Mineralogy, geochemistry, and petrogenesis of green nephrite from Dahua, Guangxi, Southern China

Abstract

A new nephrite deposit with potential for economic exploitation was recently discovered in Dahua county, Hechi City, Guangxi Autonomous Region, China. Previous studies have focused primarily on white, black, and black-cyan nephrite from Dahua, whereas few studies have been carried out on green nephrite. In this study, the mineralogical, petrological, and geochemical characteristics of nephrite from this new deposit were systematically studied using polarized light microscopy, scanning electron microscopy, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry, and the deposit genesis was discussed. The results show that Dahua green nephrite consists of interwoven microfibers, and the main mineral component is tremolite. The Mg2+/(Mg2++Fe2+) ratio ranges from 0.951 to 1.00, andthe subordinate minerals include diopside, chlorite, calcite, quartz, apatite, albite, titanite, and andradite. The contents of trace elements (Cr, Ni, and Co) are much lower than in metasomatized serpentinite-type nephrite. The rare earth element content is generally high (62.011–223.116 ppm) with a positive Eu anomaly (1.009–2.343), positive and negative Ce anomalies (0.913–1.078), enrichment in light rare earth elements, and flatness in heavy rare earth elements. The distribution curves of the rare earth elements differ between the nephrite and the surrounding rock samples, indicating multiple sources and a multiperiod superposition of the ore-forming fluids. The Fe2+/(Mg2++Fe2+) ratio in Dahua green nephrite is lower than 0.06, indicating that the nephrite deposit is surrounded by carbonates. By taking c(Ca2+), c(Mg2+), and c(Fe2++Fe3+) as endmembers for projection, amphibole can be concluded to have originated from contact metasomatism. Considering the findings of the geological survey, the Dahua green nephrite likely belongs to the magmatic hydrothermal metasomatic type, and its ore deposit is related to basic intrusive rocks such as diabase and hydrothermal fluid carried by magma. Contact metasomatism and hydrothermal metamorphism occurred with dolomitic limestone and other carbonate rocks under certain temperature and pressure conditions during the Variscan–Indosinian period. This systematic study of the newly discovered Dahua green nephrite deposit provides a theoretical basis for the prospecting of nephrite deposits in the study area.