Uraninite, detrital

Figure 7 Detrital grains of uraninite with characteristic dusting of galena, partly surrounded by PbS overgrowths. The big grain displays a typical muffin shape. Basal Reef, footwall Loraine Gold Mines, South Africa oil immersion 375X (source Schidlowski, 1966).

Ono S. (2002) Detrital uraninite and the early Earth s atmosphere SIMS analyses of uraninite in the Elliot Lake district and the dissolution kinetics of natural uraninite. Doctoral Dissertation, Pennsylvania State University, State College, PA. 

Several lines of geochemical evidence support low to negligible concentrations of atmospheric O2 during the Archean and earliest Proterozoic, when oxygenic photosynthesis may have evolved. The presence of pyrite and uraninite in detrital Archean sediments reveals that the atmosphere in the earliest Archean contained no free O2 (Cloud, 1972). Although Archean-age detrital pyrites from South Africa may be hydrothermal in origin, Australian sediments of the Pilbara craton (3.25-2.75 Ga) contain rounded grains of... 

The writer has also been impressed by the repeated occurrence of fine uraninite within thucholite bodies as illustrated on Plate 23 of Schidlowski (1970). These small crystals of uraninite give the impression of having crystallized in situ before the remains of the organism became compacted. It thus seems probable that this fine uraninite was trapped either during the life, or no later than very soon after the death, of the organisms. Certainly detrital minerals, including uraninite, are present, but a large part of the uranium appears to have been extracted from solution. 

Phillips et al. (2001) discussed the central evidence on which the reduced model of the Archaean atmosphere is based. This evidence includes the composition of detrital gold grains, inferred detrital uraninite, inferred detrital pyrite, and inferred palaeosols, especially in the Witwaters-rand succession. In a careful detailed review. Philips et al. concluded that the geological evidence for a reducing atmosphere remains ambiguous. In particular, post-depositional processes may need far more examination. Phillips et al. pointed out that some of the mineralogical and field evidence can be interpreted as supporting an oxidized Archaean atmosphere. 

Unlike iron and other common metals, the higher oxidation state of uranium, U(VI), is water soluble, and its lower oxidation state, U(IV), is insoluble. So during the deposition of detrital uraninites, which are mostly older than 2 Gyr, the level of atmospheric oxygen must have been low, <0.01 PAL (Kasting 1993 Rasmussen Buick 1999). Large-scale deposition of detrital pyrite (FeS2) similarly mostly occurred before 2 Ga. 

The mineralization age is close to that of widespread igneous activity in South Africa, such as intrusion of the Bushveld Complex and metamorphism of the Witwatersrand system, at 1,950 m.y. (Nicolaysen et ai, 1958). The lead isotopic data support the petrologic evidence presented by Liebenberg (1955) that the lead was largely derived from detrital uraninite in the Dominion Reef conglomerate. 

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