Stable isotope investigation of gold quartz veins at the Oriental mine, Alleghany District, California

Böhlke, J.K.-, Coveney, R.M. Jr., Rye, R. O., Barnes, Ivan · 1988 · Report

GEOLOGICAL SURVEY STABLE ISOTOPE INVESTIGATION OF GOLD QUARTZ VEINS AT THE ORIENTAL MINE, ALLEGHANY DISTRICT, CALIFORNIA by J.K. Bohlke 1 , R.M.

This 1988 document, Stable isotope investigation of gold quartz veins at the Oriental mine, Alleghany District, California, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY STABLE ISOTOPE INVESTIGATION OF GOLD QUARTZ VEINS AT THE ORIENTAL MINE, ALLEGHANY DISTRICT, CALIFORNIA by J.K. Bohlke 1 , R.M. Coveney, Jr.2, R.O. Rye3, and Ivan Barnes4 Open-File Report 88-279 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S.G.S. Park, California Present address: Argonne National Laboratory, Argonne, Illinois 60439 Department of Geosciences, Univ. of Missouri, Kansas City, Missouri, 64110 Denver, Colorado Menlo Park, California

Table of Contents INTRODUCTION OXYGEN ISOTOPES HYDROGEN ISOTOPES CARBON ISOTOPES SULFUR ISOTOPES CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES TABLES 1. Oxygen isotope analyses of vein quartz and calcite, and hydrogen isotope and CO2/H2O analyses of fluid inclusions, Carrolson vein 2. Oxygen and carbon isotope analyses of hydrothermal minerals from the 1300 level, Oriental vein and granite 3. Averages and standard deviations of 8180 and §DHjo for several categories of vein quartz (11 samples) 4. Oxygen and hydrogen isotope analyses of serpentinites from the 2300 level, Carrolson vein 5. Sulfur isotope analyses of sulfide minerals in veins and in altered rocks FIGURES Captions 1. Geologic sketch maps 2. Locations of samples collected underground in and near the Oriental and Carrolson veins, Oriental mine 3. 8DW 0 versus the mole ratio CO2/H2O of fluid inclusions released by crushing vein quartz 4. §d values for modem meteoric waters versus distance (due east) from the Pacific coast, at 38 to 40° north latitude

STABLE ISOTOPE INVESTIGATION OF GOLD QUARTZ VEINS AT THE ORIENTAL MINE, ALLEGHANY DISTRICT, CALIFORNIA J.K. Bohlke, R.M. Coveney, Jr., R.O. Rye, I. Barnes INTRODUCTION The Oriental mine, Alleghany district, California, contains one of the geologically best documented gold quartz vein systems in the Sierra Nevada foothills metamorphic belt (Ferguson and Gannett, 1932; Coveney, 1981; Bohlke, 1986). The veins exposed in the Oriental mine are similar, chemically and mineralogically, to many others throughout the Sierran foothills, and in other metamorphic belts worldwide (e.g. Knopf, 1929; Kerrich, 1981; Phillips and Groves, 1983). The veins at Alleghany apparently were formed in Cretaceous time from CO2-bearing fluids that flowed through concordant and discordant fractures in older igneous and metamorphic rocks (Bohlke and Kistler, 1986). On the basis of oxygen isotopic data, phase equilibria, and fluid inclusion data, hydrothermal alteration of the wall rocks and much of the vein filling are estimated to have occurred at 320 ±60°C (Marshall and Taylor, 1981; Bohlke and Kistler, 1986; Bohlke, 1986) and at pressures greater than 670 bars (Coveney, 1981) or roughly 2000 ±1000 bars (Bohlke, 1986), in the presence of a fluid with roughly 10 mole per cent CO2 (Coveney, 1981; Bohlke, 1986). The ultimate of the hydrothermal fluids is not known. The CO2-bearing fluids clearly were not in chemical equilibrium with the vein wall rocks near the sites of ore deposition, as the veins are almost everywhere enclosed within metasomatic (carbonatized) alteration halos (Bohlke, 1986). Much of the disseminated gold mineralization appears to have been associated genetically with specific wall rock alteration reactions (Bohlke, 1986), but occurrences of high-grade gold in the veins may imply that other precipitation mechanisms also were locally important. Fluid inclusions containing varying amounts of CO2 and exhibiting a wide range of homogenization temperatures occur in vein quartz, but their relationships to each other and to gold deposition have not been resolved. The data reported here constitute a preliminary study, begun in the early 1970's by Coveney and Rye, of the isotopic compositions of fluids and hydrothermal minerals in the Oriental mine vein system (Figure 1). In addition to providing support for inclusion of the Alleghany veins in the general class of gold quartz (low-sulfide) veins in Archean to Phanerozoic metamorphic terranes (Bohlke, 1982), the data also yield information on possible sources of the hydrothermal fluids and dissolved species, and they constrain the interpretation of chemical changes in the hydrothermal fluids during alteration and mineralization. The major vein structures sampled for this study are the Oriental vein and the Carrolson vein (Figures 1 and 2). Both veins are composed mainly of massive to vuggy quartz, with minor amounts of carbonates, sulfides, chlorite, mica, feldspar, carbon, and gold (Ferguson and Gannett, 1932; Coveney, 1981). Arsenopyrite is a distinctive accessory phase commonly associated with gold mineralization. Wall rock

