Mineralizações de fosfato paleoproterozoico no nordeste do cráton do São Francisco: condições paleoambientais e o evento Lomagundi-Jatuli.

Abstract

In the Northeast region of Bahia, important metasedimentary successions associated with the São Francisco Cráton. They are elongated N-S and NW-SE bodies, intruded by granitoids and metamorphosed in the high amphibolite to granulite facies. The siliciclastic, pelitic and chemical successions stand out, such as marble, calcisilicate rock, quartzite, graphite-schist, iron formations and aluminous rocks from the Tanque Novo – Ipirá Complex (TNIC), the Vale do Jacurici region (JV) and the Rio Salitre Complex (RSC). TNIC marble and calcisilicates are hosts of primary phosphate mineralization (apatite) with a disseminated form and P2O5 contents of up to 3.2%. Despite metamorphism, it is possible to identify records of primary signatures based on the behavior of rare earth elements and yttrium. When normalized to the Post-Archaean Shale (PAAS), they are marked by light rare-earth element depleted patterns, true negative Ce anomalies, positive Y and Gd anomalies. Eu anomalies are variable and inherited from the source composition or altered by metasomatic fluids. The formation of the carbonate sequence from TNIC and JV occurred on the continental margin of a paleo ocean and in an open sea, both with continental inputs, coming from the margins or magmatic arcs. In the RSC, they are related to an open sea developed into a continental rift. The positive δ13C excursions in TNIC marbles have values between +6.13 to + 7.36%. Two samples have δ13C from +9.38 to +9.69 and one sample with negative δ13C (- 5.72%) was reduced by metasomatic effects in the skarn zone. The δ13C isotopic data are similar to marine carbonates from the Paso Severino Formation (Uruguay), with δ13C of +9.0%. On RSC the marbles also present δ13C excursions of +5.38 and +6.78%. The global Lomangudi-Jatuli event (~2.3 to 2.1 Ga) has positive δ13C excursions between +5 and +10% in marine carbonates from several global cratons of Paleoproterozoic age that harbor phosphate mineralization, such as the Rio de La Plata (Argentina), Slyudyanka Complex (Russia) and marbles from the Jacurici Valley in the São Francisco Craton. The maximum age of deposition of the TNIC is 2128 Ma (U/Pb – LA-ICPMS) obtained in quartzite from the upper metasedimentary sequence. Thus, the deposition of phosphate and iron of continental origin are associated with a record of higher oxygenation rates of the atmosphere and oceans and biogenic activity, being compatible with the paleoclimatic changes that occurred in the Paleoproterozoic. The carbonate sequence were precipitates at the end of the Great Earth Oxygenation Event and at the end of the Lomangudi-Jatuli event, after the fourth and final (2.2 Ga) of a series of glaciations beginning in 2.42 Ga. The marbles are correlated with the metasediments in the Jacurici Valley terrain in terms of paleoenvironmental conditions, marine origin and phosphate mineralization. These data are compatible with the earliest phosphogenetic event in Paleoproterozoic basins and record atmospheric and oceanic oxygenation.