Geochemistry and petrology of nephrite from Alamas, Xinjiang, NW China

Abstract

The Hetian nephrite deposit is one of the largest nephrite deposits worldwide. Located within the Hetian deposit, Xinjiang, Northwest China, the Alamas ore body occurs as veins on the contact between the Late Variscan granodiorite and Precambrian dolomitic marble. Petrographic studies and electronic microprobe analyses reveal two possible nephrite formation processes: (1) dolomitic marble → tremolite and (2) dolomitic marble → diopside → tremolite. Nephrite veins show continuous coloration zones, ranging from white and white-green zones to a green zone towards the granodiorite, with increasing concentrations of Cr, FeO and TFe 2O 3. All these nephrite samples have quite low Cr and Ni contents (8.95–178.7 ppm and 0.05–3.95 ppm, respectively) relative to serpentinite-related nephrite (Cr 2O 3, 0.07–0.43 wt.%; NiO, 0.08–0.36 wt.%). Their bulk-rock REE patterns exhibit strong negative Eu anomalies (0.03–0.21) with declined LREE, flat HREE and low ΣREE concentration (2.84–84.81 ppm), similar to those of host dolomitic marble samples with negative Eu anomalies (0.15–0.23), declined LREE, flat HREE and lower ΣREE concentration (8.48–11.1 ppm), indicating a close genetic relationship between them. Homogenization temperatures analyses on the fluid inclusions in tremolite yield a minimum temperature 293 °C for the nephrite formation. Nephrites have oxygen and hydrogen isotope compositions in the range from +3.2 to +6.2 per mil and −83.0 to −94.7 per mil, respectively. The corresponding equilibrated fluids have isotope compositions of δ 18O = +3.1 to +6.1 (293 °C). Combined with field observation, there are at least three possible fluid sources: meteoric water, magmatic water and CO 2 derived from decarbonation of dolomite, and a metasomatic and dolomite-related origin is suggested for the Alamas ore body in the Hetian nephrite deposit, Xinjiang, NW China.