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Alunite in the Pascua-Lama High-Sulfidation Deposit: Constraints on Alteration and Ore Deposition Using Stable Isotope Geochemistry

C. L. Deyell

Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia

R. Leonardson

Barrick Gold Exploration, 293 Spruce Road, Elko, Nevada 89801

R. O. Rye

U.S. Geological Survey, Mail Stop 963, Denver Federal Center, Denver Colorado 80225

J. F. H. Thompson

Teck-Cominco, 200 Burrard Street, Vancouver, British Columbia, Canada V6C 3L9

T. Bissig

Universidad Católica del Norte, Depto. Ciencias Geológicas, Av. Angamos 0610, Antofagasta, Chile

D. R. Cooke

Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania, 7001, Australia

Corresponding author: e-mail, cdeyell{at}utas.edu.au

	Abstract

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

 
The Pascua-Lama high-sulfidation system, located in the El Indio-Pascua belt of Chile and Argentina, contains over 16 million ounces (Moz) Au and 585 Moz Ag. The deposit is hosted primarily in granite rocks of Triassic age with mineralization occurring in several discrete Miocene-age phreatomagmatic breccias and related fracture networks. The largest of these areas is Brecha Central, which is dominated by a mineralizing assemblage of alunite-pyrite-enargite and precious metals. Several stages of hydrothermal alteration related to mineralization are recognized, including all types of alunite-bearing advanced argillic assemblages (magmatic-hydrothermal, steam-heated, magmatic steam, and supergene). The occurrence of alunite throughout the paragenesis of this epithermal system is unusual and detailed radiometric, mineralogical, and stable isotope studies provide constraints on the timing and nature of alteration and mineralization of the alunite-pyrite-enargite assemblage in the deposit.

Early (preore) alteration occurred prior to ca. 9 Ma and consists of intense silicic and advanced argillic assemblages with peripheral argillic and widespread propylitic zones. Alunite of this stage occurs as fine intergrowths of alunite-quartz ± kaolinite, dickite, and pyrophyllite that selectively replaced feldspars in the host rock. Stable isotope systematics suggest a magmatic-hydrothermal origin with a dominantly magmatic fluid source. Alunite is coeval with the main stage of Au-Ag-Cu mineralization (alunite-pyrite-enargite assemblage ore), which has been dated at approximately 8.8 Ma. Ore-stage alunite has an isotopic signature similar to preore alunite, and ³⁴S_alun-py data indicate depositional temperatures of 245° to 305°C. The D and ¹⁸O data exclude significant involvement of meteoric water during mineralization and indicate that the assemblage formed from H₂S-dominated magmatic fluids. Thick steam-heated alteration zones are preserved at the highest elevations in the deposit and probably formed from oxidation of H₂S during boiling of the magmatic ore fluids. Coarsely crystalline magmatic steam alunite (8.4 Ma) is restricted to the near-surface portion of Brecha Central. Postmineral alunite ± jarosite were previously interpreted to be supergene crosscutting veins and overgrowths, although stable isotope data suggest a mixed magmatic-meteoric origin for this late-stage alteration. Only late jarosite veinlets (8.0 Ma) associated with fine-grained pseudocubic alunite have a supergene isotopic signature.

The predominance of magmatic fluids recorded throughout the paragenesis of the Pascua system is atypical for high-sulfidation deposits, which typically involve significant meteoric water in near-surface and peripheral alteration and, in some systems, even ore deposition. At Pascua, the strong magmatic signature of both alteration and main-stage (alunite-pyrite-enargite assemblage) ore is attributed to limited availability of meteoric fluids. This is in agreement with published data for the El Indio-Pascua belt, indicating an event of uplift and subsequent pediment incision, as well as a transition from semiarid to arid climatic conditions, during the formation of the deposit in the mid to late Miocene.

	Introduction

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

 
THE PASCUA-LAMA Au-Ag-Cu deposit is situated in the Cordillera Principal of Chile and Argentina at the north end of the El Indio-Pascua belt (Fig. 1). Widespread zones of hydrothermal alteration occur throughout the district and are visible as distinct color anomalies and topographic features at elevations of 4,500 to 5,100 m above sea level (asl). Recognition of these zones led to the discovery of high-sulfidation gold mineralization at Nevada (now Esperanza) in the Chilean region in 1977 by Companía Minera San José S.A. The property was explored by a number of different operators, including Lac Minerals Ltd., until the acquisition of this company by Barrick Gold Corporation in 1994. Continued exploration by Barrick Gold has uncovered several zones of both sulfide-rich and oxidized mineralization to the east of the original discovery and across the international border into Argentina. Estimated reserves for the deposit total 16.8 million ounces (Moz) Au and 585 Moz Ag.

View larger version (16K): [in this window] [in a new window]   FIG. 1. Location of the El Indio-Pascua belt relative to other major mineral districts in the south-central Andes (modified from Bissig et al., 2002) and the flat subduction zone (Barazangi and Isacks, 1976).

View larger version (38K): [in this window] [in a new window]   FIG. 2. Simplified geology and major faults of the El Indio-Pascua belt, showing locations of major ore deposits in the region. Geologic information is taken from Martin et al. (1995) and Ramos et al. (1989), as summarized in Bissig et al. (2002). The upper Miocene Pascua Formation is not shown due to its restricted occurrence (see text). BdTF = Baños del Toro fault.

