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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia
Barrick Gold Exploration, 293 Spruce Road, Elko, Nevada 89801
U.S. Geological Survey, Mail Stop 963, Denver Federal Center, Denver Colorado 80225
Teck-Cominco, 200 Burrard Street, Vancouver, British Columbia, Canada V6C 3L9
Universidad Católica del Norte, Depto. Ciencias Geológicas, Av. Angamos 0610, Antofagasta, Chile
Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania, 7001, Australia
Corresponding author: e-mail, cdeyell{at}utas.edu.au
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Abstract |
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 Top Abstract IntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
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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 
34Salun-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.
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Introduction |
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TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
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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.
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Geology of the Pascua-Lama Region |
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TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
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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.
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Mineralization |
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TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
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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).
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Alteration |
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TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
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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).
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Analytical details are provided in Table 2.
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.
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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.
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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.
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Stable Isotope Study |
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TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
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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.
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34S
value of bulk sulfur in the magma (34S
S)
cannot be determined directly since no large bodies of synore volcanic rocks
have been identified in the Pascua district, with the exception of the 7.84 Ma
rhyodacite dike noted previously. Estimation of 34SS
(2.8 ± 2
) is therefore based on data for magmatic steam alunite (see below).
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34S values
that range from 13 to 25 per mil (Fig. 9). Pyrite could not be separated in
sufficient quantity for analysis from preore advanced argillic alteration, but
R. Beane (unpub. report for Barrick Chile, Ltda., 1988) reported a 34S
value of –3.5 per mil for disseminated pyrite intergrown with alunite in
quartz monzonite from the Brecha Central area. This result is plotted for
comparison in Figure 9. The range of 34S values for
alunite associated with the alunite-pyrite-enargite assemblage is identical to
that for alunite in preore advanced argillic alteration, and data for ore-stage
barite (22.1) also fall within this range. Ore-stage enargite and pyrite have
34S values that cluster around cluster around –3 to –6 per
mil, consistent with the value for pyrite from preore advanced argillic
alteration.
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 34Salun-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.
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34S values of 0 to 6
per mil, which is slightly higher than 34S values
of sulfides in the deposit (Fig. 9). These results are consistent with a
steam-heated origin for this stage of alteration (e.g., Rye et al., 1992),
although the range of 34S data indicates that
limited sulfur isotope exchange occurred between aqueous SO4 and H2S
during this alteration event.
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 34Salun-py values or 18OSO4-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).
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18OSO4
values of 14 to 21 per mil (Fig. 10). The composition of preore alunite fluids
overlaps with that of typical felsic magmatic fluids (Taylor, 1988) and
indicates a dominant magmatic component in the source fluids (cf. Rye et al,
1992). The trend of the data in Figure 10, which shows gradual decreasing Dalun
nearly parallel to the meteoric water line, suggests some mixing of these
magmatic fluids with exchanged meteoric water, resulting from either water-rock
interaction and/or boiling or evaporation. Depositional temperatures calculated
from 18OSO4-OH range from –50° to
+140°C and are attributed to retrograde isotopic exchange in the OH site.
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DH2O
ranging between –30 and –47 per mil (Fig. 11). Again, these data overlap
with the DH2O of magmatic water (Taylor, 1988) and
indicate a dominantly magmatic source for the mineralizing fluid. The 18OSO4-OH
temperatures (–50° to +790°C) indicate disequilibrium, as noted above for
preore advanced argillic alunite.
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 18OSO4-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 18OSO4-OH
temperature, given the shallow depth of formation, although 18OOH
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 18OSO4-OH may have
been affected by postdepositional exchange for this sample. Calculated fluid
compositions for steam-heated alunite, as shown in Figure 11, have DH2O
values of –38 to –54 per mil and 18OH2O
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 18OSO4-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 DH2O 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 18OOH values range from 5.5 to
13.5 per mil, and 18OSO4 values range
from 9.7 to 20.7 per mil. Two of the three 18OSO4
values fall outside of the supergene alunite sulfate field and have large
positive 18OSO4-OH values that are not
consistent with a supergene origin (Rye et al., 1992).
Calculated DH2O values for late-stage alunite
range between –35 and –75 per mil, with 18OH2O
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 18OOH data for this
jarosite (Fig. 12B) is slightly heavier than the supergene jarosite OH ("SJOH")
zone as defined by Rye and Alpers (1997), but 18OSO4
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.
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Constraints on Alteration and Ore Deposition |
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TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
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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 DH2O
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.
|
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 |
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TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
|---|
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 |
|---|
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TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
|---|
) 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 ± 0.17 Ma (2; Sandeman et al. 1999). Age spectrum plots
for all samples are shown in Figure A1.
|
|
|
Acknowledgments |
|---|
April 8, 2002; October 26, 2004
April 8, 2002; October 26, 2004
|
References |
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|
TopAbstractIntroductionGeology of the Pascua-Lama...MineralizationAlterationStable Isotope StudyConstraints on Alteration and...ConclusionsAPPENDIXReferences |
|---|
Arribas, A., Jr, 1995, Characteristics of high sulfidation epithermal deposits, and their relation to magmatic fluids: Mineralogical Association of Canada Short Course Series, v. 23, p. 419–454.
