Golden Sunlight geology, Montana

Ecor•omw Geology Vol. 91, 1996, pp 507-526

Evidence for a Genetic Link between Gold-Silver Telluride and Porphyry Molybdenum Mineralization at the Golden Sunlight Deposit, Whitehall, Montana:

Fluid Inclusion and Stable Isotope Studies PAUL G. SPRY, M. MILAGROS PAREDES,

Department of Geological and Atmospheric Sciences, 253 Science I, lotca State University, Ames, Iowa 50011

FESS FOSTER, JACK S. TRUCKLE, Golden Sunlight Mines, Inc., 453 Montana Highway 2 East, Whitehall, Montana 59759

AND TOM H. CHADXVICK

17600 Rocky Mountain Road, Belgrade, Montana 59714

Abstract

The Golden Sunlight gold-silver telluride deposit, hosted primarily within the Mineral Hill breccia pipe, is spatially related to a high-level, Late Cretaceous multiphase, alkaline to subalkaline porphyry system (latite porphyries, quartz monzodiorite, and lamprophyres). Base metal veins and manganese (rhodochrosite) mineralization occur up to 2 km from the Mineral Hill breccia pipe and form part of a regional mineral zonation pattern genetically related to a low-grade porphyry molybdenum system. Alteration associated with this system consists of a proximal zone, pervasive in style (disseminations and veinlets), that is core to the alkaline igneous rocks and a distal zone that is vein controlled. Proximal alteration is composed of an early barren zone of quartz, hematite, pyrite, and barite as well as a pervasive zone composed of quartz, sericite, K feldspar, base metal sulfides, tellurides, molybdenite, and native gold. Distal alteration is characterized by the same mineralogy as the precious and base metal zone of the proximal zone except that it is vein controlled and does not contain molybdenite; a base metal and rhodochrosite zone occurs farthest from the center of the porphyry system. K feldspar alteration occurs as selvages to quartz-molybdenite mineralization and as a pervasive replacement of clasts in the Mineral Hill breccia pipe (early), whereas quartz-sericite alteration is dominantly associated with auriferous veins and the breccia pipe (late).

Molybdenite occurs in high veinlet-density quartz veins at the margins and deeper portions of the breccia pipe, in clasts in prebreccia pipe quartz-pyrite-K feldspar veinlets, as fine-grained disseminations in breccia fragments, and as rims to fragments of xenolithic latite. Previously four periods of hypogene mineralization in the breccia pipe were identified. Stages I and IV constitute •99 percent of the mineralization; native gold, calavertite, and Bi tellurides-sulfosalts occur in stage I whereas Au-Ag tellurides developed in stage Ill. Minor amounts of base metals are present in stage II whereas barite, dolomite, magnesite, trace kaolinitc, and sericite develop in stage IV. Proterozoic rocks of the LaHood Formation and the informally named Bull Mountain group host the breccia pipe and contain strata-bound sulfides-sulfosalts (up to 50% pyrite with minor to trace amounts of chalcopyrite, tennantite, pyrrhotite, sphalerite, galena, and molybdenite).

Fluid inclusion homogenization temperatures of primary and pseudosecondary aqueous liquid-vapor inclusions (constant liquid to vapor ratios) in quartz-pyrite-molybdenite veins range from 131.8 ø to 398.2øC. Primary and pseudosecondary inclusions with highly variable liquid-vapor ratios that coexist with rare three-phase COa-HaO and multiphase inclusions (one or more solid phases in addition to liquid and vapor) also occur in quartz-pyrite-K feldspar-molybdenite veins and indicate periods of intermittent boiling. Homogenization temperatures for fluid inclusions in stage I auriferous pyrite veins range from 145 ø to 345øC with salinities of • 1 to 10 wt percent NaC1 equiv. Values of/•34S for sulfides in the Mineral Hill breccia pipe, quartz-pyrite- K feldspar-molybdenite veins, Proterozoic sedimentary rocks, and distal base metal veins range from -12.2 to 3.1 per rail, -8.1 to 0.8 per rail, 0.7 to 6.1 per rail, and -8.9 to 9.5 per rail, respectively, and suggest a mixed magmatic-sedimentary sulfur source. Calculated/•SO values for water in equilibrium with late-stage sericite at 170øC vary from 0.7 to 2.8 per rail, whereas/•D water values range from -67 to -8 per rail.

Geologic, paragenetic, fluid inclusion, and stable isotope studies are consistent with an early magmatic fluid, associated with quartz-pyrite-K feldspar-molybdenite veins, that subsequently mixed with meteoric water during stages I to IV breccia pipe-hosted and auriferous pyrite vein formation. Ore-forming components (e.g., Au, Ag, Te, Cu, Bi, Mo, and much of the S) were most likely derived from the Late Cretaceous intrusive system with possible contributions from the Proterozoic host rocks.

Introduction

T•E Golden Sunlight gold-telluride deposit, one of the largest gold mines in the northwestern United States, contains total estimated reserves of 70.8 million short tons grading 0.054 ounces per ton gold (i.e., approximately 3.8 Moz of gold; Foster et al., 1993). Although gold has been mined

intermittently in the Golden Sunlight mine area for more than a century, the present open-pit operation, developed by Golden Sunlight Mines, Inc. (a subsidiary of Placer Dome U.S.), has been in continuous operation since 1983 (Roper and Lusty, 1983). Gold mineralization is hosted primarily within the Mineral Hill breeeia pipe and is spatially related

0361-0128/96/1827/507-2055.00 507

508 SPRY ET AL.

to a Late Cretaceous multiple intrusive, subalkalic to alkalic system that crosscuts Proterozoic sedimentary rocks. The Golden Sunlight deposit is one of several alkaline or subalkaline-related gold (e.g., Landusky-Zortman, Wilson and Kyser, 1988; Gies, Zhang and Spry, 1994b; Kendall, Kurisoo, 1991) and/or molybdenum porphyry systems (e.g., White Cloud, Cannivan, Big Ben, Armstrong et al., 1978) related to the Great Falls tectonic zone, which has been active since the Proterozoic (Foster and Childs, 1993).

Previous studies of the gold mineralization were conducted by Lindquist (1966), who described the structural setting of the gold-bearing quartz-pyrite veins, and by Porter and Ripley (1985), who evaluated paragenetic, stable isotope, and fluid inclusion characteristics of the deposit. Porter and Ripley (1985) showed that epithermal mineralization could be divided into four stages with stages I and III containing native gold and calaverite, and Au-Ag tellurides, respectively, and that sulfur, oxygen, and hydrogen isotope analyses supported a magmatic source for the ore-forming fluids. Fluid inclusion studies of quartz from stage I mineralization yielded a temperature of deposition of •200øC, and a single salinity datum of i vet percent NaC1 equiv.

Since the completion of Porter and Ripley's (1985) study, Golden Sunlight Mines, Inc., discovered a low-grade porphyry molybdenum system in and adjacent to the operating pit, and extensive accumulations of strata-bound sulfides in Proterozoic rocks that intersect the Mineral Hill breccia pipe. Furthermore, detailed mapping in and around the pit by Chadwick (1992) established a regional zonation of alteration and mineralization whereby quartz-pyrite-K feldspar-molyb- denire and epithermal auriferous pyrite veins are centered on the pit, and base metal veins as well as manganese mineralization occur distally, up to 2 km from the pit (Fig. 1). In light of these studies, the present investigation includes:

1. Characterization of a structural setting for the Late Cre- taceous alkaline intrusive rocks and spatially related porphyry molybdenum system and associated precious and base metal mineralization.

2. Fluid inclusion studies of porphyry molybdenum mineralization and deeper portions of the breccia pipe that were not available to Porter and Ripley (1985) at the time of their study. Additional salinity data of fluid inclusions in stage I quartz have also been obtained. A question to be addressed is why Porter and Ripley's fluid inclusion data yielded conditions that appear to be characteristic of typical heated meteoric waters, yet stable isotope data indicated a magmatic contribution to the ore-forming fluid.

3. A sulfur, oxygen, and hydrogen isotope study. In view of the detailed sulfur isotope investigation of sulfides (150 analyses) from the Mineral Hill breccia pipe by Porter and Ripley (1985), the present study has focused on sulfides in Proterozoic sedimentary rocks, a distal base metal vein deposit (Saint Paul), and porphyry molybdenum mineralization. Due to the diverse nature of sulfide types in the Golden Sunlight mine area, it was considered important to evaluate whether there were sulfur sources other than the magmatic source suggested by Porter and Ripley (1985) that may have contributed to the ore-forming fluids. Fifteen oxygen and hydrogen isotope determinations of late-stage sericite have been obtained to complement five oxygen and hydrogen iso-

tope pairs collected previously on sericite and quartz by Por- ter and Ripley (1985).

4. A genetic model for the formation of the porphyry molybdenum system, breccia pipe, and precious and base metal mineralization.

Geologic Setting The regional and local geology is summarized here and has

been described in detail by Foster and Chadwick (1990, 1996), Childs and Foster (1993), and Foster et al. (1993, 1996). Studies by Foster et al. (1993, 1996) suggest that Pro- terozoic rocks in the Golden Sunlight mine area can be subdivided into basin plain, outer fan, middle fan, inner fan, submarine canyon, and slope-shelf facies associations that pro- graded in a northerly direction. These studies also suggest that earlier correlations between Proterozoic formations in

the central Helena embayment and Proterozoic rocks in the mine area made bv McMannis (1963) are tenuous. Chadwick (1992) proposed (hat the Proterozoic sedimentary rocks can be divided into two units; the older LaHood Formation, and the younger, informally named, Bull Mountain group (Fig. 2). The following description of the local stratigraphic units is summarized from Chadwick (1992). Over 1,000 m of in- terbedded arkosic and poorly sorted sandstones of the La- Hood Formation with minor calcareous interbeds crops out on the southern end of Bull Mountain. These beds are fre-

quently conglomeratic and/or intraclastic. Sandy beds rarely exhibit complete Bouma sequences. The lower part of the LaHood Formation is considered to be the submarine fan

facies of a submarine fan-slope-shelf complex. South and east of the mine, outer fan lobe and fan fringe sandstones are underlain by basin plain carbonaceous black shales.

