Geological features of the Fresnillo Ag deposit in Mexico

The Fresnillo base metal-silver ore deposit consists of replacement chimney and manto orebodies, disseminated sulfide orebodies, and vein orebodies hosted mainly in Cretaceous marine sedimentary and volcanic rocks. The deposit exhibits southeastward zoned distribution away from a small quartz monzonite stock, and shows increasing silver content and decreasing base metal values with the distance from the stock. Wall rocks adjacent to the stock exhibit silicification and calc-silicate alteration that have been partially replaced by the sulfide minerals. Wall rocks of the vein orebodies exhibit potassic, phyllic, argillic, and propylitic alteration with increasing distance from the orebodies. Homogenization temperatures of quartz, calcite and spharelite in chimney, manto, and deep vein orebodies range from 240℃ to 350℃, while their homogenization temperatures in shallow and distal vein orebodies are between 140℃ and 275℃. Galena δ34S values from all orebodies at Fresnillo vary between -4.1‰ and -8.0‰, while sphalerite δ34S values range between -2.0‰ and -5.0‰. The contrasting δ34S values between galena and sphalerite might have resulted from a slight variation in ore fluid pH (as indicated by the wall rock alteration assemblages) or by increased log fo2.