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


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Deyell, CL, Leonardson, R, Rye, RO, Thompson, JFH, Bissig, T and Cooke, DR 2005 , 'Alunite in the Pascua-Lama High-Sulfidation Deposit: Constraints on Alteration and Ore Deposition Using Stable Isotope Geochemistry' , Economic Geology, vol. 100, no. 1 , pp. 131-148 , doi: 10.2113/100.1.0131.

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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 alunitepyrite-
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 Δ34Salun-py data indicate depositional temperatures of 245° to 305°C. The δD and δ18O data exclude significant
involvement of meteoric water during mineralization and indicate that the assemblage formed from H2Sdominated
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.

Item Type: Article
Authors/Creators:Deyell, CL and Leonardson, R and Rye, RO and Thompson, JFH and Bissig, T and Cooke, DR
Keywords: advanced argillic thermodynamic modelling copper gold
Journal or Publication Title: Economic Geology
ISSN: 0361-0128
DOI / ID Number: 10.2113/100.1.0131
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