Petrology and geochemistry of alteration associated with uranium mineralization at Jabiluka, Northern Territory, Australia

An aureole of retrogressive metamorphism in former amphibolite facies schists extends at least 200m from uranium ore zones at Jabiluka. Chloritization of biotite, and replacement of feldspar by sericite-chlorite aggregates, are the most distinctive reactions. Similar but more intense alteration occurs in the immediate vicinity of uraninite-bearing veins and breccia zones. This retrogressive metamorphism was not isochemical. Compared to their amphibolite facies parents, the altered schists are enriched in Li and Mg, and dramatically depleted in Na, Ca, Sr and Ba. There is little significant geochemical difference between schists in the outer portion of the retrogressed aureole and those closer to ore. However the immediate hosts to ore, within about 1m, are distinctly enriched in uranium associated with chlorite-septechlorite alteration products rather than with disseminated uraninite. Further but mild enrichments in Li and Mg occur, and the immediate hosts are also distinctly depleted in K and Rb. Compared to the country rocks, Jabiluka uraninite ores are significantly enriched in Ca, Sc, Y, and rare earth elements, and also in radiogenic Pb and certain chalcophile trace metals including Cu, As, Mo, Ag and Sn. Uranium appears to be its own best pathfinder at Jabiluka for exploration in unweathered rocks. Other geochemical characteristics of the ores and their host rocks may have applications to advanced stages of exploration programs, and they also provide a guide to possible pathfinder elements useful in weathered situations. (author)