Geochemistry and petrogenesis of placer nephrite from Hetian, Xinjiang, Northwest China

Abstract

The Yurungkash and Karakash rivers, also known respectively as the White Jade and Black Jade rivers, located in Hetian, Xinjiang Province, Northwest China, are the two main sources in China of white, green, and black placer nephrite, with a long history (~5000years) of exploration and mining. The twenty-nine placer nephrite samples collected from both rivers and analyzed in the present study possess fine-grained and compact microstructures. The mineral assemblages in the samples provide clues to the metamorphic/metasomatic processes that formed the nephrite, which was the result of one of the following reactions: dolomitic marble→tremolite, or dolomitic marble→diopside→tremolite. White nephrite from the Yurungkash River and green nephrite from the Karakash River are predominantly tremolite. Based on electron probe microanalytical data, backscattered electron images, and Raman spectra, two kinds of black nephrite from the Karakash River are identified: one dominated by actinolite aggregates, and another consisting of tremolite aggregates with graphite crystals up to 2mm in length. Compared with black nephrite, white and green nephrites contain fewer mineral inclusions and have lower FeO and MnO contents. All the amphiboles in the nephrites have very low contents of Cr2O3 (0.00–0.07wt.%) and NiO (0.00–0.05wt.%) relative to serpentinite-related nephrite (0.07–0.43wt.% Cr2O3, 0.08–0.36wt.% NiO). Most of the nephrite samples have low total rare earth element (ΣREE) contents, ranging from 12.22 to 49.40ppm. In two nephrite samples, relatively high ΣREE concentrations (161 and 190ppm) are related to the presence of REE-bearing minerals. Whole-rock REE chondrite-normalized patterns of all samples are characterized by strong negative Eu anomalies (0.16–0.48), moderate light-REE enrichment (La/NdN=1.8–5.0), and nearly flat heavy-REE distributions (Gd/YbN=0.3–1.7). Nephrite samples from both river locations have δ18O and δD isotope compositions ranging from 1.1‰ to 5.6‰ and −55.7‰ to −72.4‰, respectively. These values are closer to those recorded in dolomite-related nephrites than those in serpentinite-related deposits. Importantly, δ18O and δD values correspond to fluid isotope compositions of δ18O=1.6‰ to 6.1‰ (330°C), 1.8‰ to 6.3‰ (350°C), and 2.5‰ to 7.0‰ (430°C), and δD=−34.9‰ to −52.5‰ (350°C to 650°C). These values are close to or within the field of magmatic water. Geochemical and petrographic characteristics point to a dolomite-related metamorphic/metasomatic origin for nephrite at both locations. The placer nephrite is likely to have been derived from primary nephrite deposits in the Kunlun Mountains around Hetian, based on the geological occurrence of the deposits.