Stable carbon isotopic ratios of a suite of uraniferous hydrocarbons (Lower Jurassic) from the northeast Wales ore field and related deposits have been determined. The isotopic compositions (–11.41 to –29.36% PDB) become increasingly heavy with increasing uranium content (0.001–35.78wt%). Fractionation is thought to occur by repeated preferred bond breaking between 12C–12C atoms with subsequent volatile production and loss, removing 12C atoms. The extent of fractionation may also be controlled by the amount of stable aromatic material in the organic matter. Petrographic features of these uraniferous hydrocarbons, and others from occurrences in South Africa and Scandinavia, suggest reaction of radiation with organic matter. Alpha particles may be responsible for the development of high reflectance haloes around uraninite grains in uraniferous hydrocarbons from the Witwatersrand region. Bitumen reflectance falls in a non-linear trend from 3.75 to 1.75% (in oil) in a 20 μm radius round the grains. During irradiation, bitumen anisotropy (measured as bireflectance) is developed in response to increasing aromaticity and the development of a short range ordered structure. Bireflectance measurements vary from 0.047%–0.173% in samples from around the Irish Sea Basin, and 0.001%–0.294% in a suite of samples from Scandinavia. The degree of anisotropy is dependent on the bitumen composition, and varies between sample suites of differing geological histories.

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