The coprecipitation of alunite with precious metal-bearing sulfides is an unusual feature of the Pascua-Lama system. In most high-sulfidation deposits, advanced argillic alteration and alunite deposition clearly precede mineralization, regardless of how the ore was emplaced (e.g., Stoffregen, 1987; Rye 1993; Cooke and Simmons, 2000, and references therein). The occurrence of significant alunite as gangue to mineralization is seldom documented, with the exception of epithermal veins at la Mejicana, Nevados del Famatina district, Argentina (Losada-Calderon and McPhail, 1996), late alunite-enargite veins at El Indio (Jannas et al., 1999), and breccia- and vein-hosted mineralization at Tambo (Jannas et al., 1999; Deyell et al., in press). In this study, we describe the occurrence of alunite-sulfide-gold mineralization at Pascua-Lama and associated advanced argillic alteration assemblages. Stable isotope data and ⁴⁰Ar-³⁹Ar alunite ages are used to constrain the origin of both alteration and ore fluids and detail the multistage evolution of this richly mineralized epithermal system.

	Geology of the Pascua-Lama Region

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

 
Regional framework
The Pascua-Lama Au-Cu deposit is located at the northern end of the El Indio-Pascua belt in the Central Andean Cordillera Principal, straddling the Chile-Argentina border (Fig. 1). The belt occurs in the middle of the flat-slab segment of the Andes between two other well-known mineralized areas: the Maricunga porphyry Au and high-sulfidation Au-Ag belt to the north (26°–28° S) and the Los Pelambres-El Teniente porphyry Cu-Mo belt (32°–34° S) to the south (e.g., Kay et al., 1999). In addition to Pascua-Lama, the El Indio-Pascua belt hosts widespread zones of hydrothermal alteration and several world-class epithermal deposits and prospects (Fig. 2), including El Indio, which has recently closed, and Veladero, with estimated resources of 9.4 Moz Au. Mineralized systems in the El Indio-Pascua belt formed in the late Miocene with all significant high-sulfidation mineralization occurring between 6.2 and 9.4 Ma, although only minor associated magmatism is recognized (Bissig et al., 2001).

The El Indio-Pascua belt occurs within a north-south–striking tectonic depression bound by the steeply dipping, east-verging, high-angle reverse Baños del Toro-Chollay fault to the west (Fig. 2) and opposing west-verging structures (informally named the El Indio high-angle reverse fault zone) farther to the east. Within this block, an Upper Paleozoic to Lower Jurassic basement composed predominantly of calc-alkaline felsic intrusions and volcanic rocks (Martin et al., 1999) is overlain by up to 1,500 m of Tertiary subaerial volcanic strata (Fig. 2). The latter are extensively preserved in the southern part of the belt, where the Tambo and El Indio mines are located, but only limited exposures are preserved in the northern extremity of the district, near Pascua-Lama and Veladero (Maksaev et al., 1984; Martin et al., 1995). The Tertiary volcanic sequence comprises predominantly volcanic and volcaniclastic rocks of the Tilito Formation (27–23 Ma), the Escabroso Group (21–17.5 Ma), and the 17 to 14 Ma Cerro de las Tortolas Formation (Martin et al., 1995; Bissig et al., 2001). Magmatism decreased markedly after the deposition of these units. Isolated dacitic tuffs of the Vacas Heladas Formation erupted between 12.7 and 11 Ma. A single 7.84 Ma dacite dike, the only igneous rock coeval with the mineralization in the district, is reported from Pascua (Bissig et al., 2001). The Vallecito Formation (6.2–5.5 Ma) is only exposed in the Valle de Cura region (Ramos et al., 1989) and the El Indio-Tambo area. Volcanism ceased in the upper Pliocene after the eruption of the 2 Ma Cerro de Vidrio rhyolite dome in the northeast part of the belt (Bissig et al., 2002a).

Local geology
The geology of the immediate Pascua area (Fig. 3) is dominated by large volumes of Mesozoic granitic rocks that include fine- to coarse-grained porphyritic granite and small stocks of dacite porphyry and granodiorite. These granitic units have been dated at approximately 190 Ma (Bissig et al., 2001) and may be related to the regionally extensive and slightly older (212 Ma) Colorado Group intrusive rocks identified by Martin et al. (1999). Crystal and lithic ash flow tuffs of probable Lower Permian age (265 Ma; Martin et al., 1995) are the oldest exposed rocks in the region and occur on the western edge of the Esperanza sector and the southern and southeastern margins of the property. Small leucocratic diorite stocks and a larger dacite porphyry of Tertiary age, as well as various small younger dikes of andesitic to rhyolitic composition (not shown), have intruded the older intrusive and extrusive units.

View larger version (39K): [in this window] [in a new window]   FIG. 3. Generalized geology of the Pascua-Lama deposit (based on regional mapping by Barrick geologists and data presented in Bissig et al. (2001) and Chouinard (2003); see Figure 2 for location. Mesozoic granitic rocks (the Pascua-Lama Complex) include several bodies of similar composition, ranging from porphyritic granite to crowded granite porphyry. Several generations of Tertiary hydrothermal breccias have been grouped together and include Brecha Central, the largest breccia body in the region. Also shown are the trace of the Alex Tunnel (4,680 m asl) and line of section CA-00 (section A-A') illustrated in Figure 5. Locations of 40Ar/39Ar samples described in the text and Table 2 are given (projected to surface; abbreviations as given in Table 1).

View larger version (57K): [in this window] [in a new window]   FIG. 5. Pascua-Lama property map showing the simplified distribution of alteration zones at surface (using unpub. data from D. Heberlein, 1999).

