Abstract

Uranium deposits cumulatively in the 100,000 tons U 3 O 8 range occur within ductile shear zones transeeting the Proterozoie Lagoa Real granitie complex, enclosed in Archcan and Proterozoie basement gneisses of the Sao Francisco craton, at the Lagoa Real region of south-central Bahia, Brazil. The gneisses have been subjected to amphibolite and granulite metamorphic facies metamorphism, and the gneissose granites to amphibolite facies. To the west, the gneisses overlie the Proterozoie Espinhaco metasedimentary sequence along a thrust fault.Petrography and mineral chemistry show that in the zones of metasomatie alteration and mineralization, the original K feldspar + quartz + oligoelase + hastingsite assemblage is replaced by albite + aegirine-augite + andradite + hematite assemblages, with or without uraninite. Uraninite precipitation has occurred as a response to water-rock interaction via reduction of the metasomatie fluid, with mineralizing phases being controlled by the mafic mineral transformations. This information along with oxygen isotope, whole-rock geochemistry, and fluid inclusion studies indicates that the alteration process involved removal of Si, K, Rb, and Ba and addition of Na under oxidizing conditions. V, Pb, and Sr were introduced along with U via interaction with saline, isotopically light fluids under varying water/rock ratios and at temperatures of 500 degrees to 550 degrees C. The 87 Sr/ 86 Sr systematics suggest that it is unlikely that Sr, and by extension uranium, was introduced by fluids originating from the gneisses.Geologic constraints and the general alteration pattern are consistent with release of the mineralizing fluids in response to overloading of the basement and granitic rocks onto the sedimentary Espinhaco via a thrust mechanism.

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