Fluid inclusion and carbon isotope studies of quartz-graphite veins, Black Hills, South Dakota, and Ruby Range, Montana

00167037/90/$3.00

Geochimica t-t Cosnmchimico Acfa Vol. 54, pp. 683-698

+ .Gu

plc.Printed in U.S.A.

Copyright 0 1990 Pegamon Prcsa

Fluid inclusion aud carbon isotope studies of quartz-graphite veins, Black Hills, South Dakota, and Ruby Range, Montana* Institute

EDWARDF. DUKE, KEVIN C. GALBREATH,and KANE J. TRUSTY for the Study of Mineral Deposits, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA (ReceivedApril I, 1989; accepted in revisedformSeptember 25, 1989)

Abstract-Fluid inclusions and graphite are intimately associated in quartz veins that cut high grade metamorphic rocks in the Black Hills, South Dakota and at the Crystal Graphite Mine in the southwestern Ruby Range, Montana. Measured fluid inclusion compositions and volumetric properties were compared with cakmlated ambitions of ~phit~~tumt~ fluids and with estimates of rne~rno~~c P-Tconditions and carbon isotope ratios of graphite were measured to evaluate possible sources of carbon in veinforming fluids. Fluid inclusions from the two areas contrast markedly in their reliability as recorders of metamorphic fluid compositions and metamorphic conditions. The 613Cof graphite associated with the veins indicates that the source of carbon was also different in the two areas. In the Black Hills veins, fluid inclusions are dominantly H&-CO* mixtures with 24-96 mol% CO* and a maximum of -5 mol% Nz and - 13 mol% CI-L+.Isochores for the highest density inclusions pass near estimated peak me~mo~hic ~nditions (55O*~OC, 4.5-6.5 kbar) and fluid inclusion ~rn~~tions are compatible with thermodynamic predictions for fluids in equilibrium with graphite in the stated PT range at geologically reasonable fo,. Graphite in a 12-cm wall-rock alteration zone adjacent to one of the veins has uniform dr3C of -20.8 + 0.2%0, indicating that carbon in the vein-forming fluid was derived largely from reduced organic carbon. In the Ruby Range, peak metamorphic conditions were higher---750”-850°C, 5-8 kbar. In contrast to the Black Hills veins, fluid inclusions are almost all COz-CH4 mixtures (with unknown Nz content). Many contain >20 equivaient mol% CH4 and mixed HrO-CO2 inclusions were observed in only one sample. Inclusions in one vein have -84-97 mol% CH4. Virtually all inclusion compositions are incompatible with computed graphite equilibria and inclusion isochores likewise do not pass through estimated metamorphic conditions. The density and composition of most, if not all, inclusions have been modified subsequent to original trapping, possibly through H,O loss. The range of 6r3C values of vein graphites (-5.8 to -8.66) is nearly indistinguishable from values for graphite in dolomitic marble near the veins (-4.8 to -7. I %o).Carbon was probably mobilized through devolatilization reactions in the marble and precipitated as ‘3C-rich graphite in the veins at fairly constant temperature and from fluid of fairly constant composition. INTRODUCITON THE EXISTENCEOF crosscutting graphite veins in high grade metamorphic rocks provides unequivocal evidence for transport of carbon-bearing fluids along fractures at mid-crustaI depths (e.g., RUMBLEet al., 1986; RUMBLEand HOERING, 1986). In this paper we present fluid inclusion measurements and carbon isotope data on graphite-bearing veins in middle to high grade metamorphic rocks from two localities: the Black Hills, South Dakota, and the Crystal Graphite Mine and su~oundi~ area in the southwestern Ruby Range, Montana. Ours are the first data on fluid inclusions from either location and the first carbon isotope measurements reported from the Black Hills occurrences. This work also represents the first comprehensive study of the carbon isotope composition of graphite at the Crystal Graphite Mine, the only major occurrence of vein-type graphite in the United States. The appropriateness of using fluid inclusion data to infer metamorphic fluid compositions has been a question of considerable debate (e.g., KREULEN, 1987) and in some cases

the fluid inclusion compositions have been shown to be incompatible with metamorphic fluid compositions inferred from mineral equilibria (e.g., AHRENSet al., 1985; LAMB et al., 1987; VRY and BROWN, 1988). A distinctive aspect of the samples examined in this study is the close association, on both a field and petrographic scale, of fluid inclusions with megascopically or microscopically visible graphite. Because speciation in C-O-H fluids in equilibrium with graphite can be predicted as a function of T, P, and f& (e.g., FRENCH, 1966; EUGSTER and SKIPPEN, 1967), and because P-V-T-X properties of the dominant C-O-H species are known or can be approximated at both cryogenic and metamorphic conditions, these inclusions allow evaluation of the extent of attainment of chemical equilibrium between graphite and fluid as well as the degree of preservation of high-temperature ~uilib~um ~rn~tion~ and volumetric properties in fluid inclusions in me~mo~hic rocks. Because the maximum metamorphic conditions are substantially different in the two areas-middle to upper amphibolite facies in the Black Hills versus upper amphibolite to granulite facies in the Ruby Range-comparison of the two sets of fluid inclusion results has a bearing on the question of whether fluid inclusions in middle to high grade me~mo~hic rocks are representative of actual metamorphic fluids.

* Presented at PACROFI II, Second Biennial Pan-American Conference on Research on Pluid Inclusions, held in Blacksburg, VA, January 4-7, 1989. 683

684

1 F. I>uhc. h. (‘. Galhreath.

The second objective of this study is to address specificall\ the possible mode of origin of these graphite-bearing veins and temperature and pressure conditions during their formation. We attempt this by examining the possible compositions and trapping conditions of fluid inclusions in the veins and critically evaluating evidence for post-entrapment compositional and volumetric changes. Carbon isotope data on graphite associated with the veins are used to identify possible sources of carbon and thereby characterize the origin of the vein-forming fluids. GEOLOGY

OF

THE

STUDY

AREAS

Quartz-graphite veins have been recognized at several localities in the Precambrian core ofthe southern Black Hills, South Dakota (Fig. I ). REDDEN (I 963. 1968) described the distribution. character, geologic setting, and mineralogy of different types of quartz veins in the area including the graphite-bearing type considered in this report. Precambrian rocks in the area of Fig. 1 are dominantly Lower Proterozoic strata deposited in the interval 2.2-I .88 Ga but include minor Archean granite (2.5 Ga) and older schist (REDDEN et al., 1990). The three quartz-graphite veins examined in this study occur in the Lower Proterozoic rocks; these consist chiefly of schistose metasedimentary

14000’

and K. .I. 7 rwt!

strata derived from graywackes and shales, with subordinate amounts of quartzite, amphibolite. marble. and iron formation. Kegtonal metamorphism. inferred to have occurred at - I .XS Ga (ZAK I MAR and Sl-ERN, 1967). affected all rock types and is intcrprcted as a relatively high pressure. low temperature event. This \L;LSfollowed at I.72 Ga by lower pressure. higher temperature metamorphism accompanying emplacement of the Harney Peak Granite and associated pegmatites (DEWITI et al.. 1986; REDDEN et al.. 1990).Metamorphic grade increases generally toward the granite. progressing through the garnet. staurolite. and sillimanite-muscovitc Lanes to the sillimanite-K-feldspar zone in a restricted area along the southwestern flank ofthe Harneq Peak Granite dome. Most of the graphite-bearing veins examined to date occur in the upper staurolitc and lower rillimanite zones; however, additional veins may exist in lower or higher grade rocks. The veins studied here are discordant with respect to wall rock bedding and foliation and dip steeply. Where granite or pegmatite intersect the veins, the granitic rocks are iniariahly younger. The age of the veins is thercforc considered to he synmetamorphic or late metamorphic but older than most of the Harney Peak Granite. The likelihood of a minor. late thermal event in portions of the Black Hills is indicated by Rb-Sr whole rock ages of - I.54 Ga (RED~XN et al., 1990); however. neither thr extent nor the effects of this event are well known. Details of the timing and conditions of’metamorphlsm in the Black Hills are only generally known. GOSSELIN et al. ( 198X) estimated peak Proterozoic metamorphic conditions in the Bear Mountain dome. north of the sample localities (Fig. I), at 550”-6’5°C and 5.6.5 kbar. Unlike the sample localities in this study. the Bear Mountain dome contains kyanite-bearing assemblages. so the maximum pressure for our samples was likely somewhat lower than that estimated b? GOSSELIN et al. ( 1988). Their estimates of metamorphic conditions arc. nonetheless. in reasonable agreement with regional garnet-biotite isotherms of 47S”-600°C (1.. M. FRIBERG, unpublished data. 1988) and garnet-plagioclase-sillimanite-quart7 pressure estimates of 4.46.5 kbar for an area - IO km northeast of the Fourmile location on the southwestern flank of the Blarney Peak dome (E. t DIIKI:. unpublished data. 1988). Graphite occurs principally in wall rock alteration lanes adjacent to the veins shown in Fig. I. The veins are essentially pure quartz with traces of graphite. tourmaline. or sulfide. and the alteration assemblages consist of tourmalinc-graphite-quartz or quartz-graphitepyrrhotite (DIJKE and GALBRElhrIi, 1987: GALBRE4TI-1 et al.. 1988). Many of the graphitic quartz veins have been developed to varying degrees as gold prospects or mines

I 3046’

0

Phanerozolc Proterozoic L

5

10

and younger

15 r0cK.s

Harney Peak Granite

I Proterozoic metamorphic

20 km

SOUTH DAKOTA

rocks

Archean granite and metamorphic

rocks

FIG. I. Generalized geologic map of the southern Black Hills. South Dakota, showing the location of the three quartz-graphite veins studied and the principal Proterozoic tectonic and metamorphic features of the region. PT, Pine Tree vein: NP, North Pole vein; FM, Fourmile vein. Based on REDDEN et al. (1982) and DEWITT et al. (1986).

