BACKGROUND The Hetian nephrite belt in Xinjiang is the largest nephrite ore belt in the world, with a length of about 1300km. In addition to white, gray, black and brown nephrite, the Qiemo—Ruoqiang zone, Xinjiang also produces a yellow-green nephrite. There are few studies on the color origin and characterization of this kind of Hetian nephrite, which affects the understanding of the color and genesis of this kind of Hetian nephrite.

OBJECTIVES To understand the color genesis, genetic types and formation age of the yellow-green nephrite in Qiemo—Ruoqiang deposit.

METHODS Electron probe microanalysis (EPMA), backscattered electron (BSE) image, X-ray fluorescencespectrometry (XRF), and high resolution inductively coupled plasma-mass spectrometry (HR-ICP-MS) were used to investigate mineral and chemical composition. The FeO content was determined by titration method to explore its color origin and genetic type, and the zircon U-Pb isotope dating by sensitive high-resolution ion microprobe (SHRIMP) was used to constrain its mineralization age.

RESULTS Yellow-green nephrite in Qiemo—Ruoqiang was predominately composed of apatite, calcite, dolomite, diopside, epidote, sphene and zircon. The yellow-green nephrite whole rock had low total REE (∑REE=2.61-19.1μg/g) with obvious Eu negative anomaly (δEu < 0.05), LREE right dipping, and HREE flat pattern. According to the distribution pattern of rare earth and the content of Cr and Ni, it was inferred that it was a magnesia skarn type Hetian nephrite. Compared with other nephrite in the world, the average value of Fe³⁺/Fe²⁺ of the yellow-green nephrite in Qiemo—Ruoqiang (average=0.16) overlapped with nephrite in other regions. The ratio is slightly higher (0.07-0.26), while the content of Fe³⁺ and Fe²⁺ was not significantly different between the yellow-green nephrite and other color nephrite. The SHRIMP U-Pb dating of zircon in the yellow-green nephrite yielded ages of 461.7±6.1Ma (MSWD=1.6) and 498.1±4.6Ma (MSWD=1.16).

CONCLUSIONS The color genesis of the yellow-green nephrite in Qiemo is probably related to the ratio of Fe³⁺/Fe²⁺, but not to the content of Fe³⁺ and Fe²⁺. The SHRIMP U-Pb dating results of zircon in yellow-green nephrite represent the formation age of the yellow-green nephrite. These new data provide geochronological constraints for the magmatism of Hetian nephrite belt and the tectonic evolution of the West Kunlun orogenic belt.