PROXIMAL TO DISTAL GEOCHEMICAL SIGNATURES OF HYDROTHERMAL MINERALIZATION IN THE CANDELARIA CU-AU DISTRICT, CHILE

The Candelaria-Punta del Cobre district is a NE trending Cu-Au-(Zn-Ag) mineralized zone in northern Chile. Ores in the NE and central parts of the district are hosted in andesitic volcanic rocks of the Punta del Cobre Formation as structurally controlled breccias (Santos, Alcaparossa) or mantos (e.g. Candelaria). In the south, ore is mainly in skarns hosted in the calcareous sedimentary units of the Chañarcillo group stratigraphically above the volcanic rocks. The Chañarcillo group is well exposed in the eastern and southern parts of the district. Developed in the calcareous units are calcite-dominant veins that are the distal expressions of mineralizing fluids. In this study, samples of the veins and wall rock were collected along traverses that extend from known orebodies to barren rock up to 8.5 km away. The samples were analyzed for geochemistry, calcite trace elements (LA-ICP-MS), and C-O-Sr isotopes in search of distal signatures and trends.

Vein and wallrock geochemistry show high Cu concentrations (>1000 ppm) closest to ore (<100 m), then Zn (>600 ppm), Pb (>100 ppm), and Mn maxima increasingly farther away. The Mn maximum (>10,000 ppm) occurs 1-3 km from ores; farther along faults. The Mn concentrations return to background (~1,300 ppm) at distances of ~5-6 km. Calcite composition shows a similar trend, but without Zn and Pb. The Sr isotope composition of near ore vein calcite is close to primitive magmatic signature (~0.704) and increases to be similar to that of the carbonate wallrocks (~0.707; open ocean carbonate signature) at ~4.5 km distance. Wallrock ⁸⁷Sr/⁸⁶Sr ratios decreases towards ores, likely due to the addition of hydrothermal calcite closer to ores. O isotope compositions of water in equilibrium with the vein calcite are higher (~13.0-2.5‰) near ores, due to contributions from magmatic and meteoric waters and dissolved wallrock carbonates. It decreases away from ores, down to ~-10‰ at ~5 km, then remains at such values 5-8 km away from ores, indicating more meteoric water component and less dissolved wallrock carbonate. The water C signature is dominated by wallrock carbonates near ores (~1.4 to -2‰) and decreases away to -8 to -4‰ at ~5 km, then remains at such values at 5-8 km distances, probably due to less wallrock contribution and more organic components.