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High-Sulfidation Deposit Types in the El Indio District, Chile

By
Raymond R. Jannas
Raymond R. Jannas
Los Carrera 380, Of 425, La Serena
,
Chile
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Teresa S. Bowers
Teresa S. Bowers
Gradient Corporation
,
44 Battle Street, Cambridge, Massachusetts 02138
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Ulrich Petersen
Ulrich Petersen
Harvard University
,
20 Oxford Street, Cambridge, Massachusetts 02138
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Richard E. Beane
Richard E. Beane
BHP Copper
,
P.O. Box M, San Manuel, Arizona 85631
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Published:
January 01, 1999

Abstract

The El Indio district, Chile, contains two types of high-sulfidation, precious metal deposits hosted in intensely altered Tertiary rhyodacitic volcanic rocks: El Indio, with enargite-pyrite and gold-quartz mineralization in complex vein systems, and Tambo, with alunite-barite-gold, mainly in tectonic breccia pipes. This single, world-class district contains more than 10 Moz of gold, 100 Moz of silver and 1 Mt of copper. At El Indio, the banded alunite and enargite + pyrite veins, peripheral to the main copper and gold veins, suggest alternating fluid conditions prior to the spectacular high-grade gold mineralization accompanied by sericitic-argillic alteration. The δ34S, δ18O, and δD ratios indicate the 250° to 300°C, moderate to low salinity (<5 wt % NaCl equiv), weakly acidic (pH = 3.5–4.5), reduced mineralizing fluids for both El Indio ore types had a dominantly magmatic water component (>60%), with no evidence of boiling. Copper deposition is attributed to decreasing temperature while precipitation of large quantities of gold is ascribed to mixing with an acid-oxidized fluid. At Tambo, gold was deposited with early barite and again after intermediate-stage alunite, from 200° to 250°C, low-salinity, intermittently boiling fluids. The δ34S ratios at Tambo indicate that the barite fluids were mildly reduced (sulfide/sulfate ratio of 10–25) and contained disproportionated magmatic SO2. The δ18O and δD ratios indicate that alunite formed from condensed, δD-depleted magmatic vapor between gold stages. Calculations show that, within a limited range of dissolved sulfur and pH conditions, a single magmatic fluid could have evolved to produce the multiple mineral assemblages seen at both El Indio and Tambo, with the former in a deeper, more reduced, hydrothermal environment and the latter in a near-surface setting.

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Contents

Special Publications of the Society of Economic Geologists

Geology and Ore Deposits of the Central Andes

Brian J. Skinner
Brian J. Skinner
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Society of Economic Geologists
Volume
7
ISBN electronic:
9781629490311
Publication date:
January 01, 1999

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