Anomalous concentrations ofgraphite have been reported at several locations in crystalline Precambrian rocks of southwestern Montana (PERRY, 1948). The Crystal Graphite Mine near Ditlon is the only major occurrence of vein-type graphite known in the United States. with more than I 100 m of developed underground workings spanning a vertical interval of over 90 m (CAMERON and WEIS, 1960). A number of investigations have been conducted at the mine because of the many similarities with graphite deposits ofSri Lanka and the potential for commercial development (e.g.. WINCHELL. I9 10, I9 I I : BASTIN. 1912: HIJM. 1943: PERRY, 1948: HFINRICH, 1949: ARMSTRONG and FULL.. 1950: FORD. 1954). The predominant host rocks in the area are quartzofeldspathic ortho- and paragneisses metamorphosed to upper amphibolite facies about 2.75 Ga (JAMES and HEWE, 1980) along with granitic pegmatite of uncertain age (OKUMA. 1971: KARASEVICH et al., 1981). KARASEVICH et al. (1981) point out that the area was also subject to a widespread retrograde thermal event between I .85- I .60 Ga (GIL LETTI, 1966). KARASEVICH et al. (I 98 I) reviewed data recorded by several geothermometers and geobarometers in an area 7 km northnortheast of the Crystal Graphite Mine and estimated peak metamorphism at 675” t 45°C and 5.6 ? 0.2 kbar. Pelitic samples COIlected in the mine area contain the high grade assemblage sillimaniteK-feldspar-garnet-cordierite-plagioclase-bio~i~e-qua~z. In our study we obtained additional pressure-temperature estimates on three @itic samples using garnet-biotite thermometry (FERRY and SPEAR, 1978) and garnet-plagioclase-sillimanite-quartz geobarometry (NEWTON and HASELI-ON. 19X I : Koz~cn and NEWTON. 1988). Two samples came

Fluid inclusions in quartz-graphite veins from the immediate mine area and one was collected 1.2 km to the west. Garnet is nearly an ideal Fe-Mg solid solution [(Ca + Mn)/(Ca + Mn + Fe + MgrR = 0.02-0.06]; biotite has Al” + Ti slightly in excess of the limit suggested by FERRY and SPEAR (1978) [(Al” + Ti)/(Af’ + Ti + Fe + Mg)bio’i”= 0.14-O. 181.Our results indicate peak metamorphic conditions in the range 750”-800°C and 5.4-7.7 kbar. DAHL (1979) estimated retrograde metamorphic conditions in the central and northern Ruby Range, which were presumed to record the 1.85-1.60 Ga thermal event, at 570” + 7O”C, 4.0 f 1.0 kbar on the basis of cordierite coronas on garnet. At the Crystal Graphite Mine (Fig. 2), the host rocks are chiefly quartzofeldspathic gneisses with less abundant pegmatite, granite, talc-silicate gneiss, and pelitic schists. The graphite veins occur along the axis of a major isoclinal synform and cut host rock lithologies indiscriminately (HUM, 1943; HEINRICH, 1949). Dolomitic marble crops out in the core of the fold but closes off 120 m northeast of the main workings. The fold axis in the marble plunges north-northwest at -45”, approximately parallel to the postulated plunge of the ore zone as defined by underground development (PERRY, 1948; ARMSTRONGand FULL, 1950; CAMERONand WEIS, 1960). It is important to note, however, that no marble was intersected in the mine. Marble is present at the nearby Bird’s Nest Claim; however, graphite veins do not occur in marble but are restricted to pegmatite, gneiss, and schist (PERRY, 1948). The graphite veins occur as a stockwork of fractures concentrated around the main fold nose and disseminated graphite is a minor constituent of the intervening wall rocks. Graphite is the only important vein mineral although quartz occurs locally in association with the graphite. The veins are sharply bounded and largely undeformed. Wall rock alteration is inconspicuous other than as some addition of graphite near the veins. Vein formation apparently postdated most ofthe regional metamorphism and deformation although the coarse-grained “needle lump” variety of graphite characteristic of many ofthe veins indicates some movement subsequent to graphite precipitation (CAMERONand WEIS, 1960). Vein formation may have overlapped, in part, with the final stages of pegmatite consolidation (HEINRICH, 1949). METHODS

OF STUDY

Sample collection and description

In the Black Hills, samples were collected from surface exposures or shallow prospect pits. Minor graphite occurs disseminated in the

685

quartz veins, frequently associated with planes of fluid inclusions, but most graphite occurs in wail rock alteration zones adjacent to the veins. Carbon isotope measurements of graphite were performed on samples from only one of the three Black Hills sample localities. At the Fourmile vein graphite was sampled horn four alteration zones flanking each side of the vein extending to a distance of 12 cm. The samples from the Ruby Range came largely from mine dumps representing three separate levels of the Crystal Graphite Mine and from dumps at two adits on the Bird’s Nest Claim. Fluid inclusion samples from the Ruby Range are either from small quartz-rich segregations in otherwise nearly pure graphite veins or are from quartzrich veins containing a few percent disseminated graphite. One sample was collected from a graphite-free quartz vein near the Crystal Graphite Mine in order to compare its fluid inclusion population with that in the graphite-bearing veins. Carbon isotopes

Graphite concentrates were prepared from massive vein material and from graphite disseminated in silicate or carbonate wall rocks. Vein graphite from the Crystal Graphite Mine was chipped or scraped from the sample with stainless steel dental tools. Many vein samples showed textural evidence of sequential crystallization of different generations of graphite inward from the vein walls. In two such cases multiple samples were taken over distances of 0.5-2.5 cm in order to test for isotopic heterogeneity related to texture or sequence of deposition. Disseminated graphite from wall rock samples in the Crystal Graphite Mine and Black Hills areas was concentrated by first crushing the samples in hardened steel to less than 0.15 mm followed by flotation of graphite on water and recovery on glass fiber filters (DUKE and RUMBLE,1986). All graphite concentrates were further purified by sequential treatment with concentrated HNOJ, HF, and HCIO, at 200°C. Black Hills samples were also treated with HCI. Graphite is insoluble in these acids; however, “accordian-like” expansion of some graphite flakes parallel to (0001) occurred, perhaps due to dissolution of intercalated silicate material. The purity of the separates was visually estimated at 50-80s with the principal contaminant being an insoluble white precipitate formed during the acid treatment. Graphite concentrates were converted to CO2 by loading approximately l-5 mg of graphite in precombusted quartz tubes with an excess of CuO and Cu metal. Tubes were evacuated to 3-5 X 10m2

DIASASE

DIKES

MARBLE

UNDIFFERENTIATED METASEDIMENTARY AND METAIGNEDUS ROCKS (Pm-Cherry Creek Group and Dillon quarfm-feldspafhic

PRE-CHERRY

CREEK

gnefss)

GNEISSES

FIG. 2. Generalized geologic map of part of the southwestern Ruby Range, Montana. The locations of the Crystal Graphite Mine and the Bird’s Nest Claims are indicated as is the location of samples AC25 and AC25A. Geology based on KARASEVICHet al. (198 I).

686

E. F. Duke, K. C. Galbreath. and K. J. Trusty

torr, combusted l-2 h at 9OO”C,and allowed to cool to room temperature over an -8-h period. The CO1 gas was analyzed on a Finnigan MAT delta E mass spectrometer at the University of MissouriColumbia. Isotope reference materials NBS-2 I (spectroscopic graphite) and NBS-19 (TS limestone) have 6°C values of -27.8 rl- .05X0 and -1.9 + .05%0,respectively, in that laboratory (cf. -28.13%0 and -~1.93%0,1. L. BARNES,written commun.). A quartz blank was combusted in the same manner as the graphite samples but yielded only about 1% as much CO,; the &13Cvalue of the blank was m-23.68%0.

method, however. is not applicable for the majority of our inclusions because they homogenize to liquid at temperatures greater than that of clathrate melting. In addition, application of the method of SEITZ et al. (1987) to several inclusions did not appreciably change the compositional determination. Bulk compositions and molar volumes for H20-CO2 inclusions were calculated from visual estimates of phase volumes at -25°C or -40°C and the equations of RAMBOZet al. (1985) ignoring the negligible solubility ofCO;! in the aqueous phase at these temperatures.

CARBON ISOTOPE Doubly-polished plates of vein quartz, 0.3-0.7 mm thick, were prepared for fluid inclusion studies using a low-speed wafering saw. followed by hand polishing. Microthermometric measurements were performed on a USGS-design heating and cooling stage. Synthetic fluid inclusions were used to calibrate the system at temperatures of --56.6”C. -6.6”C. O.O”C,and 3141°C. Eight inclusions representative of the fluid inclusion types found in the Black Hills samples were also analyzed by laser Raman microanalysis (LRM) at the Branch of Petroleum Geology, U.S. Geological Survey, Denver, Colorado. The Raman microprobe is a Ramanor U-1000 manufactured by Instruments SA. Exciting radiation was provided by the green line of an argon laser (5 14.5 nm). LRM spectra were reduced using the formulae of PLACZEK(1934) and the cross sections of Raman scattering given in TOURAYet al. (I 985). Analytical uncertainties introduced by this treatment of the LRM data are discussed in a later section (cf. WOPENKAand PASTERIS, 1986; PASTERISet al., 1988). During microthermometric analysis of the fluid inclusions the temperatures of key phase changes were recorded. In the case of carbonic fluids (C02- and/or CH&earing) these measurements were the temperature of final melting of solid COz (or CH& Tm COz, and the temperature of homogenization of the carbonic liquid and vapor phases, Th CO2 L-V. In mixed C02-H20 inclusions it was also possible in some samples to record the temperature of final melting of CO&h clathrate, Tm clath. No measurements of the total homogenization temperature of the COz-rich and H20-rich fractions of the inclusions were possible owing to decrepitation of most inclusions between 200°C and 375°C. In the case of aqueous inclusions, measured phase changes were the final melting temperature of ice, Tm ice, and the temperature of homogenization of aqueous liquid and vapor, Th. All aqueous inclusions homogenize to the liquid phase. Salinity of the aqueous inclusions was calculated following HALL et al. (1988, Eqn. 3) and density was estimated from the salinity and Th using ZHANC and FRANTZ(1987. Eqn. 16). Compositions and molar volumes of COz-rich phases showing melting point depressions were calculated using the graphical method of HEYEN et al. (1982) for C02-CH4. Using microthermometry it is not possible to differentiate the effects of CH, and other gaseous species (i.e., N2, CO, H$, SO2)on depression of COz melting. We therefore follow RAMBOZ et al. (1985)in expressing the diluent gases as CH, equivalent. denoted CH.,(equiv.). Inclusions with Tm CO2 < -6 I “C plot in the uncontoured area ofthe V-XCH, .nlot of HEYEN et al. ( 1982); compositions and molar volumes of these inclusions were calculated using the data of DARIMONT and HEYEN (1988) for C02-N2. For calculated XCH,,(equiv.) up to about 0.30 the average relative deviations using the two techniques are usually less than 50/oin molar volume and less than 10% in XCH,(equiv.). The salinity of the aqueous portion of mixed H20-CO2 inclusions was calculated in terms of NaCl(equiv.) from the melting point depression of CO2 clathrate, neglecting the possible countereffect of CH, (BOZZOet al., 1973; COLLINS, 1979). The determination of salinity for all the mixed H20-CO2 inclusions was not possible, however, because ofthe difficulty in observing Tm clath. In addition, Tm clath only provides a valid estimate of salinity when observed in the presence of CO, liauid and CO1 vawr 1i.e.. Tm clath < Th CO, L-VI. The-difi‘erential paAitio&ng of volatile species between ciathrate and coexisting fluid. documented by RAMBOZet al. (1985) and SEITZ et al. ( 1987).was disregarded in our microthermometric compositional determinations. SE~TZet al. (1987) presented a correction method for the clathrate partitioning effect which requires that Th CO2 L-V be measured both in the presence and absence of clathrate. This

COMPOSITION

OF GRAPHITE

Black Hills The 613C values of eight samples from the alteration zones of the Fourmile vein show a total range of -20.5 to -2 1.O%U, with an average of -20.8 f 0.2%~ There is no indication of a systematic relationship between 613C and alteration zone assemblage or distance from the vein. Southwestern

Ruby Range

Carbon isotope ratios were determined on vein graphite and graphite dispersed in quartz veins, including 13 samples from the Crystal Graphite Mine and two samples from the nearby Bird’s Nest Claim (Fig. 3a, b). With one notable exception all vein graphite falls in a narrow range of 6°C of -5.8 to -8.6%0. Although there are only two samples, the Bird’s Nest veins appear to have slightly less negative values of d13C. These values for the veins are in close agreement with four previous determinations which range from -5.7 to -7.2%0 (CRAIG, 1953;WEISet al., 1981; JXUTHITT, 1982). Samples from the surface to a depth of approximately 90 m in the Crystal Graphite Mine show no systematic variation with depth. Samples of different textural varieties of graphite within single veins on the other hand indicate 613C variations of at least l.O%o on a scale of 0.5-2.5 cm. In four of the vein samples, graphite was dispersed as a minor constituent of quartz-dominant veins in contrast to the nearly pure graphite vein type. Three of these have 613C indistinguishable from the pure graphite veins; however, one sample (AC4A) is distinctly depleted in 13C with 613C of - 19.5%~ The sample is from a concordant quartz-graphite vein in graphitic sillimanite-garnet schist near the main mine workings; flake graphite from the enclosing schist is even more strongly depleted in “C with 613C of -3O.O’lo (Fig. 3b). Disseminated graphite from two remaining silicate wall rock samples from the immediate mine area yielded 6°C values nearly identical to those of vein graphites: -4.0%0 and -5.6%0 for granitic gneiss and sillimanite-garnet schist, respectively (Fig. 3b). One other sample of sillimanite-garnet schist collected 2 km west of the mine area has graphite with 613C of -16.1%o. Flake graphite from three marble samples collected 250 m northeast of the mine workings gave 613C values of - 5.5 to -7. I %o, nearly identical to the range for vein graphite in the Crystal Graphite Mine (Fig. 3b). Graphite from one marble sample on the Bird’s Nest Claim measured -4.8%0. WEIS et al. (I 98 I) analyzed graphite from marble in the area with 6°C of -4.9 to -5. I%0 and CRAIG (1953) reported a value of -2.7%0 (precise locations not known).

