闽西行洛坑钨矿流体演化过程与成矿机制:白钨矿原位微量元素、Sr同位素的制约

Ore-forming mechanism and fluid evolution processes of the Xingluokeng tungsten deposit,western Fujian Province:Constraints form in-situ trace elemental and Sr isotopic analyses of scheelite

行洛坑钨矿位于武夷山成矿带中部,是该带目前规模最大的钨矿床。钨矿体主要产于强烈蚀变的似斑状黑云母花岗岩岩株顶部,发育细脉浸染状、网脉状及大脉状多种矿化类型,黑钨矿与白钨矿含量近1∶1。关于矿床的流体演化过程与成矿机制目前仍不清楚,成因存在较大争议。本文在详细成矿阶段划分的基础上,对不同阶段产出的不同世代白钨矿开展了系统的原位LA-ICP-MS微量元素和Sr同位素、以及流体包裹体和H-O同位素的研究工作。行洛坑钨矿由早至晚可以划分为3个成矿阶段:细脉浸染状白钨矿-辉钼矿阶段(阶段Ⅰ)、钾长石-白钨矿-黑钨矿-绿柱石阶段(阶段Ⅱ)及硫化物-黑钨矿-白钨矿-碳酸盐阶段(阶段Ⅲ)。流体包裹体研究显示成矿流体为中高温、低盐度的Na Cl-H_2O-CO_2体系。H-O与Sr同位素表明成矿流体主要为岩浆流体,仅成矿晚期阶段有少量的大气降水加入。阶段I白钨矿相对富REE、Mo、Na和Nb,贫Sr;而随着流体演化,白钨矿REE、Mo、Na、Nb含量逐渐降低,Sr含量显著升高。阶段I白钨矿呈自形-半自形粒状,CL图像显示细密的、均匀的震荡环带,稀土元素主要与Na和Nb结合进入白钨矿晶格;阶段Ⅱ、阶段Ⅲ白钨矿呈半自形-他形,不发育或仅发育宽缓的、不规则震荡环带,稀土元素与Ca(Ca的离子空位)结合置换白钨矿中的Ca。结合蚀变与矿化特征,认为阶段I白钨矿形成于低水岩比环境,由初始岩浆流体沿微小裂隙渗透交代而形成;而阶段Ⅱ、Ⅲ白钨矿形成于高水岩比环境,CO_2的不混溶作用伴随强烈的水岩反应导致了钨的富集沉淀。结合矿床地质特征,认为行洛坑钨矿属于广义的斑岩型钨矿,细脉浸染状矿化构成了钨成矿的基础,而网脉状、大脉状矿化的叠加是钨进一步富集的关键。

The Xingluokeng tungsten deposit is located in the middle area of the Wuyishan metallogenic belt,which is the largest tungsten discovered in the belt by now.The orebody mainly occurs in the roof part of an extensively altered granite stock.Multiple mineralized types have been recognized,including veinlet-disseminated,stockwork and vein-type mineralization,with scheelite/wolframite ratios at 1∶1,approximately.The ore genesis of the Xingluokeng deposit is still debated,and ore-forming mechanism and fluid evolution processes remain enigmatic.In this study,we describe the detailed ore-forming stages,and carry on fluid inclusions,HO isotope and in-situ LA-ICP-MS trace elements and Sr isotope on multi-generation scheelites from different mineralization stages.The mineralized processes of this deposit could be divided into three stages:veinlet-disseminated scheelite-molybdenite stage(stageⅠ),K-feldspar-scheelite-wolframite-beryl stage(stageⅡ)and sulfide-wolframite-scheelite-carbonate stage(stageⅢ).Microthermometric analyses of fluid inclusions indicate the ore-forming fluids belong to Na Cl-H2O-CO2 system,with medium-high temperature and low salinity.H-O and Sr isotopic compositions suggest the ore-forming fluids dominantly originated from magma water,and limited meteoric water involved in late mineralization stage.The scheelites in early stage have higher contents of REE,Mo,Na and Nb,but lower Sr content;whereas REE,Mo,Na and Nb were gradually depleted and Sr was enriched,with the evolution of ore-forming fluids.The scheelites of stage I are characterized by euhedral or subhedral crystals,with dense and homogeneous oscillation zonation.In this type of scheelites,the REE substituted into the Ca site by Na-and Nd-dominated substitution mechanism.Whereas the scheelites in stageⅡandⅢ,the incorporation of REE into scheelites can be coupled with□Ca(where□Cais a Ca site vacancy).Together with mineralization and alteration features,we propose that the scheelites in stageⅠcrystallized in a low fluid/rock ratio condition,by way of pervasive replacement of primary magmatic fluids along micro fractures.Whereas the scheelites in stageⅡandⅢwere precipitated by extensively fluid-rock interaction accompanying by immiscibility of CO2.Coupled with the geological characteristics of ore deposit,we consider the Xingluokeng deposit as generalized porphyry tungsten deposit.The veinlet-disseminated tungsten mineralization constitutes the metallogenic base,and the superposition of stockwork and vein-type tungsten mineralization is a key process for the Xingluokeng tungsten deposit.