The overlying Bull Mountain group is >300 m thick and consists of an assortment of fine-grained siliciclastic sediments including laminated siltstones, shales, rhythmically laminated siltstone and shale, synsedimentary sulfide (primarily pyrite), carbonate and silicate concretions, and olistostromes. The Bull Mountain group was interpreted by Foster et al. (1996) as being primarily marine slope and shelf facies, with inner fan channel or submarine canyon fill deposits near its basal contact with the LaHood Formation. In the north-

eastern part of the map area, quartzites, arkoses, and glauco- nitic sandstones of the Cambrian Flathead Formation uncon-

formably overlie the Proterozoic sedimentary rocks. A high-level, multiple intrusive alkalic-subalkalic suite, of

Cretaceous age, is located in the vicinity of the mine and comprises dikes, sills, and stocklike bodies of quartz monzodiorite, latite porphyry, and lamprophyres. Quartz monzodior- ires are very altered and consist of feldspar, in which plagioclase exceeds potassium feldspar, quartz, hornblende, biotite, and occasionally clinopyroxene. In thin section they exhibit seriate, hypidiomorphic textures (Swanson, 1989). The latite porphyry is generally highly altered and composed of sube- qual amounts of groundmass and phenocrysts of plagioclase and potassium feldspar, also in roughly equal proportions (Porter and Ripley, 1985). Major element compositions of weakly altered latite porphyries and quartz monzodiorite indicate subalkaline to alkaline affinities (Swanson, 1989). Based on further major and trace elements studies, DeWitt et al. (1996) consider these rocks to be rhyolitic in composition. The latite porphyries are clearly crosscut by the Mineral Hill

Quat•m• - Tertiary Racks

E]l•om Mountain Valoanica

Uafio Intrueive Roc• (CreL)

Breeola (hy4rothermal & intmalon)

Mineral Hill Brecc•

Parph•t• Lathe (CreC)

Quarlz Wonzodlorite (CreL)

Paleozok• Sedlmont•ry Rocks

Proterozoic Sedlmentary Rodin

-r-"-- ........ Fault, ball on downthrown aide .......... -'--:' ...... Mineralized Structure

- • S)•ciine

Nteratlon Border PE- pervoa• e=dy .tege PM- pervae• m•U.tage V- vein controlled D - dlrl:ol

Idlneralized Structuree (Alphabetical) •- Ape• van B- Bonne Breccla-Veln System E- Exmnlner Vein F- Roron•e Vein S3•n •- Rotenee Dike G$- Golden Sunlight

Vein-Dike S•t•m /N - kllpketk•n • Vekl /.H- Lucky Heot Vein L V- LetRe Valley Vein L I•- Lefite Vdle• Breccla O- Ohio Vein R- Ridge Vein $- Suneet Vein-DEe •/- Shmnroek Vein .•- S•int Poul Velne

0 feet 3OOO

Fault, (Alph•betkml) C-Corridor /.•- Lone Eagle M- Midee NO- New Deal •- Surdight Zone SN•- Sunllght--Mldae

S•rn •- S41nt Paul

FIG. 1. Generalized geologic map of the Golden Sunlight mine area. The distribution of alteration zones is indicated. PE = a zone of pervasive early-stage quartz, pyrite, and hematite characterized by sporadic molybdenite mineralization and low gold concentrations; PM = a zone of pervasive multistage mineralization dominated by quartz, K feldspar, and sericite as alteration minerals and characterized by sporadic molybdenite and elevated gold concentrations. This zone hosts the Mineral Hill breccia pipe and the auriferous veins: V = a vein-controlled zone that contains the same mineralogy as the PM zone, but the mineralization is restricted narrow faults and fractures; and D = a distal zone where there is a paucity of gold and molybdenite but an increase in vein controlled base metal (galena, sphalerite, and chalcopyrite) and manganese (rhodochrosite) mineralization. Shown also are the major mineralized structures and faults.

509

510 SPRY ET AL.

Cambrian Flathead Quartzite

Shelf Facies Shelf Deposits Undivided Upper Slope •

Slope Facies •- Lower Slope

Deep Sea

Fan Facies

FIG. 2. Schematic stratigraphic section for the Golden Sunlight mine area. IFCF = inner fan channel fill facies; MHBP = Mineral Hill breccia pipe; SC = submarine canyon facies; Ybbs = cherty black shale; mineralogically consists of quartz, phlogopite, pyrite and minor ferroan carbonate (Bull Mountain group); Ybss = finely laminated sulfidic siltstone facies (Bull Mountain group); Ylic = massive sandstones; Yo = olistostromes.

breccia pipe locally. Clasts of latite porphyry are common in breeeia below a large sill in the breeeia pipe doeally referred to as the "Main sill") indicating that the intrusion of latite porphyry partially preceded formation of the breeeia pipe. However, in drill core taken from deep within the Mineral Hill breeeia pipe, there are several examples of intrusion breeeia with a latite porphyry matrix (herein referred to as "xenolithie latite") and latite porphyry dikes, demonstrating that phases of latite porphyry were coeval with and intruded the breeeia pipe. Given the eontemporaneity of latite porphyry and the breeeiation event, their dose spatial and intimate textural relationship, it appears that the latite porphyry and breeeia pipe are genetically related. Utilizing the ISOPLOT program of Ludwig (1990), a whole-rock model 3 isoehron age of 84 _+ 18 Ma acquired for 238U/234pb versus 206 204 -- -- ß ß

Pb/ Pb from altered samples of rhyohte, a pynte separate from altered rhyolite, and altered LaHood Formation in the breccia pipe related to gold mineralization suggests that the gold deposition and attendant alteration event occurred during the Late Cretaceous (DeWitt et al., 1996).

Relatively unaltered potassic trachybasalts, basaltic andes- ires, and basalts (generally referred to as lamprophyre) crop out approximately 1.5 km south of the Mineral Hill breccia pipe (Swanson, 1989), and an altered body of quartz monzodiorite is located i km northeast of the pipe. An abnormally high concentration of lamprophyre sills is present in the breccia pipe coincident with a zone of deformation up to 30 m wide, of Late Cretaceous, low-angle dextral-slip faulting referred to as the Corridor fault. A northerly trending swarm of lamprophyre dikes that traverses the center of the Mineral Hill pit contains xenoliths of Proterozoic sedimentary rocks and quartz monzodiorite. Lamprophyre dikes are also o14-

ented parallel to northeast-trending (N 30 ø E) deformation zones. Lamprophyres exhibit a porphyritie texture, with an aphanitie to phaneritie groundmass of biotite, augrite, and plagioelase. The predominant phenoerysts are augire, up to 3 em in length, with lesser amounts of olivine, biotite, plagioelase, and rare garnet. The intimate spatial and textural relationships between the Mineral Hill breeeia pipe and latite porphyry lamprophyre led Chadwick (1992) to suggest that they formed contemporaneously. Since lamprophyres commonly crosscut latite and breccia, they must be slightly younger. Consistent with this suggestion is the fact that lamprophyres are generally fresh or only slightly altered whereas latites are highly altered. A K-Ar date of 79.8 _ 2.8 Ma and a 4øAr-39Ar plateau date of 76.9 _+ 0.5 Ma for biotite separates from lamprophyre suggest a Late Cretaceous age (DeWitt et al., 1996).

The Mineral Hill breeeia pipe plunges at an average angle of 35 ø to the southwest (Fig. 3). It has a circular plan with a diameter of up to 170 m above the Main sill. Below the Main sill it has an elliptical plan (up to 270 m in diameter) that is elongate in a northeast-southwest direction. The results of drilling show that the pipe has a vertical extent of > 700 m. Contacts between the breeeia pipe and Proterozoie country rocks are fairly sharp in the upper portions of the breeeia, but become gradational with depth. Breeeia contacts are often eomplexly fractured, veined, faulted, and intruded by latite porphyries and lamprophyres. It is often at these complex contacts where the highest gold grades are located. The elast size, as well as the degree of breeeiation and elast rotation, increases with depth in the pipe.

The breeeia pipe is comprised of fragments of Proterozoie sedimentary rocks and latite that range in size from a few

Au-Ag TELLURIDE MINERALIZATION, GOLDEN SUNLIGHT DEPOSIT, MT 511

• •**------•-•• Mineral Hill 1 /

hym dike, sills I

•-, -- -•-, ,-•-, -- L• Latite sills dik.= V3 • '

F•o. 3. Geologic cross section of the Mineral Hill breccia pipe showing downdropped relic stratigraphy of breccia types (dotted lines) (Chadwick, 1992). la = crackle texture predominates with little or no rotation of fragments; all clasts are Bull Mountain group sedimentary rocks; the matrix to clast ratio is low and the proportion of silica flooding is ve• high. lb = open-space fill breccia with an average of 10 percent matrix; rotation of Bull Mountain group clasts is evident and silica flooding is less abundant than in la. 2 = similar to lb except that clasts are commonly latite porphyry; the presence of coarse clast brecciation is common. 3 = contains clasts of LaHood Formation, Bull Mountain group, and fatitc porphyry with a wide variation in clast sizes, shapes, and degree of rotation; rebrecciated clasts are 'also present. Note the zone of xenolithic latite at the margin of the breccia pipe as well blocks of it within the breccia pipe.

millimeters to >10 m in diameter. It has been subdivided

into four main units, based primarily on clast lithology. The units reflect wall-rock stratigraphy and are zoned vertically down the axis of the breccia pipe (Fig. 3). Six other breccia subtypes based on clast lithology, breccia texture, and mineralogy (rebrecciated breccia, high pyrite matrix breccia, latite breccia, quartz-hematite breecia, tectonic breceia--found generally at pipe margins, and sanded breccia) have also been identified (Chadwick, 1992). There is a significant down- plunge depression, of up to 250 m, of clast types relative to their corresponding soume beds in the wall rocks. The breccia pipe and axis of collapse are near-vertical when bedding in the Proterozoic rocks is rotated to the horizontal. This suggests that the breccia pipe formed as a near-vertical feature and was subsequently tilted along the west limb of the north- plunging Sunlight syncline which controls the orientation of the southern portion of Bull Mountain (see Fig. 1).

The youngest sedimentary deposits (Oligocene through Pliocene) occur along the margins of the area shown in Figure 1 and consist of valley fill sediments. Fluvial, alluvial fan, and lacustrine fades have been recognized.

The Golden Sunlight deposit is located on the eastern flank of Bull Mountain, within a Tertiary horst bounded by prominent north-south- and northeast-trending faults (Fig. 1). A broad fold of probable Late Cretaceous age, known locally as the Sunlight syncline, plunges north at a shallow angle from approximately 1.5 km south of the pit to its north wall. The mine occurs on the west limb of a north-plunging syn- dine, therefore most beds strike northwest and dip northeast. Northeast- and east-northeast-trending high-angle structures are common throughout the area, and many are mineralized (Foster and Childs, 1993). It is thought that these structures

are genetically related to the Great Falls tectonic zone (Foster and Chadwick, 1990). These structures are Cretaceous in age, because of their orientation and because they generally exhibit a component of right slip and may show normal or high-angle reverse slip.

Late Cretaceous fractures and veins are very common in the area; three trends of mineralized structures predominate. One set parallels the Golden Sunlight vein system and trends N 34 ø E with steep dips; another trends approximately N 70 ø E with steep dips to the south and north; and a third is northwest-southeast (N 60 ø W) oriented approximately per- pendicular to the Golden Sunlight vein system trend. The first two mineralized trends exhibit right-lateral strike-slip with minor dip-slip components; the third trend shows little documented offset but is a significant host to breccia occur- rences. Minor radial and concentric faults near the center of

the Mineral Hill porphyry system are believed to be related to the intrusion of the alkalic igneous complex.

One of the most prominent structures in the pit is the Corridor fault, located between the 5400 and 5850 levels primarily, that has displaced the upper portion of the breccia pipe 240 m to the east (Fig. 3). Several listric normal faults and fault systems with a variety of orientations are also present within, and in the immediate vicinity of, the breccia pipe. These structures are probably Tertiary to Recent in age.

Mineralization and Alteration

Three main types of mineralization are found in the Golden Sunlight mine area. In chronological order they are Protero- zoie strata-bound sulfide mineralization, the Mineral Hill porphyry molybdenum system, and the Mineral Hill breccia pipe and related auriferous veins. On the basis of crosscutting

512 SPRY ET AL.

relationships, porphyry molybdenum mineralization is interpreted to have formed prior to the emplaeement of the breeeia pipe. A fourth minor type which comprises disseminated diagenetie pyrite coated by supergene ehaleoeite and eovellite in upper slope fades of the Bull Mountain group is also present (Foster et al., 1996) but will not be discussed further. The auriferous pyrite veins can be subdMded into four stages (Porter and RipIcy, 1985).

