Epigenetic hydrothermal origin of native copper and supergene enrichment in the Vista Alegre district, Parana basaltic province, southernmost Brazil

摘要

Native copper is widespread in the Lower Cretaceous Parana basaltic province, southern Brazil, both as films in fractures and as massive balls in amygdules. The focus of this investigation is on the large concentration of occurrences (n = 85) that forms the Vista Alegre district in the border region of Rio Grande do Sul and Santa Catarina states. The high average of 220 ppm Cu content of the basalts resulted in ore of native copper, Cu oxides, abundant chrysocolla in the top of mineralizations, and minor malachite and azurite. Native copper is associated with dioctahedral and trioctahedral smectites, zeolites (heulandite and clinoptilonite), quartz, and calcite, typical of a low-T (100–150℃) hydrothermal alteration assembly. The PGE distribution shows enrichment in Pd in relation to Pt both in basalts and in native copper, supporting the hypothesis of hydrothermal origin of the mineralization. No evidence was found of direct precipitation of copper from the lava; based on field and petrographic evidence, integrated with BSE images, EPMA analyses, EGP contents of native copper, and bulk rock analyses, this is an epigenetic hydrothermal copper mineralization, followed by supergene enrichment.