Mineral textures and fluid inclusion petrography of the epithermal Ag–Au deposits at Guanajuato, Mexico: Application to exploration

Abstract

Fluid inclusion petrography and vein mineral textures indicative of boiling have been characterized in 855 samples from epithermal precious metals deposits along the Veta Madre at Guanajuato, Mexico. Mineral textures and fluid inclusions characteristic of fluid immiscibility or boiling, including colloform quartz, plumose/feathery/flamboyant quartz, lattice-bladed calcite and lattice-bladed calcite replaced by quartz, as well as coexisting liquid-rich and vapor-rich fluid inclusions and assemblages of vapor-rich only inclusions, have been identified in mineralized samples from the Veta Madre. Most samples studied were assayed for Au, Ag, Cu, Pb, Zn, As and Sb, and were divided into ore grade and sub-economic samples based on the gold and silver concentrations. For silver, samples containing >100ppm were classified as ore grade, and ore grade gold samples contained >1ppm Au. The feature that is most closely associated with ore grades of both gold and silver is colloform quartz that was originally precipitated as amorphous silica, and this feature also shows the largest difference in average grade between samples that show colloform texture (178.8ppm Ag and 1.1ppm Au) and those that do not exhibit this texture (17.2ppm Ag and 0.2ppm Au). Statistical analysis of the data confirmed the petrographic observations that indicated that colloform quartz is the feature that has the greatest predictive power for distinguishing between ore grade and sub-economic samples. For both Ag and Au, there is no significant difference in average grade of samples containing coexisting liquid-rich and vapor-rich fluid inclusions or assemblages of vapor-only inclusions and those that do not, suggesting that fluid inclusion evidence for boiling is not correlative with ore grades. This result is consistent with the fact that most forms of silica that are precipitated during boiling do not trap useful fluid inclusions. The results of this study suggest that mineral textures and fluid inclusions provide complementary information that should both be used in exploration for epithermal precious metal deposits.Metal grades and boiling intensity of samples collected along a traverse perpendicular to the Veta Madre and above known economic mineralization are both low at relatively short distances away from the vein and increase as the vein is approached. This suggests that mineralogical and fluid inclusion evidence for boiling are restricted to the immediate vicinity of, and increase in the direction of, mineralized veins and may be used in exploration to establish vectors towards vein systems that may host precious metal mineralization. Previous studies of epithermal systems show that the Ag and Au mineralization zone is most often located at or above the bottom of the boiling zone. In this regard, the presence of abundant evidence for boiling that is observed in the deepest levels of the Veta Madre that have been sampled suggests that additional precious metal mineralization may be present beneath the deepest levels that have been explored.