Although the gold mineralization has not been dated directly, Foster (1991) suggested that it is Late Cretaceous in age because lamprophyre dikes (dated at 77-79 Ma) erosscut the Mineral Hill breeeia. Attempts at dating the fine-grained serieite alteration associated with gold-bearing veins have been unsuccessful, but the zø6pb/23sU date of 84 + 18 Ma (DeWitt et `A., 1996) from altered and mineralized Belt Su- pergroup rocks and latite supports a Late Cretaceous age. In addition, igneous rocks in the vicinity of the Golden Sunlight mine are associated with the Laramide orogeny and are no older than Late Cretaceous (e.g., Boulder batholith, Meyer et al., 1968; Tilling et al., 1968; Elkhorn Voleanies), implying that the latite (which is contemporaneous with mineralization) is also Late Cretaceous in age.

Details of the mineralogy of each of the major sulfide- bearing types, including detailed electron microprobe analyses, have recently been reported by Spry et ̀ A. (1995). How- ever, the •najor findings of this investigation, in addition to recent mineralogieal studies by Chadwick (1992), are summarized below.

The Mineral Hill porphyry molybdenum system

A districtwide alteration system (6 km 2) has been recognized and is spatially related to two of the largest latite bodies, the Main sill and the North Arm stock (Fig. 1). Within the central portions of the alteration system, disseminated and stockwork-controlled intercepts of molybdenite have been encountered in drill hole (subeconomic to economic grades locally over >70 m).

The Mineral Hill breccia pipe is 'also located centrally within the altered area. Clasts of typical porphyry-style •no- lybdenite mineralization are found within the Mineral Hill breccia, indicating that although the formation of the Mineral Hill breccia pipe postdates the bulk of the molybdenite mineralization they are spatially associated. Mapping by Chad- xviek (1992) identified a center or most intensely 'Atered area that extends from the Mineral Hill breccia pipe northward to the contact between Proterozoic and Paleozoic rocks. To

the south and west, the system diminishes in intensity. East of the open pit, bedrock is buried below Tertiary-Quaternary cover.

The core of the altered area is characterized by pervasive alteration which may be subdivided into two zones:

1. A zone of rocks do•ninated by paragenetically early phases of quartz, pyrite, and hematite (designated as "PE" in Fig. 1, this mineralization corresponds to stages Ia and Ib in Fig. 4) as well as sporadic molybdenite mineralization (• 100 veinlets/m2), particularly in xenolithie latite, and low-grade gold concentrations.

2. A zone of rocks containing multistage mineralization dominated by quartz, K feldspar, and serieite ("PM" in Fig. 1, and stages I to IV in Fig. 4). This zone is characterized by

Mineral Stage Ia Stage lb Stage H Stage III Stage IV

Quartz Pyrite Ruffle

Hematite

Pyrrhotite Tetradymite Chalcopyrite Bornite

Chalcocite

Acanthite

Arsenopyrite Gold

Tellurium

Calaverite

Tellurobismutite

Buckhornite

Coloradoite

Melonite Tennantite

Sphalerite Galena

Aikinite

?Benjaminite Lindstromite

Friedrichite

Gladite

Krupkaite Barite

Gypsum Strontianite

Marcasite

Covellite

Krennerite

Sylvanite Petzite

?X-phase Magnesite Dolomite

Kaolinite

Fluorite

DickAte

Sericite

? ........ ?

? ....... ?

FIG. 4. Paragenetie sequence for the mineralogy of the Mineral Hill breeeia pipe (derived from Porter and Ripley, 1985; Spry, unpublished data).

sporadic molybdenite mineralization, and by the Mineral Hill breeeia pipe with attendant gold-bearing quartz veins (i.e., elevated gold concentrations).

Molybdenite occurs as disseminations at the margins of the deeper portions of the breeeia pipe, in elasts in prebreeeia pipe quartz-pyrite-K feldspar veinlets, as fine-grained disseminations in breeeia fragments, as rims to fragments of xenolithie latite, and along the •nargins of quartz veins. The K feldspar consists of orthoelase and microcline. Microscopic studies show that the •nineralogy of quartz-pyrite-K feIdspar- molybdenite veinlets consists of, in approximate order of abundance: quartz, pyrite, K feldspar, hematite (or magnetite in deeper portions of the pit), molybdenite, fluorite, mtile, and pyrrhotite.

Sulfide miner'Aization, and associated alteration, in the dis- triet is further subdivided into two nonpervasive zones:

3. A vein-controlled portion (designated "V" in Fig. 1) where alteration and mineralization are similar to (2) above but are restricted to relatively narrow structures (up to 3 m


wide). Veins are commonly composed dominantly of pyrite, galena, and sphalerite, with lesser amounts of ehaleopyrite, arsenopyrite, bornitc, eovellite, aeanthite, tetrahedrite, ten- nantire, and rare pyrargyrite. Supergene minerals include eovellite, ehaleoeite, and digenite. Alteration minerals include bleached unidentified white days and micas.

4. A distal portion (designated "D" in Fig. 1) that is distin- guished by only trace amounts of gold and molybdenite, by abundant Mn in the form of rhodoehrosite and pyrolusite, and by an extremely sparse vein density.

Mineral Hill breccia pipe and related auriferous pyrite veins

About 70 percent of the gold reserves occurs in the Mineral Hill breccia pipe, with the remaining ore in the surrounding host rock. Mineralization occurs as disseminations and as

structurally controlled veins and breccias along faults and joints, with both styles of mineralization commonly overlapping. Disseminated mineralization is located in the sulfide- rich matrix of the breccia, the latite porphyry, and in Protero- zoic sedimentary rocks. Several vein generations, ranging from pre- to postbreccia, are most abundant near the porphyry center. Three dominant steeply dipping structural trends are present. They are, in decreasing order of abundance: northeast, northwest, and north-northeast (Chadwick, 1992). High-grade mineralization may occur along all three main structural trends.

Some brecciated veins show rotation and/or transport of clasts. Veins range from several centimeters to tens of meters thick and are composed of quartz and auriferous pyrite, pre- dominantly, with several minerals occurring in minor amounts (Fig. 4). Porter and Ripley (1985) identified four stages of hypogene mineralization in the breccia pipe. Stages I and IV comprise about 99 percent of the total volume of the breccia pipe matrix. Pyrite is by far the most abundant metallic mineral, constituting up to 20 percent of some rocks. Free gold occurs interstitially to pyrite as 5- to 100-b-sized particles in stage Ib. Calaverite along with bismuth tellurides and sulfosalts are also present in this stage, whereas gold- silver tellurides developed in stage III. It should be noted that whereas the paragenetic sequence shown in Figure 4 generally conforms to that reported by Porter and Ripley (1985), several additional sulfides, sulfosalts, and tellurides have also been identified. Nonmetallic minerals include seri-

cite, kaolinitc, quartz, barite, and minor amounts of dolomite, fluorite, magnesite, and dickitc. These minerals generally occur late in the paragenetic sequence, as indicated by Porter and Ripley (1985). Supergene minerals associated with auriferous veins include covellite, chalcocite, chalcanthite, quartz, hematite, chert, and unidentified white clays.

Proterozoic strata-bound sulfide mineralization Strata-bound sulfide mineralization is located in inner fan

and submarine canyon facies of the LaHood Formation and slope deposits of the Bull Mountain group (Foster et al., 1996). Massive and semimassive sulfides are best developed in submarine canyon deposits. Sulfidic units are up to 16 m thick and consist of rhythmic bands of millimeter-scale layers of alternating K feldspar and ferroan carbonate with disseminated pyrite. Pyrite content ranges from 15 to >50 vol percent. Trace element studies of this mineralization indicate

elevated concentrations of Ba (220-1,300 ppm), F (1,200- 4,200 ppm), Te (0.1-8 ppm), and up to 500 ppb Au.

Pyrite (5-15 vol %) also occurs as blebs, disseminations, and laminae in lower slope facies interlaminated shale and siltstone near the base of the Bull Mountain group (Fig. 3) and as disseminations in overlying upper slope facies siltstone and mudstone. Interstitial cement of the upper slope facies rocks contains pyrite, ferroan carbonate, and minor chalcopyrite. Pyrite which contains trace amounts of tennantite is coated by supergene chalcocite and covellite. An average Cu content of 100 ppm occurs throughout the slope facies and locally exceeds 300 ppm. Primary oxides, sulfides, and sulfosalts in strata-bound sulfide mineralization include, in approximate order of decreasing abundance, pyrite, chalcopyrite, hematite, rut fie, tennantite, pyrrhotite, sphalerite, galena, and molybdenite.

Fluid Inclusion Studies

Introduction

Porter and Ripley (1985) obtained 50 fluid inclusion homogenization temperature (Th) measurements of 25 primary and 25 pseudosecondary inclusions in six samples of stage I quartz from the upper part of the breccia pipe. The Th range for primary inclusions was 130 ø to 230øC (median = 170øC), whereas the Th range for pseudosecondary inclusions was 100 ø to 250øC (median = 170øC). One single freezing point depression yielded a salinity of < 1 wt percent NaC1 equiv.

Doubly polished wafers, approximately 0.2 to 0.4 mm thick, of arkoses and siltstones from the LaHood Formation and

Bull Mountain group, quartz from stage I, quartz-pyrite-K feldspar-molybdenite veins, and sulfide-free quartz veins (most are probably related to stage I) were prepared for the present fluid inclusion study. Microthermometric measurements were made on a Fluid Inc.-adapted U.S. Geological Survey gas-flow heating-freezing stage calibrated with syn- thetic fluid inclusions. The estimated accuracy is _ 0.1øC bet•veen -56.6 ø and 100øC, and ___2 ø at 374.1øC. Reproduc- ibility is within the estimated accuracy of the temperature determination.

Nature of the inclusions

One of the aims of the fluid inclusion study was to expand the database of Porter and Ripley (1985) by including suitable samples from deeper parts of the breeeia pipe. However, the number of new data acquired herein (see also Porter and Ripley, 1985) was hampered by the paucity of fluid inclusions in most samples and by the fact that fluid inclusions were, in general, <5/• in diameter. Large areas in nearly all samples were either inclusion free (Fig. 5a) or contained secondary inclusions along grain boundaries. Attempts were made to collect data from all four quartz-bearing paragenetie stages recognized by Porter and Ripley (1985) and from the breeeia pipe, as well as from the prebreeeia quartz-pyrite-K feldspar- molybdenite stage. Only stage I quartz, some sulfide-free (primarily stage I) quartz, and quartz in the quartz-pyrite-K feldspar-molybdenite stage were transparent enough to allow the fluid inclusions to be visible.

