Low-sulfur epithermal gold mineralization at Inca de Oro, northern Chile: Mineralogy and fluid inclusions

Abstract

Gold mineralization during the Late Cretaceous produced an epithermal deposit of the low-sulfur (adularia-sericite) type, consisting of disseminated gold, gold-copper veins, and stockworks in Jurassic volcanic rocks and Upper Cretaceous quartz monzodiorite and quartz diorite intrusions. The host rock of the disseminated deposits is a silica cap, which represents the near-surface expression of the epithermal system, and the ore mineralogy consists of pyrite, gold, and minor cinnabar. Silicic alteration is represented by cryptocrystalline quartz, and argillic and advanced alteration by kaolinite, jarosite, and minor alunite. Fluid inclusion studies indicate that alteration occurred at 100–200°C from fluids with a salinity of 0.5–5 wt% NaCl equiv. These were probably acid-sulfate waters formed by oxidation of H 2S at the water table. In veins and stockworks, the ore assemblage consists of pyrite, gold, sphalerite, argentite, galena, electrum, chalcopyrite, copper-sulfosalts, bismuthinite, molybdenite, and cinnabar. Adularia-illite-chlorite alteration is closely connected with the stockorrk and vein mineralization. Alteration assemblage and fluid inclusion studies suggest mineralization by near-neutral alkali-chloride boiling fluids at 225–250°C, ranging in salinity from 1 to 14 wt% NaCl equiv., and pressure estimates based on fluid-inclusion data suggest a depth of 300–400 m below the paleosurface. Fracture permeability, combined with a large geothermal gradient, favored the circulation of fluids and development of the geothermal system that caused the mineralization and associated alteration.