-2alteration is conspicuous within centimeters to meters of the veins. The dominant alteration reactions involved varying amounts of carbonatization, de-silicification, chloritization, sericitization, albitization, and sulfidization, depending on distance from the veins and on the initial wall rock Hthology (Coveney, 1981; Bohlke, 1985; 1986). Ultramafic rocks are pervasively serpentinized throughout the district. Locally (near the veins) serpentinite was progressively altered to talc + magnesite + chlorite, to magnesite + quartz + chlorite, and finally to dolomite + magnesite + quartz + mica (mariposite). High grade gold ore commonly occurs within the quartz veins near their intersections with altered ultramafic rocks in the district (Ferguson and Gannett, 1932). The altered serpentinite resembles the carbonate-and fuchsite-bearing "listwanites" that accompany gold quartz mineralization in many parts of the world (e.g. Kashkai and Allakhverdiev, 1965; Boyle, 1979). A distinctive body of porous albitized, mineralized granite is exposed in the footwall of the Oriental vein on the 1300 level. Altered Oriental granite is weakly carbonatized, and portions of it contain a few per cent of arsenopyrite and pyrite. Disseminated gold is most abundant in samples with the highest arsenic contents, but the gold occurs largely as inclusions in pyrite (Coveney, 1981; Bohlke, 1986). Porosity was created by leaching of igneous quartz near subhorizontal fractures. Desilicified granitic rocks ("episyenites") are commonly associated with hydrothermal gold and uranium deposits elsewhere (e.g. Leroy, 1978; Harris, 1980). The Oriental mine affords an exceptional opportunity to study a variety of important geochemical processes related to gold transport and precipitation in relatively deep CO2-nch hydrothermal systems. Samples of quartz, sulfides, carbonates, albite, rutile, and graphite for this study were taken from veins and from hydrothermally altered wall rocks (see Figure 2). Vuggy and massive vein quartz samples were collected near and far from high-grade gold ore pockets, in order to determine whether 8D (of fluid inclusions) or 8180 (of quartz) could be used as guides to ore. Isotopic evidence was sought in part to test also (1) Coveney's (1971,1972) suggestion that high-grade gold ores precipitated from fluids undergoing reduction by H2 from serpentinite (cf. Barton's (1970) discussion of Hg ores), and (2) the suggestion that episyenites lose silica to high pH fluids that are created by extensive CO2 exsolution (e.g. Maisonneuve et al., 1984). Serpentinite and altered (talc-bearing) serpentinite were also sampled, to see if the same fluid might have caused both serpentinization and carbonatization, as implied recently by Nesbitt et al. (1986) for some gold districts in the Canadian cordillera. The bulk of the samples were collected in 1970-1973 by R.M. Coveney, Jr. and analysed in the laboratory of R.O. Rye in 1974. The others were collected in 1980- 1982 by J.K. Bohlke and analysed in the laboratory of I. Bames. Some of the data were discussed in summary fashion by Bohlke and Kistler (1986). All isotope ratios presented here are given in conventional delta (8) notation as per mil deviations from the ratios of standards (SMOW for oxygen and hydrogen; PDB for carbon; CDT for sulfur) (see O'Neil, 1979). OXYGEN ISOTOPES The 818O values of quartz from veins and veinlets range from +17.3 to +19.8 per mil (Tables 1 and 2). The highest values are from clear euhedral crystals that grew on