687

Fluid inclusions in quarts-graphite veins

inclusions formed by processes (1) and/or (2) in response to later tectonic or thermal events. No relative chronology of different secondary fluid inclusion types could be established using textural criteria such as those of ROEDDER(1984, pp. 62, 343-346). The inclusion classification adopted here relies on compositional and volumetric characteristics determined by microthermometry supplemented by LRM. Black Hills

S’%

B’

GRAPHITE

(I)

6-

5 MARBLE. BIRD’S NEST CLAIM >4.

n=,Ll

Y

m

53.

n

SCHlSTlGNElSS GR*NTTE I PEGMATiTE

3 e2.

;,s 1

-35

-30

-25

-20 6-C

-15

GRAPHITE

-10

-5

0

(%a)

FIG. 3. Histograms of 6°C values of graphite from the Crystal Graphite Mine and Bird’s Nest Claim and surrounding areas, Ruby Range. (a) a’% values of vein graphite. (b) 6°C values of graphite disseminated in country rocks near the veins. Sample AC4 (in Fig. 3b) is the host rock of vein graphite AC4A (in Fig. 3a). Samples AC25A and AC25B are from sillimanite-garnet schist host rocks and lensoidal granitic segregations, respectively, collected 2 km west of the Crystal Graphite Mine.

Graphite from a pegmatitic granite near the Crystal Graphite Mine has 613C of -7.3% and graphite in a lensoidal granitic segregation in schist 2 km west of the mine has 6i3C

of - 13.8% (Fig. 3b). CRAIG (1953) reported graphite in pegmatite from this area of -7.8%0. FLUID INCLUSION TYPES AND MICROANALYTICAL RESULTS All fluid inclusions studied from both the Black Hills and Ruby Range localities lie along complex networks or planes in quartz (Fig. 4). These features vary from well defined, narrow planes to diffuse zones of inclusions. Along each plane or zone, inclusions exhibit relatively uniform phase ratios and compositions although two or more planes may superpose in some areas to give locally diverse inclusion populations. All inclusions are considered to be secondary, that is, formed by trapping of fluids along microfractures subsequent to crystallization of the host quartz. Trapping of the observed inclusions may have occurred at a variety of stages during or subsequent to vein formation (CRAWFORDand HOLLISTER, 1986), for example: (1) contemporaneous with continued growth of other portions of the same vein, (2) at some later time, during infiltration of external fluids unrelated to veinforming fluids, or (3) through subsequent reequilibration of

Results of microthermometric measurements on samples from three quartz-graphite veins in the Black Hills are presented in Figs. 5a-c and summarized in Table 1. Detailed descriptions of fluid inclusion types are presented in the Ap pendix. Three principal classes of secondary inclusions are recognized: (1) mixed H20-CO2 inclusions showing a wide range in HzO/C02, molar volume and abundance of additional gas components expressed as CH4(equiv.) (types FMI, PTI, PTII, NPI, and NPII), (2) low density mixed CO*CH4(equiv.) (type FMII), and (3) nearly pure CO2 at low density (type FMIII). The mixed H20-CO2 inclusions occur in all samples and are the most abundant type; the pure carbonic inclusions [CO2 t CH4(equiv.)] are rare. A fourth type of inclusion was identified microscopically in most samples but was not analyzed. These inclusions contain one or more solid phases plus vapor at room temperature and are inferred to be aqueous fluids of high salinity. The latter type characteristically have flat amoeboid shapes, occur along healed fracture planes, and display varying degrees of necking down. Southwestern Ruby Range Microthermometric data on fluid inclusions from the Crystal Graphite Mine and Bird’s Nest Claim are presented in Figs. 6a-d and summarized in Table 1. Detailed descrip tions of fluid inclusion types are presented in the Appendix. Carbonic inclusions (CO2 + CH4 or other gaseous components) are abundant in all samples although most samples also include aqueous inclusions of variable salinity. In distinct contrast with the Black Hills vein samples, mixed H20-CO2 inclusions were observed in only one sample, a graphitebearing quartz vein from the Bird’s Nest Claim. The following general types of inclusions were recognized on the basis of the microthermometric data: (1) mixed COr-CH,(equiv.) (types CII and CIII), (2) nearly pure CO1 (types CI and BNII), (3) nearly pure CH4 (types CIVa&b), (4) mixed H20-CO2 (type BNI), and (5) saline aqueous (types CV and BNIII). Unless otherwise noted, all carbonic inclusions had similar shapes-subhedral to euhedral negative crystal forms-with a maximum dimension of 5-30 pm. INTERPRETATION

OF RESULTS

Sources of carbon The isotopic composition of graphite in the veins is of particular relevance because the ratio i3C/i2C is strongly dependent on the biogeochemical heritage of the carbon and therefore places important constraints on possible sources of the carbon-bearing fluids that flowed through the veins. The

.

688

b. F. Duke. K. C. Galbreath. and K. J. Trusty

FIG. 4. Photomicrographs of some fluid inclusion types discussed in text. (a) Mixed H20-(‘OY inclusion from the Fourmile vein, Black Hills (type FMI). consisting of liquid H#. COZ liquid, and CO2 vapor at room temperature. (b) High salinity aqueous inclusions from the Crystal Graphite Mine (type CV). cc) One-phase carbonic inclusions [CO2 + CH,(equiv.)] from the Crystal Graphite Mine (type CII). (d) Mixed H20-CO2 inclusions from the Bird’s Nest Claim consisting of liquid Hz0 and C02-rich fluid at room temperature (type BNI). Note hairline crack\ or “tails” extending from several inclusions.

reduced carbon of sedimentary and metasedimentary rocks is typically depleted in 13C, reflecting its biogenic origin and giving 613C values in the range - 15 to -40%0. This depletion is recognized in rocks at least as old as 3.5 Ga (SCHIDLOWSKI et al., 1983). Marine carbonates on the other hand have 613C very near O%O;their metamorphosed equivalents, marbles, generally show a somewhat broader range, about -5 to 5%0. Primordial or mantle carbon is inferred to have b”C of about -7%0. Subsequent mobilization ofcarbon from these sources through a fluid phase and precipitation as graphite tends to preserve the isotopic signature of the source although both temperature and fluid speciation affect the magnitude of graphite-fluid isotope fractionation. Rayleigh-type fractionation during precipitation of graphite from a fluid with evolving C02/CH, may shift 6°C ofgraphite by several permil (DUKE and RUMBLE, 1986) and mixing of carbon fluids from different sources may yield a wide range of intermediate 6°C values (RUMBLE and HOERING. 1986). Once crystallized. however, graphite appears to be highly resistant to further structural or isotopic modification by fluids (GREW, 1974; WEIS et al.. 1981: DUNN and VALLEY. 1987). Black Hills. GALBREATH et al. (1988) demonstrated that the alteration zones adjacent to the Fourmile quartz-graphite vein resulted from addition of carbon and boron to the wall rocks from the vein-forming fluids: these components Stdbilized graphite and tourmaline whereas potassium silicates (K-feldspar, biotite, muscovite) were dissolved. GALBREATH et al. (1988) were unable to distinguish whether the sharply bounded metasomatic zones represented diffusion or infiltration fronts or whether a single stage or multiple stages of fluid-rock interaction were recorded. The uniform 6°C of

these graphites as determined in the present study (-20.8 + 0.2%0) suggests, though does not prove, that fluid-rock interaction was a relatively simple event occurring over a narrow temperature interval and involving fluids of fairly constant carbon speciation and isotopic composition. The relatively strong depletion of “C recorded in graphite from wall rock alteration zones adjacent to the Fourmile vein requires that a major portion of the carbon in the vein-forming fluid was derived from reduced organic carbon. Although no background 613C values are available on Black Hills metasedimentary rocks, it is likely that this source was carbon released during metamorphic devolatilization of the organic component of shale or graywacke units in the region. Mixing of dominantly ‘3C-poor fluids with minor amounts of “Crich fluid cannot be ruled out: however. the very consistent 613C values require that such mixing was highly effective and occurred in relatively constant proportions during the time of graphite deposition. Tourmaline and graphite coexist in the alteration zone of the Fourmile vein indicating that the vein-forming fluid contained boron as well as carbon. Little is known about the source of boron in the vein fluids. The close association of boron and carbon in a number of veins in the Black Hills could be interpreted as evidence for a common origin during high grade metamorphism. For example, boron may have been added to the fluids at deeper levels in the crust through metamorphic breakdown of tourmaline or release from micas (TRUSCO?T and PERCIVAL., 1988).Alternatively, boron could be derived from fluids emanating from the tourmaline-rich Harney Peak Granite and related pegmatites, which are common in the area, and subsequently mixed with carbon-bearing

689

Fluid inclusions in quartz-graphite veins

fluids in the veins. A problem with the second hypothesis is that graphite-tourmaline veins are cut by granite and pegmatite at several locations whereas the opposite case has not been observed; this indicates that the veins predate the period of fluid generation from the granitic rocks. Also, geochemical data on wall rock alteration at the Fourmile vein (GALBREATH,1987; GALBREATHet al., 1988) indicate that the vein forming fluid differed in composition from pegmatite-derived fluids in the region. The vein fluids were characterized by much higherf(HzO)/f(HF) and lower Rb content than pegmatite fluids (cf. SHEARER et al., 1986) and were strongly reduced. Southwestern Ruby Range. The isotopic composition of graphite from veins in the southwestern Ruby Range is markedly different from that associated with the Black Hills quartz veins and our interpretation of the source of the carbon differs accordingly. Graphite-bearing marble, which is in close spatial association with graphite veins at both the Crystal Graphite Mine and on the Bird’s Nest Claim, clearly represents the largest local reservoir of carbon. This observation, coupled with the nearly identical values of 613Cof graphite in the marble and veins, strongly suggests that vein forming fluids were derived from, or equilibrated with, the marble. Carbonate-graphite 13Cpartitioning data for marble near the mine (WEIS et al., 198 1) and empirical calcite-graphite 13C exchange thermometry (VALLEYand O’NEIL, 198 1) indicate that 13C isotope exchange equilibrium between coexisting carbonate and graphite pairs was established at 760”-825°C (cf. 745”-795°C using WADA and SUZUKI, 1983). These estimates are in remarkable agreement with our garnet-biotite temperatures of 750”-800°C. WEIS et al. (198 1) presented no data on the proportions of calcite and dolomite in their samples; however, because of the small magnitude of calcitedolomite 13Cfractionation at high-grade metamorphic temperatures (SHEPPARDand SCHWARCZ, 1970), the isotopic temperature would be shifted to higher values by a maximum of -50°C even if all the carbonate in the samples were dometamorphic

25 ti a 20 a g

15

IA. 10

0

1

-6 -4-Z

2

3

0

2

4

7 6 6 10 5 6 TEMPERATUFlE('C)

4

6

61

11

12 13

14

I5

12 14 16 16 20 22 24 26 : lATURE('C)

FIG. 5. Histograms summarizing microthermometric behavior of carbon-bearing fluid inclusions from the three Black Hills quartzgraphite veins. (a) Temperature of final melting of solid CO*. (b) Temperature of final melting of CO2 clathrate. (c) Temperature of homogenization of CO2 liquid and vapor phases.

