湘东北栗山铜铅锌多金属矿床热液萤石稀土元素地球化学特征及其指示意义

江南造山带中段幕阜山地区栗山大型铜铅锌多金属矿床中伴生规模可观的萤石资源, 储量达41.5万t。该区萤石形成过程与机制尚缺乏有效约束, 制约了对其矿床成因机制的认识和找矿实践。本文通过栗山绿色、紫色及蓝色萤石组构分析、荧光测试和ICP-MS微量元素含量分析, 探讨栗山矿床成矿过程与物理化学环境。岩相学与矿相学证据指示栗山热液活动可划分为四个阶段: 乳白色石英+少量绿色萤石(Ⅰ), 蓝色、紫色萤石+石英+绿泥石+金属硫化物阶段(Ⅱ), 石英+绿色萤石+绿泥石+金属硫化物阶段(Ⅲ)以及石英+方解石阶段(Ⅳ)。各颜色萤石Diamond View荧光测试结果显示深紫色萤石荧光最弱, 蓝色萤石次之, 绿色萤石荧光最强。绿色萤石的∑REE高于蓝色、紫色萤石, 且绿色萤石LREE相对富集而蓝色、紫色萤石HREE相对富集, 表明成矿Ⅱ阶段至Ⅲ阶段成矿温度整体降低且pH值升高。Tb/Ca-Tb/La以及Y/Ho-La/Ho图解指示不同颜色萤石属同源岩浆流体和大气降水产物, 栗山萤石的沉淀机制主要为水岩反应并伴随流体混合。

REE geochemical characteristics of hydrothermal fluorite,Lishan copper-lead-zinc polymetallic deposit,northeastern Hunan Province and its indicative significance

Lishan copper-lead-zinc polymetallic deposit, which is located in the Mufushan area in the middle section of the Jiangnan orogenic belt, is a large-scale metal deposit containing considerable fluorite resources with a reserve of 415 thousand tons. The research on the associated hydrothermal fluorite in this area is relatively weak, and the formation process and mechanism of fluorite are still lack of effective constraints, which restricts the understanding of the genetic mechanism of the deposit and prospecting in this area. In this paper, based on the results of fabric analysis, fluorescence test and ICP-MS trace element analysis of Lishan green, purple and blue fluorite, the mineralization process, physical and chemical environments of Lishan deposit were discussed. Four stages of hydrothermal processes are recognized based on petrographic and mineralogical evidences: quartz+minor green fluorite (Ⅰ); blue and purple fluorite+quartz+chlorite+sulfide (Ⅱ); quartz+green fluorite+chlorite+sulfide (Ⅲ); quartz+calcite (Ⅳ). Dark purple fluorite has the weakest fluorescence, followed by blue fluorite, while green fluorite has the strongest fluorescence. The ΣREE of green fluorite is higher than that of blue and purple fluorite, and the LREE of green fluorite is enriched while the HREE of blue and purple fluorite is enriched, indicating that the mineralization temperature decreased and the pH value increased from mineralization stages Ⅱ to Ⅲ. Tb/Ca vs. Tb/La and Y/Ho vs. La/Ho diagrams indicate that fluorite of different colors is the product of cognate magmatic fluid and meteoric water. The precipitation mechanism of Lishan fluorite is mainly water rock reaction accompanied by fluid mixing.