Ore genesis at the Camaquã copper mine, a neoproterozoic sediment-hosted deposit in Southern Brazil

The Camaquã copper mine includes the Uruguai and the São Luiz orebodies, hosted by sandstone and conglomerate of the Neoproterozoic to Early Paleozoic Camaquã basin. Despite a great controversy regarding the evolution of the Camaquã basin, it is generally accepted that the basin was initially marine and progressively changed into a definite continental environment. The ore consists of massive sulfides in veins, pipes and stringers, and disseminated sulfides. The sulfide paragenesis in the primary ore consists of chalcopyrite, bornite, chalcocite, and pyrite. Other sulfides, such as wittichenite, idaite, molybdenite and carrollite are minor phases. Native gold is closely associated with hematite. A number of supergene minerals occur in the weathering zone, including covellite, digenite, antlerite, chrysocolla, brochantite, cuprite, malachite, azurite, tenorite, and native copper. Traces of gold and silver have also been found in the oxidized zone close to surface. Primary ore formation could be the result of multiple mineralization events. In a discrete mineralizing event, quartz was the first mineral to form, followed by pyrite, chalcopyrite and bornite. Carbonate and barite appeared by the end of the process. Sulfur isotope analyses of pyrite, chalcopyrite and bornite show little variation, δ34SCDT clustering around 0‰, indicating a magmatic origin for the sulfur. On the other hand, δ34S values of +10.30‰ to +14.24‰ for barite indicate probable oxidation due to the mixing of magmatic and meteoric SO4−2-rich fluids. Analyses of Pb isotopes in chalcopyrite, pyrite and bornite from the Camaquã deposit plot conformably with the reference isochron of 592 Ma built for regional igneous rocks (SHRIMP U/Pb zircon analyses) specifically the Lavras-Caçapava granite. Microthermometric measurements in fluid inclusions of quartz veins indicated temperatures varying from 236 to 60 °C. Fluid inclusion in carbonate displayed trapping temperatures between 150 and 50 °C, whereas the trapping temperatures in barite are below 80 °C. The fluid salinity varies largely from one mineral to another: 2.7 wt.% eq. NaCl to near pure water in quartz; 14 wt.% eq. NaCl in carbonate; 8 to 10 wt.% eq. NaCl in barite. The structural control of mineralization, the occurrence of superimposed mineralizing events, the temperature of deposition of the early-formed ore minerals (330 to 190 °C), and the sulfur isotope ratios, indicate that the Camaquã Mine may be considered as a hydrothermal type deposit, resulting of distal magmatic fluids focused into a relatively oxidized clastic succession.