Abstract

The middle Tertiary Pacencia Group of Antofagasta Province, northern Chile, consists of over 2 km of continental sediments. These include alluvial fan, playa, and lacustrine facies, representing deposition in an arid, closed basin. Copper mineralization is hosted by sheet and ribbon sandstone bodies, which were deposited in a high-energy playa-marginal sandflat environment. Ore minerals occur dominantly as a matrix to these lithic arkoses, replacing earlier carbonate and sulfate cements.The main ore minerals are native copper, cuprite, and atacamite, although there is an along-strike passage into copper sulfide cements. In a structurally complex zone, copper-iron and copper sulfides were deposited in crosscutting fractures, commonly replacing evaporite veins. Primary chalcopyrite and bornite exhibit progressive oxidation to anilite, digenite, and djurleite. Chalcocite is not found. Much of this material has oxidized further to covellite of normal and blaubleibender varieties. Galena and native silver are present in uneconomic amounts. Atacamite has a complex paragenesis, representing, at least locally, the earliest copper-bearing phase.Provenance studies of the host sediments and lead isotope data for the sulfides indicate that both the sandstones and the metals were derived from a calc-alkaline volcano-plutonic complex to the east of the basin, an area now obscured by the recent volcanic Andes. Sulfur isotope results for copper sulfides show very light delta 34 S values (-33ppm), indicative of biogenic sulfate reduction under conditions of unrestricted sulfate supply.

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