Abstract

Petrographic observations, backscattered electron imagery (spatial resolution = + or -0.1 mu m), and semiquantitative energy dispersive electron microprobe analysis indicate that postdepositional modification of the uraniferous conglomerates occurred in three stages. The first involved complete fluid-mediated leaching of Fe from detrital ilmeno-magnetite grains and mobility of U, Th, rare earth elements, Y, PO 4 (super -2) , and SiO 2 . Uraninite was replaced by coffinite ([U, Th]SiO 4 ) and quartz, detrital monazite was dissolved and slightly altered to urano-thorite ([Th, U]SiO 4 ), U reacted with TiO 2 derived from detrital grains to form composite grain aggregates composed of "brannerite" ( approximately UTi 2 O 6 ) and other alteration products, and coffinite together with a Y-rare earth elements-U phosphate precipitated in the matrix. The next stages involved the precipitation of the secondary pyrite and the occlusion of the conglomerate pore space by secondary quartz and sericite. The most probable Eh-pH fields for the ground waters involved in postdepositional modification are low to moderate Eh and slightly acid pH for ilmeno-magnetite leaching, and low Eh and near-neutral pH for pyrite precipitation. Both inferred Eh-pH fields fall within the thermodynamic stability fields of uraninite and coffinite in which uranium solubilities are low. Phosphate dissolved from monazite and apatite could have enhanced uranium solubility at the higher inferred Eh values. Conclusion that neither the deposition nor the preservation of detrital uraninite can logically be used to draw conclusions about the oxygen content of the Precambrian atmosphere at approximately 2,350 m.y. ago.--Modified journal abstract.

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