	Mineralization

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

 
Several styles of Au (+ Ag-Cu) mineralization are recognized at Pascua. The greatest volume of gold mineralization is contained within an assemblage of alunite, pyrite, and enargite that is concentrated in the Brecha Central area. Other styles of mineralization, described in detail by Chouinard (2003), include either pyrite, pyrite-szomolnokite (FeSO₄ • H₂O), or native gold-dominated assemblages, which typically occur above and lateral to alunite-pyrite-enargite ore. High-grade Ag-rich mineralization also occurs in the upper parts of the deposit (>4,700 m) but formed late and overprinted the main-stage Au-Ag-Cu ore (Chouinard, 2003). Although these peripheral mineralized zones contain significant metal accumulations, this paper focuses exclusively on alunite-pyrite-enargite–type mineralization, where the precious metal deposition is directly associated with coeval alunite.

Alunite-pyrite-enargite mineralization
Alunite-pyrite-enargite mineralization occurs as disseminations and open-space filling in the Brecha Central pipe and as banded alunite-sulfide veins (Fig. 4A). Textural evidence from mineral intergrowths and inclusions indicates that alunite was deposited synchronously with the sulfides and precious metals. Gold and silver are hosted primarily in enargite and pyrite, predominantly bound within the sulfide crystal lattices (Chouinard, 2003) but also as inclusions of native gold, calaverite (AuTe₂), and minor electrum. Alunite is intimately intergrown with, and locally host to, precious metal-bearing sulfides (Fig. 4B) and locally contains native gold. Accessory phases in the alunite-pyrite-enargite assemblage include quartz, diaspore, anglesite, pyrophyllite, stibnite, cassiterite, goldfieldite, covellite, galena, and trace chalcopyrite. Native sulfur was also deposited locally with the alunite-pyrite-enargite assemblage although most is paragenetically late and was precipitated near the end of the mineralizing event (Chouinard, 2003).

View larger version (115K): [in this window] [in a new window]   FIG. 4. Examples of alunite-pyrite-enargite mineralization. A. Banded veins in the stockwork zone surrounding Brecha Central. Alunite (white bands) alternates with pyrite-enargite (dark bands). B. Backscattered electron image showing alunite intergrown with pyrite and enargite. The latter hosts inclusions of calaverite (AuTe2). DDH-111, 200.5m.

	Alteration

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

At a broad scale, intense silicic and advanced argillic alteration zones are associated with major breccias and mineralized centers (Fig. 5). Silicic alteration is dominated by patchy vuggy silica zones, generally above about 4,750 m asl, consisting of fine-grained secondary quartz, rare primary quartz phenocrysts, and relict rutile grains (Chouinard, 2003). Local silicification is also evident as fine-grained granular quartz that occurs as veinlets, infilling voids and vugs, or as a pervasive replacement of advanced argillic-altered rocks. These zones of intense alteration grade outward to peripheral argillic and propylitic assemblages. Thick blankets of steam-heated advanced argillic alteration (Table 1) are also preserved at upper elevations, particularly above the Brecha Central zone (Fig. 6).

View larger version (49K): [in this window] [in a new window]   FIG. 6. Section CA-00 (see Fig. 3 for location). A. Distribution of major lithological units. B. Distribution of alteration assemblages and Au-grade contours. C. Distribution of phyllosilicates (kaolinite, dickite, pyrophyllite) within the advanced argillic alteration assemblage. Mineral abbreviations: alun = alunite, dick = dickite, ill = illite, jar = jarosite, kao = kaolinite, pyrophyllite (pyl), quartz (qtz).

Preore advanced argillic alteration
The earliest recognized advanced argillic alteration in the Pascua area is characterized by fine-grained intergrowths of alunite ± quartz, kaolinite and/or dickite and/or pyrophyllite, and pyrite. This assemblage is widespread and extends from surface to the maximum depth investigated by drilling (between 5,200- and 4,100-m elev). Alteration formed prior to brecciation at about 9.1 Ma (Bissig et al., 2001) and is superimposed by the main mineralizing event, although no reliable ⁴⁰Ar/³⁹Ar ages have been obtained for this alteration.

Alunite in the preore advanced argillic assemblage occurs as aggregates that replaced feldspar phenocrysts, as disseminations in the wall-rock matrix, and rarely as irregular alunite veinlets and stringers. Only primary quartz phenocrysts and minor rutile remain from the original wall rock. Alunite is the main constituent of the assemblage and comprises 10 to 40 vol percent of the altered rocks. Preore alunite contains rare inclusions of aluminum-phosphate-sulfate minerals (primarily walthierite, svanbergite, woodhouseite, and florencite), pyrite, and diaspore (Fig. 7A). Zunyite also occurs in association with alunite but exclusively at depths below a 4,500-m elevation and only where pyrophyllite is present.

View larger version (161K): [in this window] [in a new window]   FIG. 7. Photographs of alteration minerals. A. Backscattered electron image showing irregular alumino-phospho-sulfate grains (bright zones) in cores of alunite associated with advanced argillic alteration (alun). DDH-116, 289m. B. Steam-heated alteration in drill core sample DDH 119-47m above the Frontera zone. Alunite + quartz replaces feldspar phenocrysts. C. Backscattered electron image of ore-stage alunite containing REE-rich alumino-phospho-sulfate (APS) inclusions (florencite). Sampled from DDH-111, 189.9m. D. Backscattered electron image showing oscillatory PO4 ± Sr-enriched bands (light bands) in coarse-grained magmatic steam alunite (sample PS-26c). E. Backscattered electron image of pseudocubic alunite (gray) with overgrowing jarosite (white) in late-stage vein. F. Backscattered electron image showing supergene alunite, jarosite, and intermediate alunite-jarosite solid solution. Sample PM-33, Alex Tunnel.

