Characteristics of organic matter in uranium-rich coaly shales from miocene sequence at Kanamaru

We obtained a continuous 45 m-long core from the Miocene sedimentary sequence and basement Cretaceous granite at Kanamaru, northeast Japan. The Miocene sequence intercalates with a uranium-rich seam (U=25-100 ppm; Th=23-42 ppm). We analyzed the kerogen and biomarkers in the core to characterize the organic matter. Visual kerogen analysis indicated that coaly and woody kerogen is abundant in relatively organic-rich samples, while amorphous kerogen is abundant in organic-lean samples. Pyrolysis gas chromatography-mass spectrometry demonstrated that the organic matter in pyrolysates comprises mainly alkyl-aromatic hydrocarbons (alkyl-benzenes, alkyl-indenes, alkyl-naphthalenes, etc.) and n-alkanes. Acyclic isoprenoid alkanes (mainly pristane), n-alkenes, and n-fatty acids were detected as minor components. Most of these compounds are characteristic of the type-III kerogen that derived from terrestrial higher plants. These results suggest that amorphous kerogen originated from terrestrial higher plants, and we speculate that they were heavily degraded by oxidation and lost their woody texture. Thermal alteration index (TAI) of Pinus pollen was about 2.6, and Tmax values of Rock-Eval pyrolysis range from 441degC to 444degC, which indicate that the thermal maturation of the coaly shale reached the stage of early catagenesis. This maturity was also suggested by high abundance of diagenetically generated isomers of hopanes in pyrolysates. In the sediment sequence, a peak of uranium and uranium/thorium ratio is ca. 2 m shallower than the peak of organic carbon content, but it corresponds to a lithological boundary. This inconsistency suggests that organic matter was not involved in concentrating uranium. Uranium was possibly concentrated from groundwater at the lithological boundaries between different redox levels during the period of high groundwater level. (author)