Hydrothermal Alteration, Gold Distribution and Geochronology of Epithermal Gold Mineralization in the Copiapo Volcanic Complex, Chile

Abstract

This thesis is a pilot project of an ongoing investigation into the gold metallogenesis in the Copiapo volcanic complex, Chile. The study is aimed at understanding the pattern of hydrothermal alteration and its relation to the gold mineralization in a local geological context at latitudes 27o08' to 27o24' South and longitudes 69o00' to 69o20' West.

Petrographic, chemical and X-ray diffraction studies have led to the recognition of three types of altered mineral assemblages: I. Montmorillonite + jarosite + kaolinite, II. Sericite + quartz + kaolinite + chlorite and III. Alunite + quartz + dickite + prehnite. Field relations of these assemblages do not suggest any direct spatial and/or temporal relationships. Type III assemblage flanks a gold bearing quartz-alunite vein system. The most significant geochemical transfer are: 1. Addition of silicon, potassium and iron in alteration type I; 2. Removal of magnesium, manganese and calcium in all assemblages, and 3. Exchange of other elements for silicon in type III.

Gold, arsenic, lead, antimony, selenium, molybdenum and sulfur are enriched in all altered rocks. Gold is progressively enriched from fresh to altered rocks and to veins. The highest gold grade is 4.5 ppm, compared with 10 ppb in fresh rocks. Alteration-mineralization systems are restricted to north-south trending fracture zones, which appear to be the structural control for hydrothermal activity.

On the basis of new radiometric dates on volcanic rocks and altered/mineralized rock, the volcanological evolution of the complex during the last 14 Ma or so is characterized by explosive volcanism, in which major eruptions occurred at about 14, 10 and 8.6 Ma. Two lava flows were erupted in different localities, but almost at the same time: 13.8 + 0.6 and 13.29 + 0.05 Ma. Caldera formation and resurgence, alteration and gold mineralization are related to volcanism. A radiometric date on alunite, which is associated with gold mineralization, yields a K/Ar age of 12.0 + 0.6 Ma.

The Geologic characteristics, alteration and gold mineralization patterns are comparable to those recognized elsewhere in the world.

Keywords: Pages: 176 Supervisor: Marcos Zentilli