Neoproterozoic Re–Os systematics of organic-rich rocks in the São Francisco Basin, Brazil and implications for hydrocarbon exploration

Abstract

The São Francisco Basin contains a remarkable archive of Neoproterozoic strata and its hydrocarbon-bearing strata are receiving increasing attention as global oil and gas exploration targets progressively deeper and older rocks. New Re–Os geochronology for the Paracatu Slate Formation of the Canastra Group, Brazil, yields a depositional age of 1002 ± 45 Ma. This age represents the first successful application of the Re–Os system to rocks of this group and indicates excellent agreement with a previously published U–Pb detrital zircon age (Rodrigues et al., 2010). Together with TOC values of ca. 2 wt.% (despite greenschist metamorphism), it might be argued that the São Francisco Basin has had the potential for hydrocarbon generation since the Tonian (1000–850 Ma). In addition, we also report an imprecise Re–Os age (1304 ± 210 Ma) for the Serra do Garrote Formation, a further potential source rock of the Vazante Group. We suggest, based on petrological evidence, that the Re–Os systematics were disturbed by post-depositional fluid flow that was most likely associated with Vazante ore deposit mineralization. An attempt to determine a Re–Os date for the Sete Lagoas Formation, a putative post-Sturtian cap carbonate, is precluded owing to low Re abundances (≤100 ppt). Major environmental changes in the aftermath of the Jequitaí glaciation, particularly the development of palaeotopography such as subglacial tunnel valleys, may account for the apparent random distribution of TOC enrichment in these Cryogenian/Ediacaran post-glacial deposits. This scenario might thus have major implications for the hydrocarbon prospectivity of this post-glacial succession.