Minerals and potentially hazardous trace elements in marine oil shale: new insights from the Shengli River North surface mine, northern Tibet, China

The Shengli River-Changshe Mountain oil shale zone, including the Changliang Mountain-Shengli River oil shale, the Shengli River North oil shale, and the Changshe Mountain oil shale, represents potentially the largest marine oil shale resource in China. New geochemical data from the Shengli River North oil shale are presented to evaluate potentially hazardous trace elements in oil shale and to discuss possible factors controlling the geochemical anomalies. Oil shale samples from the Shengli River North area are characterized by high ash yield (59.25-66.27 %) and TOC content (4.77-9.35 %) with low total sulfur content (0.1-1.56 %) and intermediate shale oil content (3.60-16.30 %). Minerals in the Shengli River North oil shale include calcite, clay minerals, quartz, dolomite, and Fe-bearing minerals (e.g., pyrite, limonite, and hematite), and trace amounts of zeolite, apatite, and barite. Potentially hazardous trace elements in oil shale samples include As, Se, Mo, F, Ni, Cd, and Hg. Arsenic is present probably in both organic and inorganic matter. Selenium is controlled mainly by Fe-bearing (probably pyrite), while F and Cd occur mainly as apatite. The organically bound Mo and Ni are dominant, and Hg probably occurs as a substitution in the structure of pyrite, apatite and calcite. The elevated concentrations of F and Cd in the Shengli River North oil shale may be related to sediment source. Water chemistry may have played an important role for Ni and Mo enrichments, while the enrichment of Se in the oil shale samples is possibly related to hydrothermal fluids. Other elements including Hg and As are probably related to several factors.