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硅孔雀石和透视石稀土元素地球化学特征及其成矿环境研究

Geochemical Characteristics of Rare Earth Elements in Chrysocolla and Dioptase and Their Metallogenetic Environments

  • 摘要: 硅孔雀石和透视石是在热液成矿作用下经过水-岩反应形成的产物, 但这一认识缺少稀土地球化学研究成果的支持。本文通过电感耦合等离子体质谱分析获得两种矿物的稀土元素含量, 其中硅孔雀石的稀土含量较低(∑REEs=81.2 mg/kg), 透视石的稀土含量极低(∑REEs=0.65 mg/kg); 两者轻重稀土间分馏较为显著, LREEs/HREEs=3.01和6.05, (La/Yb)N=3.08和7.91。球粒陨石标准化的稀土元素配分模式图均表现为轻微“负铕异常”(δEu=0.565和0.702)、轻稀土富集、重稀土稳定的右倾特征, 表明两者物质来源于同一物体。硅孔雀石显著“负铈异常”(δCe=0.219), 透视石无“负铈异常”(δCe=0.949), 应为两者晶体结构差异所致。研究认为, 硅孔雀石和透视石中的稀土元素继承了成矿岩石的稀土特征, 证实了两者是在同一氧化条件下通过水-岩反应形成的产物。

     

    Abstract: Chrysocolla and dioptase were considered to be formed from hydrothermal solutions by fluid-rock interaction, but this view was not verified by REE geochemistry. In this study, the REEs contents in these two minerals were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Results show that chrysocolla has total REEs of 81 mg/kg, whereas dioptase has total REEs of 0.65 mg/kg. There is obvious fractionation of LREEs from HREEs for chrysocolla and dioptase with LREEs/HREEs ratios of 3.01 and 6.05, and (La/Yb)N ratios of 3.08 and 7.91, respectively. The chondrite-normalized REEs patterns of two minerals show weak Eu anomalies with δEu of 0.565 and 0.702, respectively, with LREEs enrichment and flat HREEs, indicating their similar source. Obvious Ce anomaly (δCe=0.219) of chrysocolla and no Ce anomaly (δCe=0.949) of dioptase are possibly due to their different crystal structures. Chrysocolla and dioptase inherit the REEs characteristics of the source rocks. These two minerals were formed by fluid-rock interaction under the same oxidized condition.

     

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