Uranium enrichment in Archaean crustal basement associated with overthrusting

Abstract

Uranium deposits cumulatively in the 100,000 tonne U₃O₈ range occur within ductile shear zones transecting Archaean basement gneisses at Lagoa Real, north-east of Caetité, State of Bahia, Brazil^1–3 (Fig. 1). The gneisses, dated at 2,600–3,000 Myr, are at amphibolite to granulite facies of meta-morphism. There is a general concensus that such rocks form near the base of the crust, and typically exhibit pronounced depletion of U relative to the upper crust^5,6. To the east and west of Lagoa Real the Archaean gneisses overlie Proterozoic meta sediments and volcanics of the Espinhaço supergroup along well documented thrust surfaces^7,8. Limited U–Pb dating of the deposits indicates that U-mineralization occured at ∼820 Myr (ref. 9), involving removal of Si and K coupled with Na addition in oxidizing conditions. V, Pb and light rare-earth elements (LREE) were introduced along with uranium, from hypersaline CO₂-hydrocarbon-rich fluids, with δ¹⁸O–4‰ at temperatures of ∼540°C, depths of ∼15 km, and P_fluid > P_load. We show here that the geological constraints are consistent with release, through hydrofracturing, of ∼1,000 km³ of over-pressurized-meteoric water-recharged formation brines in the Proterozoic sediments up through the hot overriding gneisses.