Mineralogy and geochemistry of El Dorado epithermal gold deposit, El Sauce district, central-northern Chile

Abstract

The El Dorado Au-Cu deposit is located in an extensive intra-caldera zone of hydrothermal alteration affecting Upper Cretaceous andesites of the Los Elquinos Formation at La Serena ( 29 470S Lat., 70 430W Long., Chile). Quartz-sulfide veins of economic potential are hosted by N25W and N20E fault structures associated with quartz-illite alteration (þsupergene kaolinite). The main ore minerals in the deposit are pyrite, chalcopyrite fahlore (As=(AsþSb): 0.06–0.98), with electrum, sphalerite, galena, bournonite-seligmanite (As=(AsþSb): 0.21–0.31), marcasite, pyrrhotite being accessory phases. Electrum, with an Ag content between 32 and 37 at.%, occurs interstitial to pyrite aggregates or along pyrite fractures. Pyrite commonly exhibits chemical zonation with some zones up to 1.96 at.% As. Electron probe microanalyses of pyrite indicate that As-rich zones do not exhibit detectable Au values. Fluid inclusion microthermometry shows homogenization temperatures between 130 and 352 C and salinities between 1.6 and 6.9 wt.% NaCl eq. Isotope data for quartz, ankerite and phyllosilicates and estimated temperatures show that 18O and D for the hydrothermal fluids were between 3 and 10ø and between 95 and 75ø, respectively. These results suggest the mineralizing fluids were a mixture of meteoric and magmatic waters. An epithermal intermediate-sulfidation model is proposed for the formation of the El Dorado deposit.