericite) throughout. The SD and S’b values of the micas range from -90 to -83%0, and -0.4 to +2.1%0, respectively (Table 5). Fluid-inclusion temperatures (quartz veins) increase with depth (200-250°C to 270-3lO“C, Yajima, 1979), accompanied by a decrease in the 6’*0 values of the micas. The hydrothermal fluid required to crystallize these micas has &B values of -63 to -56%0 (box 8 in Fig. 6). The 6”O values of the hydrothermal fluid are virtually identical over the depth and temperature ranges sampled: -4.4 to -2.4360 at 200-250°C (surface) and -3.9 to -2.8%0 at 270-310°C ( -420 m level) (box 8 in Fig. 6). The mica-fluid hydrogenisotope compositions fall within the range of local meteoric water (-76 to -50%0), as do those calculated for formation

Isotope

MINA.WISHIRAOI 566-5-70.5m

composition

of hydrothermal

clay in Japan

(Kurokot~pe de dl ) driNholcP65.70.5m deph

2555

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molltinonik M/s (mia

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drilllmle56-8.99mdqlh drillkale57-2.6lm &ptb dtiltbole576. S5m depth

MIS (miu kyars4m) MS (mica kycw80%) MIS hlica kycn-60%)

57-8445m

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I646 (da

kysuso%)

57-&Elm

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cis(cbbritc kyom-50%) c/s6zldaicclayas-50%)

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83102602

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K. Marumo, F. J. Longstaffe, and 0. Matsubaya

2556

of secondary kaolinite at the surface of this deposit (-76 to -75%0). However, fluid 6 “‘0 values are substantially higher than local meteoric water (- 10 to -8%0), perhaps the result of an ‘*O-shift caused by hydrothermal interaction between country rocks and local meteoric water. An alternate explanation is mixing between local meteoric water and small amounts of “magmatic water”. The compositions of the Chitose hydrothermal fluids plot just off the trend for hot-spring water from Noboribetsu, and may have a similar origin. Noboribetsu

(active geotheml

area)

The Noboribetsu geothermal area hosts several native sulfur deposits in Pliocene dacitic tuffs and overlying Pleistocene andesite lavas, the biggest of which (Horobetsu) is 1,000 m long, 150-300 m wide, and 15-30 m thick. The hydrothermal systems are still active; water in fractures in Miocene volcanic rocks has a temperature of >29O”C at 1500 m depth. Kaolingroup minerals, smectite, chlorite-smectite, mica-smectite, and mordenite occur near the fractures at shallow depths; chlorite, mica, laumontite, and wairakite are found at greater depths and higher temperatures (Marumo, 1985). The6’80valuesoflMmicas(+l.2to+l.4%~)and2M’ micas (-4.2 to -3.2%~) are quite different, consistent with the change in crystallization temperatures indicated by fluid inclusions ( 180-230°C for 1 M mica vs. 320-380°C for 2 M’ mica, Table 1). In contrast, the 6D values of the two mica polytypes are very similar (-91 to -86%~), despite the large range in crystallization temperatures. This outcome suggests for hydrogen is only minimally variable over that crmic~-~ussr this temperature range, an observation consistent with the small range of &D values described earlier for 1 M and 2 M’ mica polytypes from the Chitose deposit, as well as earlier results of Marumo et al. ( 1980). Hydrothermal fluid 6D and 6”O values of -62 to -5%0 and -6.3 to -3.%0 can be calculated for the 1 M micas, and -61 to -60%0 and -6.8 to -4.6%0 for the 2 M’ micas (box 9 in Fig. 6). These values are very similar, both to each other, and to fluids responsible for crystallization of the Chitose secondary micas. Direct temperature estimates are not available for formation of the kaolin-group minerals (SD = -91 to -86%0; 6’*0 = +0.3 to +3.3%0) that occur at <300 m depth (Table 1). However, assuming crystallization over a declining temperature gradient from 200 to 150°C (consistent with the other deposits examined, and with the variation in kaolin oxygenisotope values), hydrothermal fluid compositions of 6D = -77 to -72%0 and 6”O = -6.1 to -5.8%0 can be calculated (box 10 in Fig. 6). The hydrothermal fluid SD values calculated for both the micas and the kaolin-group minerals lie within the range of local meteoric water, but the 6”‘O values are shifted to higher values. Except for one peculiarly high value (residual interlayer water?), the 6D values of smectite range from -71 to -65%0. The 6 “0 values of these samples range from +4.5 to +6.1%0, higher than the micas and kaolinite, and point to crystallization at lower temperatures, consistent with smectite’s texturally later appearance within the clay alteration zone. Assuming formation between 100 and 15O”C, 6D values of -50 to -40%0 and 6 “0 values of -8.3 to -5.7%0 can be calculated for the hydrothermal fluid (box 11 in Fig. 6). These results

