Abstract

Maureen is the largest among several U (-Mo-F) prospects occurring along a Late Devonian unconformity in the Georgetown area, northern Queensland, Australia. Mineralization is structurally controlled by the intersection of steep east-west fractures with an unconformity between a Proterozoic basement and a Paleozoic cover sequence of continental sedimentary rocks and abundant rhyolitic volcanic rocks. The mineralogical composition of high-grade ore (pitchblende + Fe-rich molybdenite + arsenopyrite + arsenian pyrite + fluorite + dickite + chlorite + goyazite ± graphite or hematite), postlithification brecciation, and quartz dissolution indicate a strong chemical gradient and disequilibrium during mineralization between the reduced basement and the largely oxidized cover sequence. Elemental and mineral zonation centered to faults, and fractures cutting the unconformity, indicates locally reducing and quartz-dissolving conditions during the formation of tabular U-Mo orebodies, which are surrounded by a halo rich in fluorite.

A detailed petrographic study, microthermometry, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of quartz- and fluorite-hosted fluid inclusions provide concentrations of ore-forming components in fluids related to hydrothermal uranium mineralization. Three fluids were involved in the mineralizing process at Maureen: a likely oxidized saline (L3) fluid, an aqueous (L1) fluid, and a CH4-bearing carbonic vapor (cV). The oxidized saline (L3) fluids trapped in fluorite and dickite-bearing hydrothermal quartz veinlets crosscutting detrital quartz show the highest average concentrations of U (10–47 ppm), Mo (489–888 ppm), and As (318–777 ppm), with element ratios close to those of high-grade mineralized rocks. We suggest that aqueous (L1) fluids mixed in the cover sequence with oxidized saline fluids (L3) to precipitate fluorite as a distal halo surrounding the east-west fractures. Their mixture, an oxidized moderately saline fluid (L2), reacted with methane-bearing carbonic vapor (cV) to precipitate U and Mo by reduction, close to the intersection of the fractures with the unconformity. Geochemical analysis of coexisting vapor and liquid assemblages indicate mixing of oxidized saline fluids with reduced carbonic vapors to be the driving force for uranium precipitation. The Maureen deposit shares essential geologic characteristics and hydrothermal processes with Proterozoic unconformity-related U deposits, but probably owes its unusual element association of U with (equally redox sensitive) Mo and abundant F to a source region in the volcano-sedimentary cover sequence that was enriched in these elements, due to the presence of highly fractionated, easily leachable, and oxidizable felsic volcanics.

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