-3albite after de-silicification of the altered granite. Within the Carrolson vein, there is no significant difference in 8180 between quartz collected from gold ore pockets and that collected far from ore pockets (Table 3), though it is not yet known whether any of the gold-bearing quartz actually was precipitated at the same time as the gold. Clear drusy crystals from vugs have a slightly higher average 8180 (+18.3 per mil) than massive milky quartz (+17.7 per mil) in the Carrolson vein. This may reflect a slightly lower temperature of deposition, or deposition from slightly higher 618O fluids; however, more detailed work is needed to establish whether or not the difference is significant. The bulk of the vein quartz probably was deposited at 320 ±60°C from a fluid with 8180tf2o between +8 and +14 per mil (Marshall and Taylor, 1981; Bohlke and Kistler, 1986). These large positive 8180W2o values are similar to those expected for "metamorphic" fluids (Taylor, 1974); however, they are interpreted in part to reflect extensive interaction between the vein-forming fluids and metamorphic wall rocks along their flow paths, such that the fluid "source" characteristics may be masked. The range of vein quartz 8180 values from the Oriental and Carrolson veins is restricted to the middle of the range of values obtained from other veins near Alleghany (+16 to +22 per mil, e.g. Bohlke and Kistler, 1986). The hydrothermal fluids that formed the Oriental and Carrolson veins interacted locally with mixed metamorphosed igneous and volcano-sedimentary rocks. Fluids with higher 8180W20 values elsewhere in the district probably interacted with more siliceous (chert-rich) metasedimentary rocks before depositing vein quartz with 8180 values as high as +22 per mil; whereas fluids with lower 8180/,2o values apparently interacted with more ultramafic to intermediate metaigneous rocks before depositing vein quartz with 8180 values as low as +16 per mil (see Marshall and Taylor, 1981; Bohlke et al., 1984; Rosenbaum and Taylor, 1984; Bohlke and Kistler, 1986). Crystals of rutile (crystal structure confirmed by X-ray diffraction) occur in albitized granite and on the walls of some veinlets with ankerite ± albite. Needles of rutile were commonly overgrown by euhedral quartz crystals in open vugs and veinlets. The average 818O values for rutile (+3.4) and albite (+16.0) (Table 2) yield Aaibite-ruriie +12.6 per mil. The albite and rutile could have been in equilibrium at approximately 260-280°C with a fluid with 8180//20 +9 to +10 (rutile-water fractionation from Matthews et al., 1979; albite-water fractionation from O'Neil and Taylor, 1967, or Bottinga and Javoy, 1973), or at approximately 360-370°C with a fluid with &*OHO (rutile-water fractionation extrapolated from Addy and Garlick, 1974). Similar ranges of temperatures may be obtained from the fractionation between rutile and hydrothermal quartz from the granite (quartz-water fractionation from Bottinga and Javoy, 1973), indicating that quartz, albite, and rutile all could have been approximately in isotopic equilibrium. The calculated equilibrium temperatures for rutile, albite, and quartz in veinlets and vugs in altered porous granite span the range calculated by Bohlke and Kistler (1986) from oxygen isotope analyses of mica and quartz in veinlets in altered serpentinite from other mines in the district (320 ±60°C). In both instances, the observed fractionations are fairly constant, and most of the uncertainties in temperatures are due to uncertainties in the experimental or theoretical fractionation factors.

-4HYDROGEN ISOTOPES Four samples of serpentinite collected at various distances from the Carrolson vein on the 2300 level have not been obviously altered mineralogically by the veinforming fluid. These serpentinites have fairly uniform 8Z> values (-107 to -124 per mil) (Table 4). The low 8Z> values suggest that these "unaltered" serpentinites last equilibrated with meteoric water (cf. Wenner and Taylor, 1974). Samples of serpentinite showing progressive degrees of hydrothermal alteration (increasing talc contents) near the vein have increasingly higher 8£> values. The difference between the 8Z) values of "unaltered" and "altered" samples (>50 r mil) is probably larger than the hydrogen isotope fractionation between serpentine and talc (e.g. Bowers and Taylor, 1985); therefore, the fluid that caused the hydrothermal alteration probably had a significantly higher 8Z)#20 value than the fluid that equilibrated with the serpentinites. The 8Z)//20 values of fluid inclusions released from quartz by crushing vary between -39 and -103 per mil (Table 1). There is no significant difference in 8Z)//2o between quartz fluids collected from gold ore pockets and quartz fluids collected far from ore pockets (Table 3). However, there apparently is a substantial difference between the 8/>//2o values of fluids in clear crystalline quartz and fluids in massive milky quartz; inclusion fluids from milky quartz have consistently more negative 8D//2o values than fluids from vug quartz. The fluid inclusion 8D#2o data are tentatively interpreted to reflect mixtures of two fluid types: (1) an isotopically heavier hydrothermal fluid that occupies dominantly primary fluid inclusions in unstrained (clear) quartz, and (2) an isotopically lighter fluid that occupies secondary fluid inclusions in strained, microfractured, and partially annealed (milky) quartz. This interpretation is similar to that given by Foley et al. (1982) for low $DHO values obtained from fluid inclusions in hydrothermal quartz from Creede, Colorado (see also Bohlke and Kistler, 1986; Pickthorn et al., 1987). Because all of the quartz samples probably contain mixtures of the two populations, the 8Z)#2o values of the two hypothetical fluid end members cannot be determined with certainty. However, it is possible that secondary inclusions dominate some of the milky quartz samples, and that they contain a meteoric fluid component with a $Dn2o of approximately -100 ± 5 per mil. One analysis of late (post-quartz) calcite yielded a similar 8Z) value (-105 per mil)(Table 1). The inclusion fluids with the highest §DHO values are most likely to be those responsible for altering serpentinite to talc-bearing rock, and ultimately to mica-bearing rock (Bohlke and Kistler, 1986). They also are likely to have had relatively high CO2 contents, as both talc and mica were formed during carbonatization reactions in the ultramafic rocks (Bohlke, 1986). The relationship between 8Z>#2o and CO2IH2O for the inclusion fluids sampled in the present study is shown in Figure 3. Although there is considerable scatter, it appears that there may be a positive correlation between 8Z)#2o and CO2/H2O> This is qualitatively consistent with Marshall and Taylor's (1981) statement that higher 8Z>#2o values are obtained from Alleghany fluid inclusions with higher

