Abstract

The Faride epithermal silver-gold deposit belongs to the adularia-sericite type and is associated with an eroded volcanic center. This volcanic center is located within a geotectonic environment dominated by the development of a magmatic arc from the beginning of the Upper Cretaceous to the early Tertiary.The Faride deposit consists of a series of epithermal veins, containing precious metals and associated base metals that lie within a structural system. The veins strike roughly east-west to N 45 degrees W and extend for approximately 2 km. Within this system a total of eight ore shoots presently have been recognized and explored. These deposits are deeply oxidized down to 200 m, so that the present mineralogy consists of a suite of complex chloride, carbonate, sulfate, and oxide minerals. Below 200 m the primary mineralization can be recognized, which consists of specular hematite, pyrite, chalcopyrite, galena, sphalerite, and copper-silver sulfosalts. Gangue minerals are quartz, barite, rhodochrosite, and minor siderite. Quartz textures are typical of open-space filling, showing banding or crustification.Two main paragenetic stages of veins are described: the earlier stage (I) is gold rich and consists of quartz-hematite-pyrite-chalcopyrite, and the later stage (II) is silver rich and consists primarily of barite-galena-sphalerite-chalcopyrite and Ag sulfosalts.Hydrothermal alteration is weakly developed within the intrusive host rock, forming quartz-sericite envelopes around the veins. At depth abundant quartz-adularia veinlets predominate.Fluid inclusions in quartz associated with the gold-rich stage have homogenization temperatures that range from 200 degrees to 320 degrees C; temperature increases with depth. Salinities in this stage range between 0.4 and 8.2 wt percent NaCl equiv. Inclusions associated with the late, silver-rich stage range in homogenization temperature from 170 degrees to 245 degrees C and do not vary with depth. Salinities of the second stage are similar to those of the first, however, ranging from 1.3 to 8.0 wt percent NaCl equiv.Based on mineralogic data and fluid inclusion studies, it is suggested that a combination of boiling, mixing, and conductive cooling decreased the temperatures of ascending ore-bearing fluids, causing precipitation of the primary base metal sulfides and precious metals. For metal migration and deposition within the oxidation zone, two possible mechanisms are considered to have acted at the Faride deposit: reduction by ferrous iron, and breakdown of Au, Ag, and base metal complexes by evaporation in the vadose zone.

First Page Preview

First page PDF preview
You do not currently have access to this article.