As was shmvn by Porter and Ripley (1985), fluid inclusions in stage I quartz are simple t•vo-phase liquid-vapor inclusions, consisting of 10 to 25 percent vapor, with no visible evidence

514 s•'R¾ ET AL.

b

FIG. 5. Photomicrographs of fluid inclusions in quartz in quartz-p,vrite-K feldspar-molybdeuite mineralization and stage I quartz sulfide veins from the Golden Sunlight deposit, taken in transmitted light. a. Typical stage I, type i fluid inclusions (<5/• in length) located in clusters near the center of crystals or along grain boundaries. b. Primary, type 1, aqueous liquid-vapor inclusions in stage I quartz. c. Type 2, aqueous liquid-vapor inclusions showing highly variable liquid-vapor ratios in quartz-pyrite-K feldspar-molybdeuite mineralization. d. Type 3 inclusion in quartz-pyrite-K feldspar-molybdeuite stage mineralization showing water (w), liquid CO2 (1), and vapor phase (v). e. Type 4 inclusion (bottom right), in quartz- pyrite-K feldspar-molybdeuite stage, exhibiting an anhydrite? (a) crystal and an unidentified elongate daughter crystal. The vapor bubble is located directly beneath the anhydrite? crystal and cannot be seen in the photograph. Note also the vapor- rich inclusion (v) in the top left-hand portion of the photograph. f. Multiphase, type 4 inclusion in the quartz-pyrite-K feldspar-molybdeuite stage, showing a vapor bubble and at least three unidentified daughter crystals, in addition to halite (h). Bar scale -- 10/• except in (a) where bar scale -- 40/•.

for immiscible CO2 or daughter crystals (type 1; Fig. 5b). Primary inclusions are up to 20/• in diameter and occur as isolated inclusions or as dusters of inclusions near the center

of quartz crystals. Although secondary and pseudosecondary inclusions are difficult to distinguish, a pseudosecondary as- signment was given only to those planes of inclusions that terminated within a crystal whereas secondary inclusions were recognized as those present in planes that crosscut grain

boundaries. Type I primary, pseudosecondary, and secondary fluid inclusions also occur in sulfide-free quartz and barite samples. They exhibit sizes, shapes, and liquid/vapor ratios simfiar to those of inclusions observed in stage I quartz.

In contrast to fluid inclusions in stage I quartz, those in quartz-pyrite-K feldspar-molybdenite veins are considerably more abundant and larger (commonly between 10 and 30/• in length). They are particularly common in embayments in

Au-Ag TELLURIDE MINEBALIZATION, GOLDEN SUNLIGHT DEPOSIT, MT 515

sulfides where the effects of recrystallization were apparently minimized. Three additional types of inclusions were also recognized in these veins: type 2, two-phase liquid-vapor in- elusions that exhibit highly variable liquid to vapor ratios (Fig. 5c); type 3, three-phase inclusions with two liquids and a vapor phase (Fig. 5d); and type 4, multiphase inclusions that include one or more solid phases in addition to liquid and vapor (Fig. 5e and f). Types I and 2 are common whereas types 3 and 4 are rare. Type 3 inclusions are present in samples 26-4 1107.8 and 90C-6 1835.5 only, whereas type 4 inclusions were observed exclusively in sample 26-4 1107.8. The restriction of type 3 and 4 inclusions to embayments in pyrite porphyroblasts probably enhanced their preservation whereas other inclusions were most likely swept to grain boundaries during reerystallization. It is unlikely that type 3 and 4 inclusions were sedimentary-metamorphic grains from Proterozoie host rocks since quartz grains in these rocks are devoid of inclusions.

Heating and freezing experiraents The results of heating and freezing measurements are

listed in Table 1. Two or three measurements were conducted

on every inclusion and averaged. Primary and pseudosecondary type 1 inclusions in quartz-pyrite-K feldspar-molybdenite veins homogenize into the liquid phase between 131.8 ø and 398.2øC (n = 73) and show a peak of Th values at 200øC, with a possible additional peak at 310øC (Fig. 6a). Type 2 inclusions homogenize into the liquid and vapor phases between 150.2 ø and 425.0øC (n = 48) and show a peak at approximately 230øC. Homogenization temperatures of secondary inclusions in these samples range from 103.9 ø to 297.2øC (n = 35). Freezing point depression measurements of type 1 inclusions exhibit salinities of 1.2 to 14.2 wt percent NaC1 equiv, whereas aqueous (type 2) inclusions display salinities of 4.2 to 15.6 wt percent NaC1 equiv. Daughter crystals had not dissolved in type 4 inclusions by 500øC and indicate salinities in excess of 65 wt percent NaC1 equiv (Sourirajan and Kennedy, 1962). The nature of the daughter crystals is un- known; however, one isotropic cubic mineral is likely to be halite (Fig. 5f). The orthorhombic, anisotropic daughter crystal in Figure 5e may be anhydrite. Since vapor bubbles in type 4 inclusions disappeared between 309.6 ø and 315.8øC (i.e., lower than daughter crystal dissolution), it is likely that the fluid was saturated in salts; however, there is no additional evidence to confirm the possibility of inhomogeneous trap- ping of the solid phase.

Fluid inclusion data for type 3 inclusions were limited to five inclusions only (three in sample 90C-6 1835.5 and two in sample 26-4 1107.8). The CO.• content of most inclusions was visually estimated as 45 to 60 vol percent. Melting of solid CO2 between -57.0 ø and -56.5øC indicates a paucity of dissolved CH4 and Ns. The COs homogenizes into the gas phase between 27.4 ø and 30.9øC. Densities for CO.• range from 0.53 to 0.67 g/cc and were calculated utilizing the equation of state of Brown and Lamb (1986) and the program FLINCOR (Brown, 1989). Clathrate-melting temperatures vary from 7.6 ø to 9.4øC yielding salinities of the aqueous phase of 1.2 to 4.7 wt percent NaC1 equiv. Two type 3 inclusions in sample 26-4 1107.8 decrepitareal upon heating whereas those in sample 90C-6 1835.5 hmnogenized into the vapor phase between 307.8 ø and 313.8øC.

Forty-nine homogenization temperatures were measured on primary and pseudosecondary type 1 inclusions in six samples of stage I quartz (Fig. 6b). Although these inclusions vary in size from 5 to 10/.z and contain approximately 10 to 20 vol percent vapor (Fig. 5b), most inclusions in these samples are <5/.z in diameter (Fig. 5a). The absence of melting events at approximately -56.5øC suggests the lack of significant quantities of CO2 (i.e., <2 mole % CO•). Homogeniza- tion temperatures range from 156.4 ø to 376.1øC, with a peak at 220øC. Several values obtained herein are higher than those temperatures obtained by Porter and Ripley (1985) and are, in part, probably related to the fact that some samples se- lected here were taken from depths greater than were available to Porter and Ripley during their study. With the excep- tion of two inclusions in sample 93C-1 1954.9 (salinities of 5.5 and 6.9 wt % NaC1 equiv), none of the inclusions appeared to freeze despite supercooling to temperatures as low as -120øC. A similar phenomenon was reported also by Porter and Ripley (1985) for stage I inclusions. They ascribed the absence of freezing behavior as due to the lack of nucleation sites associated with a fluid with a salinity of < 1 wt percent NaC1 equiv. However, the cause of the unusual freezing behavior remains in question. Where visible, eutectic temperatures recording the initiation of ice melting for type 1 inclusions range from approximately -20 ø to -30øC suggesting that the inclusions contain NaC1 or KC1, with minor amounts of CaCI•.

Most of the sulfide-bearing quartz veins belong to stage I; however, the sulfide-free quartz veins in the paragenetie sequence, particularly those in drill core, are more difficult to assign. It is conceivable that some veins belong to stage I, or to the prebreeeia quartz-pyrite-K feldspar-molybdenite stage, whereas others may be associated with later mineralizing or barren postmineralizing stages. Homogenization temperatures were obtained on 72 primary and pseudosecondary inclusions in 16 sulfide-free quartz samples taken from various depths within the breeeia pipe. The absence of feldspar alteration and the presence of type 1 inclusions only argues against these veins being assigned to the quartz-pyrite-K feldspar-molybdenite stage. Homogenization temperatures of primary and pseudosecondary inclusions range from 124.0 ø to 412.8øC (n = 68) and exhibit at least two groups of data with thermal peaks at 170 ø and 290øC (Fig. 6e). Secondary inclusions display homogenization temperatures that range from 108.2 ø to 401.2øC (n = 33). The salinity determined from the freezing point depression of one primary inclusion is 1.6 wt percent NaC1 equiv.

Three samples of barite, sample 92C-1 887.1 from stage I, and samples 93C-21 1464.5 and 28-3 147.5 from stage IV, were collected for fluid inclusion studies (Table 1). Homoge- nization temperatures of six primary type 1 inclusions in stage I barite range from 204.8 ø to 290.5øC and are within the range of T}, values obtained for type 1 primary-pseudoseeondary inclusions in stage I quartz. Although few in number, Th values of primary, pseudosecondary, and secondary inclusions in stage IV barite are generally <200øC (except for one outlier which yielded a value of 206. IøC). Considerable caution must be used to interpret the values of Th in barite, in general, because of the likelihood of postentrapment leakage (Boed- der, 1984). Notwithstanding this problem, replicate measurements of T}, of samples studied herein yielded essentially

516 seR•' ET AL.

TABLE 1. Results of Fluid Inclusion Microthermometry Studies

Inclusion Te range a Tm range 4 Salinity 5 Th range 6 Elevation Sample no. Mineral 1 type s (øC) (øC) (wt % NaC1 equiv) (øC) (ft)

90C-6 2352.6 m 1, p 1, ps 1, s

90C-6 2340 m 1, p 1, s

92C-14 1571.8 m 1, p 1, ps

26-4 1287.2 m 1, p 1, ps 1, s -26

26-4 1253.8 m 2, p 26-4 1253.5 m 1, p

1, ps -21 1, s

92C-14 1442 m 2, p 1, s

26-4 1147.4 m 2, p -24 to -23 1, s

26-4 1107.8 m 2, p 3, p7 4, pS

90C-6 1835.5 m 1, p 2, p 3, p9 ],s

92C-3 1054.2 m 2, p -24 to -20 2, ps 1, s

93C-22 551.1 m 1, p -22 1, ps -26 to -18 1, s

93C-22 952.8 m 1, p -33 to -25 1, ps ],s

92C-2 619.4 m 1, p 1, ps

DDH 157 822 m 1, ps 1, s

DDH 233 344 m 1, p 1, ps 1, s

93C-1 1954.9 py 1, p -19 to -18 1, ps

92C-10 1084 py 1, p 1, ps 1, s

90C-8 1256 py 1, p 1, ps 1, s

93C-22 952 py 1, p 93C-19 709 py 1, ps

1, s 90C-7 504.4 py 1, p

1, ps 1, s

26-4 1178.6 q 1, s 90C-7 1273.6 q 1, p

1, ps 1, s

26-4 631 q 1, p 1, ps ],s

28-1 583.8 q 1, p 1, s

28-1 551.1 q 1, p 1, ps 1, s

28-6 565.5 q 1, p 28-6 540.5 q 1, s

-10.6 14.6

-10.2 14.2

-11.5 to -7.2 10.7-15.6

-3.3 to -2.5 (3)

1.2-3.6

4.2 -5.4

-4.3 6.8

-4.5 to -1.0 (5) 1.2-7.2 -15.2

-3.1 to -1.2 (2) 2.6-5.1

-3.9 to -3.4 (2) 5.5-6.9

310.1-332.7 (2) 3,924 212.3-266.1 (2) 150.1-184.5 (6) 196.2-208.1 (2) 3,935

124.2 (1) 202.4-316.7 (7) 4,089 141.5-176.1 (2) 205.4-315.1 (5) 4,102

204 129.1-173.6 (3) 150.2-360.0 (5) 4,133 167.4-271.6 (8) 4,134 169.2-186.5 (2) 132.5-167.4 (3) 259.8-274.5 (2) 4,205 143.2-211.4 (6) 220.1-425.0 (15) 4,237 145.5-156.4 (3) 195.1-357.6 (10) 4,275