-5CO2 contents. Five hydrothermal micas in altered ultramafic rocks from the Alleghany district yield 8D values of -49. ± 9. per mil (B&hlke and Kistler, 1986). The heaviest inclusion fluid (8DWz0 -39 per mil) could be isotopically similar to the ones that formed some of the micas (T 320 ± 60°C; B&hlke and Kistler, 1986; B&hlke, 1986; Bowers and Taylor, 1985); however, it should be noted that the measured CO2/H2O ratio of this sample is almost two orders of magnitude lower than that inferred for the metasomatic fluid, and observed in many individual fluid inclusions (Xco =0.1) (Coveney, 1981; B&hlke, 1986). Paleomagnetic data from plutonic rocks of Late Jurassic to Early Cretaceous age in the Sierra Nevada indicate that the Early Cretaceous gold quartz veins in the foothills metamorphic belt probably formed at approximately 40 to 50° N latitude (Irving, 1979). Stratigraphic studies of onlapping sedimentary sequences in the Great Valley indicate that the metamorphic belt was exposed subaerially, and that the shoreline was approximately 110-120 kilometers west of Alleghany, between 100 and 125 m.y. ago (Ingersoll, 1982). The shoreline moved east to approximately 60 kilometers from Alleghany by 75 m.y. ago (op cit.). Present-day groundwaters in the vicinity of 38 to 40° N along the California coast decrease in 8D away from the shoreline (White et al., 1973; Ingraham and Taylor, 1984) (see Figure 4). According to analyses of springs reported in White et al. (1973), modem 8D values range from approximately -35 to -65 per mil at 30 to 100 kilometers from the Pacific Ocean. According to Ingraham and Taylor (1984), modem 8/> values of "ground waters" range from -50 to -75 per mil at 0 to 55 kilometers from the Pacific. Modem meteoric waters in the Sierra Nevada are between approximately -80 and -130 per mil, generally decreasing with elevation (Friedman, et al., 1964; Bames et al., 1981; Ingraham and Taylor, 1984). From these data (which appear partly to be in conflict) it is not possible to state with certainty if the inclusion fluids with the lowest 8DW 0 values could be Early Cretaceous meteoric water that entered the veins during or soon after the high 8DWz0 fluids, or if they must have entered much later when the coastal configuration and the elevation of the Sierra Nevada were more like they are today. Note that the inferred $Du2o values for secondary fluid inclusions in the Oriental mine (-100 ±5 per mil) are close to those expected for modem meteoric water at Alleghany (Figure 4). It is possible that the highest 8/>w 0 values observed in quartz inclusions (and implied by the micas) are higher than those of Cretaceous meteoric waters. They could well have been in equilibrium with low-to medium-grade hydrous metamorphic rocks (i.e. "metamorphic fluids") (e.g. Taylor, 1974). Fluids which entered the system in a variety of ways (e.g. degassed from magmas or prograding metamorphic minerals, or as deeply circulating meteoric water) may be isotopically indistinguishable if they equilibrated with metamorphic rocks at low waterrock ratios. Alternatively, it is possible that the higher 8DWj0 values are representative of Early Cretaceous meteoric water (Nesbitt et al., 1986). This would require that the inferred secondary inclusions were trapped considerably later, possibly within the last few tens of millions of years, when the Great Valley was exposed above sea level (Ingersoll, 1982), or within the last ten million years, when the Sierra Nevada underwent rapid uplift (e.g. Bateman and Wahrhaftig, 1969; Huber, 1981). Fluid inclusion studies have not been detailed enough yet to determine if both fluids were present in the veins at the same time.

