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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

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.

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