Steam-heated alteration
Blanketlike zones of silica minerals (cristobalite, opal, chalcedony) ± kaolinite, alunite, and native sulfur occur at or near the present-day surface at elevations greater than approximately 4,900 m (Fig. 6B). This alteration extends to greater depths along structures, locally crosscutting preore advanced argillic and peripheral argillic assemblages. The tabular, near-surface distribution of this assemblage, in addition to the alteration mineralogy, is consistent with the steam-heated environment (e.g., Schoen et al., 1974). Alteration is typically pervasive, occurring as powdery, friable masses, and original textures are only locally preserved. Alunite is a minor constituent in the surficial blanket assemblage and typically occurs toward the base of the alteration zones. Where present, alunite occurs as extremely fine grained (<5-µm), pseudocubic, tabular, and anhedral crystals in irregular patches and disseminations in veinlets and open spaces (Fig. 7B). Fine-grained jarosite occurs locally as overgrowths on alunite.

The timing of steam-heated alteration is unresolved. Attempts to date alunite from this zone were mostly unsuccessful and only an approximate ⁴⁰Ar/³⁹Ar age of 9.14 ± 1.98 Ma was obtained (Table 2). Textural relationships suggest that steam-heated assemblages formed after preore advanced argillic wall-rock alteration, but these zones are never observed in contact with alunite-pyrite-enargite–stage mineralization. Based on the overlap in age data with this event (Fig. 8), steam-heated alteration may have been, at least in part, coeval with mineralization.

View larger version (23K): [in this window] [in a new window]   FIG. 8. Range of 40Ar/39Ar age data (with 2 error bars) for alunite from the alunite-pyrite-enargite (APE)-stage ore, steam-heated alteration (SH), magmatic steam (MS), late-stage veins (LV), and one sample of supergene jarosite (SP). Also indicated is the approximate minimum age for preore advanced argillic alteration (AA1), based on age dates for advanced argillic alteration in the Frontera and Lama areas from Bissig et al. (2001). The age of the Pascua Formation rhyodacite dike is also reported in Bissig et al. (2001). The cross-hatched area represents the possible age range of APE mineralization, given constraints from preore AA1 alteration and postore MS alunite.

Minor alunite + quartz also occur in restricted alteration halos peripheral to veins hosting alunite-pyrite-enargite mineralization. This is thought to be a wall-rock alteration envelope related to the alunite-pyrite-enargite veins, although the extent of these envelopes is difficult to determine because they grade into the more pervasive preore advanced argillic assemblages. For this reason, we restricted our sampling and analysis to alunite clearly related (as gangue) to alunite-pyrite-enargite ore.

The ⁴⁰Ar/³⁹Ar ages for alunite range from 8.78 ± 0.63 (this study, Table 2) to 8.1 ± 0.1 Ma (Bissig et al., 2001). We consider the 8.78 Ma age to be a better representation of the alunite-pyrite-enargite assemblage because the younger samples reported in Bissig et al. (2001) were taken from a smaller oxidized breccia center that is now interpreted to have formed after the peak of the mineralizing event. However, due to the relatively large analytical error on the reported alunite sample, the age of mineralization can only be constrained to between about 9.1 and 8.2 Ma (Fig. 8).

Magmatic steam alunite
Another style of alunite deposition is recognized in isolated surface exposures of coarse vug- and matrix-filling alunite above Brecha Central. This alunite is medium to coarse grained and clear to golden yellow in color. Electron microprobe data indicate that this alunite is nearly stoichiometric [KAl₃(SO₄)₂(OH)₆] with minor Na and P substitution in oscillatory growth zones (Fig. 7D). Rare inclusions of barite and jarosite are present. The coarse-grained nature of this alunite and lack of coeval sulfides (in addition to stable isotope characteristics described below) are consistent with a magmatic steam origin. The magmatic steam environment was initially defined by Rye et al. (1992) on the basis of data for veins of coarsely crystalline alunite at Marysvale, Utah (Cunningham et al., 1984).

The reported age of 8.38 ± 0.17 Ma (Table 2) indicates that magmatic steam alunite may be younger than the alunite-pyrite-enargite mineralizing event, although the two stages overlap within analytical error.

Late-stage alunite veins
A late stage of alunite occurs as veins, veinlets, and local disseminations throughout the Pascua district. Alunite is typically fine to medium grained (5–20 µm) and occurs as tabular to anhedral crystals locally intermixed with silica. A pseudocubic crystal habit is observed locally in finer grained (<5-µm) alunite (Fig. 7E). Jarosite is also common in the veins, but in all examples it has overprinted alunite. This assemblage occurs as either a fine-grained, granular to powdery pervasive alteration, cryptocrystalline alunite (±jarosite, kaolinite) veins, or as medium- to coarse-grained, euhedral crystals in veins and disseminations. Crosscutting relationships indicate that this stage of alteration postdates both preore advanced argillic alteration and alunite-pyrite-enargite mineralization. Repeated attempts to obtain a reliable ⁴⁰Ar/³⁹Ar age for late-stage alunite were unsuccessful due to insufficient release of argon, although a rough age of 7.97 ± 1.59 Ma is reported (Table 2).