-6CARBON ISOTOPES With one exception, the 813C values of calcite, dolomite, and ankerite are between -1.5 and -3.1 per mil (Table 2). The values for three ankerite samples from altered granite are identical (813C -2.9 per mil). The relative constancy of these values suggests that major fluid reduction did not accompany the deposition of these carbonates, and it is consistent with fluid inclusion evidence that CO2 CH4 in the vein fluids (e.g. Coveney, 1981; Bohlke and Kesler, unpublished data). The data are consistent with the lack of fluid inclusion evidence for fluid unmixing in these veins (Coveney, 1981; Bohlke, unpublished data). Deposition of dolomite and ankerite apparently was not accompanied by large degrees of CO2 - H2O unmixing, as exsolution of a CO2-rich vapor phase at >200°C would have left the remaining dissolved carbonate species depleted in 13C (Friedman and O'Neil, 1977). The 813C values of all but one of the Oriental mine carbonates are within the relatively narrow range of values (-7 to -1 per mil) reported previously for vein carbonate minerals from Alleghany, the Sierra Nevada foothills, and similar deposits worldwide (e.g. Marshall and Taylor, 1981; Bohlke and Kistler, 1986; Colvine et al., 1984; Burrows et al., 1986; Weir and Kerrick, 1987). These 813C values are ambiguous with regard to the source of carbon (e.g. Colvine et al., 1984). The narrow range of 813C values, their rough similarity from place to place, and their similarity to those of "magmatic carbon", have been used to argue in favor of a magmatic source for CO2 in Archean gold quartz vein fluids (e.g. Burrows et al., 1986). However, many of the values (including those in Table 2) are also close to those expected from sedimentary carbonate sources, and all reported 613C data from similar veins could be explained by mixtures of oxidized and reduced sedimentary reservoirs in proportions not much different from the crustal average (e.g. Ohmoto and Rye, 1979). A juvenile or magmatic source for much of the carbonate may be consistent with the geochronologic and isotopic data from Alleghany, but so could a carbonate-bearing sedimentary source that was tapped during metamorphism below (at higher temperature than) the mineralized veins. The one exceptional 613C value (-10.5 per mil) is from a sample of late calcite filling vugs in a quartz veinlet in porous altered granite. Earlier ankerite on the walls of the same veinlet has 813C -2.9 per mil. The late calcite could have precipitated from a main-stage fluid that had either lost an immiscible CO2 vapor phase or incorporated organic carbon from the vein wall rocks. It is also possible that the late fluids were unrelated to main stage fluids, and either mixed with or followed the main stage fluids in the vein fractures. One sample of graphite from an altered wall rock (crystal structure confirmed by X-ray diffraction) was analysed (813C -26.2 per mil). This carbon is probably too light to have been in isotopic equilibrium with any of the carbonates in the veins at 320 ±60°C (Friedman and O'Neil, 1977), but it is isotopically similar to sedimentary organic carbon (Schwarz, 1969). The 813C value of this graphite sample is slightly lower than those reported by Taylor (1981) for carbon in veins at Alleghany (813C -20 to -25).

-7SULFUR ISOTOPES With one exception, the 834S values of pyrite are between 1.6 and 3.1 per mil (Table 5). The values themselves, and the fairly small variation, are consistent with the interpretation that the pyrites were precipitated from a relatively homogeneous, reduced fluid, and that no major oxidation events accompanied their deposition (e.g. Ohmoto, 1972; Rye and Ohmoto, 1974). The 834S values of the Oriental mine pyrite are indistinguishable from the majority of values reported for pyrite in Archean gold quartz veins from Western Australia (+1 to +4 per mil, Lambert et al., 1984) and from around the world (+1 to +6 per mil, Colvine et al., 1984). These values are generally slightly more positive than those of most magmatic sulfides, but could indicate various sources within the metamorphic belts (e.g. Ohmoto and Rye, 1979; Lavigne and Crocket, 1984; Lambert et al., 1984). The one exceptional 834S value (-25.8 per mil) is from drusy pyrite crystals which grew on the surfaces of coarse-grained (Type I) arsenopyrite crystals relatively near the present-day ground surface. Relatively late pyrite with a similar habit is commonly associated with calcite ± quartz or chalcedony throughout the Alleghany district, and could have formed from relatively oxidized ground waters some time after the main flux of deep-source hydrothermal fluids through the veins. Isotopically anomalous iron disulfides are a common feature of many hydrothermal deposits, and are believed to be related to downward flooding of systems by overlying high f0 , low pH waters (e.g. Plumlee and Rye, 1987). Arsenopyrite 834S values are all between 2.1 and 3.2 per mil, except for one at 5.5 per mil. Two arsenopyrite-pyrite assemblages from altered granite yield apparent values of tkmtn0fyrile_Vyrite equal to 0.76 and 0.92. Another pair of samples from the 200 level of the vein yields bruno?yrlte_pyrlte of 1.02. The consistency of the arsenopyrite 834S values, and of the apparent arsenopyrite-pyrite fractionations, and the general absence of cross-cutting or replacement textures, suggest that both minerals formed under roughly the same physical-chemical conditions and possibly precipitated contemporaneously. The differences in 834S between the galena sample and the three vein pyrite samples (not from the same mine level) yield apparent fractionation temperatures of approximately 245, 305, and 375°C (Ohmoto and Rye, 1979), which are roughly consistent with the range of possible temperatures suggested by 8180 fractionations between hydrothermal quartz and mica in the Alleghany district (320 ±60°C, Bohlke and Kistler, 1986), and with the compositions of coexisting calcite and siderite from the Oriental mine (<350°C, Bohlke, 1986). The sphalerite could have been in isotopic equilibrium with coexisting pyrite (near the heavy end of the observed pyrite range) at similar temperatures. No consistent variations in fluid 8345 with elevation are indicated, nor are there systematic differences between vein and wall rock sulfides (all of which are interpreted to be hydrothermal, not indigenous). The heaviest sulfur was found in a single sample of acicular arsenopyrite (Type IE), which is interpreted by Coveney (1981) to have formed somewhat later than Types I and II.

