湖南锡田钨锡多金属矿田燕山期NE向断层演化历史及其成矿意义

Yanshanian NE-Striking Fault Evolution and Its Implications on Mineralization in the Xitian W-Sn Polymetallic Ore Field,Hunan Province

为了确定湖南锡田矿田燕山期NE向断层演化历史、蚀变特征及与钨锡成矿关系本文调查了矿田内燕山期断层野外宏观特征,对断层中充填石英脉进行了显微鉴定、阴极发光、含矿元素分析,结果显示:(1)断层以NE60°~70°平行分布于茶汉盆地南北两侧,均倾向茶汉盆地,断层的陡倾、棱角状角砾及梳状石英等指示了断层的张性性质;(2)断层经历了3期流体活动,第一期的石英脉中石英颗粒相对粗大(0.5~15.0 mm),局部发育破裂与变形纹,石英可为断层角砾;第二期与第三期沿断层中张性裂隙充填的石英脉,粒径较小(0.01~2.00 mm),第二期石英发育微裂隙,而第三期石英为自形长柱状,无变形;(3)第一期与第三期石英脉中不含云母,第二期则富含绢云母,其W、Sn元素含量高,并有少量白钨矿、黄铁矿等矿物,其组分演化特征与区内矿床热液组分演化趋势相似;(4)锡田地区燕山期钨锡矿化分布与NE向系列断层关系密切,显示了茶汉盆地两侧由南向北分带格局.作者认为在燕山期华南伸展背景控制下,地幔上隆及重熔岩浆上侵产生的综合伸展是NE向系列断层形成的可能机制,NE向系列断层的张性环境是岩浆热液聚集的有利因素,为成矿热液运移提供了通道.

In order to determine the fault activity history and its relationship with hydrothermal alteration and W⁃Sn mineralization,we investigated the field characteristics of the Yanshanian faults in the Xitian ore field and carried out microscopic identification,cathodoluminescence and ore⁃forming element analysis on quartz in faults.The results show that:(1)The faults are distributed in northern and southern sides of the Chahan basin and dipping to the basin,with striking directions of NE 60°-70°.High dip angles,angular breccias and pectination quartz veins in the faults indicate extensional characteristics.(2)Three stages of magmatic fluid activities occurred in the faults.The stage I is characterized by quartz veins occurring as coarse quartz particles(0.5-15.0 mm)with local deformation and fragmentation structures.The stage II and stage III are typically quartz veins filling in tensile fractures with small quartz particles(0.01-2.00 mm).Moreover,stage II quartz develops microcrack structures while stage III quartz is idiomorphic and unreformed,suggesting a progressively weakening trend of fault deformation.(3)In contrast to stage I and stage III mica-free quartz veins,stage II quartz veins involve more micas with the high contents of W and Sn elements and minor scheelites and pyrites,which show similar compositional evolutionary characteristics to that of the W⁃Sn deposits in the Xitian ore field.(4)The distribution of Yanshanian mineralization in the Xitian ore field is closely related to the normal faults,showing clear and regular mineralizing zonation of W⁃Sn and Pb⁃Zn deposits from the south to the north on each side of the Chahan Basin.Under the control of extensional tectonic setting of the South China Block during Yanshanian,the comprehensive extension resulting from mantle uplift and remelted magmatic intrusion may be a potential mechanism for the formation of normal faults.The extensional environment of the normal faults was favorable to accumulation of magmatic hydrothermal fluids;and the normal faults provided a channel for the migration of ore-forming hydrothermal fluids.