Espectroscopia micro-Raman e MEV/EDS aplicados a minerais do Craton São Francisco: evidência da formação de pirita mediada biogenicamente e suas conexões com a origem da vida

Abstract

The occurrence and isotopic composition of carbonaceous components in ancient rocks are strong evidence of life forms on the Earth's surface since the Eoarchean (> 3.7 Ga). These organic components are found in rocks formed in different contexts and the earliest examples of organic matter are found in Isua Greenstone Belt rocks (> 3.7 Ga) in Greenland; in the Pilbara Craton, at the North Pole Dome region (> 3.4 Ga), Australia; and in South Africa, in the Greenstone Belt of Barberton (> 3.2 Ga). In these examples, organic matter is associated with rocks formed in oceanic environments, in the vicinity of exhaling fumaroles, as well as in coastal environments. Another important example is the Witwatersrand Basin (2.9 to 2.7 Ga) in South Africa, where the organic matter is found widespread as bitumen, oil fluid inclusions and graphite inclusions in detrital components of conglomerate horizons. Organic matter in this basin has an intrinsic relationship with gold mineralization, uranium, and pyrite. Specifically, in the São Francisco Craton, State of Bahia, there are two deposits that have similarities with those mentioned above, although there are differences between their ages of formation. The first of these occurs in the Mundo Novo Greenstone Belt. The second deposit, represented by the conglomerates of the Jacobine Basin, is in many ways similar to the Witwatersrand Basin and its mineralization of gold, uranium, and pyrite. In both deposits are reported the occurrence of organic matter. For this reason, analyzes were performed via micro-Raman spectroscopy and SEM/EDS on pyrite grains to identify and characterize the association of the organic matter with gold deposits of Jacobina Basin and the Fazenda Coqueiro VMS deposit, of the MNGB, Bahia, Brazil. The results clearly show the presence of carbonaceous agglomerates (characteristic of organic matter) - peaks between 1400 and 1660 cm-1. Moreover, the presence of cline and calcochlorites also identified in the micro-Raman spectra potentiate the presence of organic matter. In addition, it will be held later dating (direct or indirect) of the material in order to include it within the Archean Era. This result is of great importance to science because it would record that primitive life forms on Earth's surface would be dated more than 3 Ga.