-8CONCLUSIONS Though limited, the isotope data from the Oriental mine suggest the following general conclusions: (1) The carbon, sulfur, and oxygen isotope data are similar to those reported from many other gold quartz vein systems in metamorphic terranes worldwide, and are consistent with the majority of such systems having formed in similar physical and chemical environments. (2) The sources of carbon and sulfur are not uniquely determined, but could include metamorphic rocks and (or) juvenile components. (3) The 8180W2o of the hydrothermal fluids must have been buffered in large part by exchange with the metamorphic rocks. Oxygen isotope ratios in vein quartz from the Oriental mine have a narrower range than the ratios in all vein quartz from various parts of the Alleghany district (cf. Marshall and Taylor, 1981; Bohlke and Kistler, 1986). This is consistent with the interpretation that vein-forming fluids in different parts of the district exchanged oxygen with different wall rocks along their flow paths. (4) Hydrogen isotope data indicate that vein quartz samples contain mixed populations of fluid inclusions. There is some evidence that inclusions with higher CO2/H2O ratios have higher 8DW2<? values. The highest values are consistent with a metamorphic origin for the C02-bearing fluids, but a meteoric origin cannot be ruled out on this basis alone; however, if the higher 8DW2o values do correspond to unexchanged Cretaceous meteoric water, then the fluid inclusions with low 8DW 0 values must have been trapped a long time after the main stages of alteration and vein formation. (5) The fluids responsible for hydrothermal alteration (carbonatization and steatization) had higher $DH2o values than the fluids responsible for much of the serpentinitization This is consistent with, but does not confirm, the interpretation that serpentinization and gold mineralization occurred independently (cf. Bohlke, 1986). (6) Most of the isotopic data are consistent with the interpretation that the "main stage" hydrothermal fluid had concentrations of sulfide sulfate, and CO2 C7/4, and that the bulk of the sulfide and carbonate deposition was not accompanied by large excursions in the oxidation state of the fluid (see Bohlke, 1986). (7) No direct isotopic evidence can be cited yet to support the contention that H2 gas from serpentinite had a role in precipitating high-grade gold ores in the veins (see Coveney, 1981). (8) There is no isotopic evidence for extensive fluid unmixing during or immediately following de-silicification of the Oriental mine granite to form episyenite, or during precipitation of vein dolomite. It is possible (but certainly not proven) that CO2 exsolution accompanied or preceded relatively late calcite deposition in open cavities in vein quartz. (9) Sulfur, carbon, and hydrogen isotope data all are consistent with the possibility that fluids from shallower reservoirs entered the veins after much of the hydrothermal alteration and vein filling by C02-bearing fluids from deeper sources took place. It remains to be demonstrated whether or not the different fluids mixed and caused precipitation of some of the gold in the veins. It is possible that fluid mixing and wall

-9rock alteration caused high-grade and disseminated mineralization, respectively (cf. Bbhlke, 1986; Bbhlke et al., 1987; Kirschbaum et al., 1987). ACKNOWLEDGEMENTS This work could not have been done without the consistent and generous support of D.R. Dickey (Dickey Exploration Company). Isotope analyses were performed in part by J.F. Whelan, M. Bums, and S. Hauser. Discussions with J.R. O'Neil, B.E. Taylor, and W. Pickthom were helpful. Early versions of the manuscript were improved by comments from W. Pickthom, N.C. Sturchio, and R.P. Ashley.

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- 12Kerrich, R., 1981, Archean gold-bearing chemical sedimentary rocks and veins: a synthesis of stable isotope and geochemical relations: Ontario Geological Survey Miscellaneous Paper 97, p. 144-175. Kirschbaum, C, Irwin, J.J., Bohlke, J.K., and Glassley, W.E., 1987, Simultaneous analysis of halogens and noble gases in fluid inclusions in neutron-irradiated quartz veins: a laser microprobe study: EOS, American Geophysical Union Transactions, v. 68, p. 1514. Knopf, A., 1929, The Mother Lode system of California: U.S. Geological Survey Professional Paper 157, 88p. Lambert, I.E. Phillips, G.N., and Groves, D.L., 1984, Sulphur isotope compositions and genesis of Archaean gold mineralization, Australia and Zimbabwe: in Foster, R.P., ed., Gold '82: The geology, geochemistry, and genesis of gold deposits, A.A. Balkema, Rotterdam, p. 373-387. Lavigne, M.J., Jr., and Crocket, J.H., 1984, Sulphur sources in the Dickenson gold mine as suggested by sulphur isotopes: in Foster, R.P., ed., Gold '82: The geology, geochemistry, and genesis of gold deposits, A.A. Balkema, Rotterdam, p. Leroy, J., 1978, The Margnac and Fancy Uranium deposits of the Lacrouzille district (Western Massif Central France): geologic and fluid inclusion studies: Economic Geology, v. 73, p. 1611-1634. Maisonneuve, J., Mergoil-Daniel, J., and Labemardiere, H., 1984, Contributon a la metallogenie de L'Uranium. Genese des episyenites par corrosion alcaline du quartz: Annales Scientifiques de L'Universite de Clermont-Ferrand n, Numero 81, 12 p. (Author-supplied translation) Marshall, B., and Taylor, B.E., 1981, Origin of hydrothermal fluids responsible for gold deposition, Alleghany district, Sierra County, California: U.S. Geological Survey Open-File Report 81-355, p. 280-293. Matthews, A., Beckinsale, R.D., and Durham, J.J., 1979, Oxygen isotope fractionation between rutile and water and geothermometry of metamorphic eclogites: Mineralogical Magazine, v. 43, p. 405-413. Nesbitt, B.E., Murowchik, J.B., and Muehlenbachs, K., 1986, Dual origins of lode gold deposits in the Canadian Cordillera: Geology, v. 14, p. 506-509. Ohmoto, H., 1972, Systematics of sulfur and carbon isotopes in hydrothermal ore deposits: Economic Geology, v. 67, p. 551-579. Ohmoto, H., and Rye, R.O., 1979, Isotopes of sulfur and carbon: in Bames, H.L., ed., Geochemistry of hydrothermal ore deposits, John Wiley and Sons, p. 509-567. O'Neil, J.R., 1979, Stable isotope geochemistry of rocks and minerals: in JSger, E., and Hunziker, J.C., Editors, Lectures in Isotope Geology, Springer-Verlag, Berlin, p. 235-263. O'Neil, J.R., and Taylor, H.P., Jr., 1967, The oxygen isotope and cation exchange chemistry of feldspars: American Mineralogist, v. 52, p. 1414-1437. Phillips, G.N., and Groves, D.I., 1983, The nature of Archaean gold-bearing fluids as deduced from gold deposits of Western Australia: Geological Society of Australia

