Supergene Alteration In the Capricornio AU-AG Epithermal Vein System, Antofagasta Region, Chile

Abstract

Abstract The Capricornio epithermal Au-Ag vein system is located within the Chilean Paleocene–Eocene metallogenic belt, 60 km ENE of Antofagasta in the physiographic Central Depression, between the Coastal Cordillera and the Domeyko Cordillera. Gold-silver mineralization occurs in multiple epithermal quartz-adularia-carbonate mineral veins, which extend from the surface down to ∼400 m depth. The veins are hosted in Cretaceous volcanic rocks belonging to the Paradero del Desierto Formation. The hypogene ore association at Capricornio is represented by Au-Ag alloy (“electrum”, containing on average 60 wt.% Au), Ag-sulfoarsenides, Ag-sulfoselenides, Ag-sulfoantimonides, Ag-sulfosalts, and acanthite, as well as by base metal sulfide minerals including galena, pyrite, chalcopyrite, sphalerite, and bornite. The hypogene mineralization is deeply weathered down to 200 m. The surficial oxidized blanket (0–150 m depth) is underlain by a secondary covellite-chalcocite-bearing zone, which exhibits a variable thickness up to 50 m in each of the veins. In the oxidized zone base metal sulfides have been replaced by several other metallic minerals: (oxy-hydroxy) chlorides (atacamite, pseudoboleite, cumengeite, herbertsmithite), chlorocarbonates (phosgenite), chloroiodates (seeligerite), chlorophosphates (pyromorphite), chloroarsenates (mimetite), and halides (iodargyrite). In this zone Au still occurs as “electrum”, but after the remobilization of Ag during the process of supergene alteration it is slightly enriched in Au (up to a maximum of 78 wt.% Au) and occurs together with Fe-oxy-hydroxides. Gold has also been locally re-mobilized and may occur as veinlets of native Au. The major Ag-bearing mineral in the supergene zone is iodargyrite. The abundance of Ag-halide minerals, together with the higher Au contents of “electrum” in this zone (compared with that in the hypogene zone) suggests a weathering-related Ag remobilization. Silver was not leached from the deposit and was locally re-precipitated as Ag-oxidized compounds. The abundance of Ag-halide minerals in the uppermost part of the supergene zone suggests a large availability of halide ligands during weathering. This is expected, since Cu- and Ag-halide minerals commonly occur in the supergene weathering zones of porphyry Cu deposits in the Atacama region.