Study of the minerogenetic mechanism and origin of Qinghai nephrite from Golmud, Qinghai, Northwest China

Abstract

Electronic microprobe analysis showed that all QN samples are mainly composed of tremolite and minor accessory minerals, such as diopside, calcite, serpentinite, and magnetite. According to the cation coefficients, the crystallo-chemistrygenesis illustration demonstrates that all QN deposits are contact metasomatic. Depending on the mole percent of Fe2+(3+)/(Mg2++Fe2+(3+)) and the content of Cr, Co, and Ni in all QN samples measured by X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), green and azure-green QNs are characterized as serpentinite-related contact metasomatic deposit (S-type), whereas white, green-white, brown, blue-violet, yellow, and viridis QNs are dolomite- related contact metasomatic deposit (D-type). The assemblages and chemical composition of accessory minerals of the eight-color QN samples show evident characteristics, which reveal four possible ore-forming processes. We also measured trace and rare earth elements (REEs) in these samples through ICP-MS to deduce the origin of and the changes in metallogenic conditions. The chondrite-normalized REE patterns of D-type QN exhibit moderately negative Eu anomalies with moderate light REE enrichment, flat heavy REE (HREE), and low ƩREE concentrations, similar to dolomitic marble. Green QN samples of S-type show enrichment in HREE and moderately negative Eu anomalies, which is consistent with characteristics of dunite. Whereas azure-green QN samples of S-type exhibit a right-dipping V-type curve with severe depletion in Eu (δEu=0.36–0.47), in accordance with the characteristics of gabbro from Yushigou ophiolite in North Qilian mountains. Furthermore, this finding is consistent with the content of trace elements and the petrographic analysis results. On the basis of several significant differences in the characteristic elements, which may have been affected by the metallogenic environment, we inferred the differences in oxygen fugacity and basicity of mineralization environments in different-colored QNs.