陕南铜厂铜矿床成矿物质来源探讨

陕南铜厂铜矿床是"勉—略—宁"矿集区内最具代表性的铜矿床,通过黄铜矿等单矿物及矿区地质体的微量与稀土元素地球化学对比研究发现:1)黄铜矿以富Ni、Zn和贫Co为特征,与晚元古代郭家沟组细碧岩类似,较闪长岩和钠长岩不同;矿床中存在轻稀土富集和稀土配分模式相对平坦两类黄铜矿,岩体内外接触带黄铁矿Co/Ni值差异表明其成矿物质具多源性;由矿区各地质体成矿元素背景可见,矿床成矿物质来源应以细碧岩为主、闪长岩为辅。2)黄铜矿Eu负异常明显,其δEu值明显低于闪长岩和细碧岩,这与成矿过程中富挥发分流体所形成云英岩化、钠长石化造成的Eu亏损有关,且黄铜矿Y/Ho值与钠长岩较为相似,暗示铜矿化与钠质交代作用关系密切。3)黄铜矿Co、Ni含量一般大于黄铁矿几倍到几十倍,与矽卡岩、斑岩、火山-次火山热液及火山-喷气型铜矿中黄铜矿差异明显,而与铜镍硫化物型矿床中黄铜矿类似,这可能与成矿作用继承了富Ni源区有关。该矿床成矿模式为:海西期,伴随着勉略洋盆闭合俯冲-碰撞形成勉略宁地区韧-脆性逆冲推覆构造、走滑断层,在区域变质流体与天水混合形成富碱和CO2的混合热液作用下,使地层——细碧岩和部分闪长岩中Cu等成矿物质大量析出,形成低温、低盐度成矿热液,沿矿区发育EW向与NE向两组韧性走滑断裂充填沉淀成矿。

Source of Ore-Forming Materials of Tongchang Copper Ore Deposit in Southern Shaanxi Province, China

The Tongchang deposit is the most typical copper deposit in Mianxian-Lueyang-Ningqiang(Mian-Lue-Ning) area in southern Shaanxi,China.By studying on trace elements and REEs geochemistry of chalcopyrite(pyrite) as well as geological bodies in the deposit,we can draw some conclusions as followings.Firstly,the chalcopyrite is characterized by enrichment of Ni and Zn,depletion of Co,which is similar to the spilite of Late Proterozoic Guojiagou Formaion,while different from diorite and albite rocks in the mine.There are two kinds of chalcopyrites with different REE patterns,one is enriched in LREE and the other has relatively flat REE pattern.The pyrite grains hosted in internal and external contact zones of the diorite intrusion have different Co/Ni ratios,suggesting multiple sources of ore-forming materials.Analyzing on the background values of those ore-forming elements in various geological bodies indicates that ore-forming material is mainly from the spilite and minor from the diorite.Secondly,the chalcopyrite is characterized by obvious negative Eu anomalies and its δEu value is much lower than those of the diorite and spilite.The depletion of Eu is likely related to greisenization and albitization caused by volatile fluids during mineralization.Furthermore,Y/Ho ratios of the chalcopyrite are similar to those of albite rocks,implying the close relationship between copper mineralization and Na replacement.Thirdly,due to the inheritance of the trace elements feature from source of ore-forming mineralizing materials,the Ni and Co content in the chalcopyrite is n-n×10 times than in the pyrite,corresponding to that in Cu-Ni sulfide type deposit and different from that in skarn-type,porphyry-type,volcanic-subvolcanic hydrothermal type as well as VMS Cu deposit.The metallogenic model of Tongchang copper deposit can be listed as following.During the Hercynian period,with the closure,subduction and collision of Mian-Lue ancient oceanic basin,the dutile-brittle thrusting nappe structure and strike-slip fault were formed in Mian-Lue-Ning area.The metasomatism of mixed hydrothermal by regional metamorphic fluid and rainwater,which rich in Na+,K+ and CO2,resulted in the translation of ore-forming materials(e.g.Cu and Ni) from strata of Guojiagou Group and spilite(and diorite) to form the metallogenic hydrothermal fluid.The hydrothermal fluid was characterized by low temperature and salinity,and enriched in ore-forming materials.The Tongchang deposit was resulted from filling and metasomatism of the hydrothermal fluid in the EW-and NE-trending dutile strike-slip faults in this area.