Supergene jarosite (± alunite)
Jarosite is present as a late mineral in association with advanced argillic, silicic, and argillic (quartz-illite) alteration assemblages. It occurs interstitial to alunite, quartz, and illite as fine-grained disseminations, granular crystals in vugs and open spaces, and locally as veinlets that dissect alunite grains or crosscut preore advanced argillic and argillic alteration. The distribution of jarosite is strongly fault and fracture controlled and extends to the maximum depths investigated by drilling (4,100-m elevation). Accessory hematite is present locally. Alunite rarely occurs intergrown with jarosite, in thin veinlets and fine-grained aggregates in the rock matrix. The two cannot be distinguished in thin section, but SEM-EDS analysis shows pseudocubic alunite grains (<5 µm) overgrown by successive bands of jarosite and compositional zones intermediate to alunite-jarosite end members (Fig. 7F). An age of 7.98 ± 0.43 Ma (this study, Table 2) for jarosite of this type is the youngest alteration event recorded in the Pascua-Lama district.

	Stable Isotope Study

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

 
Stable isotope data for alunite and associated alteration minerals are used to characterize the type and source of alteration and alunite-pyrite-enargite mineralizing fluids. Alunite from each of the assemblages described above and two samples of jarosite was analyzed for ³⁴S and a subset of samples was selected for D and ¹⁸O analysis. Sulfur data were also obtained for ore-stage pyrite, enargite, and barite. All analyses were conducted at the U.S. Geological Survey isotope laboratory in Denver. The ³⁴S analyses were carried out by an online method, using an elemental analyzer coupled to a Micromass Optima mass spectrometer, following the method of Giesemann et al. (1994). Hydrogen isotopes were determined by a step-heated and cryogenic technique modified from Godfrey (1962), as described in Wasserman et al. (1992). Oxygen isotope data were collected for alunite (both sulfate and hydroxyl groups) and barite, using the conventional BrF₅ method described in Wasserman et al. (1992). Several samples were also analyzed by pyrolysis with a Finnigan TC/EA coupled to a Finnigan Delta Plus XL mass spectrometer, using continuous-flow methods modified from Kornexl et al. (1999). Details of sample preparation and analytical methods are reported in Deyell (2001). Brief sample descriptions and all isotope data are listed in Table 3.

View larger version (15K): [in this window] [in a new window]   FIG. 9. Range of 34S values (in ) for all stages of Pascua alunite. Also shown are data for associated sulfides, jarosite, and barite. The estimated 34SS ~ 2 to 4 per mil is indicated by the shaded region. Range of 34Salun-py temperatures for preore advanced argillic (AA1) alteration and alunite-pyrite-enargite (APE) mineralization are given, as well as estimated H2S/SO2 ratios (based on 34S values for coexisitng pyrite and alunite). See text for discussion.

Alunite ³⁴S data for the preore advanced argillic- and alunite-pyrite-enargite assemblages are significantly higher than all other stages of alunite and jarosite in the deposit (Fig. 9). The 14 to 30 per mil difference between these alunites and associated sulfides is consistent with sulfate derived from the disproportionation of SO₂ (Rye et al., 1992; Rye, 1993) and is interpreted to reflect a magmatic-hydrothermal origin. The large variation in ³⁴S values for alunite is attributed to deposition over a range of temperatures. These temperatures can be estimated using the isotopic fractionation equation of Ohmoto and Lasaga (1982) for coexisting alunite and pyrite. Data for the one alunite-pyrite pair from a preore advanced argillic sample reported by R. Beane (unpub. report, 1988) give an equilibrium ³⁴S_alun-py temperature of 380°C. Using an average ³⁴S value for pyrite of -4 per mil, depositional temperatures for other preore advanced argillic alunite samples are estimated to be 190° to 350°C. Likewise, equilibrium temperatures calculated for three alunite-pyrite-enargite assemblage alunite-pyrite pairs (Table 4) range from 245° to 305° C. The H₂S/SO₄ ratios for these same pairs average about 2.

View this table: [in this window] [in a new window]   TABLE 4 TABLE 4. 34S Data for Alunite and Coexisting Sulfides from the Alunite-Pyrite-Enargite Ore Stage

Magmatic steam alunite was sampled from coarse-grained, euhedral crystals obtained near surface in the matrix of Brecha Central. This alunite has a ³⁴S value of 2.8 per mil, which is slightly higher than sulfides in the underlying deposit but significantly lower than alunite from either the preore advanced argillic or alunite-pyrite-enargite assemblages (Fig. 9).

The ³⁴S values for late-stage alunite range from 4 to 7 per mil. These values are higher than the ³⁴S values of sulfides in the deposit and suggest that the sulfate was derived from either the oxidation of sulfides or H₂S degassed during the collapse of the hydrothermal system. In either case, limited sulfur isotope exchange between sulfate and sulfide species is inferred based on the positive ³⁴S values for this stage of alteration.

Only two samples of jarosite were separated for analysis. One sample, taken from a series of fine-grained veinlets approximately 300 m below the present-day surface, has the lowest ³⁴S value (–2.5) of all sulfates in the deposit. This value approaches the ³⁴S values for pyrite associated with the alunite-pyrite-enargite assemblage and is consistent with sulfate derived from the oxidation of precursor sulfides. A second sample of jarosite, from veinlets that crosscut preore advanced argillic-altered wall rock, has a ³⁴S value (3.3), similar to that of late-stage alunite.

