Eco•lo.tic {;eolo•.t I Vol. •'L IDa8. pp. 1•,30-1•10 Hydrothermal Evolution in the Formation of Silver-Gold Veins

The Tayoltita mine, located in the states of Durango and Sinaloa, Mexico, is a major example of a Tertiary volcanic-hosted silver-gold epithermal vein deposit. Significant silver and gold values and base metal sulfides as well as most of the vein quartz and accompanying chlorite, calcite, rhodonite, and adularia are restricted to a vertical interval in the veins between ap-proximately 500 and 1,000 m beneath the original surface. Fluid inclusion studies indicate that ore deposition was from fluids averaging approximately 260øC and was accompanied by some boiling. The salinity of the fluid inclusions i estimated at 0.1 m NaC1 equivalent; greater apparent salinities indicated by lower freezing temperatures probably reflect some dissolved COa. Correlation of Ag/Au values with the apparent salinities and with vein mineral paragenesis shows that during ore deposition the fluids evolved in time and space from relatively concen-trated to dilute conditions owing to gas loss, and possibly, some fluid mixing. Ag/Au ratio zoning, orebody distribution, and vein mineral zoning patterns are all consistent with vein formation from fluids flowing predominantly in horizontal flow patterns