FLUID INCLUSIONS ANALYSIS AT CHARCAS MINE, CHARCAS MINING DISTRICT, MEXICO

The Charcas Mining District located at the center-west part of the Sierra Madre terrain, is a tertiary Zn skarn, genetically related to a magmatic event of 47 Ma. The Charcas intrusive complex (CIC) was emplaced in Triassic to upper Cretaceous sedimentary rocks, but only few dikes from the CIC developed metamorphism halo and related zinc mineralization. The inner and outer alteration patterns and the mineralogical sequence found are compatible with the description of distal Zn skarn type.

In CIC, fluid inclusions from the proximal to distal phases were found within garnets, quartz and calcite. All minerals show up the presence of primary and secondary fluid inclusions generation. The diminute size of the fluid inclusions in garnet precluded their study. Quartz and calcite crystals show a plethora of primary fluid inclusion types. These fluid inclusions are found either isolated or in clusters, mostly displaying well developed negative crystal shapes. They present two-phase (L+V) to multi-phase (L+V+S) at room temperature. They display heterogeneous L+V volumetric ratios (from 0.4 to 0.95) within a given crystallographic plane, indicative of heterogeneous trapping. Few solids are observed (NaCl and/or KCl) and homogenization occurs in liquid phase. The fluid inclusions show a wide size range, from 10 to >100 µm and presence of CO_2. Only primary inclusions were studied in the proximal to distal phases. The salinity of chloride-dominated aqueous fluid inclusions is calculated from the melting temperature of ice during a cycle of freezing and heating, although some uncertainty remains due to possible variation in cation chemistry (Oakes et al., 1990; Becker et al., 2008). In CIC, primary fluid inclusions in proximal and distal minerals present homogenization temperatures (Th) ranging from 143,3 to 412,1ºC. Salinity for these inclusions ranges between 0,6 to 46,9 wt%, with a mean NaCl/CaCl₂ ratio of 0,6 to 7,8.

The magmatic origin of the fluids and the evolutionary history of the Charcas Zn skarn deposits of the inner calcite zone is highlight by hight temperature - hight salinity fluids and CO₂. In the outer zone, the mixing of the degassed rich magmatic brines with meteoric water may be responsible for boiling, dilution, and cooling of the resulting solution, processes that could cause the deposition of the mineralization.