江西大湖塘超大型钨矿似伟晶岩壳成因研究

Most pegmatoid shells refer to a special type of rocks, which have granitic composition and own distinguished coarse/ skeletal/ graphic or unidirectional solidification textures. They generally occur at the top or margin of porphyries, and are often associated spatially with porphyry-type W, Cu and Mo deposits. So, extensive attentions are paid on the pegmatoids by economic geologists and geological prospectors. Previous studies show that pegmatoid shells are usually thought to form during the magmatic-hydrothermal transition, recording the formation, accumulation, and excretion of the early ore-forming fluids in porphyry deposits. However, their genesis still remains the subject of much debate. Two hypotheses have been proposed to account for formation of pegmatoid shells, they are igneous origin and hydrothermal models. To test these hypotheses, as well as to investigate nature and source of primitive ore-forming fluids in porphyry W systems, pegmatoid shells from the super-large Dahutang tungsten deposit in the northwestern Jiangxi Province have been selected for the following studies. Based on the petrography, especially cathodoluminescence petrography, we will mainly use EMPA and recently-emerged in situ analysis techniques (e.g., LA-ICPMS, SHRIMP, and laser Raman spectroscopy) to study the trace elemental and oxygen isotopic geochemistry of quartzs in pegmatoids and the melt/fluid inclusions.The aims of this study are to: (1) clarify genesis of pegmatoid shell; (2) identify nature and source of initial ore-forming fluids in the porpyry W deposit; and (3)improve the mtallogenic theory and prospecting for porphyry W system.

大多数似伟晶岩壳是具有花岗岩质成分，且发育粗晶/骸骨/文象结构或单向固结结构的一类岩石，常分布于含矿斑岩体的顶部或边缘。它与斑岩型W、Cu、Mo矿床关系密切，常受到矿床学家和地质勘探者的广泛关注。已有的研究显示，似伟晶岩壳形成于岩浆-热液过渡阶段，记录着斑岩矿床早期成矿流体的生成、聚集和排泄等过程。但其成因却有争议，存在岩浆和热液两种起源观点。为了查明似伟晶岩壳成因，同时也为了调查斑岩钨矿早期成矿流体的性质及来源，项目选择赣西北大湖塘超大型钨矿中的似伟晶岩壳开展立典式研究，拟在详细的岩相学、特别的阴极发光岩相学的基础上，主要采用电子探针、激光消融等离子质谱、激光拉曼光谱、离子探针等原位分析技术，开展似伟晶岩壳中石英的微量元素及原位氧同位素地球化学、熔体/流体包裹体地球化学等方面的研究，以期查明似伟晶岩壳的成因机制，约束斑岩钨矿早期成矿流体的性质及来源，完善斑岩钨矿成矿理论和促进找矿。

江西大湖塘地区是江南造山带中段九岭矿集区内的一个世界级超大型钨矿产地，白钨矿矿体主要呈细脉浸染状缓倾于中粗粒黑云母花岗闪长岩（赋矿围岩）中。位于该区一矿带的似斑状黑云母花岗岩（成矿母岩）与赋矿围岩之间的内接触带，发育完好的似伟晶岩壳，是该区超大型钨矿重要的标志层，壳体中W含量很低，但对巨量W的迁移富集有重要意义。壳体主要由长石、石英和白云母等组成，在空间上具有显著的组构和成分变异特征，由母岩向外，依次发育长英质细晶岩，石英长石似伟晶岩，长石石英似伟晶岩和石英体。石英长石似伟晶岩为斑马纹状构造，巨晶长石垂直于接触带生长，代表了岩浆早期流体出溶的方向。阴极发光岩相学研究显示，长石石英似伟晶岩中的石英包括巨晶、大斑晶、小斑晶、基质和石英脉5种类型。各类石英大多分别先经历了早阶段结晶石英和中阶段扩散交代石英，最终遭受了晚阶段统一的渗滤充填石英叠加作用。原位氧同位素和微量元素研究表明，各类石英早阶段的结晶是一个生长加速的过程，中阶段扩散交代作用促进了18O的富集，而晚阶段渗滤充填作用降低了18O石英。各世代石英均表现为富碱金属元素、低Li/Al（大多数＜0.35）的地球化学特征，指示了壳体中的石英主要为岩浆成因，局部为岩浆-热液过渡或热液成因。各类石英形成于中高温（≥483℃）、富18O（11.05‰≤18O石英≤15.07‰）条件，大多数18O石英-水＞10‰，反映了成矿流体为过铝质花岗岩岩浆水。微量元素研究表明，晚期流体pH值降低。石英巨晶中有明显的熔融包裹体和气液流体包裹体共生，流体包裹体富集气相CH4，说明流体氧逸度较低。综合研究表明，一矿带似伟晶岩壳记录了过铝质花岗岩浆流体出溶的过程，是含矿岩浆-热液过渡过程的产物，流体出溶具有连续性、脉动性和多阶段性，其反映的高温、富水、富18O、富碱金属元素、低氧逸度以及晚期酸性的流体性质促进了巨量成矿元素W向围岩的搬运和富集。