309.6-315.8 (2) 171.1 (1) 4,385

165.6-238.1 (5) 307.8-313.8 (3) 136.1-165.5 (7) 153.4-334.7 (9) 4,777 176.2-198.5 (2) 214.0-297.2 (2) 308.2-356.9 (3) 4,797 203.4-280.7 (7) 103.9-158.1 (3) 139.8-398.2 (10) 4,799 162.2-169.9 (3) 134.0-153.9 (2)

147.1 (1) 5,013 133.4

131.8-228.1 (5) 5,228 107.5-156.6 (5) 156.8-275.8 (10) 5,758

158.6 (1) 135.8 (1)

169.6-238.8 (6) 3,918 175.1-238.9 (8) 207.1-364.0 (6) 4,412 331.6-376.1 (2) 197.5-225.2 (4) 180.1-310.1 (4) 4,564

195.3 (1) 203.4-205.2 (2) 161.1-361.5 (7) 4,800

216.0 (1) 4,802 123.2-135.2 (3)

156.4 (1) 5,322 175.9-295.6 (3)

125.1 (1) 119.0-125.0 (2) 4,207 178.5-279.0 (3) 4,591 143.7-162.5 (3) 111.2-122.9 (3) 125.0-208.5 (4) 4,736 176.2-318.5 (3) 157.0-401.2 (6) 180.7-208.4 (2) 4,759

12s.o (1) 228.3 (1) 4,792 124.0 (1) 127.7 (1)

217.0-332.2 (6) 4,812 132.0 (1) 4,835

Au-Ag TELLURIDE MINERALIZATION, GOLDEN SUNLIGHT DEPOSIT, MT

TABLE l. (Cont.)

517

Inclusion % range 3 Salinity 5 Elevation Sample no. Mineral I type g (øC) T,,, range 4 (øC) (wt % NaCI equiv) Th range • (øC) (ft)

28-2 374.3 q 1, p 148.1-360.0 (5) 5,019 1, ps 128.2-179.1 (3)

89C-16 952.2 q 1, ps 165.1-227.3 (3) 5,027 1, s 138.2-173.7 (3)

92C-7 548.5 q 1, p 274.8-412.8 (4) 5,074 1, ps 134.8-168.8 (3) 1, s 174.8-202.7 (3)

28-2 229 q 1, p 140.7-308.3 (5) 5,145 1, ps 167.2 (1) 1, s 108.2-183.2 (3)

92C-4 349.3 q 1, ps 153.1-220.1 (2) 5,235 GSM 52 q 1, p -0.9 1.56 170.2-276.5 (9) 5,284 GSM 21 q 1, p 200.8-360.0 (3) 5,500

1, s 111.5 (1) GSM 2 q 1, p 146.8-360.0 (7) 5,500 GSM 24 q 1, s 120.3-255.8 (9) 5,500 93C-21 1464.5 bIV 1, p 147.2 (1) 4,162

1, ps 184.3 (1) 1, s 144.7 (1)

92C-1 887.1 bI 1, p 204.8-290.5 (6) 4,843 1, s 106.6 (1)

28-3 147.5 bIV 1, s 120.3-206.1 5,239

•Mineral = mineral used for fluid inclusion analyses; m = quartz in quartz-pyrite-K feldspar-molybdenite mineralization, py = quartz in pyrite-bearing stage I breccia pipe-hosted mineraliation, q = quartz in sulfide-free ? stage I breccia pipe-hosted mineralization, bI = barite in stage I breccia pipe-hosted mineralization, bIV = barite in stage IV breccia pipe-hosted mineralization

g Inclusion type: p = primary fluid inclusions, ps = pseudosecondary inclusions, s = secondary inclusions; I = type I inclusion, 2 = type 2 inclusion, 3 = type 3 inclusion, type 4 = type 4 inclusion

3 % = eutectic temperature 4 Tm= melting temperature ,5 Salinity determined by clathrate melt (8.2ø-9.4øC); COg homogenized between 27.6 ø and 30.9øC • Th = homogenization temperature; numbers in parentheses = numbers of measurements 7 Solid COg and clathrate melted at -57.0øC(2) and between 8.2 and 8.3øC, respectively; COg homogenized between 28.4 ø and 28.5 ø s Homogenization to liquid phase (309.6 ø to 315.8øC); daughter c .rystals had not melted by 500øC • Solid COg melted between -56.6 ø and -56.5øC(3)

identical values (no difference > _0.2øC was recorded for any single inclusion). Pressure correction

Based on maximum post-Tertiary displacement measured on the Golden Sunlight fanIt (presently named the Midas fault), and estimated overburden thicknesses, Porter and Rip- ley (1985) estimated a maximum lithostatie pressure of 200 bars. They also indicated that fluctuations between lithostatie and hydrostatic pressures may be expected for the Mineral Hill breccia pipe but assumed a maximum pressure of 200 bars and proposed a 20øC pressure correction to values of Th of type 1 fluid inclusions in stage I quartz, utilizing the calculations of Potter (1977). A similar correction has been applied here to values of Th of stage I inclusions.

Type 2, coexisting liquid-rich and liquid-poor inclusions in quartz-pyrite-K feldspar-molybdenite veins occur between 4,100 and 4,700 ft of elevation and possibly suggest a zone of boiling that is restricted in space. No pressure correction is required for these inclusions since they occur along the two-phase liquid-vapor boundary. Although it is uncertain whether a 20øC pressure correction to type 1 inclusions in the quartz-molybdenite-pyrite stage is appropriate, it should be noted that a correction of this magnitude to the peak of type 1 Th values at 200øC corresponds reasonably well with the peak of Th values of type 2 inclusions at 230øC.

Stable Isotope Studies

Sulfur, hydrogen, and oxygen isotope studies of ore and alteration minerals can be used to elucidate the origin and evolution of hydrothermal fluids, and to determine conditions of ore formation (Ohmoto, 1972, 1986; Taylor, 1974, 1979). All but two sulfide samples for isotopic analysis were either handpicked or drilled with a Dremel tool and inspected under a binocular microscope to insure a purity of >95 percent. The two exceptions were fine-grained molybdenite-quartz and pyrite-quartz samples which were crushed and analyzed. The resultant isotopic values were assumed to derive exclusively from molybdenite and pyrite. Sericites were also exam- ined for purity with a binocular microscope and several samples were analyzed by X-ray diffraction techniques to check for clay intergrowths. The presence of clays other than sericite was not detected by this technique, despite the fact that some samples deerepirated in water. Perhaps this suggests the presence of a minor amount (<5 vol % as expected from X-ray analysis) of an expandable clay.

Sulfur, oxygen, and hydrogen isotope analyses were per- formed by the Stable Isotope and Fluid Inclusion Labora- tories of the Department of Geology, University of Missouri- Columbia. Standard techniques for extraction and analysis were used as described by Grinenko (1962) and Rye (1966). Isotope data are reported in standard 6 notation relative to

518 SPRY ET AL.

12

• io

16

14

12

i0

4

2

Quartz + Pyrlte

IllPrimary (•ype 1) 171Seconc/•ry (type

TemperaCure (øC)

FIO. 6. Histogram showing homogenization temperatures of fluid inclusions in quartz in quartz-pyrite-K feldspar-molybdenite veins (a), stage I, quartz sulfide veins (b), and sulfide-free quartz veins (c). Most data are from stage I veins but may include stages II to IV or even postbreccia pipe- forming veins.

the Canyon Diablo troilite (CDT) standard for sulfur, and the Vienna SMOW standard for oxygen and hydrogen. The standard error for each analysis is approximately +_0.1 per rail for oxygen and sulfur, and +2 per rail for hydrogen.

Sulfur isotope studies Sulfur isotope analyses were obtained from 32 samples of

pyrite, galena, sphalerite, and molybdenite from quartz-pyrite-K feldspar-molybdenite veins, Proterozoic massive sulfides, and from a distal sulfide vein deposit (Saint Paul; Fig. 1). The results are listed in Table 2 and presented in Figure 7 along with data obtained by Porter and Ripley (1985). Pyrite and molybdenite from quartz-pyrite-K feldspar-molybdenite veins exhibit 6a4S values of -8.1 to 0.8 per rail (n = 10). Values of 6a4S of pyrite in Proterozoic massive sulfides range from 0.7 to 6.1 per rail (avg = 4.2%0; n = 7), which is slightly higher than the average of 2.0 per rail (n = 4) obtained by Porter and Ripley (1985). Sulfides from distal veins exhibit a broad range of values of 6•4S from -8.8 to 9.5 per rail (n = 14). Of these samples, five show values isotopically heavier than sulfide samples in stages I to IV obtained by Porter and Ripley (1985) from the Mineral Hill breccia pipe (-15.8 to

3.1%o, n = 150). One additional value of stage I pyrite obtained herein yields a value of -2.6 per rail. Disequilibrium between sulfides is indicated because values of 5a4S of coexisting sulfides show the expected fractionation trend (i.e., •32[Spyrite > •34Ssphtderite > •34-Sgalena) for only a few sulfide pairs. Oxygen and hydrogen isotope studies

Values of 5•sO and 5D of 15 samples of stage IV sericite range from 9.0 to 11.1 per rail and -112 to -53 per rail, respectively, and are in general agreement with 5•SO (9.7- 15.5%o) and 5D (-87 to -74%0) values obtained previously by Porter and Ripley (1985) from four samples of sericite (Table 3).

Based upon the upper stability limit of coexisting sylvanitc and petzite in stage III as indicated by the experimental studies of Cabri (1965), and making the reasonable supposi- tion that the ore-forming system is cooling throughout its life, we have assumed a maximum depositional temperature of 170øC for stage IV mineralization. Calculated 5D values of water at 170øC are -105 to -48 per rail (extrapolating the data from the isotope fractionation equation of muscoxdte- quartz of Lawrence and Taylor, 1971). Calculated 5•sO values range from 0.73 to 2.81 per rail utilizing the muscovite-water equation of Clayton eta]. (1972). Isotopic values of water in equilibrium with muscovite at temperatures <400øC should be treated with caution because the fraetionation curve for

hydrogen becomes less sensitive to temperature changes at

TABLE 2. Sulfur Isotope Data for the Golden Sunlight Deposit

Sample no. Mineralization type • Mineral •5a4S %0

PS93-1 St. Paul PS93-1 St. Paul PS93-2 St. Paul PS93-3 St. Paul PS93-5 St. Paul PS93-5 St. Paul PS93-5 St. Paul PS93-6 St. Paul PS93-7 St. Paul PS93-7 St. Paul PS93-8 St. Paul PS93-8 St. Paul PS93-9 St. Paul PS93-9 St. Paul

90C-11 701.22 Stage 1, 93C-6 853.9 QPKM 93C-6 1484.4 QPKM PS94-1A QPKM PS94-4 QPKM PS94-1B QPKM PS94-2 QPKM PS94-3 QPKM PS94-5 QPKM PS94-6 QPKM PS94-7 QPKM DH30-8 238 Massive

