Geochemical characteristics and ore-forming mechanism of Luodian nephrite deposit, Southwest China and comparison with other nephrite deposits in Asia

Abstract

A nephrite deposit was recently discovered in Luodian, Guizhou Province, which belongs to the South China Plate and is located in the eastern Tethys tectonic domain. This study examines systematically the geochemical characteristics and mineralization mechanism of Luodian nephrite using various analytical techniques including polarized light microscopy, scanning electron microscopy, electron probe microanalysis, and laser ablation-inductively coupled plasma-mass spectrometry. The results show that the main mineral of Luodian nephrite is tremolite, and the secondary minerals include diopside, calcite, quartz, albite, talc, and iron and manganese oxides. The tremolite was present in three morphological forms which have the characteristics of multi-period mineralization. The Mg2+/(Mg2++Fe2+) ratio indicates that the Luodian nephrite deposit is a dolomite-related type deposit. The trace elements of Cr, Ni, and Co, and the ratios of Rb/Sr and Rb/Ba are much lower than those serpentinite-related type nephrite, and are within the range of those for basic magma contact metasomatic nephrite. The rare earth elements (REEs) show that the formation of Luodian nephrite originates from metasomatism of sedimentary rocks. Curves for the distribution of the chondrite-normalized REE pattern and the characteristics of the REEs in nephrite are identical to those for marble or limestone, and are clearly different from those for diabase, indicating that the formation of nephrite occurrs closer to the surrounding rock. Combined with chemical analyses and a geological survey, the results show that Luodian nephrite was formed by the contact metasomatism of basic diabase and carbonate rock during the Indosinian-Yanshannian (Late Triassic to Cretaceous) period. The diabase magma provided a high-temperature environment and the Si hydrothermal environment; the presence of the carbonate rock facilitated Ca and Mg to form nephrite under specific physical and chemical conditions. The comparison of geochemical compositions can help to distinguish the genetic type, reaction mechanism and origin characteristics of nephrite deposits. These results provide theoretical guidance for the search for nephrite deposits of dolomite-related type origin.