UPPER JURASSIC AU-ZN MINERALIZATION IN EL FALDEO DISTRICT, CHILEAN PATAGONIA

The Jurassic Au-Zn mineralization at El Faldeo district is hosted main ly in felsic volcanic and subvolcanic rocks. Early epithermal gold min eralization is related to silicic alteration and deposition of pyrite, arsenopyrite, and minor sphalerite, galena, gold, and chalcopyrite, i n veinlets and dissemination. The ore bodies are stratiform horizons ( 10-30 m thick and 150-240 m long) hosted in dacitic to rhyolitic lapil li to ash tuffs and lavas. Grades vary between 0.2 and 3.5 ppm Au. Flu id inclusion data indicate homogenization temperatures varying between 140 and 171 degrees C, salinities ranging from 0.2 to 3.4 wt.% NaCl e quivalent, and gold deposition during boiling events. Late Zn minerali zation consists of stockwork and dissemination of quartz, sericite, ca lcite, barite, pyrite, arsenopyrite, sphalerite, Ag-bearing galena, an d minor gold. The ore bodies are irregular or stratiform horizons host ed in felsic volcanic and subvolcanic rocks. Ore grades vary from 2 to 8% Zn, 0.4 to 3% Pb, 20 to 110 ppm Cd, 10 to 100 ppm Ag, and 40 to 11 00 ppb Au. Fluid inclusion data indicate ore deposition during boiling at homogenization temperatures varying between 250 and 330 degrees C, and salinities ranging between 4 and 23 wt.% NaCl equivalent. S, Pb, C, and O isotopic data support a magmatic and meteoric source for hydr othermal fluids with a significant role played by leaching of Paleozoi c basement rocks. Field observations, radiometric dating, and structur al interpretations support that mineralization occurred synchronously with magmatic and tectonic activity in an upper Jurassic intra-arc pul l-apart basin environment. Finally, it is proposed that the Au-Zn mine ralization at Fl Faldeo district corresponds to a hydrothermal system peripheral to porphyry copper.