DH30-8 229.5 Massive DDH191 573.5 Massive

DDH190 624.2 Massive 93C-16 110 Massive

93C-16 134 Massive

93C-16 109.7 Massive

vein

vein

vein

vein

vein

vein

vein

vein

vein

vein

vein

vein

vein

vein

vein

sulfides sulfides sulfides sulfides sulfides sulfides sulfides

Galena 7.41

Sphalerite 9.47 Pyrite -2.24 Pyrite 7.02 Sphalerite - 1.73 Pp'ite 1.31 Galena 0.70

Pyrite -8.83 Pyrite -0.37 Galena -0.33

Pyrite 4.98 Galena -2.02 Galena 0.67

Pyrite 6.01 Pyrite -2.61 Pyrite - 8.11 Pyrite 0.76 Molybdenite - 1.85 Molybdenite -0.23 Pyrite -6.97 Pyrite - 7.14 Pyrite -6.84 P?'ite -6.80 Pp'it e - 7.64 Pyrite -6.09 Pyrite 4.74 Pyrite 4.90 Pyrite 0.74 Pyrite 3.78 Pyrite 5.54 Pyrite 3.44 Pyrite 6.14

QPKM = quartz-pyrite-K feldspar-molybdenite mineralization Plotted with Porter and Ripley's data given in Figure 7

Au-Ag TELLURIDE MINEBALIZATION, GOLDEN SUNLIGHT DEPOSIT, MT 519

16 I 12

8

4

0,

-12 -10 -8 -6 -4 -2 0

[] Moly veins []Vein (sph)

[] Vein (py)

[] Vein (gn)

2 4 6 8

2ø t

-12 -10 -8 -6 -4 -2

[] Breccia (tet III)

[] Breccia (py la)

[] Breccia (py IV)

[] Breccia (bar IV)

[] Breccia (cpy II)

[] Breccia (py [-II)

0 2 4 6 8 10

16

• 12

• 8

I,,1. 4

-12 -10 -8 -6 -4 -2

[] Massive sulfides (py)

[] Country Rock (py)

0 2 4 6 8 10

834 S ø/oo

F•G. 7. Sulfur isotope histograms. Pyrite in quartz-pyrite-K feldspar-molybdenite mineralization (moly veins), and sphalerite (sph), pyrite (py), and galena (gn) in the distal Saint Paul vein. b. Breccia pipe-hosted mineralization for tetrahedrite in stage III (tet III), pyrite in stage Ia (py Ia), pyrite in stage IV (py IV), barite in stage IV (IV), chalcopyrite in stage II (cpy II), and pyrite in stages I or II (py I-II). All data are derived from Porter and Ripley (1985) except for a sample of stage Ib pyrite, obtained in this study, which yielded a •5a4s value of -2.6 per rail. c. Pyrite in Proterozoic strata- bound sulfides (massive sulfides, this study) and from Porter and Ripley (1985; country rock).

low temperature (see Taylor, 1979; and O'Neil, 1986). For example, if the fractionation curve for muscovite behaves in the same manner as the experimentally determined curves obtained by Lambert and Epstein (1980) and Liu and Epstein (1980) for kaolinire at T < 400øC, hydrogen isotope values of water in equilibrium with sericite at 170øC may be at least 45 per mil heavier than values given above. Notwithstanding this concern, calculated values of 6D and 61sO for water derived from the four sericites analyzed by Porter and Bipley (1985), using Lawrence and Taylor's (1971) and Clayton et al.'s (1972) fraetionation curves, are -42 to -11, and 1.43 to 7.20 per mil, respectively (Table 3, Fig. 8a).

Discussion

Variations in temperature of hydrothervml fluids The wide range of homogenization temperatures of pri-

mary and pseudosecondary inclusions in quartz most likely

indicates a continuum of hydrothermal events rather than a series of individual episodes. However within this continuum of events, periods of more vigorous thermal activity, possibly related to intermittent movement along the strike-slip system, may have taken place. Whether the Th peak at 230øC for fluid inclusions in stage I quartz (Fig. 6b) corresponds to the same thermal event recorded in quartz-pyrite-K feldspar-molybdenite veins is unclear. However, if it is related to the same hydrothermal event, the pressure correction to stage I Th values may be smaller (i.e., <5øC rather than 20øC) than that suggested by Porter and Ripley (1985), or alternatively, hydrostatic (especially during transtensional periods) rather than lithostatic conditions dominated. Even though field studies suggest that quartz-pyrite-K feldspar-molybdenite mineralization formed prior to breccia pipe formation and the contained auriferous pyrite veins, the presence of quartz, pyrite, hematite, pyrrhotite, and possibly rutile in quartz-pyrite-K

520 SPRY ET AL.

TABLE 3. Oxygen and Hydrogen Isotope Data for the Golden Sunlight Deposit

8•O•i,.i• 8D ..... te 8180 8DH•o 8•O 8DH•o H20 H20

Sample no. (%0) (%0) (%0, 200øC) • (%0, 200øC) '2 (%0, 170øC) • (%0, 170øC) 2

93C-7 402.5 9.65 -55 1.15 - 12 1.35 - 10 93C-5 1440.8 10.41 -80 1.91 -37 2.11 -35 93C-7 374.0 10.33 -57 1.83 - 14 2.03 - 12 93C-1 1956.6 9.03 - 78 0.53 -35 0.73 -33 92C-7 422.0 9.55 -78 1.05 -35 1.25 -33

92C-1 136.5 10.35 -83 1.85 -40 2.05 -38 DDH157 827.8 9.78 -72 1.28 -29 1.48 -27

93C-16 1378.8-1380 9.61 -53 1.11 -10 1.31 -8 90C-6 1870.3 9.62 -88 1.12 -45 1.32 -43

92C-14 812.3 9.77 - 62 1.27 - 19 1.47 - 17 93C-13 1463.5 9.29 -63 0.79 -20 0.99 - 18 GSM-28 9.30 -98 0.80 -55 1.00 -53

93C-7 600 11.11 -74 2.61 -31 2.81 -29 93C-6 2286 10.09 -63 1.59 -20 1.79 -18 GSM 27 10.76 -112 2.26 -69 2.46 -67 186-19153 10.59 -74 2.09 -31 2.29 -29 187-10063 15.50 - 85 7.00 - 42 7.20 - 40 187-1060.53 9.73 -87 1.23 -44 1.43 -42 175-303.33 11.87 -82 3.37 -39 3.57 -37

Based on muscovite-water hydrogen isotope fractionation equation of Lawrence and Taylor Based on muscovite-water oxygen isotope fractionation equation of Clayton et al. (1972) Froin Porter and Ripley (1985)

(1971)

feldspar-molybdenite veins and stage Ia mineralization, as well as overlapping fluid inclusion homogenization temperatures, suggests similar physico-chemical conditions early in the hydrothermal ore-forming stages. A continuation of hydrothermal activity below 200øC, to as low as approximately 100øC, is reflected by the homogenization temperatures of secondary inclusions in nearly all samples studied herein, as well as by primary inclusions in sulfide-free quartz veins that show a thermal maxima at around 170øC (Fig. 6c). The obser- vation of Porter and Ripley (1985) that the coexistence of sylvanitc and petzite in stage III suggests a maximum temperature of 170øC is consistent with the proposed temperature decrease during the waning stages of hydrothermal activity. It should be noted that no apparent vertical temperature zonation could be recognized in molybdenite-bearing or auriferous veins.

Sources of sulfur and metals, and composition of the hydrothermal fluids

The most likely sources of sulfur for sulfides in the Mineral Hill breccia pipe and the quartz-pyrite-K feldspar-molybdenite veins are Proterozoic strata-bound sulfides in the La-

Hood Formation and the Bull Mountain group or the spatially related alkaline-subalkaline Cretaceous intrusive complex. On the basis of the sulfur isotope compositions of four pyrite- dominant samples of Proterozoic country rock (634S = 0.1- 4.0%o), Porter and Ripley (1985) argued that the sulfur isotope composition of country-rock sulfur was essentially indis- tinguishable from magmatic sulfur. The average 634S value of 4.2 per mil for pyrite in Proterozoic rocks obtained in the present study, although 2.1 per rail heavier than the average of 2.1 per mil obtained by Porter and Ripley, is within the 0 + 5 per rail range normally assigned to magmatic sulfur values (Ohmoto, 1986). Sulfur isotope compositions of pyrite and molybdenite in quartz-pyrite-K feldspar-molybdenite veinlets (634S = -8.1 and 0.8%o) are within the broad range of isoto-

pie values obtained by Porter and Ripley (1985) for sulfides in breeeia pipe mineralization. Although it is likely that sulfur associated with porphyry molybdenum mineralization was derived from a magmatie fluid soume, the isotope data do not rule out contributions from Proterozoie sedimentary sulfur sources.

The observed variations in the sulfur isotope composition of sulfides in the Mineral Hill breeeia pipe were attributed by Porter and Ripley (1985) to differences in X;SO4/X;H2S of the ore-forming solution, near the hematite-pyrite boundary, at relatively constant values of temperature, pH, fs.,, total sulfur, and ionic content. A temperature decrease from 200 ø to 150øC across stages I to IV was proposed by Porter and Ripley. Based upon fluid inclusion studies herein, the formation temperature of the quartz-pyrite-K feldspar-molybdenite stage and stage I veins was closer to 230øC (and possibly >300øC for molybdenite mineralization) than to 200øC, with an ionic content probably closer to 2, than 0.4. For these higher values of T and the ionic content of stage I fluids, calculated values of log fo2 and pH utilizing the program FO2PH (Zhang and Spry, 1994a) show that they are approximately the same as those suggested by Porter and Ripley (1985, fig. 15) (logfo.z = -40 to -37, pH = 3.5 to 4.5). If the inK- of the ore-forming solution is >0.02, values of pH will also be higher.

Since K feldspar, rather than sericite, is the potassium silicate in equilibrium with members of the system Fe-S-O in the prestage I quartz-pyrite-K feldspar-molybdenite veins, the pH of fluids responsible for the formation of porphyry molybdenum mineralization, assuming the same T, ionic content, and total sulfur content as estimated for stage I mineralization, would be higher than the pH associated with stage I mineralization. However, logfo2 values would remain approximately the same (i.e., near the hematite-magnetite buffer), since magnetite and hematite coexist in porphyry molybde-


o

-25

-50

,-, -75

c• -10(;

-125

-150

-20

a Cretaceous?? / .... •1 ß I

'"'-----k .•7• ø'ø•o.oo• • I I

Mid & Late /••o•1 • I I TeHia• •-• ' /

•/-•o.1 MAGMATIC

••-, -o.• WATER ,x• Present

....................... -15 -10 -5 0 5 10

0

-25

-50

-75

• -10C_ -125

-150

-20

Cretaceous?? o7o_:oøo

Mid & Late / . _ = - o • 1 I I • • 0.5 o.•.o• I I TeHia• •-/ /

AOaATC /•' WATER

,x• Present

-15 -10 -5 0 5 10

•'o(?-)

FIG. 8. a. Calculated O and H isotope compositions of water in equilibrium with stage IV sericite from the Golden Sunlight deposit. Open squares are data collected in this study whereas dosed triangles are taken from Porter and Ripley (1985). Also shown are the range for average magmatic waters derived from crustal felsic magmas, arc magmas, and midocean ridge basalts (Taylor, 1986, 1992; Hedenquist and Lowernstern, 1994), the meteoric xvater line (MWL), and ranges for present-day meteoric water, Late Cretaceous, and mid- and late Tertiary waters in southwest Montana (Shep- pard and Taylor, 1974). a. Magmatic water exchanged at 170øC with 50 percent magmatic and 50 percent sedimentary rocks at various water/rock ratios passes through the calculated isotopic compositions whereas trajectories determined for Late Cretaceous (upper curve), and mid- and late Tertiary meteoric water (lower curve) do not pass through the calculated values. b. Exchange trajectories have been derived for magmatic, Late Cretaceous, and mid- and late Tertiary meteoric water at 170øC with 80 percent sedimentary and '20 percent igneous rocks. Note that the calculated values intersect all three curves at very low water/rock ratios.

num mineralization (Fig. 9). It should be noted that fluid inclusion studies indicate that higher salinities, and possibly higher temperatures, and rare boiling were associated with molybdenite deposition. Part of the reason for highfo2 conditions during the porphyry molybdenum stage may be due, in part, to boiling which leads to an increase of the •SO4/•H2S ratio, because of the fractionation of H2S into the vapor and SO4 into the fluid (Ohmoto and Rye, 1979; Drummond and Ohmoto, 1985). The loss of H2S will also cause an increase in oxygen fugacity that will lead to an attendant increase in the XSO4/XH2S and the isotopic composition of H2S relative to the fluid, and result in significant variations in the isotopic composition of precipitating sulfides. In calculating Figure 9, a •34Szs of 0 per rail was utilized. Note that the hachured area indicated for the proposed stability of this stage of mineralization overlaps the range of sulfur isotope values obtained for pyrite.

