江西铜坑嶂斑岩钼矿床成矿流体特征与成矿作用研究

Characteristics of ore-forming fluid and metallogenesis of the Tongkengzhang porphyry molybdenum deposit,Jiangxi Province

江西铜坑嶂钼矿是新近发现的中型斑岩型钼矿,矿体主要分布在白垩纪花岗斑岩体内。根据矿物组合和穿插关系可将该矿床分为三个矿化蚀变阶段:钾硅酸盐化阶段、萤石-黑云母(白云母)-钾长石-辉钼矿阶段和绢云母-石英-碳酸盐阶段。流体包裹体研究表明早阶段成矿流体为富含碱质、挥发份的高氧化性岩浆流体,该成矿流体形成压力较大(约1000bar),温度较高(550～>600℃之间)并发生了沸腾作用,分离出高盐度多相流体包裹体和富气相低盐度包裹体。随着温度的降低(420～440℃之间)和压力的持续释放(320～360bar)成矿流体再次沸腾并导致了钼矿的沉淀。晚阶段与绢云母化等蚀变有关的脉体中大量存在的高盐度(29.58%～44.12%NaCleqv)流体包裹体指示岩浆流体为该蚀变作用的主导,该阶段少数石英脉中富液相包裹体的广泛发育可能与岩浆水和大气降水的混合作用有关。激光拉曼和扫描电镜实验在早阶段流体中检测到Fe3O4、SO2,表明该阶段流体氧化性较高,钾长石化发育,属碱性环境。主成矿阶段流体中还原性气体CH4等的存在,以及辉钼矿与蚀变白云母共生,揭示出该阶段处于相对还原的酸性环境,亦表明氧逸度、pH值变化可能共同导致了钼的沉淀。

Tongkengzhang molybdenum deposit is a newly discovered deposit,which is mainly hosted by the Cretaceous granitic porphyry.Based on mineral assemblage and vein crosscutting relationships,the ore-forming process could be divided into three mineralization-alteration stages：Potassic stage,fluorite-biotite（muscovite）-K-feldspar-molybdenite stage,sericite-quartz-carbonate stage.The study of fluid inclusions shows that early ore-forming fluids are high salinity magmatic fluids,containing alkalic components and volatile phase,which formed in condition of high pressure（～1000bar）and high temperature（550～600℃）,and were separated to multiphase high salinity fluid inclusions and lower salinity vapor-rich fluid inclusions by boiling.With cooling of oreforming fluids（420～440℃）and release of pressure（320～360bar）led to re-boiling of ore-forming fluids,which caused precipitation of molybdenite.Give the evidence of widespread occurrence of high salinity（29.58%～44.12%NaCleqv）fluid inclusions in the sericite-quartz alteration related veins,indicating that the fluids caused the later alteration was dominated by the magmatic fluids,only a few quartz veins are primarily occupied by aqueous inclusions suggesting the mixture of magmatic fluids and meteoric fluids.Meanwhile,the Laser Raman and SEM measurements detected Fe 2 O3,SO 2 in the early fluids,indicating the early fluids were highly oxidized,the abundance of K-feldspar suggests alkalic environment.The existence of CH 4and intergrowth of molybdenite and muscovite reveals fluids were in a relative reduced environment,suggesting redox potential fluctuations and the change of pH value could lead to deposition of molybdenite.