Geologic setting and concepts on origin of uranium deposits in the East Alligator River region, N.T., Australia

Pancontinental Min. Ltd., Sydney, Australia

Major uranium deposits, including Jabiluka, Ranger, Koongarra, and Nabarlek, plus numerous smaller prospects, occur in metamorphosed lower Proterozoic psammatic, pelitic, and chemical sediments of the Cahill Formation (South Alligator Group). These transitionally overlie migmatic metasediments of the Mt. Partridge Formation that occur marginal to Archean granitic complexes. The migmatites are the result of anatectic rejuvenation of the Archean complexes during the late lower Proterozoic and these events were accompanied by regional metamorphism of the Cahill Formation. After metamorphism, pegmatites were intruded, followed by basic intrusives. A period of retrograde metamorphism, uplift, and erosion preceded middle Proterozoic deposition of the Kombolgie Formation. There is a marked spatial relationship of discovered uranium occurrences to the erosional margin of the lower Proterozoic-middle Proterozoic unconformity. Mineralization typically occurs in Cahill Formation chlorite and/or graphite schists which have undergone structural disruption. The main ore mineral is pitchblende which typically occurs in a massive, amorphous form with lesser amounts of disseminated crystalline uraninite. There is pervasive chlorite alteration of the host rocks and chlorite, quartz, and sericite are the main gangue minerals. Associated sulfides include pyrite with lesser chalcopyrite and galena. Gold occurs locally in economic concentrations. The ore deposits are considered to be of postprograde metamorphism in origin. The main stages of uranium concentration consisted of remobilization and introduction into open-space environments. The initial stage of uranium remobilization coincided with regional metamorphism around 1,700 m.y. ago. The main uranium mineralizing events are dated at approximately 900 and 500 m.y. and postdate deposition of the Kombolgie Formation. The latter sediments appear to have protected the deposits from surface oxidizing and dispersing processes until the present time.

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