Coupling of sedimentary environment and hazardous trace element enrichment in lacustrine oil shale: evidence from the Lunpola Basin, Tibetan Plateau, China

The Lunpola Basin, developed during the Cenozoic era on the Tibetan Plateau, serves not only as a sedimentary record of plateau uplift but also as a significant reservoir of oil shale. This study focuses on the sedimentary environment of the oil shales in the Lunpola Basin and the enrichment mechanisms of trace elements, particularly hazardous elements, within them. A total of 38 samples were analyzed for major elements, trace elements, total organic carbon (TOC), and total sulfur (TS). Results show that several elements are significantly enriched relative to upper continental crust values, notably As (up to 45×), Mo (5.2×), Pb (3.8×), and U (3.3×). Geochemical proxies indicate deposition in a brackish to saline lacustrine setting, under predominantly anoxic to suboxic conditions, and in a warm–humid paleoclimate with moderate to low weathering intensity. Correlation analysis reveals that hazardous elements (As, Mo, Pb, U) are positively associated with TOC and TS, suggesting that organic matter and sulfide phases play key roles in their enrichment. Redox conditions appear to exert a primary control, while salinity and clay minerals contribute to secondary regulation. These results indicate that the enrichment of hazardous elements in the Lunpola oil shales is controlled by a combination of depositional environment and organic matter preservation, providing new insights into element cycling in continental lacustrine systems and implications for environmental risk during oil shale exploitation.