Abstract

ln the Lagoa Real region of the Sao Francisco craton altered basement, amphibolite-granulite facies felsic gneisses host significam uranium deposits. Retrograde alteration under epidote-amphibolite facies conditions as revealed by mass balance calculations is dominated by sodium addition, oxidation, silica leaching and uranium enrichment. Oxygen isotope data indicate that uranium was precipitated from isotopically light fluids at temperatures near 500°C. Fluid inclusion data indicate similar temperatures at variable salinities. It is proposed that the geochemical observations are in accord with a model where the basement has been thrust over Proterozoic sediments and processes are controlled by an inverted thermal gradient.

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