Reflected-light microscopy of uraniferous bitumens

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Abstract
Uraniferous bitumens from Great Britain, Scandinavia and South Africa have been studied by oilimmersion reflected-light microscopy and categorised into those formed either by replacement of preexisting uraninite and pitchblende or by complexation/reduction mechanisms in pre-existing hydrocarbons. The former are characterised by displaying normal replacive textures, and containing high concentrations of non-mineral-bound uranium, or later, occasionally exotic, uraniferous fracturefilling phases. Uraniferous bitumens formed during complexation/reduction reactions display monotonous mineralogies and ordered mineral-inclusion distributions.Radiolytic alteration of uraniferous bitumens induces both chemical and mechanical alteration. Early alteration is marked by the generation of mobile hydrocarbons during 'cracking reactions' with subsequent within-sample migration to form globular bitumens and dendritic interspersions of mineralrich and -poor uraniferous bitumen. Mobile hydrocarbons may act as lubricants during mechanical deformation. Advanced organic alteration is characterised by well-documented increased reflectance around uraniferous grains, and by fracturing of the bitumens.