The Nabarlek uranium deposit, Northern Territory, Australia; some petrologic and geochemical constraints on genesis

Abstract

The Nabarlek uranium deposit, Northern Territory, Australia, is confined to a shear zone in contorted and metasomatized early Proterozoic schists and occurs near an unconformity with overlying middle Proterozoic sandstone. Massive chlorite + or - sericite + or - hematite rocks, breccias, and intensely altered schists are characteristic of the ore zone, and at least three generations of chlorite have been recognized and described. The primary ore mineral assemblage is dominated by uraninite intergrown with chlorite. The orebody was sericitized at or after 920 m.y. ago, resulting in the widespread replacement and breakdown of chlorite, the formation of hematite, and the solution and redeposition of uraninite. Redox reactions involving iron and uranium are evident in sericitized rocks containing hematite and residual uraninite. High U/Th ratios in the ore zone suggest that uranium was transported to the site of deposition as a uranyl complex. Although no carbonaceous material occurs in the ore zone, isotopic data on minor carbonate associated with uraninite suggest that organic material was originally associated with the mineralization and indicate that these carbonates have evolved from the interaction of hydrothermal fluids with this material. Erosion of the middle Proterozoic cover rocks in the recent past has exposed themore » deposit to the effects of weathering.« less