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- 14Table 1. Oxygen isotope analyses of vein quartz and calcite, and hydrogen isotope and CO2IH2O analyses of fluid inclusions, Carrolson vein Sample # Mineral 618O 6D//2o CO2/H2O Remarks Q-62 n 19-7E 19-2W Q-63 Q-17 ti it 23-1W 23-4W 23-6W 23-9W quartz

quartz quartz quartz quartz quartz it it quartz quartz quartz quartz calcite

1900 level, east end; vug crystal w 1900 level, east end; massive milky quartz 1900 level, near gold pocket; massive milky quartz 2300 level, near gold pocket; vug crystal 2300 level, east end; vug crystal '2300 level, high grade gold pocket; massive milky quartz 2300 level; massive milky quartz 2050 level; vug crystals, associated with pyrite; 813C is -3.1 per mil Note: analysed by R.O. Rye, using techniques described by Rye (1966) and Rye and Rye (1974); S18O and SO in per mil relative to SMOW; 813C in per mil relative to PDB; CO2/H2O is the molar ratio.

Table 2. Oxygen and carbon isotope analyses of hydrothermal minerals from the 1300 level, Oriental vein and granite Sample # Mineral OG-1' OG-1 5 OG-1 5 OG-1 1 527* 559* 356£ 356£ 356£ 356£ 356£ 552* quartz albite ankerite rutile ankerite albite quartz quartz ankerite rutile calcite quartz dolomite 3.5 (2) 17.1 (2) 15.7 (2) 16.2 (2) Remarks euhedral clear crystals in veinlets euhedral clear crystals in veinlets brownish (partially oxidized), mixed with silicates 2-3 mm long acicular crystals from vugs -2.9 (2) mixed with silicates euhedral clear crystals in veinlets euhedral clear crystals in veinlets massive milky vein filling (2) vein lining (on rutile?) vein lining (under ankerite?) late vein filling veinlets in altered serpentinite (2) veinlets in altered serpentinite (trace magnesite ± quartz?) a b Note: 1300 level, veinlets and porous alteration zones in granite; 1300 level, Oriental vein, near west end; data followed by (2) are averages for two splits of the sample analysed separately; "ankerite" has the approximate composition analysed by I. Barnes, M. Burns, and S. Hauser, using techniques summarized by Bfthlke and Kistler (1986) for silicates and carbonates, and those of Borthwick and Harmon (1982) for rutile; yields of CO2 from "ankerite", mixed with silicates, are not known, but the isotopic data may be assumed to be accurate to within ±1 per mil (see Becker and Clayton, 1972; Rosenbaum and Sheppard, 1986); 8180 relative to SMOW; 613C relative to PDB.

- 16Table 3. Averages and standard deviations of 8180 and 8Z>//20 for several categories of vein quartz (11 samples) Quartz (group) 8180 (SMOW) bDo (SMOW) from gold ore pockets 18.0 ±0.6 (5) -79 ± 10 (4) from barren zones 18.0 ±0.8 (6) -80 ±24 (7) vug crystals 18.3 ±0.9 (4) -54 ±14 (3) massive milky quartz 17.7 ±0.3 (7) -90 ±9 (8) Note: compiled from data in Table 1.