lie about the meteoric water line, with just slightly higher 6D and S”O values than local meteoric water, including hot springs. The 6D values of mica-smectite and chlorite-smectite (550% mica layers in each) suggest formation from similar fluids, although the 6 “0 value obtained for one sample of chlorite-smectite (+6.7%0) may indicate crystallization at slightly lower temperatures. Trioctahedral, Fe-Mg Ib and III, chlorites also form part of the clay alteration zone at Noboribetsu, crystallizing at about 230-280°C (Table 1) (i.e., temperature intermediate between the 1 M and 2 M’ micas). The chlorite S”O values range from -3.8 to - 1.5%0,but not in a fashion expected for higher temperature crystallization of IIb vs. Ib polytypes. At best, similar temperatures are indicated for formation of both polytypes. Some variation also exists in the 6D values of these chlorites (-104 to -91X0) (see below). From these compositions and fluid-inclusion temperatures, 6D values of -65 to -52%, and 6”O values of -6.1 to -2.1%0 can be calculated for the hydrothermal fluid (box 12 in Fig. 6)) similar to the compositions determined for 1 M and 2 M’ micas from this area (box 9 in Fig. 6). The variability in chlorite SD values (13%~ compared to 5%0 for the micas with which they are associated at both higher and lower temperatures) may be a function of Fe content. The ferrous iron content of the chlorite samples is shown in Fig. 7a (estimated from the XRD intensities of basal diffractions) as a function of y in the simplified structural formula: ( Mkx,FeYAl,) ( Si&l,)O’O( 0H)r. There. is a tendency for the 6D values to decrease with increasing Fe content (y) from -91960 for a y value of 1.5 to -104%0 for y > 2.5. The general pattern of decreasing chlorite 6D values with increasing Fe content ((Fe” + Fe’+)/(Fe*+ + Fe’+ + Mg”) ratios; ATEM data) is illustrated further in Fig. 7b, together with additional data for similar, secondary chlorite from the Ohnuma geothermal area (located near Hokuroku; see Fig. 1) (Marumo et al., 1980). Fluid Evolution The preceding discussion shows that all clay minerals in the alteration haloes, including smectite and mixed-layer smectitic clays, generally have hydrogen- and oxygen-isotope. compositions diagnostic of the fluids from which they formed. Using the clay isotopic data, hydrothermal fluids associated with the Kuroko-type deposits can be distinguished unambiguously from those responsible for the alteration haloes about the Au-Ag-Cu-Pb-Zn and Au-Ag vein type deposits. Hydrothermal fluid SD and 6 I80 values for mica ( sericite) , mica-smectite (silicified breccia zone) and nacrite from the Kagenosawa and Minamishiraoi deposits (boxes 5, 1, and 3, respectively, in Fig. 6) approach compositions typical for Kuroko-type deposits (Urabe, 1987). These values are consistent with mixing between seawater (SMOW) and felsic magmatic water (Taylor, 1992) (Fig. 6). More seawater was involved in the Minamishiraoi than the Kagenosawa deposit; in fact, mica-smectite and smectite in lower temperature portions of the Minamishiraoi alteration halo have compositions identical in D and only slightly enriched in “0 relative to seawater (boxes 2 and 4 in Fig. 6). Perhaps if the hydrothermal fluid contained a sizeable fraction of magmatic water (i.e., Kage-

// 1 /

Isotope composition

(4

/ y=6

of hydrothermal clay in Japan

\

Y=4

30

30

Y=3

/ ,&.-;104

,Y=2

-100 /

;‘)lj.!T,Y=< -99

50 /

‘/

50

?!

