Abstract

Hydrothermal fluid circulation in the Tayoltita geothermal system evolved from pervasive to fracture-controlled flow. The early, pervasive circulation produced the widespread propylitic alteration around the mine. In the lower levels of the system, this pervasive propylitization is dominated by chlorite, whereas in the upper levels epidote and albite are also major components of the alteration. Fluid temperatures associated with this stage ranged from >360 degrees C in the lower level of the mine (based on the alteration assemblage oligoclase + actinolite + chlorite + sphene) to <270 degrees C in the upper level (estimated from the composition and expandability of chlorite). As the system evolved, the fluids were concentrated into fractures, probably due to the growth of hydrous alteration minerals (especially chlorite) that reduced the permeability of the rock. Coupled with cooling and collapse of the system, this resulted in the epidote-rich, upper level alteration assemblages being overprinted on lower level rocks around fracture zones. As the system continued to develop, fluid flow became concentrated into major vein structures. This is evidenced by average homogenization temperatures in narrow, isolated quartz veinlets approximately 20 degrees C higher than those of fluid inclusions in the major veins. Economic Au-Ag mineralization in the mine is primarily associated with the earlier stages of this late, vein-forming episode of fluid circulation. The final and most voluminous stage of vein formation is characterized by massive quartz with little or no economic mineralization. Fluid inclusions in both stages of vein quartz are predominantly low salinity (<2 wt % NaCl equiv), although there are a significant number of inclusions with higher salinities (>2.0 wt % NaCl equiv) in the earlier quartz associated with ore. The higher salinity inclusions (also present in the early, isolated quartz veinlets) are believed to represent early mixing of a saline (magmatic?) fluid component.

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