(Au-Fe) Skarn Deposits of the Mezcala District, South-Central Mexico: Adakite Association of the Mineralizing Fluids

Abstract

Geochemical evidence is presented that the auro-ferriferous (Au-Fe) mineralized belt of southcentral Mexico (Sierra Madre del Sur-Mezcala region) is directly linked to a regional post-Laramidic tectono-magmatic event, which was the product of interaction between the Farallon oceanic plate and the North America continental plate during the Middle to Late Tertiary. The Fe-rich skarn-type ore deposits are located near to the coast (80 to 100 km from the paleo-trench), whereas the Au-iron rich deposits of the Mezcala district are located 270 km from the paleo-trench where the crustal thickness is approximately 45 km. The magmatic record of the Tertiary rocks from the "Sierra Madre del Sur" extends from the Paleocene to Miocene, constituting an extended calc-alkaline magmatic province. Adakite-type intrusive rocks are reported in the area, and their relationship with the Au-Fe mineralization in Mezcala is well established. The geochemical anomalies of MgO, Cr, and Ni observed in the intrusive adakites are higher than the experimentally generated adakites; these geochemical features are interpreted to represent an adakite magma that reacted with a mantle wedge. The Sr/Y vs. Y plot shows a clear overlapping relationship between intrusions associated with Au and Cu ore deposits and adakites worldwide. The high La/Sm and Sm/Yb ratios are consistent with an amphibolite-garnet source for these Mezcala adakite rocks. The Au mineralization at Mezcala is spatially related to quartz porphyritic granodiorite (adakite) sills, and developed peripherally on adjacent wall rocks. Gold abundance in wall rock correlates very well with the composite thickness of the adjacent sills. The fluid inclusion analyses reveal that homogenization temperatures (Th) and exo-oxidizing high-salinity magmatic fluids are related to the gold- mineralizing event. The fluid inclusions in late magmatic quartz phenocrysts are multi-component (L + V + S1-5). RAMAN microspectrometric results indicate the presence of halite (S1), sylvite (S2), hematite (S3), anhydrite (S4), and an unidentified crystal (S5). The late fluids have L + V and L-rich fluid inclusions. The earliest fluid identified has the higher salinity (63% total salinity, NaCl + KCl) and Th between 480 and 675°C. This early fluid exhibits a large Th variation from 420°C (decompression-boiling) to final dilution processes at 100-127°C and salinities of 0.25 wt% NaCl equivalent. Most likely, this late fluid dilution is the product of mixing of nearly neutral meteoric water (pH = 7) with hot NaCl-rich fluids transporting gold in solution. The main mineralization in the area (hematite-gold) is the product of these late hydrothermal processes. The original magmatic fluids are dense and oxidizing, with boiling and late dilution linked to the retrograde evolution of the Au-Fe-rich skarn.