Mineralogy and geochemical anomalies of Lower Cretaceous marine oil shale from Changshe Mountain West, northern Tibet, China

• Enriched elements in the Lower Cretaceous oil shale are Se, Mo, Cd, Pb, and As.
• Hydrothermal barite, celestite, sphalerite, Fe(Si,Al)-oxysulfate, and pyrite
• Terrigenous-relative trace elements exhibit low enrichment factors.
• Seawater and hydrothermal activity influenced oil shale geochemical compositions.

The Shengli River–Changshe Mountain oil shale zone in northern Tibet represents a large marine oil shale resource in China. Thirty-four samples of oil shale were collected from the Changshe Mountain West oil shale, a new oil shale deposit discovered in this oil shale zone, to evaluate the minerals and geochemical anomalies in the oil shale. Minerals identified in the oil shale include calcite, dolomite, quartz, illite, and trace amounts of siderite, hematite, feldspar, apatite, zinc oxide, pyrite, sphalerite, barite, celestite, and Fe(Si, Al)-oxysulfate minerals. The oil shale samples are rich in trace elements compared to post-Archean average shales including Co (up to 15.8 μg/g), Ni (up to 83.6 μg/g), Cu (up to 101 μg/g), Zn (up to 775 μg/g), As (up to 255 μg/g), Se (up to 6.91 μg/g), Sr (up to 738 μg/g), Mo (up to 87.9 μg/g), Pb (up to 17.4 μg/g), Cd (up to 13.0 μg/g), Bi (up to 0.38 μg/g), and U (up to 7.10 μg/g), and to a lesser extent, Li, F, Sc, V, Cr, Ga, Sn, Hg, and rare earth elements. Trace elements of probable terrigenous origin in the oil shale were derived from two possible sources: a felsic volcanic rock source and a limestone source. High concentrations of lithophile elements (e.g., Li, Sc, Ga, and REEs) were derived from these provenances. However, the enrichment factor for these elements also is low. In addition, seawater and hydrothermal activity may have influenced the geological and geochemical compositions of the oil shale. Minerals such as calcite, dolomite, and a large proportion of pyrite are of marine origin, and the enrichment of V, Cr, Co, Ni, Cu, Se, Sr, Mo, Pb, and U in the oil shales was probably due to marine influence. Barite, celestite, sphalerite, Fe(Si, Al)-oxysulfate minerals, and fracture-filling pyrite are of hydrothermal origin, and F, Zn, and Cd were derived from hydrothermal fluids.