Ocorrências de grafita no complexo Tanque Novo - Ipirá, Nordeste do cráton do São Francisco, Bahia, Brasil: caracterização e potencial metalogenético

Abstract

Brazil is the third biggest producer of graphite worldwide, with the state of Bahia being the second largest producer in the country, as most of the production comes from the Graphite Province of Bahia-Minas. The study area is situated at 350 km to the north of this producing region, in the Tanque Novo-Ipirá Complex, Itabuna-Salvador-Curaçá Orogen, São Francisco Craton. The area is comprised of host rocks with occurrences of graphite such as schists, marbles, and granulites as well as country rocks characterized by quartzites, calc-silicate rocks, marbles, and iron formations. The petrographic analysis supported by MEV/EDS indicates two generations of graphite: the first one, connected to the graphitization process generated by metamorphism (syngenetic), exhibit flakes with three distinct associations related to the greenschist, amphibolite, and granulite facies in which the host rocks were subjected; and the second generation, is related to the percolation of hydrothermal fluids (epigenetic) due to retrograde metamorphism in the greenschist facies that promoted the deposition of graphite in venules or as a result of carbonation. The graphite bodies show a high total graphitic carbon (TGC), analyzed by LECO infrared, ranging from 8.70 to 15.98wt% and they occur in two distinct lithologies: aluminous and carbonate. The geochemical characteristics of the host and country rocks indicate sedimentary protoliths with signatures influenced by diagenetic alterations and detrital input in a marine paleoenvironment. However, they were not sufficient to change the anomalies of Ce/Ce* (0.64 to 1.49) that alongside the ratios of U/Th (0.04 to 1.30) suggest a paleoenvironment dominantly oxic. The analyzes of TGC with P/Ti, Ba/Ti, Ni and Cu show correlations that suggest the deposition of organic matter in a context of high organic paleo productivity. This depositional setting combining with the phosphorus anomalies in the country rocks, deposited during the Paleoproterozoic in an oxic paleoenvironment, suggest that the deposition of organic matter that originates the graphite might be related to the increase in paleo productivity generated following the Great Oxygenation Event (GOE), which culminated in the first major phosphogenesis event in the geological record