The origin and properties of uranium-niobium-tantalum mineralised hydrocarbons at Narestø, Arendal, southern Norway.

The mineralogy, chemical and physical properties of uraniferous hydrocarbons (0.86 wt.% uranium) from a pegmatite at Narestø, near Arendal have been investigated. The hydrocarbon contains irregular inclusions of uraninite and grains of xenotime and monazite, relics of minerals formerly in the pegmatite mass. The uraninite has been corroded by reaction with the hydrocarbon. Aeschenyte and quartz fill fractures and form overgrowths around other minerals within the hydrocarbon. The properties of the hydrocarbon (specific gravity = 1.51-1.55, R0 = 1.63, hardness = 5-6, H/C = 0.52-0.63 and ?13C = -26.6% to -26.9%. PDB) compare with those of thermally altered hydrocarbons from Kongsberg and Åmot. These differ only in having lighter stable carbon-isotopic compositions and lower H/C ratios. In comparison, thermally immature bitumens from Sweden have high H/C ratios (1.04-1.46), low specific gravities (1.0-1.2) and reftectances (R0 = 0.01-0.17). The isotopic compositions of the Swedish bitumens compare with the non-uraniferous hydrocarbons from Norway. A biogenic source for the Narestø hydrocarbon is suggested on the basis of carbon-isotopic evidence.