Metamorphism, Metasomatism And Mineralization At Lagoa Real, Bahia, Brazil

摘要

Uranium deposits cumulatively in the 100,000 tonne U(,3)O(,8) range occur within ductile shear zones transecting Archean basement gneisses of the Sao Francisco Craton, at the Lagoa Real region of south-central Bahia, Brazil. The gneisses, dated at 2.6-3.0 Ga, are at amphibolite and granulite facies and overlie to the west, the Proterozoic Espinhaco metasedimentary sequence along a thrust fault.;Petrography and mineral chemistry show that in the zones of alteration/mineralization, the original k-feldspar + quartz + albite/oligoclase + hastingsite assemblage, is replaced by albite + aegirine-augite + andradite + hematite assemblages, with or without uraninite. This information along with oxygen isotope, whole rock geochemistry and fluid inclusion studies indicate that the alteration process involves removal of Si, K, Rb, Ba and addition of Na under oxidizing conditions. V, Pb, Sr were introduced along with U via interaction with saline CO(,2)-rich, isotopically light fluids under varying water/rock ratios and at temperatures of 500-550(DEGREES)C. (\u2787)Sr/(\u2786)Sr systematics suggest that it is unlikely that Sr, and by extension uranium, were introduced by fluids originating from the basement gneisses.;Geological constraints and the general alteration pattern are consistent with the release of the mineralizing fluids in response to overloading of the basement rocks onto the sedimentary Espinhaco via a thrust mechanism.