Low-temperature, hydrothermal base and precious metal deposits hosted by volcanic-sedimentary sequences of the Camaquã basin, southernmost Brazil

Abstract

This paper reviews the most important Au and base-metal ores hosted by Neoproterozoic volcanic–sedimentary sequences in the Camaquã basin, mined for half a century in southern Brazil (Camaquã Cu-Au; Santa Maria Cu-Pb-Zn and Cerro dos Martins Cu deposits). All deposits contain ore sulfides that are mainly fracture-controlled and also disseminated in the matrix of siliciclastic and volcanic rocks. The Camaquã deposits consist of NW veins, stockworks and disseminated ores enclosed in conglomerates and sandstones. These contain chalcopyrite, bornite, chalcocite, pyrite, gold and silver accompannied by chlorite, white mica, quartz, albite, carbonate and later barite and hematite. The ores of the Santa Maria deposit are disseminated in arenites and conglomerates or occur as massive lodes crosscutting the stratigraphy. Sphalerite and galena intergrowths are the dominant texture in the ores while chalcopyrite and chalcocite are minor acompanied by carbonate, chlorite, adularia and barite. The ore lodes of the Cerro dos Martins consist of a set of Cu-sulfide NW-trending veins and disseminations within the volcanic-sedimentary sequence. Chalcocite and bornite are the main ore minerals with minor chalcopyrite, galena and sphalerite, whereas carbonates, barite, quartz and hematite are the gangue. The volcanic host rocks show an alkaline affinity and the age of mafic–related dyke is 550 Ma. Geothermometry of the three deposits show deposition temperatures within the hydrothermal field (maximum of ca. 215 to 300 C). Sulfur isotope @34S results of sulfides show homogeneous composition, with values in the range of -1.8 to +0.6h (Camaquã); -7.1 to+1.1h (Santa Maria) and -6.2 to +0.9 h (Cerro dos Martins) which indicates a magmatic origin for the sulphur of the deposits