Influence of paleoenvironment and radiolytic alteration on the geochemistry of organic matter from Autunian shales of the Lodeve uranium deposit, France

Abstract

Thirty-three samples of shales from the Lodeve Permian uranium deposit were examined by different analytical techniques. Samples were analyzed by Rock-Eval pyrolysis and by gas chromatography-mass spectrometry after extraction and fractionation. The determination of uranium content was carried out by atomic emission spectrometry. The composition of the organic extracts allows two groups of shales to be distinguished: the first one is marked by a high proportion of aliphatic hydrocarbons and the second by the predominance of aromatic compounds. These two groups exhibit distinctive biomarker characteristics. One group is characterized by (1) pristane/phytane ratios higher than 1, (2) distributions of terpanes dominated by the tricyclic terpanes and (3) the predominance of C 29 and C 28 steranes. The second group is marked by (1) pristane/phytane ratios lower than 1, (2) the predominance of the pentacyclic terpanes over the tricyclic terpanes and (3) the predominance of C 27 and C 29 steranes. It is suggested that the differences observed are mainly related to different depositional environments, although the uranium content may also influence some parameters such as the hydrogen index and the extraction yield.