Type

n

vol. frac. carbonic fluid

Tm CO2 (“C)

Th CO2 L-V (“a

Tm clath Tm ice (“C) (“0

Th (“C)

bulk density (g/cc)

lll0lar VOlllIIE (cc/m01

BIackHiII&BouthD Pine Tree Vein (I sample) 5 .30-.35 -56.9--53.0 PTI 5 .55 - 5.5 -60.7 - -61.9 PTII

12.8 - 17.3 -0.6 - 2.7

7.4-9.0 13.9- 14.2

---

---

.88 - .94 .54 - .75

23-24 31-46

North Pole Vein (2 samples) 28 .lO -.50 -57.4 - -58.6 NPI 2 .90- 1.0 -58.0--58.2 NPII

9.3 - 19.0 -1.3. +6.5

9.0- 10.3 -9.4

---

-_ _-

.86 - .97 .81- .83

19-28 44-50

Fourmile Vein (2 samples) 56 .25 - .85 -56.5 - -57.4 FM1 4 1.0 -57.4 - -57.5 FM11 1.0 -56.7 --57.2 FM111 14

2.4 - 29.5 4.0- 10.7 21.5 - 30.6

2.1-7.9 ---

----

----

.70 - .97 .77 - .84 .56 _ .76

22-49 50.55 58-78

--.63 - .84 -.48 - 94 ---.91- 94 -. --.28 ---.32 -1.5 - -43.5 108~205 .90 - 1.18

58-62 52-89 41-43 -57-W -63 18-21

BmlthwestenlRuby~MontauEl E,,tal CII CIII CIVa CIVb cv

Graphite 32 1.0 Mine-56.5 (5 samples) - -57.5 64 1.0 -57.1 --66.4 6 1.0 -62.8 --64.8 35 1.0 --99 - -107 7 1.0 -72.1 --77.0 -_ 51 0

Bird’s Nest Claim (I sample) BNI ZL .15- .45 -56.9--57.3 31 1.0 BNII -56.8 - -57.3 9 -_ BNIII 0

6.3. 21.8 29.5. 45.7 -30.7 -45.7 -80.3 --96.4 -73.1 --77.6 .. -l.O- +26.8 -4.0 - +12.8 __

---_ _-.-

-9.7 -+9.0 .-.-1.2 ::26.0

--

.88 - 1.08 .80 - .91 67 I-l54 94 - 1.26

M - 26 ;;:;

E. F. Duke, K. C. Galbreath. and K. J. Trusty Data on variations in 6°C of the vein graphite in the Ruby Range are more comprehensive than are those on the Black Hills localities. The very restricted overall range in 613C values. 2.6%0 for the Crystal Graphite Mine (12 samples) and 0.1 %O for the Bird’s Nest Claim (2 samples), indicates that fluids depositing graphite were of fairly similar composition and origin. Minor secular fluctuations in composition or source are supported by fine scale heterogeneity of vein graphite amounting to at least 1.O%, over distances less than 2.5 cm (cf. RUMBLE and HOERING, 1986). Our data and that of WEE et al. (198 1) suggest that, on average, 613C values in vein graphite are slightly lower (- 1%o) than in graphite from marble at both the Crystal Graphite Mine and the Bird’s Nest Claim. If this observation is valid, the variation could be explained in several ways. Small shifts in 613C would occur if graphite precipitated from a fluid of constant ‘3C/‘2C but at progressively lower temperatures. For example, between 800°C and 500°C the temperature dependence of COz-graphite 13C fractionation (BOTTINGA, 1969) forces graphite toward lower 613C values by about 1x0 per 100°C. Even at constant temperature, the 613C of graphite precipitated from a fluid of constant ‘3C/‘2C may be shifted to lower values if the fluid evolves to higher CH4/C02 between the source and the site of deposition. Such a change in composition could occur due to interaction with wall rock minerals or mixing with fluids of different composition. Even minor changes in CH4/C02 are capable of inducing shifts of several permil in graphite precipitating from a fluid whose composition is evolving in such a manner (DUKE and RUMBLE, 1986). More drastic reductions in 6’“C of vein graphite relative to graphite in a marble source rock could be effected by mixing of 13C-rich marble-derived fluid with 13C-poor fluid such as might be in equilibrium with graphitic metapelites. As noted previously, the presence of 13C-depleted graphite FIG. 6. Histograms summarizing microthermometric behavior of carbon-bearing fluid inclusions from five vein samples from the Crystal Graphite Mine and one vein sample from the Bird’s Nest Claim. (a) Temperature of final melting of solid C02. (b) Temperature of final melting of carbonic solid which is interpreted as CH.,. Note break in scale between -78°C and -98°C. Observed in only one sample. (c) Temperature of final melting of CO2 clathrate. Occurs in only one sample. (d) Temperature of homogenization of carbonic liquid and vapor phases.

lomite (cf. WADA and SUZUKI, 1983). The observations of DUNN and VALLEY (1987) in a polymetamorphic amphibolite terrane indicate that this high temperature 13C partitioning would be effectively locked in even if later, lower grade metamorphism occurred in the area provided the graphite did not recrystallize during the later event (e.g.. PLUIJM and CARLSON, 1989). The absolute magnitude of 613C in the vein graphite is important for the insight it provides about carbon sources; however, equally important is a knowledge of 613C variations within and between veins and between vein graphite and the carbon source. These observations make possible a better understanding of the ultimate source of the carbon but also permit evaluation of possible mechanisms of graphite precipitation and provide additional constraints on estimates of fluid compositions and temperature of graphite precipitation.

in one of the vein samples supports the existence of such “Cdepleted fluids and these should have higher CH4/C02 as well. If these fluids were actively moving through the network of fractures at the same time as the more abundant “C-rich fluids, then mixing may have occurred in some fractures. The scarcity of the “C-poor vein type in the area, the uniformity of 6°C in all remaining veins, and the absence of intermediate 13C vein types are factors that weigh against mixing as a dominant mechanism controlling the slight isotopic variability. Accordingly, the importance of mixing as a cause of graphite precipitation is questionable at this location in contrast to some graphite veins in New Hampshire which have up to 6.0%0 variation in 6r3C within a single deposit (RUMBLE and HOERING, 1986).

Calculated compositions of fluid inclusions derived from microthermometric data are presented in Table 2 along with available compositional estimates based on LRM measurements. In Figs. 7 and 8 the results are summarized in graphical form by plotting in terms of the end-member components COZ, HZO, and CH,(equiv.). As emphasized previously, inclusions in all samples except one, sample AC3 (inclusion type CI, Table 2, Fig. 8) occur in intimate association with graphite in quartz veins. Thus. despite the secondary origin

691

Fluid inclusions in quartz-graphite veins

CH4 equiv.

Table 2. Summary of estimated fluid inclusion compositions and carbon isotope composition of associated graphite. UM S13C rv graphite XC& XNaCl n XI&O XC02 (equiv.) (equiv.) n XCH4 XN2 (%) TYpa Bleek~souti~ta PTI 5 .85-A3 .lO-.13 PTII 5 .65-.78 .15-27 NPI 28 .75-z% .03-23 NPII 2 C-.25 .70-92 FM1 56 AK.92 .08-.75 FM11 4 : Ax.95 FM111 14 96-1.00

5.01 s.01 __ .07-.08 .Ol-.02 s.01 .0&.08 s.03 .01;02 __ .05-06 __ o-.04

SouthweatemRuby~Montaua ::I CIII

32 64 6

0 0

cv CIva CIVb BNI

51 .86-99 35 0 7 0 !ZZ .73-94

..

.95-98 .53-95 .i4-.72

.05-.47 .02-.05 .26-.36

-PO3 -. 16 .05-.21

-!I7 -.64 LO1

._ ..

.Ol-.14 ._ -<.Ol-.07

BNIII 31 9 .90-99 0 0 ,Ol;lO BNII 0 .X-.98 .02-.04

0 ;

__ .- n.d. .08-.13 .Ol-.02 n.d. .Ol .03-.05 n.d. __ __ n.d.

: 0 2 0 0

: 0 0 0

FMII FM _. --

I-J 1: 0

__ --

-518

::

.:9

n.d.: not determined

ofthe inclusions, all inclusion fluids are inferred to have been in chemical equilibrium with graphite at the conditions of trapping or to be derived from such graphite-saturated fluids through subsequent modification. It is instructive, therefore, to compare the calculated fluid compositions of the inclusions with compositions of graphite-saturated fluids in the C-O-HN system computed from thermodynamic predictions. Any incompatibilities in the two types of compositional estimates would indicate either: (1) the thermodynamic data and assumptions are incorrect, (2) kinetic barriers prevented attainment of equilibrium between graphite and fluid, or (3) the fluid inclusions reequilibrated subsequent to trapping so that their present compositions do not accurately represent the compositions of actual fluids which flowed along fractures in the geologic past. Graphite saturation boundary curves are plotted in Figs. 7

and

8

for two temperature-pressure conditions: 5OO”C,6 kbar and 7OO”C, 3 kbar. Because temperature and pressure have opposing effects on the position of the graphite boundary, this range of conditions-low temperature, high pressure and high temperature, low pressure, respectively-serves to bracket possible configurations of the boundary for middle to high grades of metamorphism. The limits of graphite saturation are calculated as described in DUKE and RUMBLE(1986)

and assume CH4(equiv.) to be identical to true CH4. The only other species that was detected in the fluid inclusions in this study and which might contribute to the calculated CH&quiv.) is N2. Addition of N2 to H20-C02-CH4 mixtures serves only to reduce the mole fractions of the others species in constant proportions because N2 is essentially inert and acts only as a diluent (HOLLOWAY,1981). Thus in Figs. 7 and 8 the position of the graphite saturation boundaries do not shift appreciably with addition of Nz and the boundaries can be viewed as projected from Ns onto the H20-COz-CH4 plane. The plotted positions of fluid inclusion compositions, however, will be incorrect if they contain Nz in place of some or all of their CH4. In such cases the projected compositions would be shifted away from the CH4(equiv.) apex in proportion to the N&H., of the fluid. Examination of Table 2 shows qualitative agreement between CH&quiv.) determined by microthermometry and overall CH, + N2 content determined by LRM on selected samples from the Black Hills veins. The comparison shows, however, that in some samples N2 may be more abundant than CH, and our microthermometric methods tend to underestimate the total amount of CH4 + Nz as determined by LRM. For example, LRM analysis of the relatively CH,(equiv.)-rich PTII inclusions indicates substantially greater CH&quiv.) component than determined by microthermometry. This

co2 2o mole 4opercent 6o . FMI QFMII

0 PTI lPTll

*O Hz0

b NPI ANPII

FIG. 7. Molecular compositions of Black Hills fluid inclusion types as determined by microthermometry. Compositions are plotted in terms of HzO, CO*, and CH4(equiv.) (CH4 + Nz), along with curves representing graphite saturation at selected temperature-pressure conditions. Compositions on the low-Hz0 side of these curves should be saturated with graphite at the stated conditions and be represented by two phases-graphite plus fluid lying on the boundary. Compositions lying on the Hz0 side of the curves are unsaturated and exist as a single supercritical fluid at the stated conditions. (a) Ranges of individual inclusion compositions for each inclusion type. (b) Distorted view of lower portion of Fig. 7a showing average compositions (symbols) and ranges (dashed fields) for different planes of each inclusion type. See text for discussion of accuracy of measurements and correspondence with LRM data.

observation is consistent with the differential partitioning of CH4 into clathrate, which may lead to significant underestimation of CH4 in HzO-bearing inclusions (SEITZ et al., 1987). Additionally, PASTERIS et al. (1988) pointed out several shortcomings ofcalculating molecular

CH4 equiv.

mole percent

H20

FIG. 8. Molecular compositions of Ruby Range fluid inclusion types as determined by microthennometry. See Fig. 7 for explanation.