A

VIA-MA

P

q 0

0

I

1

0.2

0

(Fez++

’

0.4 Fe3*)/(Fe2+

1

1

0.8

0.8

+ Fe3++

Mg3+)

FIG. 7. (a) Relationship between 6D values and Fe content of trioctahedral Fe-Mg chlorite from the Noboribetsu geothermal area. The Fe content of chlorite is represented by y values in the ideal structural formula, (Mgax,Fe~‘Al,)(S&_,Alx)OIBo,r as estimated from the intensities of 1.4, 0.7, and 0.47 nm basal diffraction intensities. 1 = sa. IB2-980 m; 2 = sa. IB2-1030 m; 3 = sa. IB2-1478 m; 4 = sa. IB3-329.5 m; 5 = sa. 1B3-379.5 m. (b) Relationship between 6D values and (Fe’+ + Fe’+)/(Fer+ + Fe3+ + Mg*‘) ratios of trioctahedral Fe-Mg chlorite from the Noboribetsu and Ohnuma geothermal areas (see Marumo et al., 1980).

nosawa), rich, polymetallic deposits were able to form. If seawater was overwhelmingly dominant (i.e., Minamishiraoi), bar&e-rich but base-metal poor deposits resulted (Urabe and Marumo, 1991). The hydrothermal fluids responsible for mica (sericite), mica-smectite, chlorite-smectite, and smectite alteration about the Date Au-Ag-Cu-Pb-Zn, vein-type deposit show a much stronger affinity to meteoric water (boxes 6 and 7 in Fig. 6). Nevertheless, some modification to the isotopic composition of present, local shallow groundwaters is needed. Mixing between local meteoric water and magmatic water (andesite water, Giggenbach, 1992; felsic magmatic water, Taylor, 1992; see Fig. 6) could produce the required fluid compositions. Hence, a role for magmatic fluids during the main mineralization cannot be ruled out. But evolution of local meteoric water along an evaporation trend also could produce hydrothermal fluids of appropriate composition (e.g., Noboribetsu hot-spring waters, Fig. 6). The hydrothermal fluids also could have evolved from meteoric water via the “O-shift that commonly characterizes such rock-water interaction (Craig, 1963). If so, the groundwaters at 5.2 Ma had slightly higher starting isotopic compositions than at present (see Mizota and Longstaffe, 1995); however, the low 6D values obtained for

2557

late-stage kaolin-group minerals (Table 6) in the Date alteration zone are less easily reconciled with this scenario. Similar observations are possible concerning hydrothermal fluid evolution in the still younger Chitose (3.6 Ma) and Noboribetsu ( 1.8 Ma to present) areas. In these localities, meteoric water is even more dominant. The fluids responsible for mica and chlorite alteration in both areas (boxes 8, 9, and 12 in Fig. 6) conceivably can be created by mixing local meteoric water and small amounts of magmatic fluids. But the range of local meteoric and geothermal water compositions permits mica and chlorite crystallization from fluids produced by an i80-shift alone; contribution from magmatic water is not demanded. Fluid compositions calculated for kaolin formation (box 10 in Fig. 6) also can be obtained solely from meteoric water through an “O-shift. Perhaps significantly, extrapolation of a putative mixing line between felsic magmatic and meteoric waters intersects the meteoric water line at suitable 6D values for kaolin formation. Fluid compositions for smectite formation at still lower temperatures (box 11 in Fig. 6) plot directly about the meteoric water line, at compositions not too dissimilar from local hot-spring waters. We note that the SD and 6”‘O values of hot-spring waters at Noboribetsu form a trend that closely parallels fluid compositions calculated for the Date, Chitose, and Noboribetsu deposits, and intersects the field for Kuroko-type fluids (Urabe, 1987). The trend of hot-spring water compositions is significantly shifted from the local meteoric water, presumably because of near-surface boiling and evaporation, plus rock-water interaction. The general difficulty in distinguishing such trends from those produced by mixing of magmatic and meteoric waters is well known; these ambiguities are particularly poignant in these examples from southwestern Hokkaido. CONCLUSIONS The hydrogen- and oxygen-isotope compositions of hydrothermal clay minerals have been used to identify the fluids involved in variable styles of mineralization in a small portion of southwestern Hokkaido, Japan. Mica (se&he), chlorite, mica-smectite, chlorite-smectite, smectite, and kaolin-group minerals were examined at Minamishiraoi (Kuroko-type, Ba > Zn, Pb, Cu), Kagenosawa (Kuroko-type, Cu > Zn, Pb > Au), Date (vein-type, Au-Ag-Cu-Pb-Zn), Chitose (vein type, Au-Ag), and the Noboribetsu geothermal area. Poorer reproducibility is commonly obtained for swelling vs. nonswelling clay minerals using standard procedures for hydrogen-isotope analyses. Accordingly, prior to using 6D values for smectite, mica-smectite, and chlorite-smectite from southwestern Hokkaido in genetic interpretations, several additional tests were conducted. The results showed that the standard analytical protocol generally is acceptable, but care should be paid to the following observations. First, during in vacua stepwise heating, 6D values of water released at room temperature were as expected ( 5 -70%0), but residual interlayer water extracted between 25100°C and lOO-200°C had very high 6D values (up to +29%0). In some smectitic clays, water released during still higher temperature steps then displayed more or less constant 6D values characteristic of hy-

