西藏邦铺斑岩矽卡岩矿床二长花岗斑岩Sr-Nd-Pb-Hf同位素及闪锌矿黄铁矿Rb-Sr等时线年龄研究  

Sr-Nd-Pb-Hf Isotope Compositions of Porphyritic Monzogranite and Rb-Sr Isochron Age of Sphalerite-Pyrite at the Bangpu Porphyry-Skarn Deposit

作　　者：赵晓燕[1,2] 杨竹森[2] 刘英超[3] 裴英茹[1] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]中国地质科学院矿产资源研究所,北京100037 [3]中国地质科学院地质研究所研究所,北京100037

出　　处：《地质学报》2015年第3期522-533,共12页Acta Geologica Sinica

基　　金：青藏高原南部大陆聚合与成矿作用项目(编号:2011CB403104);IGCP/SIDA-600联合资助成果

摘　　要：邦铺钼多金属矿是形成于印亚板块碰撞后碰撞伸展环境中的大型富Mo(~0.09%)、贫Cu(~0.32%),并且伴生Pb-Zn的斑岩-矽卡岩矿床。邦铺矿床位于冈底斯斑岩成矿带东段北侧,其成矿年代(~15.32Ma)与冈底斯斑岩成矿带内其它典型的斑岩矿床具有一致性(12~18Ma)。邦铺矿床除了富Mo(Cu),还含有大量的Pb-Zn矿石,其中Mo(Cu)以斑岩型矿化为特征,而Pb-Zn则主要赋存于矽卡岩之中。本文立足邦铺矿床两大问题:矿床富Mo原因和斑岩-矽卡岩两期矿化的关系,借助含矿岩体二长花岗斑岩Sr-Nd-Pb和锆石Hf同位素及共生闪锌矿和黄铁矿的Rb-Sr等时线年龄研究,为更好地了解矿床成因和冈底斯斑岩成矿带成矿规律提供依据。对矽卡岩矿区共生的黄铁矿闪锌矿进行Rb-Sr定年,87 Rb/87Sr=0.2351~7.903,87Sr/86Sr=0.714011~0.715539,闪锌矿和黄铁矿的Rb-Sr等时线年龄为13.93±0.87Ma,其成矿时间与斑岩成矿时间(辉钼矿Re-Os年龄15.32Ma)近于一致;同时,在空间上,随着距离二长花岗斑岩距离的增大,石榴子石减少,方解石及其它碳酸盐矿物增多,矽卡岩矿物组合由高温向低温转变,闪锌矿颜色由不透明向半透明过渡,暗示了温度连续下降的过程;所以无论时间还是空间上,邦铺斑岩矿化和矽卡岩矿化都属于同一成矿系统。邦铺二长花岗斑岩(87Sr/86Sr)i值介于0.707504~0.710012之间,变化范围较小;εNd(t)值为-3.96^-3.56,TDM2为1020~1050Ma;206 Pb/204 Pb、207 Pb/204 Pb、208Pb/204Pb分别为18.304~18.439、15.744~15.793、38.842~38.904;锆石176 Hf/177 Hf为0.282826~0.283009,平均值为0.282916,εHf(t)值为2.2~8.7,平均值为5.4;全岩Sr-Nd-Pb及锆石Hf同位素特征指示邦铺二长花岗斑岩为壳幔源岩浆混合作用的产物,其壳源组分的含量相对于冈底斯斑岩成矿带典型的含矿斑岩更高。幔源物质来源于上涌的软流圈地幔而壳源物质为加入大量古老地壳成分的新生下地壳;由于Mo、Pb、Zn主要来自于�Bangpu deposit in Tibet is a large but poorly studied Mo-rich（~0.089%）,and Cu-poor（~0.32%）porphyry deposit that formed in a post-collisional tectonic setting.The deposit is located in the Gangdese porphyry copper belt,and formed（~15.32 Ma）at the same time as other deposits within the belt（12~18Ma）,although it is located further to the north and has a different ore assemblage（Mo-Pb-Zn-Cu）compared to other porphyry deposits（Cu-Mo）in this belt.Two types of mineralization have been identified in the Bangpu deposit：porphyry Mo-（Cu）and skarn Pb-Zn mineralization.In this paper,based on the whole rock Sr-Nd-Pb and zircon Hf isotopic compositions of the ore-bearing porphyritic monzogranite and Rb-Sr isotopic research on the coprecipitated pyrite and sphalerite,we discuss the mechanism of Mo enrichment at Bangpu and the relationship between the two types of mineralization.RbSr isotope dating of the paragenetic pyrite and sphalerite from the Bangpu Pb-Zn skarn deposit was carried out to figure out the relationship between porphyry and skarn mineralization events. We got a Rb-Sr isochron age yielded at 13.9±0.9Ma,which is almost identical to the age of porphyry mineraliztion（~15Ma）.In addition,the account of garnet decreases and the account of both calcite and other carbonate minerals increase,the transformation from high temperature to low temperature skarn mineral compositions,the opaque to translucent sphalerite from the porphyritic monzogranite,suggesting the two types of mineralization in Bangpu deposit are formed in a signal porphyry-hydrothermal system.The（87Sr/86Sr）ivalues of the monzogranitic porphyry are 0.707504~0.710012 with a small range.TheεNd（t）are-3.96^-3.56 while the two stage depleted mantle model ages of Nd isotope are 883~938 Ma.The206Pb/204 Pb,207Pb/204 Pb,208Pb/204 Pb values are 18.304~18.439,15.744~15.793、38.842~38.904,respectively.Zircon 176 Hf/177 Hf are 0.282826~0.283009 with the mean data of 0.282916,whileεHf（t）=2.2~8.7averaging at 5.4.These data i

关 键 词：SR-ND-PB同位素 锆石HF同位素 RB-SR等时线年龄 斑岩-矽卡岩 邦铺 西藏 

分 类 号：P618.2[天文地球—矿床学] P597[天文地球—地质学]