69’

E. F. Duke. K. C. Galbreath. and K. J. T-rust!

ratios from LRM spectra of high density fluid inclusions using published Raman cross section data for pure gases at low pressure without regard for internal pressure in the inclusions and certain instrumentspecific effects. The net result of these factors will be to further underestimate CH, (SEITZ et al., 1987). Bhk Hills. Our results for the Black Hills samples are shown in Fig. 7a, b. The bulk of the fluids are nearly HzOCO* binary mixtures with only a few samples containing greater than 5 mole percent CH4(equiv.). Most compositions plot close to the graphite equilibrium curves which closely follow the HzO-CO2 sideline in much of the CH,-poor portion of the diagram. LRM data in Table 2 indicate that for several inclusion types N2 may actually exceed CH4. which would tend to shift plotted compositions away from CH,, i.e.. even closer to the graphite equilibrium curves. Type PTII inclusions have higher CHI/C02 by LRM; this would shift them to more CH4-rich compositions at constant or slightly decreasing H20 but they would maintain a position near the high-temperature, low-pressure graphite boundary. No LRM data were obtained for type NPII or FM11 so it is not known whether that additional data would result in slight adjustment of their plotted compositions. A general trend of inclusion composition fields radiating away from the HZ0 apex is seen in Fig. 7a and is even more evident in the expanded view of the low CH,(equiv.) portion of the diagram (Fig. 7b). Some of this trend may be an artifact resulting from inability to estimate precisely the relative volumes of the aqueous and carbonic phases. The overall trend of the data for different samples ranging from H,O-rich to C02-rich compositions undoubtedly reflects true compositional variability, however. The compositional variation correlates well with volumetric variation ranging from low molar volume (high density) HzO-rich fluids to high molar volume (low density) CO*-rich fluids (Fig. 9). There is no II priori reason for this apparent relationship. The observations could be explained by successive trapping of graphite saturated fluids which were evolving simultaneously to lower XH?O and lower density. Such could be the case, for example, if the rocks followed a “clockwise” I-‘-T path from 500°C. 6 kbar to 700°C. 3 kbar with,!& buffered at QFM-I (Fig. 7b). Alternatively, the correlation of lower XHzO with lower bulk density could reflect selective post-entrapment loss of H20 from inclusions under conditions of generally decreasing P/7’. Thus most of the fluid inclusion compositions for the Black Hills samples are in approximate agreement with calculated fluids in equilibrium with graphite at metamorphic conditions provided a range of temperature, pressure, or./& conditions are represented. All LRM data points fall within -5 mole percent of the graphite saturation curves in H20-CO*-CH4Nz between 7OO”C, 3 kbar and 500°C. 6 kbar. Exceptions to the apparent compatibility of measured and calculated compositions are the HZO-poor inclusion types NPII and FMII, which fall well within the two-phase graphite-plus-fluid region. Under metamorphic conditions, pure CO* fluids are unlikely to coexist with graphite. Fluids approaching pure CO2 (type FMIII) could coexist with graphite only at fairly high .f& (log .I;,, - -22 at 500°C or - -17 at 700°C). The more H20- and CHa-rich inclusions (type PTII) would require substantially lowerf& (log,f& - -24 at 500°C or - - I8 at 700°C).

10

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

MOLEFRACTIONCARBONIC (XC02 + XCH4 equiv.)

0.6

0.9

1

FLUID

FIG. 9. Correlation of bulk molar volume with carbonic fluid content for the Black Hills Buid inclusions. Inclusions with the highest mole fraction of CO* + CH4 have the highest molar volumes (i.e., lowest densities). .Suuthwestern Ruby Range. Compared with the Black Hills samples, calculated compositions of the Ruby Range inciusions are notably Hz0 deficient (Table 2, Fig. 8). It has long been recognized that H20-deficient fluid inclusions are characteristic of high grade metamorphic terranes (e.g., TOURET, 1981); these generally consist of pure COZ or nearly binary mixtures of C02-CH4, C02-N2, or C&-N2 (KERKHOF, 1988). The majority of our data conform to this trend with the predominant inclusions represented by mixtures of COz and CH4(equiv.). No LRM data are available to determine the prevalence of Nz in the inclusions; however, the phase behavior of CHd(equiv.)-rich type CIVa&b inclusions indicates that their CH4(equiv.) component is indeed true CH4 (see Appendix). The only HzO-bearing inclusions are the pure aqueous inclusions (types CV and BNIII), which occur in most samples and have generally high salinity in terms of NaCl(equiv.) and H20-rich HzO-CO2 inclusions (type BNI), which occur in only one sample. Predictions of graphite stability in the system HZ0CO*-CH4-N2 indicate that the bulk of the fluid compositions measured in the laboratory are not compatible with graphite-saturated fluids at metamorphic conditions. Because the geologic setting of the samples dictates that the original fluid inclusions were in equilibrium with graphite, these results require significant chemical reconstitution of the inclusion population since trapping of the original fluids. The simplest mechanism to derive the observed fluids from compositions originally constrained to lie along the graphite boundary is selective loss of H2 or Hz0 from the inclusions (e.g., CRAWFORD and HOLLISTER, 1986; KREULEN, 1987). These constituents may be lost from the inclusions by diffusion (cf. STERNER et al., 1988; STERNER and BODNAR, 1989) or, in the case of H20, by capillary extraction along microcracks (“wicking,” CRAWFORD and HOLLISTER, 1986: WATSON and BRENAN, 1987), a situation that could occur if inclusions were ruptured at conditions below the H20carbonic fluid solvus (-4OO”C, HOLLOWAY, 1984; up to - 500°C with added NaCI, BOWERS and HELGESON, 1983). Although migmatites are not characteristic of the local wall rocks, it is possible that anatectic melts were produced in this region and these may have selectively extracted Hz0 from

Fluid inclusions in quartz-graphite veins metamorphic pore fluids at the time of melting (e.g. TOWRET, 1981). Because our samples are restricted to crosscutting veins, which appear to postdate the peak of metamorphism and deformation, this explanation does not appear to account for the dominance of carbonic inclusions in this case. Selective loss of HZ0 from the inclusions is an attractive mechanism because it provides a simple means of deriving compositions along the C02-C&(equiv.) sideline from fluids originally lying along the graphite boundary with varying C02/CH4. Observed low to moderate CH4(equiv.) and high CH.,(equiv.) compositions could be derived by subtracting Hz0 from geologically reasonable fluids along the graphite curve. Loss of HZ would have similar effects but it would be difficult to explain the generally high CH4(equiv.) component in this fashion (KREULEN, 1987). If H20 loss by diffusion or wicking were the dominant process in deriving the observed compositions from fluids along the graphite curve, then some inclusions record a net loss of Hz0 amounting to -60 volume percent of the original fluid (KREULEN, 1987). Although arguments have been presented that fluid compositions calculated from our microthermometric data do not represent original crustal fluids at the time of trapping, there are important differences among some of the inclusion types that can be related to differences in geologic setting. The inclusions do, therefore, retain some compositional characteristics acquired at early stages of their evolution. For example, inclusions in the only graphite-free vein sampled are nearly pure CO2 (type CI) in contrast to the vast majority of inclusions in the graphite veins, which have substantial CH4(equiv.) components. The only other inclusions that approximate pure CO* are type BNII from the Bird’s Nest Claim. The Bird’s Nest Claim also has abundant H20-CO2 inclusions (type BNI) in marked contrast to the Crystal Graphite Mine, which has none. Some of these differences between the Bird’s Nest and Crystal samples may be attributable to the fact that marble is intimately associated with the Bird’s Nest veins (although no veins actually occur in marble), whereas the Crystal veins are one hundred meters or more from the nearest outcrops of marble. Perhaps the most distinctive inclusions are the high CH4(equiv.) types CIVa and CIVb. These occur together in sample AC4A, a graphite-bearing quartz vein in graphitic sillimanite-garnet schist associated with the main graphite veins at the Crystal Graphite Mine. Graphite in the schist has b13C of -30.0% and graphite separated from the quartz vein has d13C of -19.5%0. The high CHJCOI which characterizes these inclusions is what would be expected for reduced fluids in equilibrium with the graphitic schist. Like the fluid composition, the 613C of graphite in the vein also indicates exchange between the vein fluid and the wall rock assemblage. We conclude that, although few if any of the fluid inclusions from the Ruby Range presently contain fluid that could have been in equilibrium with graphite at metamorphic conditions, at least some inclusion types retain a vestige of original compositional characteristics such as C02/CH4. Most or all inclusions have experienced post-entrapment compositional changes that may be best attributed to selective loss of H20. The observation of hairline cracks extending from a small percentage of most inclusion types raises the possibility that

693

some of the chemical modification may have been effected by physical processes (discussed below) leading to rupture of inclusions. A problem with equating Hz0 loss with rupturing of inclusions lies in the fact that the sample containing the most pervasive evidence for physical modification of inclusions, sample BN2, is also the only sample containing H20COZ inclusions and many of these have conspicuous cracks or tails surrounding them (Fig. 4d). Conditions of trapping and vein formation

Isochores based on microthermometric determinations of the molar volume or density of individual fluid inclusions can be used to characterize possible pressure-temperature trajectories along which the inclusions were trapped. In rocks from metamorphic terranes, which commonly contain multiple generations of secondary inclusions, isochores can be compared with estimates of “peak” metamorphic conditions calculated on the basis of mineral equilibria to help evaluate a rock’s pressure-temperature path during cooling and uplift (CRAWFORD and HOLLISTER, 1986). In this section we present isochores for fluid inclusions from graphite-bearing veins of the Black Hills and Ruby Range. The isochores for different fluid inclusion types are examined in light of independent estimates of the P-T history of the study areas and evidence discussed above for variable degrees of post-entrapment modification of the inclusions. Isochores for inclusions from the Black Hills and Ruby Range are shown in Figs. IOa, b. Average isochores were computed for groups or planes of compositionally similar inclusions. Ranges of these average isochores are shown in the figures for each compositional type of inclusion. lsochores for aqueous inclusions were calculated using the formulation OfZHANGand F~ANTZ(1987) for H20-NaCI. The isochores are extrapolated above 3 kbar, the limit of the authors’ experiments. Substitution of NaCl by CaC12results in a minor increase in isochore pressure, - 100 bars at 3 kbar. Extrapolation of H20-NaCI isochores of POTTERand BROWN(1977) and BOWERSand HELGESON(1983) 8ive significantly lower pressures: at 600°C -5 kbar lower and -3 kbar lower. resnectivelv. The data of ZHANG and FRANTZ(1987). however, appears to fit-better with available experimental results. Isochoresfor mixed H20-C02 inclusions were calculated following the method of BROWNand LAMB(1986). Calculations accounting for NaCI content estimated from clathrate melting temperatures and the equation OfBOWERSand HELGESON(1983) yield generally similar results with some scatter to both higher and lower pressures. For inclusions containing only carbonic species, isochores were calculated using the equation of state of HOLLOWAY(198 I) because it explicitly accounts for CH, and N2 in the inclusions and fits experimental data for nonaqueous compositions (KERKHOF, 1988). For more H20-rich fluids, the HOLLOWAY(198 1) equation predicts isochores that lie at substantially higher pressure than the experimental data (BROWN and LAMB,-1986). The H20-CO2 and carbonic isochores are dashed below about 300°C because of the likelihood of unmixing of H20-

C02-CH4-N2mixtures at these temperatures. Black Hills. Average isochores calculated for the Black Hills samples are in reasonable agreement with postulated P-T conditions during regional and thermal metamorphic events (Fig. IOa). It is possible to conclude therefore that the inclusions reflect successive trapping of different generations of fluids along a P-T-time path that passes through the regional and granite-related metamorphic conditions. The higher density inclusions have &chores that pass near in-

694

E,.F. Duke. K. C. Galbreath. and K. J. Trusty

A.

