Geochemistry of platinum group elements in marine oil shale from the Changshe Mountain area (China): Implications for modes of occurrence and origins

摘要

The Shengli River–Changshe Mountain oil shale zone, located in the North Qiangtang depression, northern Tibet plateau, represents a potentially largest marine oil shale resource in China. Eighteen samples, including oil shale and micritic limestone, were collected from the Changshe Mountain oil shale to determine the concentrations, the modes of occurrence, and origins of platinum group elements (PGEs) in marine oil shale. The oil shale samples from the Changshe Mountain area exhibit low total PGE contents ranging from 3.196 to 8.071 ng/g with a weighted mean value of 5.383 ng/g, while the micritic limestone samples from the Changshe Mountain area exhibit a little lower ranging from 0.471 to 5.669 ng/g. PGEs in oil shale samples are characterized by high contents in Pd (3.23 ng/g), Pt (1.60 ng/g), and Os (0.32 ng/g), as compared to Rh (0.083 ng/g), Ru (0.112 ng/g), and Ir (0.043 ng/g). The individual PGEs of oil shale samples from the Changshe Mountain area exhibit various modes of occurrence. Ru and Ir are mainly controlled by clay minerals and partly present as sulfide. Rh is mainly associated with sulfide and possibly present as clay minerals. Pt is associated with clay minerals and sulfide, and Pd is controlled by organic matter and pyrite, while Os is mainly present in organic matter. Three possible sources of PGEs were identified in oil shale seams in the Changshe Mountain area. Ru and Ir are mainly terrigenous inputs. Rh and Pt are derived from mixed sources (seawater and terrigemnous supply), while Pd and Os are mainly derived from seawater. The terrestrial source of the oil shale seams in the Changshe Mountain oil shale was the Suowa Formation limestone and the Nadi Kangri felsic volcanic rocks.