LAUSR

Abstract Organic-rich siliceous rocks of Late Permian age from western Hubei Province, China are markedly high in trace elements including V (2031 μg/g), Cr (856 μg/g), Mo (245 μg/g), and U (39.2 μg/g). The mineralogy and geochemistry of these siliceous rocks were investigated using various petrological, mineralogical, and geochemical methods. The results show that the siliceous rocks are high in total organic carbon (TOC), which range from 3.88 to 46.99%, with a weighted average of 17.83%. TOC is dominated by amorphous organic debris, resulting from intense degradation, and lesser amounts of marine microfossils (e.g. foraminifera , algae, and poriferan ). The minerals mainly consist of quartz, mixed layer illite/smectite (I/S), and small proportions of calcite, dolomite, K-feldspar, albite, pyrite, gypsum, apatite, marcasite, jarosite, kaolinite, and bassanite. Trace amounts of xenotime-(Y) ((Y,REE)PO 4 ), rhabdophane-(Nd) ((Nd, Ce, La,)PO 4 ·H 2 O), chalcopyrite, and sphalerite were observed using SEM-EDX. The SEM-EDX results indicate that V and Cr are mainly within the organic matter and mixed layer I/S; Mo is closely associated with organic matter and sulfides (e.g., pyrite, sphalerite, and chalcopyrite); U is only detected in P-bearing minerals, especially in xenotime-(Y) and rhabdophane-(Nd). The sampled interval was deposited in a transitional marine basin between a continental margin and open-ocean pelagic setting, where the anoxic environment was attributed to intense organic matter degradation and restricted water mass renewal. The enriched elements V, Cr, Mo, and U were mainly derived from submarine hydrothermal activities.