新疆奥米夏和田玉矿床成因及锆石LA-ICP-MS定年研究

张勇; 魏华; 陆太进; 陈华; 韩冬; 冯晓燕; 买托乎提·阿不都瓦衣提

doi:10.15898/j.cnki.11-2131/td.201801170007

新疆奥米夏和田玉矿床成因及锆石LA-ICP-MS定年研究

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

摘要

摘要: 新疆奥米夏和田玉矿床分布在西昆仑造山带中段，产于花岗岩与镁质大理岩之间，该玉矿规模大，开采历史悠久，然而其矿床成因和成矿年代缺乏研究。本文采用岩相学、全岩分析手段对该玉矿的成因进行了研究，利用玉石中的锆石LA-ICP-MS定年方法限定了玉矿的成矿时代。岩相学观察结果表明玉石多期次形成的现象较为明显；全岩稀土配分模式表明玉石稀土元素总量丰度低（∑REE平均值7.228 μg/g），Eu负异常（δEu=0.58~0.73），LREE下降，HREE平坦；和田玉中锆石²⁰⁶Pb/²³⁸U年龄显示该玉矿的成矿上限年龄介于411.1±5.3 Ma~489.6±10.5 Ma之间。综合分析认为，该玉矿为接触交代成因的镁质矽卡岩型和田玉矿床，玉石形成过程中经历了透闪石化、细粒化、重结晶等复杂的成矿过程，其形成时代与西昆仑地区其他玉矿一致，整体上可能形成于加里东晚期至海西早期。本研究有助于认识新疆西昆仑地区和田玉的形成机理，也可为西昆仑地区的地质研究提供新的基础数据。

Abstract:
BACKGROUNDThe 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.
OBJECTIVESTo understand the genesis and upper age of the Omixia nephrite deposit by collecting and analyzing typical samples.
METHODSPetrological 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.
RESULTSPetrographic 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.
CONCLUSIONSOmixia 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.

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