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 <5 wt percent NaCl equiv entered fractures in compositionally discrete pulses, boiling at temperatures of 250 degrees to 330 degrees C under inferred lithostatic pressures, which probably existed for only a very short period. Other fluid inclusion data indicate that dilute waters, containing <5 wt percent NaCl equiv, entered the same fractures, boiling at cooler temperatures of 190 degrees to 250 degrees C under inferred hydrodynamic pressures; these temperatures and pressures are thought to represent the prevailing steady state conditions of hydrothermal flow during infilling of veins at and beneath Cerro Proano.Regional models of the anatomy and evolution of the Fresnillo hydrothermal system were deduced from presented and published data and comparisons with modern hydrothermal systems. Two important hydrologic aspects of these models are the existence of a deep-seated brine reservoir, possibly derived from crystallizing magmas, from which metal-bearing solutions were periodically injected to shallow levels through a compositionally stratified hydrothermal system; and the water table, which controls the position of hydrodynamic boiling conditions, descended approximately 400 m to a level beneath the present-day surface before formation of silver-rich epithermal veins. Because many of the ore minerals in these veins deposited under boiling conditions at temperatures >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.

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