Hydrologic implications of alteration and fluid inclusion studies in the Fresnillo District, Mexico; evidence for a brine reservoir and a descending water table during the formation of hydrothermal Ag-Pb-Zn orebodies

Abstract

Ag-Pb-Zn mineralization in the Fresnillo district formed from a large magma-related hydrothermal system between approximately 32 to 28 Ma. A partial record of this hydrothermal activity is preserved by hydrothermal alteration mineral assemblages and fluid inclusions occurring in barren and mineralized parts of the district. These features provide constraints on the hydrologic interpretation of the mineralizing system.Surficial exposures contain alteration assemblages which reflect two distinct fluid types that are known to occur in the shallow parts of active hydrothermal systems. The hydrothermal mineral assemblage comprising quartz, calcite, illite, or interlayered illite-montmorillonite, plus or minus chlorite, pyrite, or adularia occurs in scattered exposures around the district and indicates the former widespread presence of near-neutral pH chloride waters. Fluid inclusion data in barren exposures south of the district center indicate that these fluids were dilute, containing less than 5 wt percent NaCl equiv and had temperatures in the range of 160 degrees to 240 degrees C. The assemblage of kaolinite, plus or minus alunite and natroalunite, occurs locally above blind orebodies in the southern part of the district and in outlying exposures and indicates the former presence of acid sulfate waters; geologic and fluid inclusion evidence strongly suggests that these waters originated as steam-heated condensates which formed above the water table. At Plateros, which lies north of Fresnillo, a hotter thermal environment is exposed where fluid inclusion data range from 220 degrees to 300 degrees C.In contrast to other surficial exposures in the Fresnillo district, those that crop out in the center of the district on Cerro Proano contain significant silver mineralization associated with stockwork quartz veins. Fluid inclusion data from two quartz crystals indicate part of a complex hydrothermal history in which fluids of 17, 10, and 200 degrees C, this last feature explains why the tops of late-formed orebodies are concealed at 150- to 200-m depth. This model implies that dilute hydrothermal fluids devoid of ore metals were the fluids mostly responsible for hydrothermal alteration and vein fillings at the present-day surface and that changes to the hydrology significantly altered the position of the ore depositional environment.