BLACK HILLS

8

100

200

300

400

500

600

700

800

900

TEMPERATURE (“C)

B.

RUBY RANGE

8

0

100

200

300

400

500

600

700

800

900

TEMPERATURE (“C) FIG. IO. Pressure-temperature diagrams summarizing postulated me~mo~hic conditions and average isochores determined in this study. (a) Generalized fluid inclusion types from the Black Hills. lsochores are labeled with values of XH,O which decreases from higher to lower pressure isochores. Pressure-temperature conditions shown for the Hamey Peak Granite prevailed in an area - 10 km east of the study area and are not strictly applicable to these samples (REDDEN et al., 1982). Temperatures probably did not exceed -600°C in the study area. (b)Generalized fluid inclusion types from the Ruby Range. Pre~ure-tem~mture conditions of retrograde me~mo~hism (DAHL, 1979) are estimates for areas in the central and northern Ruby Range, 7 km or more north of our study area, and may not be strictly relevant to the samples in this study. See text for derivation of &chores and discussion of their significance. Aluminum silicate stability fields after HOLDAWAY(197 1).

ferred conditions of regional metamorphism. These inclusions are H20-rich and, as discussed in the section on fluid composition, have compositions that match most closely with predicted fluids in equilibrium with graphite. Lower density inclusions were formed at relatively low pressure, are COZrich, and have compositions that suggest ~st-en~apment modification. Thus the commotion and volumetric data both indicate that these inclusions have reequilibrated possibly at quite low temperatures (below 3OO”C?) and pressures. Southwestern Ruby Range. Nearly ail carbonic inclusions from the Crystal Graphite Mine have relatively low densities and isochores that pass at substantially lower pressures than estimated prograde or retrograde P-T conditions (Fig. lob). The only exception is a group of higher density COz-

CH,(equiv.) inclusions which do pass through the postulated metamorphic conditions (type CIII): however, these represent a total of only 6 inclusions, Thus the volumetric data indicate that virtually none of the carbonic inclusions can be pristine metamorphic fluids. It is possible that formation of all the veins sampled took place at lower pressure than the metamorphic events. However, the uniformly low densities of carbonic inclusions coupled with the evidence that bas been cited for compositional reequilibration indicate that the inclusions have undergone significant density decrease probably due to diffusion, leakage, or volume change (partial decrepitation). Isochores calculated for HzO-CO2 inclusions (type BNI), present only in the sample from the Bird’s Nest Claim (sample BN2), have relatively steep slopes passing on the hip-pr~sure side of inferred metamorphic conditions. As pointed out in the Appendix, these inclusions display abundant hairline cracks extending from inclusions (see also Fig. 4d), a feature that suggests volume expansion as a result of partial decrepitation due to internal overpressure. There are no distinct density differences between inclusions containing obvious cracks and those in which cracks could not be detected. If these inclusions have reequilibrated by volume expansion, then the original isochores were at even higher pressure and therefore even further removed from the inferred metamorphic conditions. As one alternative, it is conceivable that the textures of these inclusions actually result from internal underpressure (cf. BODNAR et al., 1989). Another alternative explanation invokes natural thermal decrepitation of the inclusions in this sample as a result of intrusion of diabase

dikes. Such dikes occur throughout this portion of the Ruby Range (Fig. 2): however, it is not known whether any exist in the immediate area. Aqueous inclusions with very steep isochores clearly were not trapped at the peak metamorphic or retrograde conditions shown in Fig. lob. They may have been trapped at relatively low temperature, perhaps 300°C or less. It is possible that some of these inclusions represent the complementary highsalinity aqueous component of HzO-CO#-I.,-NaC1 inclusions that have undergone phase separation. Rupturing such inclusions below the solvus and wicking of their aqueous fluid component could result in saline aqueous fluids and carbonic fluids that are segregated into different planes (CRAWFORD and HOLUSTER, 1986).

Fluid inclusions in vein quartz associated with graphite from the Black Hills, South Dakota, and the southwestern Ruby Range, Montana, are interpreted as H~O-COZ-CH~N2-NaCl mixtures. The Black Hills veins were formed at temperatures of 600°C or less and contain dominantly HzOCOt inclusions. Pure carbonic inclusions (CO2 or COKH4) are rare. Microthermometry and LRM indicate that the inclusions contain a maximum of - 5 mol% N2 and - 13 mol% CH4 although our analytical methods may tend to underestimate the concentration ofthese components. Saline aqueous inclusions occur in the samples but were not analyzed. Graphite veins in the Ruby Range occur in host rocks metamorphosed to -800°C or higher. The inclusions there are

Ruid inclusions in quartz-graphite veins almost exclusively C02-CI-L, mixtures; the N2 content is unknown because no LRM analyses were made. HzO-CO2 inclusions were found in only one sample although aqueous inclusions with low to high salinity occur in most samples. Many carbonic inclusions contain >20 mol% CH4(equiv.), and one sample contains inclusions with -97 mol% CH4(equiv.). Molecular compositions calculated for the Black Hills inclusions are in most cases compatible with thermodynamic predictions for graphite-saturated fluids at the temperature and pressure conditions under which the veins are believed to have formed and at geologically reasonable values of&. Some of the observed range in HzO/COz in the inclusions may reflect post-entrapment modification by diffusion or selective leakage of HZ0 from the inclusions; the rare, pure carbonic inclusions are believed to represent the end-product of such a process. Calculated compositions of the Ruby Range inclusions are notably HzO-deficient compared to the Black Hills samples as is characteristic of inclusions from highest amphibolite and granulite facies terranes. With the possible exception of HzO-CO2 inclusions found in one sample, these fluids do not record equilibrium with graphite. Therefore post-entrapment modification occurred in nearly all inclusions and compositions measured in the laboratory do not accurately reflect primary metamorphic fluids. Complete loss of Hz0 from the inclusions provides the simplest process to derive observed compositions from fluids buffered by graphite equilibria at metamorphic conditions. Despite evidence for compositional modification of most or all of the Ruby Range inclusions, sampling of a number of different veins in a small area at the Crystal Graphite Mine and nearby Bird’s Nest Claim proves that the inclusion population is far from homogeneous. Distinct compositional features characterize inclusions from graphite-free versus graphite-bearing veins and ‘3C-rich versus ‘3C-poor veins. The major compositional variable in the carbonic inclusions is C02/CH4; this ratio approaches cc in graphite-free veins and graphite veins intimately associated with marble, is between 1 and 20 for 13C-rich graphite veins in quartzofeldspathic host rock, and is 0.2 or less in ‘3C-poor graphite veins in graphitic schist. The distinct compositional differences in fluid inclusions from different geologic settings in a small area probably results largely from buffering of fluids by the local wall rock. Some of the differences may reflect incomplete reequilibration of primary fluid inclusions in which case some of the characteristics of the original vein-forming fluid are preserved. Average isochores calculated for fluid inclusions from the Black Hills graphite-bearing veins are in reasonable agreement with postulated metamorphic conditions in the local area. The concentrations of nonaqueous components (COZ, CH4, N,) increase in lower density fluids trapped at relatively low pressure/temperature. This relationship is consistent with trapping of high H20/C02 inclusions in the vein at or near conditions of regional metamorphism, followed by trapping of successive generations of lower H20/C02 inclusions, and, ultimately, pure carbonic inclusions at lower pressures. The lower density, low H20/C02 and carbonic inclusions probably result from volume and composition reequilibration of earlier

695

inclusions within the veins rather than from trapping of an externally derived metamorphic fluid that was evolving to lower HzO/COz. The proposed pressure-temperature-fluid composition evolution results in a pressure-temperature path that is generally convex with respect to the temperature axis. This is in accord with the known metamorphic evolution in the region where early high-pressure, low-temperature regional metamorphism was followed by lower-pressure, highertemperature thermal metamorphism related to granite emplacement. Isochores calculated for fluid inclusions from the Ruby Range are very inconsistent with postulated prograde and retrograde metamorphic conditions, thus the inclusions were probably subject to wholesale volumetric changes since trapping in addition to proposed compositional reequilibration. The isochores, therefore, provide little useful information about the metamorphic conditions or early uplift history of the area. Pure carbonic and pure aqueous inclusions probably were trapped at temperatures below the H20-NaCl-carbonic fluid solvus, possibly below 400°C. Carbon isotope ratios of graphite from veins indicate that metamorphic fluids derived carbon from quite different sources in the two areas. In the Black Hills, graphite was deposited in quartz-mica schist wall rocks for a distance of up to 12 cm from a small graphite-bearing quartz vein. The graphite has d13C of -20.8 + 0.2% showing that reduced carbon of biogenic origin was the major contributor of carbon to the fluids. This was probably accomplished through devolatilization of organic-rich shales during upper amphibolite facies metamorphism. The uniform 613Cwith distance from the vein indicates that the source of carbon, fluid composition, and temperature of precipitation were relatively constant during the period of graphite deposition. At the Crystal Graphite Mine and the Bird’s Nest Claim graphite occurs largely within veins and is the only important vein mineral. 6°C of graphite in the veins is -5.8 to -8.6%‘~ closely matching 613Cofgraphite in nearby dolomitic marble (-4.8 to -7.1%0). The graphitic marbles are the most likely source of carbon in the vein-forming fluids. Carbon was mobilized into a fluid phase during metamorphic devolatilization reactions in the impure dolomitic marble. The devolatilization reactions may have facilitated hydraulic fracturing in the more brittle quartzofeldspathic rocks nearby, whereas the ductility of the marble prevented the development of significant fracture permeability and fluid flow may have been more pervasive. Fracturing of the quartzofeldspathic rocks was further localized in the axial region of the major fold that crosses the mine area. The narrow range observed in t;13Cand the similarity with graphite in the marble indicate that transport of the carbon took place without significant cooling or mixing with fluids of different composition although 1.O%Ovariations in 613Cof graphite samples over distances of <2.5 cm require some secular fluctuations in fluid source, composition, or temperature. Some of the fluid probably infiltrated the quartzofeldspathic wall rocks surrounding the veins, forming disseminated graphite with 613Cof -4.0 to -7.3%0 in contrast to syngenetic graphite in two pelitic wall rocks of -16.1 and -30.0%0. The existence of one graphite-bearing quartz vein which has b13C of graphite of --19.5%0 and which contains CH,-rich fluid inclusions in-