Oxygen and hydrogen isotopes
Oxygen and hydrogen isotope compositions for each stage of alteration are given in Figures 10 through 12. Fluid compositions in equilibrium with alunite of each stage are calculated from the fractionation equations of Stoffregen et al. (1994) over a range of temperatures, as specified. These temperatures are estimated from ³⁴S_alun-py values or ¹⁸O_SO4-OHisotope fractionations for each stage, where available. The D of local meteoric water is estimated to have been –110 ± 10 per mil based on paleoelevation models (B. Taylor, pers. commun.) and previous studies of the El Indio-Pascua belt (Jannas et al., 1999; Deyell, 2001).

View larger version (20K): [in this window] [in a new window]   FIG. 10. D, 18OSO4, and 18OOH values for alunite in the preore advanced argillic assemblage (AA1). Fluid compositions (DH2O and 18OH2O) in equilibrium with alunite are calculated from equations of Stoffregen et al. (1994) at 200° to 380°C (based on range of 34Salun-py for AA1 alteration). Reference lines and fields shown include: meteoric water line (Craig, 1961), kaolinite line (Savin and Epstein, 1970), typical fluids dissolved in felsic magmas (Taylor, 1988), the range of water compositions discharged from high-temperature fumaroles (volcanic vapor; Giggenbach, 1992), and the composition of paleometeoric waters in the El Indio-Pascua belt (estimated from paleotopography; B. Taylor, pers. commun., 2001).

View larger version (42K): [in this window] [in a new window]   FIG. 12. D, 18O SO4, and 18OOH values for magmatic steam alunite (MS), late vein alunite (LV), and jarosite (jar). The range of DH2O and 18OH2O for fluids in equilibrium with each sample (bars and oval) are calculated from equations of Stoffregen et al. (1994) and Rye and Stoffregen (1995). Temperatures used for calculations (LV = 50°–100°C; Jar = 25°–50°C) are derived from 18OSO4-OH data. The range of temperatures for MS alunite (150°–200°C) is based on estimates of depositional temperatures related to magmatic steam processes (Rye et al., 1992; Rye, 1993). Reference lines and fields as given in Figure 10 together with the additional supergene alunite sulfate field (SASF) from Rye et al. (1992), and the supergene jarosite OH zone (SJOZ) and supergene jarosite sulfate field (SJSF) as described in Rye and Alpers (1997).

View larger version (25K): [in this window] [in a new window]   FIG. 11. D, 18OSO4, and 18OOH values for alunite from alunite-pyrite-enargite (APE)-stage ore and steam-heated (SH) alunite. The range of calculated DH2O and 18OH2O for fluids in equilibrium with APE and SH alunite (calculated from equations of Stoffregen et al., 1994) are shown as separate fields. Temperatures used for calculations (SH = 60°–140°C; APE = 200°–340°C) are derived from 34Salun-py and 18OSO4-OH isotope data. Reference lines and fields as given in Figure 9.

Steam-heated alunite: Fluid compositions in equilibrium with steam-heated alunite (Fig. 11) were calculated over a temperature range of 60° to 140°C. These temperatures are based on ¹⁸O_SO4-OH data for three steam-heated alunite samples, which correspond to 60°, 140°, and 270°C. The lower temperatures are reasonable for the steam-heated environment, in which aqueous sulfate forms at shallow levels from the oxidation of H₂S by atmospheric oxygen (Rye et al., 1992; Ebert and Rye, 1997), and suggest that oxygen isotope equilibrium was obtained between aqueous sulfate and water in this fluid. The third sample gives an unreasonably high ¹⁸O_SO4-OH temperature, given the shallow depth of formation, although ¹⁸O_OH data are in the range of the previous two samples. These data indicate that significant sulfur isotope exchange occurred between aqueous SO₄ and H₂S, but ¹⁸O_SO4-OH may have been affected by postdepositional exchange for this sample. Calculated fluid compositions for steam-heated alunite, as shown in Figure 11, have D_H2O values of –38 to –54 per mil and ¹⁸O_H2O values between –5 and +4 per mil. These values overlap with those calculated for ore-stage alunite and are significantly higher than those for meteoric water.

Magmatic steam alunite: Depositional temperatures for alunite of this stage are poorly constrained. A calculated ¹⁸O_SO4-OH temperature of 110°C is lower than expected for magmatic steam alunite, given the coarse-grained nature of the sample and its inferred magmatic source, and may indicate equilibration with a low D_H2O fluid. Similar processes have been inferred for magmatic steam alunite from Marysvale, Utah (Rye et al., 1992), and Tambo, Chile (Deyell et al., in press). Calculation of fluid compositions in equilibrium with this alunite was therefore made over a range of assumed temperatures from 150° to 250°C. These fluid compositions (Fig. 12) overlap with those for preore advanced argillic alteration and alunite-pyrite-enargite mineralization and indicate a major magmatic component in the source fluid. Combined with a low ³⁴S value, lack of coeval sulfides, and the coarse-grained nature of the alunite, these data are consistent with a magmatic steam origin (cf. Rye et al., 1992). Alunite of this type is considered to have ³⁴S values close to that of the bulk sulfur in the magma, based on relationships determined by Cunningham et al. (1984) and Rye et al. (1992) for alunite at Marysvale, Utah.

