Physical-chemical properties of the hydrothermal ore fluid in the Faride epithermal deposit (Antofagasta Region, Chile) from a preliminary set of high-resolution fluid inclusion data

The Faride deposit of the Antofagasta Region of Chile is a medium size, low-sulfidation Au-Ag epithermal deposit whose ore bodies form a set of conjugated dip-slip, E-W/NW striking, steeply dipping faults that crosscut a granodiorite intrusion of Paleocene age. The ore bodies fill discontinuous segments of single faults, which are made of a variety of open-space filling features. A conspicuous mineral assemblage fills the vein segments during a hypogene stage of deposition, and includes argentiferous galena, sphalerite, hematite, chalcopyrite, pyrite, pyrolusite, and gold. A later supergene stage of deposition was generated by oxidation of the hypogene minerals, and corresponds to the deposition of a complex paragenesis made of chalcosite, acanthite, digenite, covellite, native silver, stromeyerite, boleite, psilomelane, pyrolusite, goethite, hematite, atacamite, cerussite, anglesite, malachite, and jarosite. The geological and minerogenetic histories of Faride are fairly well documented by previous extensive work, but the physical and chemical properties of the ore fluid needs to be documented in more detail to model accurately all stages of ore deposition. The present work shows a preliminary set of fluid inclusion data that are aimed at improving the knowledge of such properties, and a fluid-mineral equilibrium model of ore deposition. I show microthermometric data from a co-genetic group of fluid inclusions entrapped within euhedral vein quartz precipitated together with pyrolusite within a dilational jog of a major vein during the late stage of mineral deposition. Fifty measurements from these inclusions single out restricted ranges of bulk salinity (4.1±0.3 eq wt% NaCl) and total homogenisations (219±16 °C) of the late ore fluid, providing an objective improvement of fluid properties at the time of gold deposition. The thermodynamic model of ore formation captures some features of the deposit and shows that hypogenic precipitation of Au at Faride was probably controlled by negligible or moderate variations of ore fluid properties, in particular oxidation, cooling, and possibly boiling. Additional data are however necessary to evaluate the temperature-composition-time evolution of ore fluid and mineral assemblage during ore precipitation, with obvious implications for mineral exploration.