696

E. F. Duke, K. C. Galbreath. and K. J. Trusty

dicates that a second type of carbon-bearing fluid. derived from organic matter in metapelites, was present locally at some time in the evolution of the area. ,4cknowledgments-We thank D. Rumble and .I. B. Lyons for stimulating our interest in graphite veins. J. A. Redden introduced us to the Black Hills quartz-graphite veins and R. B. Berg (Montana Bureau of Mines and Geology) provided logistical help in the Ruby Range. We thank F. W. Brown for allowing us to carry out studies at the Crystal Graphite Mine; the mine is located on deeded ground and permission is required for access. P. 1. Nabelek and K. L. Shelton supervised carbon isotope analyses at the University of MissouriColumbia. R. C. Burruss and T. Ging provided access to the LRM in the Branch of Petroleum Geology, U.S. Geological Survey, Denver. Colorado. Thoughtful reviews by R. J. Bodnar, S. R. Dunn, P. I. Nabelek, and D. Rumble resulted in substantial improvement of the manuscript. We are especially grateful to R. J. Bodnar for hospitality during PACROFI II and for expert editorial handling of the manuscript. Primary financial support came from the Donors of The Petroleum Research Fund, administered by the American Chemical Society, and by a supplemental grant from the American Chemical Society’s Project SEED. R. K. Gaines provided valuable assistance in organizing Project SEED at the local level. Additional support was granted by the Institute for the Study of Mineral Deposits. J. J. Papike. Director. Ediiorial handling: R. J. Bodnar REFERENCES AHRENSL. J., SISSONV. B., and HOLLISTERL. S. (1985) CO@h metamorphic fluids, south-central Maine (abstr.). Eos 66, 389. ARMSTRONGF. C. and FULL R. P. (1950) Geology of the Crystal Graphite Mine, Beaverhead County. Montana. US Geol. Surv. Open-File Map 50-26. BASTINE. S. (1912) The graphite deposits of Ceylon. A review of present knowledge with a description of a similar deposit near Dillon, Montana: Econ. Geol. 7, 4 10-443. BODNARR. J.. BINNS P. R.. and HALL D. L. (1989) Svnthetic fluid , inclusions. VI. Quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure. J. Metam. Geol. 7, 229-242. BOTTINGAY. (1969) Calculated fractionation factors for carbon and hydrogen isotope exchange in the system calcite-carbon dioxidegraphite-methane-hydrogen-water vapor. Geochim. Cosmochim. Acta 33,49-64. BOWERST. S. and HELGESONH. C. (1983) Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H@-CO*-NaC1 on phase relations in geologic systems. Geochim. Cosmochim. Acta 47, 1241-1275. Bozzo A. T., CHENJ. R., and BARDUHUA. J. (1973) The properties of the hydrates of chlorine and carbon dioxide. Intl. Symp. Fresh Water from the Sea 3,437-45 I. BROWNP. E. and LAMBW. M. (1986) Mixing of HzO-CO2 in fluid inclusions; geobarometty and Archean gold deposits. Geochim. Cosmochim. Acta 50,847-852. CAMERONE. N. and WEIS P. L. C1960) Strategic eraphite-a survey. - - . US Geol. Surv. Bull. 1082-E 201-32 I. COLLINSP. L. F. (1979) Gas hydrates in CO*-bearing fluid inclusion and the use of freezing data for estimation of salinity. Econ. Geol. 74, 1435-1444. CRAIG H. (1953) The geochemistry of the stable carbon isotopes. Geochim. Cosmochim. Acta 3, 53-92. CRAWFORDM. L. and HOLLISTERL. S. (1986) Metamorphic fluids: the evidence from fluid inclusions. In Fluid-Rock Interactions During Metamorphism (eds. J. V. WALTHERand B. J. WOOD), Advances in Phys. Geochem.. Vol. 5, Chap. 1, pp. 1-35. SpringerVerlag. DAHL P. S. (1979) Coexisting garnet and cordietite as indicators of retrograde metamorphic conditions in the Ruby Range, southwestern Montana. Geol. Sot. Amer. Abstr. with Prog. 11, 408409. II

DARIMONTA. and HEVENG. (1988) Simulation des equilibres de phase dam le systeme C02-Nz: application aux inclusions fluides. Bull. Min&al. 11 I, I 79-l 82, DEWITT E., REDDENJ. A., WILSONA. B., and BUSCHERD. (1986) Mineral resource potential and geology of the Black Hills National Forest. South Dakota and Wyoming. C’S Geol. Sure. Bul/. 1580 135p. DOUTHITTC. B. ( 1982) Precambrian coal or anthraxolite: a source for graphite in high-grade schists and gneisses-a discussion. Econ. Geol. 17, 1247- 1249. DUKE E. F. and GALBREATHK. C. (1987) Graphite as a recorder of fluid flow in the mid-crust. Geol. Sot. Amer. Abstr. Prog. 19, 648. DUKE E. F. and RUMBLED. (I 986) Textural and isotopic variations in graphite from plutonic rocks, south-central New Hampshire. Contrih. Mineral. Petrol. 93, 409-4 19. DUNNS. R. and VALLEYJ. W. (1987) Calcite-graphite carbon isotope thermometry: recognition of polymetamorphism, Tudor Gabbro (abstr.). Geol. Sot. Amer. .4hstr. Prog. 19, 649. EUGSTERH. D. and SKIPPENG. B. ( 1967) Igneous and metamorphic reactions involving gas equilibrium. In Researches in Geochemistv (ed. P. H. ABELSON),Vol. 2. pp. 492-520. J. Wiley & Sons. FERRYJ. M. and SPEARF. S. ( 1978) Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contrih. Mineral. Petrol 66, 1l3- I I 7. FORD R. B. (1954) Occurrence and origin of the graphite deposits near Dillon, Montana. Icon. Geol. 49, 3 l-43. FRENCHB. M. (1966) Some geological implications of equilibrium between graphite and a C-H-O gas phase at high temperatures and pressures. Rev. Geophys. 4, 223-253. GALBREATHK. C. (1987) Mass transfer during wall-rock alteration: .4n example from a quartz-graphite vein, Black Hills, South Dakota. M.S. thesis, South Dakota School of Mines and Technology. GALBREATHK. C., DUKE E. F., PAPIKEJ. J., and LAULJ. C. (1988) Mass transfer during wall-rock alteration: An example from a quartz-graphite vein, Black Hills. South Dakota. Geochim. Cosmochin7. .Icta 52, 1905- I9 18. GIL.I.ETTIB. (1966) Isotopic ages from southwestern Montana. ./. Geophys. Rex 72, 4029-4036. GOSSELIND. C., PAPIKEJ. J.. ZARTMANR. E., PETERMANZ. E., and LAUI. J. C. (1988) Archean rocks of the Black Hills, South Dakota: Reworked basement from the southern extension of the Trans-Hudson Orogen. Grol. Sot. .4mer. Bull. 100, I244- 1259. GREW E. S. (I 974) Carbonaceous material in some metamorphic rocks of New England and other areas. J. Geol. 82, 50-73. HALL.D. L.. STERNERS. M.. and BODNARR. J. (1988) Freezing point depression of NaCl-KCI-Hz0 solutions. Econ. Geol. 83, 197202. HEINRICHE. W. ( 1949) Pegmatites of Montana. Econ. Geol. 44, 307-335. HEYENG., RAMBOZC., and DUBESSYJ. (1982) Simulation des equilibres de phase dans le svsteme CO?-CHd en dessous de 50°C et de 100 bar: application aux inclusions fluides. C. R. Acad. SC;.Paris 294,203-206. HOLDAWAYM. J. (I 97 I) Stability of andalusite and the aluminum silicate phase diagram. Amer. J. Sci. 271, 97- 13I. HOLLISTERL. S. and BURRUSSR. C. (1976) Phase equilibria in fluid inclusions from the Khtada Lake metamorphic complex. Geochim. Cosmochim. Acta 40, 163- 175. HOLLOWAYJ. R. (198 1) Compositions and volumes of supercritical fluids in the earth’s crust. In Short Course in Fluid Inclusions: Applications to Petrology (eds. L. S. HOLLISTERand M. L. CRAWFORD),pp. I - 12. Mineral. Assoc. Canada. HOLLOWAYJ. R. (1984) Graphite-CH4-H20-CO2 equilibria at low grade metamorphic conditions. Geology 12,455-458. HUM C. K. W. (1943) Geology and occurrence of graphite at the Crystal Graphite Mine near Dillon, Montana. B.S. thesis, Montana School of Mines. JAMESH. L. and HEDGEC. E. (I 980) Age of the basement rocks of southwest Montana. Geol. Sot. Amer. Bull. 1:91, 1 l- 15. KARASEVICHL. P., GARIHANJ. M.. DAHL P. S., and OKUMAA. F. (198 1) Summary of Precambrian metamorphic and structural history, Ruby Range, southwest Montana. In Mont. Geol. Sec. Field

Fluid inclusions in quartz-graphite veins Confr. and Symp. Guidebook to Southwest Montana (ed. T. E. TUCKER),pp. 225-231. KERKHOFA. M. VAN DEN (1988) The system C02-CH4-N2 in fluid inclusions: Theoretical modelhng and geological applications. Ph.D. dissertation, Free Univ., Amsterdam. KREULENR. (1987) Thermodynamic calculations of the C-O-H system applied to fluid inclusions: Are fluid inclusions unbiassed samples of ancient fluids? Chem. Geol. 61, 59-64. KOZIOLA. M. and NEWTONR. C. (1988) Redetermination of the anorthite breakdown reaction and improvement of the plagioclasegarnet-Al$iO$quartz geobarometer. Amer. Mineral. 73, 216-223. LAMBW. M., VALLEYJ. W., and BROWNP. E. (1987) Post-metamorphic CO+ch fluid inclusions in granulites. Contrib. Mineral. Petrol. 96,485-495.