Barazangi, M., and Isacks, B.L., 1976, Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America: Geology, v. 4, p. 696–692.
Bethke, P.M., Rye, R.O., Stoffregen, R.E., and Vikre, P., in press, Evolution of the Summitville magmatic-hydrothermal acid-sulfate system: Chemical Geology.
Bissig, T., Lee, J.K.W., Clark, A.H., and Heather, K.B., 2001, The Cenozoic history of magmatic activity and hydrothermal alteration in the Central Andean flat-slab region: New 40Ar-39Ar constraints from the El Indio-Pascua Au (-Ag, Cu) belt, 29°20'–30°30' S: International Geology Review, v. 41, p. 312–340.
Bissig, T., Clark, A.H. and Lee, J.K.W., 2002a, Cerro de Vidrio rhyolite dome: Evidence for late Pliocene volcanism in the central Andean flat-slab region, Lama-Veladero district, 29°20' S, San Juan province, Argentina: Journal of South American Earth Sciences, v. 15, p. 571–576.[CrossRef][Web of Science][GeoRef]
Bissig, T., Clark, A. H., Lee, J.K.W., and Hodgson, C.J., 2002b, Miocene
landscape evolution and geomorphologic controls on epithermal processes in the
El Indio-Pascua Au-Ag-Cu belt, Chile and Argentina: Economic Geology, v. 97, p.971
–996.
Chouinard, A., 2003, Alteration, mineralization and geochemistry of the high-sulphidation Au-Ag-Cu Pascua deposit: Unpublished Ph.D. thesis, Montreal, Canada, McGill University, 269 p.
Cooke, D.R., and Simmons, S.F., 2000, Characteristics and genesis of epithermal gold deposits: Reviews in Economic Geology, v. 13, p. 221–244.[GeoRef]
Craig, H., 1961, Isotopic variations in meteoric waters: Science, v. 133, p.1702
–1703.
Cunningham, C.G., Rye, R.O., Steven, T.A., and Mehnert, H.H., 1984, Origins
and exploration signficance of replacement and vein-type alunite deposits in the
Marysvale volcanic field, west-central Utah: Economic Geology, v. 79, p. 50–71.
Dalrymple, G.B., Alexander, E.C., Jr., Lanphere, M.A., and Kraker, G.P.,1981 , Irradiation of samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor: U.S. Geological Survey Professional Paper 1176, 55 p.
Deyell, C.L., 2001, Alunite and high sulfidation Au-Ag-Cu mineralization in the El Indio-Pascua belt, Chile-Argentina: Unpublished Ph.D. thesis, Vancouver, Canada, University of British Columbia, 300 p.
Deyell, C.L., Rye, R.O., Landis, G.P., and Bissig, T., in press, Alunite in an evolving magmatic-hydrothermal system: The Tambo high-sulfidation deposit, El Indio district, Chile: Chemical Geology.
Ebert, S.W., and Rye, R.O., 1997, Secondary precious metal enrichment by
steam-heated fluids in the Crowfoot–Lewis hotspring gold-silver deposit and
relation to paleoclimate: Economic Geology, v. 92, p. 578–600.
Giesemann, A., Jager, H.J., Norman, A.L., Krouse, H.R., and Brand, W.A.,1994 , On-line sulfur-isotope determination using an elemental analyzer coupled to a mass spectrometer: Analytical Chemistry, v. 65, p. 2816–2819.[CrossRef]
Giggenbach, W.F., 1992, Magma degassing and mineral deposition in hydrothermal systems along convergent plate boundaries: Economic Geology, v. 87, p. 1927–1944.[Web of Science][GeoRef]
Godfrey, J.D., 1962, The deuterium content of natural waters and other substances: Geochimica et Cosmochimica Acta, v. 27, p. 43–52.
Hedenquist, J.W., Arribas, A., Jr., and Reynolds, T.J., 1998, Evolution of an
intrusion-centered hydrothermal system: Far Southeast-Lepanto porphyry and
epithermal Cu-Au deposits, Philippines: Economic Geology, v. 93, p. 373–404.
Henley, R.W., and Stewart, M.K., 1983, Chemical and isotopic changes in the hydrology of the Tauhara geothermal field due to exploitation at Wairakei:Journal of Volcanology and Geothermal Research , v. 15, p. 285–314.[CrossRef][Web of Science][GeoRef]
Jannas, R.R., Bowers, T.S., Petersen, U., and Beane, R.E., 1999, High-sulfidation deposit types in the El Indio district, Chile: Society of Economic Geologists Special Publication, v. 7, p. 27–59.
Kay, S.M., Mpodozis, C., and Coira, B., 1999, Neogene magmatism, tectonism, and mineral deposits of the Central Andes (22° to 33° S latitude: Society of Economic Geologists Special Publication, v. 7, p. 27–59.