Table 4. Oxygen and hydrogen isotope analyses of serpentinites from the 2300 level, Carrolson vein Sample # 618O SD Remarks RC-1 from DDH at 35 feet; relatively unaltered RC-2 from DDH at 110 feet; relatively unaltered O-23-511 face of drift; relatively unaltered O-23-509 100 feet west of 507; relatively unaltered O-23-507 near ore pocket; minor talc veining near ore pocket, 50 feet below 2300 level; abundant hydrothermal talc (+magnesite?) Note: analysed by R.O. Rye and J.F. Whelan, using techniques described by Rye and Rye (1974); 5180 and SD relative to SMOW.

-18Table 5. Sulfur isotope analyses of sulfide minerals in veins and in altered wall rocks Sample # VEINS Mineral Yield (%) 8*5 Remarks O-16-444 9-197-132A 9-197-132B 9-237-189 WALL ROCKS O-13-514 O-l 3-420 O-23-396 O-13-514 O-l 3-420 SOW O-23-110 pyrite pyrite pyrite pyrite arsenopyrite i arsenopyrite i arsenopyrite f T_a arsenopyrite II arsenopyrite IIa galena (with gold) sphalerite pyrite pyrite pyrite arsenopyrite IIa arsenopyrite II arsenopyrite IIa arsenopyrite El* 200 level; late crystal on arsenopyrite 200 level 1300 level 1600 level 200 level 1900 level 1970 level 1970 level 2370 level 1900 level 200 level; includes -10% chalcopyrite 1300 level; in altered granite 1300 level; in altered granite 2300 level 1300 level; in altered granite 1300 level; in altered granite 1300 level; in altered granite 2300 level; near ore shoot Note: arsenopyrite types as defined by Coveney (1972): coarse (l-30cm) grains, usually with native gold inclusions (all three analysed specimens contained gold), 11= fine (l-10mm) equant euhedral twinned crystals, 111= fine (l-10mm) acicular crystals (higher As/S ratios, and possibly younger, than I and II); yields are given in per cent of sulfur expected for pure stoichiometric minerals; microprobe and X- ray diffraction analyses of arsenopyrites indicate that they are As-deficient (Coveney, 1981; Bbhlke, unpublished data); therefore, a "yield" as high as -110 per cent may be reasonable; analysed by R.O. Rye and J.F. Whelan, using techniques described by Rye and Rye (1974); 65 relative to CDT.

FIGURE CAPTIONS Figure 1. Geologic sketch maps of (A) a portion of the Alleghany district, western Sierra Nevada metamorphic belt, California, and (B) the 1300 level of the Oriental mine, showing the locations of the Oriental and Carrolson veins. The elevations of the surface outcrop (heavy solid line) and the 1300 level (heavy dashed line) of the Oriental vein are indicated on the small scale map; dotted lines indicate portions of the underground workings; figure modified from Bohlke (1986). Figure 2. Locations of samples collected underground in and near the Oriental and Carrolson veins, Oriental mine. The Carrolson vein (A) is approximately 50 meters north of the Oriental vein (B) at the 1300 level, and both veins dip toward the north (see Figure 1); the distribution of vein quartz on mined levels is shown in black, with thicknesses exaggerated for clarity; high grade gold ore shoots are indicated by asterisks whose relative sizes reflect relative production; the eastern limit of footwall serpentinite (sp) is shown on most levels; vein samples are designated by open circles; wall rock samples are designated by filled circles; see Ferguson and Gannett (1932), Coveney (1972a; 1981), and Bohlke (1986) for descriptions of the mine geology; the mine is currently flooded below the 1300 level. Figure 3. versus the mole ratio CO2/H2O of fluid inclusions released by crushing vein quartz. The data are listed in Table 1. Figure 4. SO values for modem meteoric waters versus distance (due east) from the Pacific coast, at 38 to 40° north latitude. Data sources are: circles Friedman et al. (1964), various surface waters (broken circles designate samples for which the coordinates given in the original reference do not appear to be consistent with the geographic locations of the samples); triangles White et al. (1973), creeks and springs; crosses Bames et al. (1981), "soda springs"; lines Ingraham and Taylor (1984), representation of trends for "ground waters" across northern California; parentheses enclose data for Clear Lake (Coast Ranges) and Lake Tahoe (Sierra Nevada), which appear to be shifted systematically to higher 5D values than surrounding springs or streams.

CAR*?"ON 1r'61'w Alegtny VEIN ORIENTAL VEIN GOLD CROWN VEIN N 600m Creek [ undeformed Tertiary and Quaternary units serpentinized ultramafic rocks EMJ greenschist to blueschist grade metavolcanic and metasedimentary rocks (Paleozoic-Mesozoic?) granite (Paleozoic) UUIHI amphibolite complex (Paleozoic) B Figure 1

CARROLSON VEIN 1600 Level 1900 Level

sp ORIENTAL VEIN 1300 Level S 1300 Level -Granite Stope 1200 Level 1000 Level 900 Level 800 Level 600 Level 400 Level

200 Level Figure 2B,

0 r o Q £.007 C02/H20 Figure 3

OrCOAST RANGES GREAT VALLEY Q

SIERRA NEVADA Allegheny t Kilometers East from Coast Figure 4