Supergene alteration in the Capricornio Au-Ag epithermal vein system, Antofagasta region, Chile

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. Goldsilver mineralization occurs in multiple, epithermal quartz-adularia-carbonate mineral veins, which extend from the surface down to ~400 m in 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, Agsulfoantimonides, Ag-sulfosalts and acanthite, as well as by base metal sulfides including galena, pyrite, chalcopyrite, sphalerite and bornite. The hypogene mineralization is deeply weathered down to a depth of 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. The oxidized zone has been identified because base metals sulfides have been replaced by several other metallic minerals: (oxy-hydroxy) chlorides (atacamite, pseudoboleite, cumengeite, herbertsmithite), chlorocarbonates (phosgenite), chloroiodates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 remobilized and may occur as veinlets of native Au. The major Ag-bearing mineral in the supergene zone is iodargyrite. The abundance of Ag-halides, 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-halides in the uppermost part of the supergene zone suggests a large availability of halide ligands during weathering. This is expected since Cu- and Ag-halides commonly occur in the supergene weathering zones of porphyry Cu deposits in the Atacama region.