BACKGROUNDThe Hetian nephrite belt is the longest nephrite belt in the world at 1300km. In addition to the traditional primary and placer nephrite, there is widespread Gobi nephrite in the Gobi desert of the Quoqiang district in Xinjiang.

OBJECTIVESTo identify the origin, genesis, ages and types of Gobi nephrite.

METHODSElectronic Microprobe, X-ray Fluorescence, Inductively Coupled Plasma-Mass Spectrometry and sensitive high-resolution Ion Microprobe were used to examine the mineral assemblages, chemical composition and ages of Gobi nephrite. Based on these analyses, the genesis of Gobi nephrite was constrained.

RESULTSGobi nephrite was predominantly composed of tremolite (>95%) with minor apatite, diopside, epidote and chromite (< 5%). The color of Gobi nephrite was mainly dark green, green, yellow-green and white. The samples, with the exception of white, were related to the FeO content (0.48%-2.92%). The whole rock analysis suggested that both Gobi nephrite and tremolite had a similar chemical composition. All samples displayed LREE enrichment, flat HREE and negative Eu anomaly (δEu=0.09-0.66). Totally, all these samples had low content of REE (6.93-115.93μg/g), Cr (68.8-119μg/g), and Ni (16.4-38.8μg/g). δD (-24.94‰--56.83‰) of ore-forming fluids indicated that it was composed of magmatic water and meteoric water. SHRIMP U-Pb dating of zircons showed that there were four groups of ages:40-60Ma, 480Ma, 785Ma and 1450-2460Ma. These ages could be used to constrain the formation ages of Gobi nephrite.

CONCLUSIONSThe geochemistry and ore-forming fluid composition of the Gobi nephrite is similar to the composition of nephrite in the typical Mg-skarn deposit previously reported. The ore-forming age of 400Ma is consistent with the mineralization age of most of the reported ages in the Hetian areas. The multiple age groups also indicate multi-stage mineralization of nephrite.