The Genesis and LA-ICP-MS Zircon Ages of the Omixia Nephrite Deposit, Xinjiang, China

Abstract

BACKGROUND The Omixia nephrite deposit is located in the middle West Kunlun Orogenic belt. The deposit is hosted in Mg-skarn between granite and magnesian marble. The deposit is large and has a long exploration and large scale, but the research on ore genesis and age remains lacking. OBJECTIVES To understand the genesis and upper age of the Omixia nephrite deposit by collecting and analyzing typical samples. METHODS Petrological and bulk rock methods are used to investigate the ore genesis, whereas the age of the deposit is constrained by LA-ICP-MS U-Pb zircon dating of nephrite. RESULTS Petrographic observation shows that nephrite formed at multiple stages. The REE patterns of the nephrite indicate low REE concentration with the average ∑REE of 7.228 μg/g, and strong negative Eu anomalies (δEu=0.58-0.73) with declined LREE and flat HREE. Zircon in nephrite has 206Pb/238U ages ranging from 411.1±5.3 Ma to 489.6±10.5 Ma. CONCLUSIONS Omixia nephrite deposit is a typical contact metasomatic Mg-skarn nephrite deposit. The formation of gem-grade nephrite jade has undergone multiple processes, tremolite mineralization, diapolitization, fine granulation, and recrystallization. The formation age of this nephrite deposit is similar to other nephrite deposits in West Kunlun, and they are formed at the Late Caledonian and Early Hercynian. This study is helpful to understand the formation mechanism of nephrite in West Kunlun area of Xinjiang and provides new basic data for geological research in the West Kunlun.

Keywords

• mg-skarn
• nephrite
• contact metasomatic metamorphism
• zircon dating