As sucessões carbonáticas neoproterozóicas do Cráton do São Francisco e os depósitos de fosfato: correlações e fosfogênese

Abstract

The neoproterozoic carbonate successions of the Sao Francisco craton are formed by thick carbonate and siliciclastic strata accumulated in epicontinental seaways, andon passive margins. At least two transgressive-regressive sea level cycles have been recognized during the evolution of the carbonate mega-sequences. These successions, represented by the Vazante, Bambui, and Una Groups, lie above basal glacio-marine diamictites of probable Sturtian age, and an upper glacio-marine diamictite, was observed in upper units of the Vazante Group. The broad similarities of lithofacies, as well as the presence of phosphate and sulfide (Zn,Pb) deposits restricted to narrow stratigraphic intervals, suggest that the Vazante, Bambui and Una Groups may be correlative. The Sr isotope data from well preserved carbonates and carbonate fluorapatite from each of these successions , ranging from 0.70763 to 0.70794, support the general correlation and are indicative of seawater composition around 650 Ma, although the lower 87 Sr/ 86 Sr values of 0.70614 at the Vazante carbonates may suggest that this unit is older than the Bambui and Una carbonate rocks. Carbon isotope data of carbonate fluorapatite reveal sharp negative excursions between -3.47 and -12.25‰ VPDB in the cratonic area , and between -1.3 and -9.61‰ VPDB in the carbonate fluorapatite hosted by the passive margin Vazante carbonates, suggesting that primary P concentration was accumulated in strong anoxic environments in both geotectonic settings. Carbonate hosting the phosphate deposits in these units are moderately enriched in 13 C , ranging from positive a 13 C values of +2.0 and +9.6‰ VPDB in the different sections. Primary phosphate accumulations occur above glaciogenic successions. We suggest that these concentrations may be due to high organic productivity after glacial events.