Kornexl, B.E., Gehre, M., Höfling, R., and Werner, R.A., 1999, On-line 18O
measurement of organic and inorganic substances: Rapid Communications in Mass
Spectrometry, v. 13, p. 1685–1693.[CrossRef][Web of Science][Medline]
Losada-Calderón, A.J., and McPhail, D.C., 1996. Porphyry and high-sulfidation epithermal mineralization in the Nevados del Famatina mining district, Argentina: Society of Economic Geologists Special Publication, v. 5, p. 91–118.
Maksaev, V., Moscoso, R., Mpodozis, C., and Nasi, C., 1984, Las unidades volcánicas y plutonicas del cenozoico superior en la Alta Cordillera del Norte Chico (29°–31° S): Geología, alteración hidrothermal y mineralisacion: Revista geológica de Chile, v. 21, p. 11–51.
Martin, M., Clavero, J., and Mpodozis, C., 1995, Estudio geologico regional del Franja El Indio, Cordillera de Coquimbo: Santiago, Chile, Servicio National de Geologia y Minera Registered Report IR-95-06, 232 p.
——1999, Late Paleozoic to Early Jurassic tectonic development of the high Andean Principal Cordillera, El Indio region, Chile (29–30° S): Journal of South American Earth Sciences, v. 12, p. 33–49.[CrossRef][Web of Science][GeoRef]
Muntean, J.L., and Einaudi, M.T., 2001, Porphyry-epithermal transition:
Maricunga belt, northern Chile: Economic Geology, v. 96, p. 1445–1472.
Ohmoto, H., and Lasaga, A.C., 1982, Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems: Geochimica et Cosmochimica Acta, v. 46, p. 1727–1746.[CrossRef][Web of Science][GeoRef]
Ramos, V.A., Mahlburg Kay, S., Page, R.N. and Munizaga, F., 1989, La ignimbrita Vacas Heladas y el cese del volcanismo en el Valle del Cura, Provincia de San Juan: Revista Asociación Geológica Argentina, v. 44, p. 336–352.
Rye, R.O., 1993, The evolution of magmatic fluids in the epithermal
environment: The stable isotope perspective: Economic Geology, v. 88, p. 733–753.
——in press, A review of the stable isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems:Chemical Geology .
Rye, R.O., and Alpers, C.N., 1997, The stable isotope geochemistry of jarosite: U.S. Geological Survey Open-File Report 97-88, 28 p.
Rye, R.O., Bethke, P.M., and Wasserman, M.D., 1992, The stable isotope
geochemistry of acid-sulfate alteration: Economic Geology, v. 87, p. 225–262.
Rye, R.O., and Stoffregen, R.E., 1995, Jarosite-water oxygen and hydrogen
isotope fractionations: Preliminary experimental data: Economic Geology, v. 90,
p. 2336–2342.
Sandeman, H.A., Archibald, D.A., Grant, J.W., Villneuve, M.E., and Ford, F.D.,1999 , Characterisation of the chemical composition and 40Ar-39Ar systematics of intralaboratory standard MAC-83 biotite: Geological Survey of Canada Current Research 1999-F, p. 13–26.
Savin, S.M., and Epstein, S., 1970, The oxygen and hydrogen isotope geochemistry of clay minerals: Geochimica et Cosmochimica Acta, v. 34, p. 24–42.
Schoen, R., White, D.E., and Hemley, J.J., 1974, Argillization by descending acid at Steamboat Springs, Nevada: Clays and Clay Minerals, v. 22, p. 1–22.[CrossRef][Web of Science][GeoRef]
Steiger, R.H., and Jäger, E., 1977, Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359–362.[CrossRef][Web of Science][GeoRef]
Steven, T.A., and Ratté, J.C., 1960, Geology of ore deposits of the Summitville district, San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 343, 70 p.
Stoffregen, R., 1987, Genesis of acid-sulfate alteration and Au-Cu-Ag
mineralization at Summitville, Colorado: Economic Geology, v. 82, p. 1575–1591.
Stoffregen, R.E., Rye, R.O., and Wasserman, M.D., 1994. Experimental studies of alunite: I. 18O-16O and D-H fractionation factors between alunite and water at 250-450°C: Geochimica et Cosmochimica Acta, v. 58, p. 903–916.[CrossRef][Web of Science][GeoRef]
Taylor, B.E., 1988, Degassing of rhyolitic magmas: Hydrogen isotope evidence and implications for magmatic-hydrothermal ore deposits: Canadian Institute of Mining and Mineralogy Special Volume, v. 39, p. 33–49.
Truesdell, A.H., Nathenson, M., and Rye, R.O., 1977, The effects of boiling and dilution on the isotopic compositions of Yellowstone thermal waters: Journal of Geophysical Research, v. 82, p. 3694–3704.[GeoRef]
Wasserman, M.D., Rye, R.O., Bethke, P.M., and Arribas, A., Jr., 1992, Methods
for separation and total stable isotope analysis of alunite: U.S. Geological
Survey Open-File Report 92-9, 20 p.
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(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 