Late-stage alunite veins: Data for late-stage alunite are plotted in Figure 12, which includes reference fields for supergene alunite and jarosite as given in Rye et al. (1992) and Rye and Alpers (1997), respectively. Late-stage alunite ¹⁸O_OH values range from 5.5 to 13.5 per mil, and ¹⁸O_SO4 values range from 9.7 to 20.7 per mil. Two of the three ¹⁸O_SO4 values fall outside of the supergene alunite sulfate field and have large positive ¹⁸O_SO4-OH values that are not consistent with a supergene origin (Rye et al., 1992).

Calculated D_H2O values for late-stage alunite range between –35 and –75 per mil, with ¹⁸O_H2O less than 0 per mil. The data fall on a trend that projects, at low D values, close to the meteoric water line, and suggests mixing of magmatic fluids with varying amounts of meteoric water.

Late jarosite: Oxygen isotope data are available only for one sample of jarosite. The ¹⁸O_OH data for this jarosite (Fig. 12B) is slightly heavier than the supergene jarosite OH ("SJOH") zone as defined by Rye and Alpers (1997), but ¹⁸O_SO4 data (4.5) fall within the supergene jarosite sulfate field ("SJSF," Rye and Alpers, 1997). These values may indicate limited OH exchange with low pH water (Rye and Alpers, 1997) or formation from partly exchanged meteoric water. The composition of fluid calculated in equilibrium with jarosite is close to the predicted composition of meteoric water and is consistent with formation in the supergene environment.

	Constraints on Alteration and Ore Deposition

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

 
The Pascua deposit is a large, complex system that formed from episodic magmatic-hydrothermal activity recorded over a period of about 1 Myr. The occurrence of alunite throughout the paragenesis of this deposit is atypical of most high-sulfidation systems, where advanced argillic alteration and alunite deposition clearly precede mineralization (e.g., Stoffregen, 1987; Rye 1993; Cooke and Simmons, 2000, and references therein). Constraints on the evolution of this system from preore alteration through main-stage ore deposition to postore processes can be deduced from stable isotope systematics for alunite, in combination with mineralogical data and ⁴⁰Ar/³⁹Ar dating (Fig. 13).

View larger version (35K): [in this window] [in a new window]   FIG. 13. Schematic section showing the evolution of the Pascua deposit and associated alteration assemblages over time. A. ~9.1 to 8.8 Ma: widespread, early hydrothermal alteration in the greater Pascua region created patchy vuggy silica (silicic) alteration zones and widespread advanced argillic assemblages (AA1) grading outward to argillic and propylitic (not shown) assemblages. Sericite at depth is inferred from old core logs and company reports. Minor steam-heated alteration zones may have developed at or near surface at this time. B. 8.8 to ~8.4 Ma: main-stage Au-Ag-Cu mineralization followed brecciation in the Brecha Central area and in several smaller satellite breccia bodies. Alunite-pyrite-enargite (APE) mineralization was deposited in open spaces in the breccia matrices and surrounding vein networks. Upwelling magmatic fluids (condensed magmatic vapors) overwhelmed and displaced meteoric water. Ore deposition was coincident with lowering of the water table due to regional uplift and erosion. C. 8.4 to 7.9 Ma: postore alteration and late-stage processes coincided with the waning stages of the magmatic-hydrothermal system. Pulses of magmatic vapor deposited magmatic steam alunite near surface above Brecha Central. Late-stage alunite ± jarosite formed thin veinlets and disseminations from mixed magmatic-meteoric fluids down to depths of ~600 m below the present-day surface. Isolated disseminations and veinlets of supergene jarosite ± alunite (not shown) formed from cooler meteoric waters that penetrated back into the system.

Steam-heated alteration
Steam-heated alunite forms from aqueous sulfate created by the oxidation of H₂S at or above the water table (Schoen et al., 1974). The fluids responsible for this alunite typically have a meteoric isotopic signature (e.g., Rye et al., 1992; Ebert and Rye, 1997), however steam-heated alunite at Pascua has D_H2O values up to 70 per mil higher than those of local meteoric water. Deuterium enrichments are reported in steam-heated fluids from active geothermal systems, due to the effects of subsurface boiling and/or evaporation (Truesdell et al., 1977; Henley and Stewart, 1983), but these are typically much smaller (10–20). This enrichment is typically on the order of 10 to 20 per mil, however, and is much less than that recognized at Pascua. The Pascua data, therefore, cannot be attributed to boiling or evaporation alone, and we interpret these results to indicate that there was little meteoric water involved in steam-heated alteration. In fact, much of the ground water seems to be composed of condensed magmatic fluids that effectively displaced meteoric ground water.

The timing of steam-heated alteration above Brecha Central cannot be constrained by ⁴⁰Ar/³⁹Ar methods due to the large overlap in analytical errors (see Fig. 8) but may have been coeval with either the development of preore advanced argillic alteration or the main ore stage. We suggest the latter is more likely because only during the main ore stage would renewed heat and fluid input, combined with decompression related to the eruption of multiple breccias, have caused boiling and release of sufficient volatiles to account for such widespread alteration in the steam-heated zone.

