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Uranium/thorium (U/Th)-rich bitumen has been discovered within both Palaeoproterozoic black pelites and the Archean granitic basement of the Rum Jungle Mineral Field, Northern Territory, Australia. Granite-hosted bitumen occurs as small (up to 400-mm diameter) discrete individual nodules, which exhibit many morphological similarities to those observed in Phanerozoic siliciclastic rocks. Thorium, the dominant radioelement, occurs primarily as a hydrated Th-Y-Si-P phase. Uranium-rich inclusions are rare, and correspond to a hydrated U-Th-Y-Si-P phase, identified as coffinite-thorogummite. Metasediment-hosted bitumen is more variable in morphology, occurring as massive (<2 cm in width) veins that cross-cut all foliations, as discrete individual nodules or as elongate seams (up to 500 mm in length), interpreted to represent a series of coalesced individual nodules. In all examples, uranium, the dominant radioelement in the metasediment-hosted bitumen, is present as Th-poor uraninite, with variable Y 2 O 3 contents (up to 3.21 wt.%).Raman investigation of all types of bitumen indicates that it is a poorly organized carbonaceous matter, which has not been subjected to metamorphism. Consequently, a post-metamorphic timing for hydrocarbon emplacement can be inferred and a magmatic origin can be precluded.Potential source rocks for the bitumen are black shales of the Whites Formation (up to 8 wt.% total organic carbon (TOC)) and the Koolpin Formation (approximately 13 wt.% TOC). Post-metamorphic sericitization of rocks within the Whites Formation is accompanied by a near-complete removal of organic matter. Alteration was possibly the catalyst for hydrocarbon generation.The Th-Y-Si-P phase within the granite-hosted nodules is interpreted to be the result of the alteration of antecedent monazite. During this alteration, U, LREE and P were fractionated and removed, while Th, Y and Si remained immobile, and recombined to form a hydrated Th-Y-Si phase. This pervasive alteration within the basement U/Thrich granites is proposed as a genetic model for the formation of uranium deposits in the Rum Jungle Mineral Field and possibly unconformity associated uranium deposits on a global scale.

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