NEWTONR. C. and HASELTONH. T. (198 1) Thermodynamics of the garnet-plagioclase-Al$i05-quartz-geobarometer. In Thermodynamics of Minerals and Melts (eds. R. C. NEWTON,A. NAVROTSKYand B. J. WOOD), Advances in Physical Geochemistry, Vol. 2, Chap. 7, pp. 13 1- 147. Springer-Verlag. OKUMA A. F. (1971) Structure of the southwestern Ruby Range, near Dillon, Montana. Ph.D. dissertation, Pennsylvania State Univ. PASTERISJ. D.. WOPENKAB.. and SEITZJ. C. C1988) Practical asnects of quantitative laser Raman microprobe spectroscopy for the study of fluid inclusions. Geochim. Cosmochim. Acta 52,979-988. PERRYE. S. (1948) Talc, graphite, vermiculite and asbestos in Montana. Mont. Bur. Mines and Geol. Mem. 27. PLACZEKG. (1934) Die Rayleigh and Raman Streuung. In Handbuch der Radiologie (ed. E. MARX), Vol. 6, Chap. 2, pp. 209-375. Akademische Verlagsgesellschaft, Leipzig. PLUIJMB. A. VAN DERand CARLSONK. A. (1989) Extension in the Central Metasedimentary Belt of the Ontario Grenville: Timing and tectonic significance. Geology 17, 16 1- 164. POTI’ERR. W. and BROWND. L. (1977) The volumetric properties of aqueous sodium chloride solutions from 0” to 500°C at pressures up to 2000 bars based on a regression of the available literature data. US Geol. Surv. Open-File Rept. 75-636. RAMBOZC., SCHNAPPERD., and DUBESSYJ. (1985) The P-V-T-Xf0, evolution of H20-C02-CH., bearing fluid in a wolframite vein: Reconstruction from fluid inclusion studies. Geochim. Cosmochim. Acta 49,205-2

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REDDENJ. A., PETERMANZ. E., ZARTMANR. E., and DEWITT E. (1990) U-Th-Pb zircon and monazite ages and preliminary interpretation of the tectonic development of Precambrian rocks in the Black Hills, South Dakota. In Geol. Assoc. Canada Spec. Paper on the Tram-Hudson Orogen (ed. J. F. LEWRYand M. R. STAUFF’ER)(in press). ROEDDERE. (1984) Fluid Inclusions. Reviews in Mineralogy, Vol. 12. Mineral. Sot. Amer. RUMBLED. and HOERINGT. C. (1986) Carbon isotope geochemistry of graphite vein deposits from New Hampshire, U.S.A. Geochim. Cosmochim. Acta 50, 1239-1247. RUMBLED., DUKE E. F., and HOERINGT. L. (1986) Hydrothermal graphite in New Hampshire: Evidence of carbon mobility during regional metamorphism. Geology 14, 452-455. SCHIDLOWSKI M., HAYESJ. M., and KAPLANI. R. (1983) Isotonic inferences of ancient biochemistries: Carbon, Sulfur, Hydrogen, and Nitrogen. In Earth’s Earliest Biosohere: It’s Origin and Evolution (ed.J. W. SCHOPF),Chap. 7, pp: 149- 186. P&ceton Univ. Press. SEITZJ. C., PASTERISJ. D., and WOPENKAB. (1987) Characterization of C0&H4-H20 fluid inclusion by microthermometry and laser Raman microprobe spectroscopy: Inferences for clathrate and fluid equilibria. Geochim. Cosmochim. Acta 51, 1651-1664. SHEARERC. K., PAPIKEJ. J., SIMONS. B., and LAUL J. C. (1986) Pegmatite-wallrock interactions, Black Hills, South Dakota: In-

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STERNERS. M., HALLD. L., and BODNARR. J. (1988) Post-entrapment compositional changes in fluid inclusions: Experimental evidence for water diffusion in quartz (abstr.). Geol. Sot. Amer. Abstr. Prog. 20, 100. TounAy J. C., BENY-BASSEZ C., DUBESSYJ., and GUILHAUMOUN. (1985) Microcharacterization of fluid inclusions in minerals by Raman Microprobe. Scanning Electron Microscopy I, 103- 118. TOURETJ. (198 1) Fluid inclusions in high-grade metamorphic rocks. In Short Course in Fluid Inclusions: Application to Petrology (eds. L. S. HOLLISTERand M. L. CRAWFORD),pp. 182-208. Mineral. Assoc. Canada. TRUSCOTTM. G. and PERCIVALJ. A. (1988) The boron cycle in metasedimentary rocks and derived granite, Quetico Belt, Ontario (abstr.). Geol. Assoc. Canada/Mineral. Assoc. Canada Prog. Abstr. 13, 126-127.

VALLEYJ. W. and O’NEIL J. R. (1981) 13C/12Cexchange between calcite and graphite: a possible thermometer in Grenville marbles. Geochim Cosmochim. Acta 45,4 1 l-4 19. VRY J. K. and BROWNP. E. ( 1988) Fluid inclusions: Equivocal fluid evidence in high-grade metamorphic rocks. Geol. Sot. Amer. Abstr. Prog. 20, 34 l-342.

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WEIS P., FRIEDMANI., and GLEASONJ. D. (1981) The origin of epigenetic graphite: Evidence from isotopes. Geochim. Cosmochim. Acta 45.2325-2332.

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WINCHELLA. N. (19 11) A theory for the origin of graphite as exemplified in the graphite deposit near Dillon, Montana. Econ. Geol. 6,2 18-230. WOPENKAB. and PASTERISJ. D. (1986) Limitations to quantitative analysis of fluid inclusions in geological samples by laser Raman microprobe spectroscopy. Appl. Spectrosc. 40, 144- 151. ZARTMANR. E. and STERNT. W. (1967) Isotopic age and geoloaic relationships of the Little Elk Granite, Northern Black Hills. South Dakota. US Geol. Surv. Prof: Pauer 575-D. 157-163. ZHANG Y.-G. and FRANTZJ: D. (1987) Determination of the homogenization temperatures and densities of supercritical fluids in the system NaCl-KCI-CaC12-H20 using synthetic fluid inclusions. Chem Geol. 64,335-350.

APPENDIX-DESCRIPTION OF FLUID INCLUSION TYPES Black Hills Mixed HzO-CO,. The mixed H20-CO2 inclusions are the most widely distributed inclusion type in the Black Hills quartz-graphite veins. Visual estimates at -25°C indicate consistent proportions of the H20-rich and C02-rich phases along a given healed fracture but wide variations in the ratio between different fracture planes, typically ranging from about 25-90 volume percent C02-rich phase (Table 1). These inclusions display subhedral to euhedral negative crystal forms of the host quartz ranging in longest dimension from ~10 pm to 25 urn.

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The maximum depression of Tm COZ in the Black Hills samples varies with focality from O.YC at Fourmile to 2.O”C at North Pole and 5.3”C at Fine Tree (Fig. 5af. Analysis ofthe nonaqueous portion of~~~~nta~ive inclusions by LRM (Table 2) indicated the presence afsignificantamounts ofboth CH, and N2: a trace of HPSwas detected ln one inclusion. Clathrate melting temperatures from nine fracture planes correspond to aqueous phase salinities ranging from less than 1to about 1t wt% NaCl(equiv.) (Fig. 5b). Inclusions from the Pine Tree vein with Tm clath of about 14°C require significant amounts of CHp (eg. HULLISTER and BURRUSS,1976) which is corroborated by Tm CO2 of -60.7”C to -61.9”C and LRM data (Table 2). Upon heating, the CO&h phase in all the mixed H20-CO2 inclusions homogenized to the liquid phase (Fig. SC). C’QCff4(equiv.). Only four inclusions of the CO&YH,(equiv.) type were observed (FMII), these lying on a single healed fracture. They exhibit the same textural features as described above for the mixed H20-CO2 inclusions. These inclusions are characterized by a maximum melting point depression ofCOz of 0.9”C. No LRM data were obtained on these inclusions. YUWCO*. Two healed fractures in the Fourmile sampfes contain essentially pure COZ inclusions (FMIlI). Although these inclusions displayed melting point depression of CO1 up to 0.6”C, only a suggestion of CH, was indicated in two Raman spectra. No additionaf components analyzed by LRM (N2, CO, I&S. SO,) were detected. The pure CO2 inclusions range in size from ~10 grn to 33 pm. and most have negative crystal form.

CU+_I’H~(~quiv.}.Low-density CO&H4(equiv.) (type CII) constitute the most common inclusion type, occurring in all graphitebearing vein sampfes from the Crystal Graphite Mine. Additional gaseous components (CH.+,NZ, or others) cause significant depression of the melting point of CO1, with Tm 03~ ranging from -57.1 “C to -66.4”C (Fig. 6a; Table 1). Type CIII inclusions are a rare type present in two samples consisting of high-density CO*-CH4(equiv.). Tm CO2 ranges from -62.8”C to -64.8”C and Th CO, L-V varies between -3O.7”C and -45,7”C with homogenization occurring to the liquid phase (Fig. 66) Pure c’@. Low-density, nearly pure CC& inclusions were found in only two samples. One of these, sample AC-3, is the oniy quartz sampled in the area that is not associated with graphite; it comes from a concordant, white quartz segregation in silfimanite-garnet schist and amphibofite near the graphite veins. Type CI inclusions found in this sample exhibit minor depression of Tm C02, with a minimum value of -57S”C. Sample BN2, from the &d’s Nest Claim, also contains nearly pure CO2 inclusions {type BNII); these have minimum Tm CO2 of -57.3”C. but have slightly higher density than type CT.

PWP C%I,. Inclusions with extremely low melting and homogenization temperatures were found only m sample AC-4A, a concordant quartz vein with dispersed graphite that is foliated parallel to graphitic siilimanite-garnet schist wall rock. Most inclusions in this sample exhibit complex phase behavior that is distinct from other carbonic types. Many inclusions were difficult to freeze even at - 196°C and precise measurements of the temperature of final melting were not possible. Most inclusions melted in the range -99°C to - 107°C (Fig. 6b) and most homogenized to liquid between -80°C and -97Y (type CIVa, Fig. 6d): this sequence of phase changes is similar to type H2 of KERKWCW (198X).By compa~son with KEKKHOF’SI’-X and T-X projections for CO+X,, these results indicate XCH, of roughly 0.97 f .O2. One plane of inclusions, type CIVb. showed near critical liquid-vapor homogenization at --73.l”C to -77.6”C followed by total homogenization by sublimation between -72. I “C and -77.O”C, similar to type 52 of KERKHCE(198g). These observations are consistent with KERKHOF'S phase bchavinr for ,YCH+ ofabout 0.84 i .05 in the CO,-CH, system. Mixed ff20-CY32 i~~~~{~~~~~.s, No mixed H20-CO2 inclusions were observed in the five samples from the Crystal Graphite Mine area. Such inclusions are abundant, however, in the sample from the Bird’s Nest Claim (type BNI), which also contains nearly pure CO, inclusions (type BNII) and aqueous inclusions (type BNIII). A small percentage of all aqueous and carbonic inclusion types From the Crystal Graphite Mine are characterized by what appear to be very fine hairline cracks, typically a single fracture a few micrometers in length. emanating from the inclusion. The micr~therm~met~c properties of such inclusions are not consistently different from related inclusions showing no evidence of the fractures. In contrast, a large proportion of inclusions in the sample from the Bird’s Nest Claim show such “tails” or hairline cracks (Fig. 4d). The Bird’s Nest sample has braad zones with high concentrations of large mixed H20-C02 inclusions. These are up to 40 pm across and many have irregular to amoeboid shapes or show small cracks extending tnto the host quartz. At 40°C they contain two phases, liquid HZ0 and carbonic fluid, with a variable volume fraction of carbonic fluid usually in the range 0. f 5-0.45. Also present in the Bird’s Nest sample are smaller (525 pm) one phase CC&-CH4inclusions almost all of which have “tails” or hairline cracks. I~~O-NuC~(~4trit~.l. Types C’Vand BNIII inclusions are unsaturated aqueous inclusions of variable salinity and high density relative to the carbonic types {Fig. 4b). The aqueous inclusions occur in most samples, Aqueous inclusions that we have studied range from IO25 pm in maximum dimension and have a high degree of fill [L/(L + V) about 0.90-0.95 at 25”C]. The shapes vary from slightly irregular to equant, subrounded forms. ‘Ihe aqueous inclusions show T@?ice between - I .2”C and -43.5”C and 7%(L) in the range 67°C to 205°C. Additional inclusion types that were observed but not analyzed include fairly common aqueous inclusions containing one or more solid phases. These could not be frozen during cooling runs. Rare one-phase gaseous inclusions did not freeze or nucleate a vapor bubble at 120°C.