Alunite-pyrite-enargite assemblage mineralization
Deposition of the alunite-pyrite-enargite assemblage with associated Au and Cu mineralization occurred at about 8.8 Ma. Alunite-sulfide stable isotopes indicate that alunite-pyrite-enargite mineralizing fluids were of magmatic origin with a wide range of temperatures from 200° to 350°C. Although abundant sulfate minerals are recognized during this stage, sulfur isotope data indicate that the Pascua system, as a whole, was H₂S dominant with H₂S/SO₄ averaging about 1 to 3. This dominance of H₂S is typical of many magmatic-hydrothermal systems (Rye, 1993, in press; Bethke et al., in press). At Summitville, there is evidence that rock buffering locally produced H₂S/SO₄ <1 (Bethke et al., in press), however similar shifts in the ratio of sulfur species have not been recognized at Pascua during alunite-pyrite-enargite deposition. The bulk of alunite-pyrite-enargite ore precipitated over a narrow range of redox and pH conditions (Fig. 14), although local variations in the mineralogy indicate that a broader range of depositional conditions was achieved locally.

View larger version (42K): [in this window] [in a new window]   FIG. 14. Log (O2)-pH diagram illustrating condition in the Pascua alunite-pyrite-enargite (APE) mineralizing event at 275°C and vapor saturation (59 bars). The stability fields of alunite (alun), enargite (en), hematite (hem), magnetite (mag), muscovite (musc), pyrrhotite (po), pyrite (py), tennantite (tenn), and the predominance fields of aqueous sulfur-bearing species are shown. Shaded and stippled gray areas represent total gold solubility contours for Au(HS)–2, AuHS(aq), and AuCl–2 at 1 ppb, 10 ppb, and 1 ppm, resepectively. Calculations for the distribution of species and gold concentrations are modified after Cooke et al. (1996). The white area labeled "APE ore fluids" and the black area contained within represent the chemical characteristics of alunite-pyrite-enargite mineralizing fluids. The black area represents the bulk of alunite-pyrite-enargite ore and is contained within the stability fields of alunite, pyrite, and enargite and is constrained by H2S/SO4 ratios from 1 to 2.5 (based on 34S results from this study). Local mineralogical variability (e.g., monomineralic veins of alunite, enargite, and/or pyrite and the presence of native sulfur) suggests that the composition of the ore fluid varied over a wider range locally, schematically represented by the larger white area.

Postore alteration
Late-stage alteration events record the gradual collapse of the Pascua magmatic-hydrothermal system after about 7.9 Ma. Results from the stable isotope study indicate that late alunite veins were not generated under typical supergene conditions, although they have a field appearance suggestive of a supergene origin. Instead, alunite of this stage formed from mixed magmatic-meteoric fluids at moderate temperatures of about 100°C. The aqueous sulfate required to precipitate alunite of this stage was likely derived from the oxidation of precursor sulfides, given their abundance within the Pascua-Lama system. Significant silver enrichment may have accompanied this stage of alteration, although the exact nature and timing of this event remain unclear (Chouinard, 2003).

The occurrence of jarosite with a supergene isotopic signature suggests that most magmatic activity in the region had ceased by 7.5 Ma, at the latest. Evidence for the supergene remobilization of gold is observed locally throughout the Pascua district in oxidized breccias such as Esperanza and Penelope, where visible gold occurs as coatings on high-level, oxidized fracture surfaces.

	Conclusions

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

 
The unusual occurrence of coeval alunite-pyrite-enargite with the major stage of Au, Ag, and Cu mineralization at Pascua constrains the nature and source of the ore-forming fluids. In most other high-sulfidation systems, ore fluids are considered to be of mixed magmatic-meteoric origin, resulting from the adsorption of magmatic vapors or brines by shallow meteoric water (e.g., Rye, 1993, Arribas, 1995; Hedenquist et al., 1998, Cooke and Simmons, 2000, and references therein; Muntean and Einaudi, 2001). However, at Pascua, there is no evidence for the involvement of meteoric fluid in the main ore-forming stage. Alunite-pyrite-enargite ore precipitated from fluids that were acidic, H₂S dominant, and had a wide temperature range from 200° to 350°C. Gold was likely transported as a bisulfide complex, suggesting that boiling was a viable depositional process.

The predominance of magmatic fluids is recorded throughout the paragenesis of the Pascua deposit. Significant meteoric input is only recognized in the precipitation of supergene jarosite (+ alunite) in the final stages of the hydrothermal system. Similar conditions have been recorded at the Tambo high-sulfidation deposit, which formed at nearly the same time in the southern end of the El Indio-Pascua belt (Deyell et al., in press), suggesting that the same hydrologic regime was affecting epithermal processes at a regional scale. These findings are in agreement with published data from Bissig et al. (2002b) that recorded regional uplift and subsequent pediment erosion throughout this region during the mid to late Miocene. These events, combined with a transition from semiarid to arid climatic conditions (e.g., Bissig et al., 2002b, and references therein), would have caused significant disturbances to the local water table and contributed to the limited availability of meteoric fluids in the epithermal environment. As a result, magmatic fluids were largely undiluted for the duration of magmatic-hydrothermal activity in the Pascua-Lama region.

	APPENDIX

Top Abstract Introduction Geology of the Pascua-Lama... Mineralization Alteration Stable Isotope Study Constraints on Alteration and... Conclusions APPENDIX References

 
All ⁴⁰Ar-³⁹Ar alunite analyses reported in this study were carried out at the Queen’s University laboratory. All samples were step heated and dates and errors (all given at 2) are calculated using formulae given by Dalrymple et al. (1981) and the constants recommended by Steiger and Jäger (1977). The interlaboratory standard (flux-monitor) used is Mac-83 biotite (Sandeman et al., 1997), with the most recent published age of 24.36 ±