Skarn mineralogy and fluids associated to the La Esperanza Cu-Zn-Ni-Co deposits, Sonora, México

Abstract

The La Esperanza deposits in Sonora contain a metallic Cu-Zn-Ni-Co paragenesis, which is singular in the metallogenetic setting of northwestern Mexico. The mineralization is related to a contact metamorphic halo developed between a 89–90 Ma porphyry and late Paleozoic carbonated rocks. These skarn-type deposits, are characterized by a prograde stage represented by calcic garnet and pyoxene, and a retrograde stage contaning calcite, epidote, quartz, chlorite, feldspar and sulfides. Ore minerals are chalcopyrite, sphalerite, pyrite, siegenite (Ni,Co)3S4, hematite, and magnetite. Fluid inclusions data from garnet (prograde skarn) indicate a fluid with 12.8–20.2 % NaCl eq. and homogenization temperatures (Th) of 480–680 °C. Fluid inclusions in quartz and calcite from the retrograde stage indicate a fluid with 12.8–18.6 % NaCl eq. and Th of 180–190 °C. Primary fluid inclusions in quartz phenocrystals of the porphyry, indicate a supersaturated fluid, with 36–44 % NaCl eq. and total Th of 270–370 °C. Chemical compositions of garnet and Th values suggest that the prograde skarn formed at high temperature, between 500 and 680 °C and fluid pressures lower than 600 bar. Th in quartz and calcite are consistent with a retrograde stage at temperature and pressure lower than 380 °C and 50 bar, respectively. Fluids in garnet, quartz and calcite come from the same source, of magmatic origin, mixed during a late mineralizing stage with meteoric waters. Fluid inclusion data of quartz of the porphyry are related to early magmatic fluids previous to the the skarn formation.