Several elements (e.g., Au, Te, Zn, Pb, Cu, Mo, F, and As) that occur in high concentrations in the auriferous pyrite veins are also anomalously high in Proterozoic strata-bound sulfides. Whereas the elevated concentration levels in Pro-

terozoic strata-bound sulfides are most likely due to hydrothermal enrichment related to later breccia- and porphyry- forming processes, these levels may be the result of syngenetic or diagenetic processes during the Proterozoic. The Great Falls tectonic zone is a long-lived, deep-seated structure that has been active since the Proterozoic and has been the locus

of igneous rocks with an alkaline affinity. Periodic tapping of hydrothermal fluids derived from alkaline igneous rocks over an extended period of time may account for the common elemental signature for Proterozoic sedimentary rocks and the Cretaceous age auriferous vein system at Golden Sun- light. In this context, it is significant to note Proterozoic New- land and Greyson sedimentary rocks, crosscut by a Protero- zoic gabbro at the York project, northeast of Helena, Mon- tana, are also known to contain elevated concentrations of base and precious metals, including Au and Te and are considered to have formed by either diagenetic or epigenetic processes (Baitis, 1988; Foster and Childs, 1993). This common alkaline igneous source for sulfides in Proterozoic rocks and Late Cretaceous mineralization may also account for the overlap in sulfur isotope values of sulfides among Proterozoic strata-bound sulfide, porphyry molybdenite, and auriferous pyrite veins associated with the Mineral Hill breccia pipe. Values of (534S for sulfides in the distal Saint Paul deposit are as high as 9.5 per rail and suggest a nonmagmatic, probable sedimentary sulfur contribution for sulfides at the margins of the Mineral Hill porphyry system.

- B• ' • Hem

Opy _..•• Mag Po

Kaol Musc K-spar • I I I I I I I ! ! J_ .I. !_ I

-3O

-35-

-40-

•-45-

-50-

-55

0 I 2 3 4 5 6 7 8 9 10 11 12 13 14

pH

FIG. 9. Logf%-pH diagram showing approximate conditions (hachured area) of the quartz-pyrite-K feldspar-molybdenite stage, relative to mineral stabilities in the systems Cu-Fe-S-O and K-A1-Si-O-H at 230øC for 6aSzs = 0 per mil; XS = 0.01 m, rn•+ = 1 m, m•+ = 0.1 m, and rnc• •+ = 0.1 m. Also shown are contours indicating the range of 6•4S values for pyrite. Drawn using thermodynamic data and programs FO2PH and CONSTANT in Zhang and Spry (1994a). Abbreviations: Bo = bornire, Cpy = chalcopyrite, Hem = hematite, Kaol = kaolinite, K-spar = K feldspar, Mag = magnetite, Musc= muscovite, Po = pyrrhotite, Py = pyrite.

522 spin • ET AL.

Origin of the ore-forming fluids

In evaluating the origin of auriferous ore-forming fluids at Golden Sunlight, Porter and Ripley (1985) maintained that oxygen and hydrogen isotope data collected by them for stages I and IV were similar and indicated that isotopic compositions of the implicated hydrothermal fluids varied only slightly between different paragenetic stages. Since oxygen and hydrogen isotope data collected here for stage IV are almost identical to those collected by Porter and Ripley for stages I and IV, it is also assumed here that fluids formed during stages I to IV were similar.

On the basis of mass balance calculations, Porter and Rip- ley (1985) argued that the oxygen and hydrogen isotope data were most consistent with isotopic exchange between a magmatic fluid and igneous rocks, for T = 500øC, at water to rock ratios of >-0.01. Such a scenario is compatible with sulfur isotope compositions of the ore fluid associated with early formed quartz-pyrite-K feldspar-molybdenite and stage I mineralization that also support involvement of a magmatic component to the ore-forming fluid. Furthermore, the isotopic composition of water in equilibrium with sericite at 170øC for Porter and Ripley's sample 187-1006 (6•sO = 7.2%0; 6D = -40%0) lies within the range of magmatic waters observed worldwide based on data in Taylor (1986, 1992) and Heden- quist and Lowernstern (1994).

Based on the results of additional mass balance studies, using the techniques of Ohmoto and Rye (1974) and Field and Fifarek (1985), the isotopic composition of stage IV fluids has been modeled here in terms of exchange between magmatic fluid and rocks with markedly variable igneous to sedimentary rock ratios, and temperatures as low as 150øC. As an example, an exchange trajectory shown in Figure 8a illustrates involvement of a magmatic fluid, at T = 170øC, with 50 percent igneous rock and 50 percent sedimentary rock, utilizing initial igneous rock 6D and 6•sO values of -40 and 5.5 per rail, respectively, and 6•sO and 6D values of 13 and -115 per mil for Proterozoic sedimentary rocks. The 6•sO value (13%o) was derived by averaging the isotopic compositions obtained by Porter and Ripley (1985) for the LaHood Forma- tion and Bull Mountain group (even though it is uncertain whether these data are entirely representative of all of the Proterozoic sedimentary rocks), whereas 6D was determined from the corresponding value on the kaolinitc line of Savin and Epstein (1970). The exchange trajectory passes through the range of stage IV fluids for water/rock ratios between 1 and 5. Trajectories have also been calculated for exchanges between a volume of rock containing 50 percent igneous and 50 percent sedimentary rock and Tertiary meteoric water, assumed in Porter and Ripley's study, in addition to Late Cretaceous meteoric water utilizing 6D and 6•sO values given by Sheppard and Taylor (1974). However, neither of these curves intersect stage IV xvaters (Fig. 8a). For temperatures >-200øC or for ratios of sedimentary to igneous rock >80:20, and an initial meteoric water composition rather than an initial magmatic water composition, calculated trajectories will pass through the isotopic compositions of water that has isotopically equilibrated with stage IV sericites (Fig. 8b). A sedimentary to igneous rock ratio higher than that used by Porter and Ripley (1985; i.e., 75% sedimentary rock and 25% igneous rock) is likely because at least 30 percent of the epither-

mal mineralization is hosted entirely within Proterozoic sedimentary rocks outside of the breeeia pipe, and rock fragments in the breeeia pipe are dominated by Proterozoie sedimentary rocks. It should be noted that care must exercised before

interpreting the origin of late-stage fluids associated with the formation of serieite at temperatures near 150øC because it has been shown that retrogressive isotopic exchange with the host rock will result in depletion in •sO and concentration of D (e.g., Campbell et al., 1984; Richards and Kerrled, 1993). Stage IV 6•sO and 6D values are markedly heavier than present-day meteoric water values (6D = - 140 to - 130%o; 6•sO = -19 to -17%o) and suggest that stage IV serieites were probably not affected isotopically by present-day meteoric waters.

Mass balance calculations suggest that auriferous ore-forming fluids were probably mixed magmatie-meteoric waters that exchanged with a combination of sedimentary and igneous rocks, at low water/rock ratios, rather either exclusively magmatie (as favored by Porter and Bipley, 1985) or meteoric in origin.

Spatial and genetic relationships between porphyry molybdenum and gold mineralization

The spatial and genetic relationship between alkaline intrusive rocks, porphyry molybdenum mineralization, and gold deposits in the Great Falls tectonic zone has been described briefly by Foster and Childs (1993). Examples of porphyry molybdenum deposits include Big Ben, Cannivan, ¾Vhite Cloud, and Thompson Creek (see Armstrong et al., 1978) whereas gold mineralization is known at Gies, Spotted Horse, Kendall, and Landusky-Zortman. The Golden Sunlight porphyry system appears to represent a transitional environment between the deeper molybdenum porphyry systems and the shallower alkaline-related gold systems. The association between gold and molybdenite in the Great Falls tectonic zone is not unique to the Golden Sunlight system because molyb- denitc occurs in gold ores at the Landusky-Zortman (Russell, 1991) and Beal deposits (Hastings and Harrold, 1988), and elevated Mo contents are found in the Mayflower deposit (Cocker, 1993). Furthermore, gold also occurs, for example, in the peripheral zone of porphyry systems at Butte (Meyer et al., 1968) and Big Ben (Olmore, 1991).

The spatial relationship between porphyry Cu-Mo systems and gold mineralization has been described by Sillitoe (1988). However, the spatial relationship between igneous-related molybdenum-bearing, Cu-poor systems and gold is less well documented, but includes Kidston, Australia (Mustard, 1986; Baker and Andrew, 1991); Prasolovskoye, Kuril island arc (So et al., 1995); Fort Knox, Alaska (Bakke, 1996); Aylwin Creek, Canada (Heather and Hodgson, 1984); Gilt Edge and Tertiary veins in the Homestake deposit, South Dakota (Paterson, 1990); Central City, Colorado (Rice et al., 1985); Boulder district, Colorado (Saunders, 1991); Merapi volcano, Indo- nesia (Kavalieris, 1994), and possibly Malala, Indonesia (Sil- litoe, 1994); as well as the Hemlo deposit, Canada (Kuhns et al., 1994). Petrological, mineralogical, and paragenetic similarities exist among the Golden Sunlight, Central City, and South Dakota deposits. In the Central City area, molyb- denurn, uranium, precious and base metal, and telluride mineralization was thought by Rice et al. (1985) to represent the upper part of a Laramide alkaline porphyry molybdenum

Att-Ag TELLURIDE MINERALIZATION. GOLDEN SUNLIGHT DEPOSIT, MT 523

system. The inferred parent magma is a quartz bostonite or alkali rhyolite •vhieh compositionally resembles the latite porphyry associated with the Mineral Hill porphyry system. A molybdenum porphyry preceded a late fluorite-bearing quartz-pyrite-telluride-molybdenite stage. Furthermore, the Central City area exhibits a regional distribution of mineral zonation similar to that found in the Golden Sunlight area.

