四川扎乌龙伟晶岩型稀有金属矿床的成矿机制及找矿标志

The ore-forming mechanism and geological indicators of the Zhawulong pegmatite-type rare-metal deposit in Sichuan

扎乌龙稀有金属矿床位于松潘-甘孜造山带,属于伟晶岩型锂辉石矿床,同时伴生铍和铌钽矿化。本文总结了扎乌龙矿床地球化学和矿物学特征,初步梳理了矿床成矿模式,并指出含矿性评价与找矿标志。扎乌龙伟晶岩脉在空间上围绕白云母花岗岩分布,具有类似的稀土元素和微量元素配分模式,但其稀土元素含量明显低于白云母花岗岩,表明伟晶岩脉为白云母花岗熔体分异演化的产物。扎乌龙白云母花岗岩岩石地球化学组分呈现高Rb/Sr和Rb/Ba、低CaO/Na_(2)O壳源特征,具有接近地壳的Sr-Nd-Hf同位素组成,表明其来源于松潘-甘孜大规模造山运动导致的西康群变泥质岩的深部重熔。矿物学和流体包裹体特征显示,扎乌龙伟晶岩脉的形成可划分为岩浆阶段和热液阶段。岩浆阶段主要结晶稀有金属矿物,如锂辉石、绿柱石和铌钽铁矿族矿物;热液阶段主要为原生矿物与流体发生再平衡,如原生锂辉石发生蚀变形成云母等矿物,原生绿柱石边部和内部缝隙产出相对富Cs_(2)O或Na_(2)O的次生绿柱石。锂矿物主要沉淀于相对封闭、高压(350MPa)的伟晶岩岩浆阶段。氢、氧同位素组成表明成矿流体主要来源于岩浆水,晚期流体来源于伟晶岩熔体的自身分异演化。扎乌龙锂辉石伟晶岩熔体分异程度属于中等,远低于高分异的新疆可可托海三号伟晶岩脉,而高于低分异的南爱尔兰Leinster伟晶岩脉。地质背景条件、围岩标志、断裂构造条件、矿物标型、物化探异常标志、遥感图像可作为扎乌龙重要找矿标志。

The Zhawulong rare-metal deposit,located in Songpan-Ganzêorogenic belt,belongs to the pegmatite type spodumene deposit accompanied with beryllium and niobium-tantalum mineralization.This study summarizes the geochemical and mineralogical characteristics of the deposit,preliminarily outline the ore-forming mechanism and geological indicators.The pegmatite dikes in this deposit are mainly distributed in the internal and external contact zones of the muscovite granite pluton.The spodumene-bearing pegmatites have similar REE and trace element patterns,but theirΣREE contents are significantly lower than those of muscovite granites,suggesting that the muscovite granite and its related pegmatites in Zhawulong deposit are petrogenetically linked through a fractional crystallization process.The muscovite granite in this deposit has a high Rb/Sr and Rb/Ba ratio,and a low CaO/Na_(2)O with similar Sr-Nd-Hf isotopic compositions to those of the schists,indicating that they are originated from the dehydration and melting of the Xikang Group metamorphic sedimentary rocks.Mineral and fluid inclusion observations suggest that the formation of the pegmatites can be divided into the magmatic and hydrothermal stages:the magmatic stage was characterized by the crystallization of primary beryl,spodumene and columbite-tantalite;while the hydrothermal stage was characterized by intense interaction with hydrothermal fluids which caused the alteration of spodumene and mica,and the development of Cs-or Na-enriched veinlets in beryl.The spodumene is deposited at the magmatic stage,characterized by a high pressure(350MPa)condition and a closed system.The H-O isotopic compositions indicate that the ore-forming fluids of the system were magmatic origin,whose hydrothermal fluids were residual fluids originated from pegmatite melt.The pegmatites show a medium degree of fractional crystallization,which is much lower than the high-fractionated Koktokay No.3 pegmatite but higher than the low-fractionated South Ireland Leinster pegmatite.Geological background,host rocks,fault structure,minerals,geophysical and geochemical exploration anomalies and remote sensing images can be severed as important indicators for the exploration of the Zhawulong deposit.