Epithermal fluids evolution related to an IOCG system in Cerro Colorado Chico

Abstract

Cerro Colorado Chico located in Ancash region, northern coast of Peru. Brecciated siliceous bodies occur with restricted halos of advanced argillic alteration silica-pyrophyllite-alunite and extensive silica-sericite-pyrite alteration, breccias and hematite bodies, hematitized horizons into volcano-sedimentary rocks of the Casma Group. A microthermometry study of fluid inclusions has been carried out in 4 samples with a total of 55 primary and secondary fluid inclusions in quartz. Two stages are distinguished, the first at mesothermal temperatures between 605° and 560° C with salinities of 47.66% to 44.82% NaCl eq., and the second at epithermal temperatures between 329° and 138° C with salinities of 0.7%. to 9.73% NaCl eq., with complex fluids evolution processes as cooling, dilution, mixing and boiling. This study permit to explain the processes that take place in the epithermal environment of IOCG-type magmatic-hydrothermal systems from fluids derived of silica-rich silicate melts that partition from poorly differentiated magmas, when decompressed separate into a dense saline liquid phase and a low-density vapor phase that rises to the upper levels and is responsible for the formation of the brecciated siliceous bodies and the advanced argillic alteration.