2558

K. Marumo, F. J. Longstaffe, and 0. Matsubaya

droxyl-group hydrogen. In other samples, however, 6D results for the 200-300°C aliquot were still anomalously high, indicating contamination by residual interlayer water. The net effect is a potential bias towards slightly high bulk 6D values in samples so affected. However, the possibility of such contamination can be minimized by sufficiently long preheating under vacuum at ~200°C. Second, in vacua TG patterns for some smectite samples showed a clear separation between interlayer water at <2OO”C and water evolved from hydroxyl-groups (B4OO”C). However, in other samples, water loss was gradual above 200°C. Some particularly troubling consequences were noted for smectite in the latter category. The 6D values of water released above 200°C was typical of hydroxyl-groups; no isotopic evidence exists for retention of residual interlayer water. Instead, gradual release of hydroxyl-hydrogen likely has occurred, beginning at temperatures much less than 300°C. This places limits on the maximum temperature that can be used to ensure complete removal of interlayer water prior to analysis. More importantly, a significant shift to lower 6D values (up to 1%0) was obtained for hydroxyl-hydrogen when this smectite was dispersed in low 6D water for three weeks. Whether hydrogen-isotope exchange occurred, and whether such behaviour is generally characteristic of clays exhibiting gradual (and low temperature), rather than discrete, release of hydroxyl-groups requires further investigation. Wider use of TG methods to anticipate such variability in the isotopic behaviour of swelling clays may be warranted. Nevertheless, in general, the isotopic compositions obtained for swelling clay minerals using conventional procedures remain sufficiently diagnostic of the hydrothermal fluids from which they formed. Hydrothermal fluid 6D and 6 “0 values for mica (sericite), mica-smectite, and nacrite from the Kuroko-type Kagenosawa ( 14.2 Ma) and Minamishiraoi ( 12.5 Ma) deposits are consistent with mixing between seawater and felsic magmatic water (Taylor, 1992). Seawater was the dominant hydrothermal fluid in both cases, but was most important in the barite-rich but metal-poor Minamishiraoi deposit. The putative contribution of magmatic water was greatest in the polymetaliic Kagenosawa deposit. Hydrothermal fluids responsible for clay alteration about vein-type mineralization at Date (5.2 Ma) and Chitose (3.6 Ma), and the Noboribetsu geothermal area (~1.8 Ma) were dominated by (evolved) meteoric water. Some role for magmatic fluids during the main polymetallic mineralization at Date cannot be ruled out, but evolution of local meteoric water along an evaporation trend and/or an ‘*O-shift due to rockmeteoric water interaction also could have produced hydrothermal fluids of appropriate isotopic composition. Meteoric water was even more important in clay alteration and mineralization at the Chitose deposit and Noboribetsu geothermal area. Modem hot-spring waters at Noboribetsu have 6D and 6 ‘*O values that are shifted to the right of the meteoric water line, and closely parallel fluid compositions calculated for the Date, Chitose, and Noboribetsu deposits. Such trends in isotopic composition, which can be produced by near-surface boiling and evaporation of meteoric water, and rock-water interaction, are difficult to distinguish uniquely from mixing lines involving meteoric and magmatic water.

Acknowledgments--We thank Dr. K. Okabe and M. Yahata for assistance in the field, Dr. T. Okai for assistance with wet chemistry, and Dr. H. Mumin and P. Middlestead for assistance with the oxygenisotope measurements. Funding was provided by grants from the Geological Survey of Japan, Agency of Industrial Science and Technology of Japan (to KM and OM), and the Natural Sciences and Engineering Research Council of Canada (to FJL). We also thank David R. Cole, Samuel M. Savin, an anonymous reviewer, and David J. Wesolowski for their helpful comments and suggestions on an earlier version of this paper.

Editorial handling: D. J. Wesolowski

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