GENESIS OF EPIZONAL AG VEIN MINERALIZATION AT SAN-BARTOLOME IN CENTRAL ECUADOR - TEXTURAL EVIDENCE, FLUID INCLUSIONS, AND STABLE-ISOTOPE GEOCHEMISTRY

Epizonal Ag-Pb-Zn vein mineralization at San Bartolome, in the eastern Andes of Ecuador, is one of a series of mineral occurrences which mak es up the Canar-Azuay hg belt. The mineral assemblage in the veins cry stallized in five stages: comb quartz selvages on vein margins; coarse , crystalline aggregates of sphalerite associated with subsidiary amou nts of freibergite (avg 23.9% hg) and galena, with minor stannite; spe ctacular interpenetrating networks of bladed pyrrhotite with interstit ial galena, freibergite, arsenopyrite, sphalerite, and minor stannite; coarse galena and Ag-bearing sulfosalts dominated by plumose masses o f owyheeite (Pb5Ag2Sb6S9), boulangerite, and jamesonite; and finally, a gangue assemblage of quartz, rhodochrosite, and dolomitic carbonates . Pyrrhotite is now almost completely replaced by hypogene marcasite a nd pyrite pseudomorphs. Sulfur isotope geothermometry indicates ore-fo rming temperatures of 219 degrees to 354 degrees C and delta(34)S(CDT) (Canyon Diablo troilite) values in the range -3 to +2 implies a magma tic origin for a dominantly reducing, sulfidic fluid. Relative delta(3 4)S values in the principal vein sulfides suggest that crystallization proceeded under conditions close to equilibrium. Gradual variation in delta(34)S values along individual bladed structures further suggests that steady crystal growth occurred within a system which was closed to sulfur replenishment. The delta(13)C and delta(18)O values in rhodo chrosite are consistent with carbonate-depositing fluids also having a primarily magmatic source with a minor meteoric component. Microtherm ometry of fluid inclusions in quartz, carbonate, and sphalerite shows that three fluids have contributed to mineralization: a hypersaline fl uid (30 wt % NaCl equiv), and an intermediate-salinity fluid (6-15 wt % NaCl equiv), both associated with inclusion homogenization temperatu res (T-h) in the range of 200 degrees to 400 degrees C; and a low-sali nity fluid (<6 wt % NaCl equiv) associated with lower TI, in the range of 100 degrees to 300 degrees C. On the basis of these observations, it is most likely that the San Bartolome veins formed from magmato-hyd rothermal fluids possibly linked to the intrusion of an S-type pluton, close to the palaeocontinental margin. Mineralizing fluids invaded a dilatant fracture system as a series of discrete pulses, where they we re subsequently diluted by the later introduction of cooler, meteoric ground waters.