Mineralogy, geochemistry and petrogenesis of nephrite from Tieli, China

Abstract

Concurrent with the increase in nephrite mining in recent years, many new nephrite deposits have been discovered worldwide. However, studies of these new deposits remain limited. To address this knowledge gap, the objective of this study was to systematically investigate the nephrite deposit in Tieli, Heilongjiang Province, China. Electron probe micro-analysis and laser ablation inductively coupled plasma mass spectrometry were used to measure the mineralogical, petrological and geochemical characteristics of samples from this deposit. The nephrite exhibits a fine and smooth texture, with felted blastic and fibroblastic structures, indicating high quality compared with nephrites from other deposits. The main mineral composition is tremolite, with a Mg/(Fe2++Mg) ratio of 0.982–0.992, which is purer than nephrite samples from most other deposits. Furthermore, all tremolite examined in the Tieli nephrite has low amounts of Cr2O3 (0.000–0.060 wt%) and NiO (0.000–0.061 wt%) relative to dolomite-related nephrites. The nephrite samples all have very low total rare earth element (ΣREE) contents, ranging from 2.8633 to 10.8375 ppm, and are characterized by positive and negative Eu anomalies (0.3286–1.4570), light-REE enrichment and flat heavy-REE. The varied REE patterns of the samples highlight the multiple sources of ore-forming fluids, multistage superimposed mineralization, and oxidized mineralization environment. The mineral composition of the wall rocks indicates that during formation, two typical processes results in nephrite genesis: contact and hydrothermal metasomatism. Generally, this nephrite is of a dolomite-related origin. The discovery of the Tieli deposit and this first-time systematic study provide a theoretical basis for prospecting nephrite deposits in the study area, where intermediate to felsic igneous rocks or Si-enriched hydrothermal fluids intruded limestone or dolomite marble formed during the Variscan-Indosinian epoch.