Paterson (1990) described the geologic and geochemical characteristics of igneous-, schist-, and sediment-hosted precious metal deposits in the northern Black Hills, South Da- kota. Igneous rocks are lithologieally variable but have alkalie affinities. At the Gilt Edge deposit, ore occurs in traehytes and in breeeias along the margins of quartz traehyte stocks. Groff (1990) described a vertical zonation of a deep Au-Mo- W-Ag association to a shallo•v Pb-Zn-Ag-As association. A vertical and lateral mineralogieal association of Au-Mo-Bi- (Cu-Zn-Pb-Ag) to Cu-Zn-Pb-Ag •vas also reported for precious metal veins around Tertiary intrusions by Uzunlar et al. (1990). Note that bismuth tellurides and sulfosalts are also associated with ealaverite in stage I mineralization at the Golden Sunlight deposit.

Genetic Model for Mineralization

The Golden Sunlight ore system is one of several porphyry molybdenum-eopper or alkaline igneous-related gold systems spatially associated with the Great Falls tectonic zone (Foster and Chadwick, 1990; Foster and Childs, 1993). Whereas O'Neill and Lopez (1985) proposed that the Great Falls tee- tonic zone may form the contact between Precambrian crustal terranes, Mutsehler et al. (1991) suggested that it may have been a transtensional zone or a releasing bend between transcurrent faults during the Cretaceous. Mutsehler et al. 'also proposed that a passive mantle hot spot formed where the Great Falls tectonic zone tapped alkaline mantle melts. If Foster and Childs (1993) are correct in supposing that alkaline magmatism extended from the central Montana alkalie province to south•vest Montana adjacent to the Golden Sunlight deposit, the period of alkaline magmatie activity in Montana may have commenced as early as 80 Ma. A genetic relationship among latite porphyries, the Golden Sunlight ore system, and a crustal extension related to the Great Falls tectonic zone is indicated by the prominent northeast-trending, steeply dipping, auri{brous veins and i•tults, in and adjacent to the open pit, that parallel the general northeast structural trend associated with the Great Falls tectonic zone, as well as by the orientation of a conjugate set of northeast- and northsvest-trending high-angie fractures.

Although latite porphyry is erosscut by the Mineral Hill breeeia pipe, it is important to note that the breeeia pipe is part of the Mineral Hill porphyry system. Various mechanisms that account for the formation of collapse-style breeeiation are summarized in Sillitoe (1985). Mechanisms for the formation of the Mineral Hill breeeia pipe include breeeiation caused either by exsolution of a fluid-vapor phase from an alkalie magma during decompression (Porter and Ripley, 1985) or during magma withdrawal as the alkalie magma cooled. Ho•vever, regardless of the precise mechanism, geologic evidence suggests that breeeia pipe formation •vas genetically related to intermittent dextral strike-slip movement •vhieh •vas operating in the Golden Sunlight area during the Late Cretaceous. It is envisaged that such intermittent move-

ment allo,ved deep tapping of alkaline magmas, provided space for intrusion, and extension for collapse brecciation. Subsequent to this movement, a fluid-vapor phase was ex- solved from the crystallizing magma and was accompanied by precipitation of nmltiple stages of sulfide-telluride mineralization. Recent diamond drilling also indicates significant tectonic milling of breccia clasts in addition to simple collapse. The breccia pipe initially fractured in place without significant movement of clasts, then collapsed (Foster et al., 1993). This is evident from the preservation of a relic stratigraphy in the pipe (Fig. 3). The presence of latite porphyry clasts vertically 'along the margins of the breccia pipe suggests that overlying wall rocks caved into a slightly molten pluton. Various stages of mixing of latite porphyry fragments with still plastic latite porphyry magma is indicated by the presence of latite intrusion breccia and xenolithic latite porphyry. For example, weakly xenolithic latite porphyry can grade into highly xenolithic latite porphyry (>50% clasts) and finally into intrusion breccia with clasts of sedimentary rocks from the LaHood Formation and latite porphyry matrix. Although the role of meteoric water mixing with magmatic vapor as a potential breccia-fonning process in the Golden Sunlight mine area remains unclear, it should be emphasized that fluid inclusion, hydrogen, and oxygen isotope studies are permis- sive of addition of a meteoric water component to the breccia pipe-hosted auriferous veins.

Potassic alteration and silicification is associated with mo-

lybdenite mineralization, whereas sericitic alteration is associated with late-stage veining in the Mineral Hill breccia pipe. Phyllic and propylitic alteration commonly related to other porphyry molybdenite mineralization in the Great Falls tee- tonic zone (e.g., Big Ben; Olinore, 1991) is apparently not associated with the Miner'o] Hill porphyry system. There are several possible explanations for the limited development of alteration minerals much beyond the breeeia pipe:

1. The Proterozoie sedimentary rocks may not have fractured xvell, and, thus, •vere not conducive to the development of a strong plumbing system.

2. The Proterozoie silielastie rocks •vere nonreactive.

3. The lo•v-grade Mineral Hill porphyry system may be a failed system that did not develop stockworks and alteration zones normally related to typical alkalie intrusive-related, ore- grade porphyry molybdenum systems.

Sulfur, oxygen, hydrogen, and fluid inclusion studies are consistent with a magmatic component to the auriferous ore fluids. Fluid inclusion studies suggest that porphyry molybde- hum mineralization was precipitated from near-neutral, intermittently boiling fluids with highly variable salinities (1->50 vet % NaC1 equiv), at temperatures between 160 ø and 400øC, with a population at -230øC. Phase separation promoted the presence of localized high-salinity ore fluids with minor CO2. Overlapping homogenization temperatures and salinities of porphyry molybdenum and early stage I breccia-hosted mineralization indicate a hydrothermal continuum between these mineralizing events and further emphasize the concept that the auriferous gold veins formed as part of the Mineral Hill porphyry system. Although fluid inclusion studies show that fluids •vere dominated by nonboiling, CO2-poor fluids, oxygen and hydrogen isotope studies suggest a contribution of mete- orie ,vater in the earliest stage of formation of the breeeia

524 sPRY ET AL.

pipe-hosted auriferous vein system. Such a magmatic to meteoric evolution has been described, in a general sense, for magmatic porphyry Cu and Mo-Au systems by Sillitoe (1988) and is described in detail for the Porgera gold telluride deposit by Richards (1992) and Richards and Kerrich (1993).

It can be speculated that a source of metals for the Mineral Hill porphyry system may be the sulfides in Proterozoic La- Hood Formation and Bull Mountain group which are en- riched in various metals including Au, Mo, Cu, and Te. A possible scenario is that the enrichment in metals in Protero- zoic sediments at the Golden Sunlight mine may be related to exhalations from nearby sea-floor vents in a manner similar to that envisioned by Poole (1988) for turbiditc-hosted stratiform barite deposits which contain anomalous amounts of As, Pb, Mn, Mo, and Zn. This exhalative event is considered to have been in response to tectonic activity associated with movement along the Great Falls tectonic zone. An alternative cause for the elevated concentrations of various metals in the

Proterozoic sediments in the Golden Sunlight area is due to fine-scale epigenetic replacement of Proterozoic pyrite- bearing horizons by Late Cretaceous hydrothermal fluids associated with the Mineral Hill porphyry-forming event. Note that a palinspastic reconstruction prior to Laramide movement along the Golden Sunlight fault suggests that the quartz monzodiorite intrusion (presently 1 km northeast of the breccia pipe) may have been located almost directly beneath the breccia pipe. The genetic relationship between alkaline intrusive rocks and gold-silver telluride mineralization has been described elsewhere in Montana (e.g., Zortman-Landusky, Russell, 1991; Gies, Zhang and Spry, 1994b). However, until elemental and mineralogical studies of sulfides in Proterozoic sedimentary rocks are undertaken farther away from the center of the Mineral Hill porphyry system and the effects of the Late Cretaceous mineralizing event (i.e., beyond the distal Mn mineralization), the syngenetic versus epigenetic nature of the sulfides will remain in doubt.

One of the best geologic analogues to the Mineral Hill breccia pipe is the Carache Canyon breccia pipe in the Ortiz Mountains, New Mexico, which has been described by Coles (1990). Although mineralogical differences exist between the two breccia pipes (e.g., molybdenite and bismuth minerals are absent in the Carache Canyon breccia pipe), the geologic setting, nature and localization of precious metal mineralization, breccia pipe-forming process, and geochemical conditions of ore formation at the two localities bear remarkable resemblance to each other. The similarities include: (1) the localization of both breccia pipes along northeast-trending faults that have been active since the Proterozoic; (2) the spatial relationship of both breccia pipes to Tertiary or Creta- ceous calc-alkaline to alkaline intrusions; (3) the presence of crackle breccia near the top of both breccia pipes with more rotated clasts at depth; (4) the retainment of relict stratigraphy near the center of the breccia pipes that has collapsed downward over 100 m, with poor sorting in marginal zones; (5) gold mineralization that consists of veins, stockworks, and disseminations in and adjacent to the breccia pipes; (6) the probable formation of the breccia pipes as a result of collapse due to the release of magmatic volatiles or magma withdrawal; and (7) the generation of early magmatically derived ore- forming fluids in both breccia pipes that were saline, COs- bearing, and intermittently boiling, followed by cooler, less

saline fluids that had a probable meteoric water input (see Coles, 1990).

Summary

A list of events, given in approximate geochronological order, that were ilnportant in the formation of mineralization in the Golden Sunlight is given below:

1. Proterozoic sedimentary rocks were deposited in a pro- gradational basin plain-submarine fan-slope complex. This complex contained syngenetic pyritic sediments which may have contributed metals and sulfur to the Late Cretaceous mineralization.

2. During the Late Cretaceous, pre-Laramide dextral strike-slip faulting produced a transtensional environment along the Great Falls tectonic zone. Alkaline to subalkaline magmas were emplaced at depth resulting in the formation of shallow latite porphyry and quartz monzodiorite intrusions. Decompression of the magma chamber resulted in the formation of the regional Mineral Hill porphyry system with related mineral zonation and molybdenite mineralization from magmatic fluids. The molybdenite mineralization formed pre- dominantly in quartz-pyrite-K feldspar veins but locally re- placed sulfide-rich Proterozoic sedimentary rocks.

3. Collapsing of the magma chamber due to magmatic vapor exsolution or magma withdrawal resulted in the formation of the Mineral Hill breccia pipe and related stages of dominantly northeast-trending auriferous veins and localized replacement of sulfide-rich Proterozoic sedimentary rocks. The ore-forming fluid had most likely evolved from a magmatic-dominated system to one that contained a meteoric component.

4. At approximately 79 Ma, unaltered lamprophyres, rep- resenting deep subcrustal (?) tapping by the Great Falls tectonic zone, were emplaced along bedding planes, fractures, faults, and veins.

5. During the Late Cretaceous or Tertiary, block faulting tilted the vertically emplaced Mineral Hill breccia pipe approximately 30 ø to the northeast. Subsequent low-angle dextral strike-slip faulting resulted in the formation of the Corri- dor fault and further displaced the breccia pipe to the east.

Acknowledgments

Bruce Cox and Glenn Johnson are thanked for discussing various aspects of the geology. Preliminary versions of this manuscript were improved by comments and suggestions of Ed Ripley and Scott Thieben. We thank two Economic Geol- ogy referees for their incisive and constructive reviews. This project was financially supported by Golden Sunlight Mines, Inc., and the Iowa State Mining and Mineral Resources Re- search Institute's program administered by the U.S. Bureau of Mines under allotment grants Gl1œ4119 and Gl134119.

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Golden Sunlight geology, Montana