Nephrite pebbles of the Vitim Highlands and the Hetian area (China): a comparative mineralogical and geochemical analysis

Research subject. Placer nephrites of the Vitim Highlands and the Hetian area in northwest China. Aim. Comparison of mineralogical and geochemical characteristics of nephrite pebbles from the Tsipa River in the north-east of the Republic of Buryatia and the world’s largest accumulation of placer nephrite from the Yurunkash and Karakash Rivers in the Xinjiang Uygur Autonomous Region, Northwest China. Materials and methods. The mineral composition of placer nephrite from the Vitim Highlands was studied by X-ray diffraction on a PowDix600 powder diffractometer; the diffractograms were interpreted using Almaz software. The chemical composition was studied by SEM (EDX) on a Merlin Carl Zeiss scanning electron microscope equipped with an AZtec X-Max energy dispersion spectrometer (Oxford Instruments). The content of 59 trace elements and rare earth elements was determined by ICP MS method on an iCAP Qc ThermoFisher Scientific inductively coupled plasma mass spectrometer. Statistical processing of the analytical results was carried out using STATISTICA and Excel software programmes. Results. The main mineral of light-coloured nephrite pebbles is tremolite, which can change into actinolite, accompanied by a significant (10 times) increase in the FeO content with the acquisition of dark green and black colouring of placer nephrite. Petrochemical diagrams Mg/(Mg + Fe 2+ ), MgO + FeO–FeO, Al 2 O 3 – Na 2 O + K 2 O, SiO 2 –CaO + Na 2 O + K 2 O allowed to obtain distinct differences between the studied objects. All studied nephrite pebbles have common characteristics of rare earth elements (REE) behaviour: negative Eu-anomaly, pronounced right-hand slope, moderate enrichment of light-REEs, practically flat distribution of heavy-REEs. Conclusions. The genetic similarity of placer nephrite from both nephrite-bearing areas is confirmed by the common distribution of REE in them. The distinctive features of nephrite pebbles of the Vitim Highlands are elevated content of alkalis associated with the participation of acidic intrusions in the formation of the primary source of nephrite, and a developed staining edge acquired under exogenous conditions. Black nephrite of the Karakash River consists predominantly of actinolite, which are formed at elevated levels of ferric oxide. The similarity of alluvial nephrite of the Tsipa, Yurungkash and Karakash rivers testifies to the great prospects of placers in the Vitim region and the need for specialized geological exploration.