LOW-SULFUR EPITHERMAL GOLD MINERALIZATION AT INCA-DE-ORO, NORTHERN CHILE - MINERALOGY AND FLUID INCLUSIONS

Gold mineralization during the Late Cretaceous produced an epithermal deposit of the low-sulfur (adularia-sericite) type, consisting of diss eminated gold, gold-copper veins, and stockworks in Jurassic volcanic rocks and Upper Cretaceous quartz monzodiorite and quartz diorite intr usions. The host rock of the disseminated deposits is a silica cap, wh ich represents the near-surface expression of the epithermal system, a nd the ore mineralogy consists of pyrite, gold, and minor cinnabar. Si licic alteration is represented by cryptocrystalline quartz, and argil lic and advanced alteration by kaolinite, jarosite, and minor alunite. Fluid inclusion studies indicate that alteration occurred at 100-200- degrees-C from fluids with a salinity of 0.5-5 wt% NaCl equiv. These w ere probably acid-sulfate waters formed by oxidation of H2S at the wat er table. In veins and stockworks, the ore assemblage consists of pyri te, gold, sphalerite, argentite, galena, electrum, chalcopyrite, coppe r-sulfosalts, bismuthinite. molybdenite, and cinnabar. Adularia-illite -chlorite alteration is closely connected with the stockwork and vein mineralization. Alteration assemblage and fluid inclusion studies sugg est mineralization by near-neutral alkali-chloride boiling fluids at 2 25-250-degrees-C, ranging in salinity from 1 to 14 wt% NaCl equiv., an d 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 de velopment of the geothermal system that caused the mineralization and associated alteration.