江西大湖塘超大型钨矿初始岩浆流体出溶——来自似伟晶岩壳的记录

Exsolution of primary fluids from magma in the superlarge Dahutang tungsten deposit of Jiangxi Province: Records from the pegmatoid shell

江西大湖塘地区是江南造山带中段九岭矿集区内近年查明的世界级超大型钨矿产地，白钨矿体主要呈细脉浸染状缓倾于中粗粒黑云母花岗闪长岩中。位于区内一矿带的成矿母岩顶部发育完好的似伟晶岩壳，壳层的组构特征明显，由母岩向含矿围岩，依次为似斑状黑云母花岗岩→长英质细晶岩→石英长石似伟晶岩→长石石英似伟晶岩→含白钨矿黑云母花岗闪长岩。壳内石英的阴极发光岩相学研究显示，石英巨晶、斑晶、基质和石英脉大多经历了早阶段主晶石英，中阶段扩散交代石英和晚阶段渗滤充填石英3个生长阶段。微量元素研究表明，早阶段石英的生长是一个流体加速沉淀的过程，中阶段和晚阶段流体对主晶石英均有叠加改造，其微量元素呈降低趋势。各类石英均表现为富碱金属元素、中高温（≥483℃）、低Li/Al（0.01~0.50，大多小于0.24）的地球化学特征，反映了成矿流体为岩浆水来源，石英的生长具有岩浆—热液过渡性质。综合研究表明，大湖塘一矿带似伟晶岩壳是初始含矿岩浆流体出溶的产物，其形成过程中动态的物理化学条件促进了成矿元素W由母岩向围岩搬运富集。

Located in the Jiuling ore concentration area of the middle Jiangnan orogenic belt, the Dahutang tungsten deposit discov-ered in recent year in Jiangxi Province is a famous producing area of world-class superlarge tungsten ore. Scheelite orebodies occur-ring as disseminated veins are dominantly hosted gently in the medium-coarse grained biotite granodiorite. Pegmatoid shell is well de-veloped on the top of the mother rock in the Dahutang No. 1 ore block. A vertical section from the mother rock to the wall rock shows the following sequence： porphyritic biotite granite, felsic aplite, quartz-feldspar pegmatoid, feldspar-quartz pegmatoid, and scheelite-bearing biotite grandodiorite. Cathodoluminescence petrographic studies of the quartz from the pegmatoid show that most quartz megacrysts, phenocrysts, matrices and quartz veins experienced three evolution stages, i.e., the early stage of main crystalliza-tion, the middle stage of diffused metasomatism and the late stage of filtrated metasomatism. Trace elemental studies show that the growth of quartz at the early stage was a process of accelerated precipitation of fluids. Fluids of the middle and late stages exerted su？ perimposition and transformation of the host quartz, causing decreasing trend of trace elements. Quartzs of all types are characterized by enriched alkali metal elements, middle-high temperature （t≥483℃）, and low Li/Al ratios （0.01~0.50, most less than 0.24）, which suggests that the fluids might have originated from magmatic water. The growth of quartzs shows a transition from magma to hydro-thermal system. Comprehensive studies indicate that the pegmatoid shell in Dahutang was formed by exsolution of primary magmatic fluids, and the dynamic physical-chemical environment during the crystallization was favorable for the transport of W toward the wall rocks and enrichment of this element in these rocks.