Genesis of Epizonal Ag Vein Mineralization at San Bartolom6 in Central Ecuador: Textural Evidence, Fluid Inclusions, and Stable Isotope Geochemistry

Epizonal Ag-Pb-Zn vein mineralization at San Bartolom & in the eastern Andes of Ecuador, is one of a series of mineral occurrences which makes up the Cafiar-Azuay Ag belt. The mineral assemblage in the veins crystallized in five stages: comb quartz selvages on vein margins; coarse, crystalline aggregates ofsphalerite associated with subsidiary amounts of freibergite (avg 23.9 % Ag) and galena, with minor stannite; spectacular interpenetrating networks of bladed pyrrhotite with interstitial galena, freibergite, arsenopyrite, sphalerite, and minor stannite; coarse galena and Ag-bearing sulfosalts dominated by plumose masses of owyheeite (Pb.•AgaSb6S•), boulangerite, and jamesonitc; and finally, a gangue assemblage of quartz, rhodochrosite, and dolomitic arbonates. Pyrrhotite is now almost completely replaced by hypogene marcasite and pyrite pseudo-morphs. Sulfur isotope geothermometry indicates ore-forming temperatures of 219 ø to 354øC and 634Sc•r (Canyon Diablo trollitc) values in the range-3 to +2 implies a magmatic origin for a dominantly reducing, sulfidic fluid. Relative 634S values in the principal vein sulfides suggest that crystallization proceeded under conditions close to equilibrium. Gradual variation i 6•4S values along individual b aded structures further suggests that steady crystal growth occurred within a system which was closed to sulfur eplenishment. The 61•C and 6•S