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

摘要

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 (approx = 29 deg 47'S Lat., 70 deg 43 min W Long., Chile). Quartz-sulfide veins of economic potential are hosted by N25W and N20E fault structures associated with quartz-illite alteration (+supergene kaolinite). The main ore minerals in the deposit are pyrite, chalcopyrite +- fahlore (As/(As + Sb): 0.06-0.98), with electrum, sphalerite, galena, bournonite-seligmanite (As/(As + Sb): 0.21 -0.31), marcasite, pyrrhotite being accessory phases. Electrum, with an Ag content between 32 and 37 at. percent, occurs interstitial to pyrite aggregates or along pyrite fractures. Pyrite commonly exhibits chemical zonation with some zones up to 1.96 at. percent 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 deg C and salinities between 1.6 and 6.9 wt. percent NaCl eq. Isotope data for quartz, ankerite and phyllosilicates and estimated temperatures show that delta~(18)O and delta D for the hydrothermal fluids were between 3 and l0 per thousand and between -95 and - 75 per thousand, 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.