URANIUM MINERALIZATION AND ITS RADIOACTIVE DECAY-INDUCED CARBONIZATION IN A BLACK SHALE-HOSTED POLYMETALLIC SULFIDE ORE LAYER, SOUTHWEST CHINA

The black shale in the Lower Cambrian Niutitang Formation, southwest China, which has been dated to 532.3 ± 0.7 Ma by U-Pb radiometric measurements, hosts a sedimentary layer with abnormally high Ni-Mo-PGE-Au contents. This layer is enriched in Ni (up to 3.8 wt %), Mo (up to 7.7 wt %), and U (up to 595 ppm), but its genesis is still controversial. Here we report the first direct observation of uranium-bearing minerals and their radioactive effects on the surrounding matter in the polymetallic sulfide ore. X-ray absorption fine structure analyses confirmed the reduced valence state of uranium. In combination with high-resolution electron microscopy and electron probe microanalysis, the mineral was identified as coffinite (USiO4). A strong positive correlation between the sizes of the coffinite crystals and their surrounding carbonized rings reveals that the coffinite is authigenic, and its crystallization-produced radiation resulted in the radiolysis of surrounding organic matter. The association of various biogenic metal sulfides, phosphates, and abundant organic substances within the Ni-Mo sulfide-enriched ore suggests that biological adsorption may have participated in the enrichment of soluble U(VI), and that microbial sulfate reduction might have facilitated the uranium mineralization.