西藏雄村矿区南部玢岩的地质年代学、岩石地球化学及其地质意义

摘要

西藏雄村矿区是南冈底斯带上的一个以铜为主、伴生金(银)的超大型矿集区,成矿时代为中侏罗世,侏罗纪玢岩体是矿区勘查的首选目标,因此,开展玢岩体地质年代学研究对矿区的找矿工作部署意义重大。本文在详细的野外地质调查基础之上,首次对矿区出露较多的含粗粒石英斑晶的角闪石英闪长玢岩(简称南部玢岩)开展锆石SHRIMP U-Pb定年和岩石地球化学研究,以期为矿区找矿工作部署、岩浆演化与成矿作用研究提供有益的技术支撑。研究结果表明：(1)南部玢岩形成于中侏罗世,锆石SHRIMP U-Pb年龄为174.4±1.6 Ma,侵位于II号矿体形成之后、I号矿体形成之前；(2)南部玢岩的岩石地球化学特征具有岛弧岩浆的亲和性,表明其形成于岛弧构造环境,可能是新特提斯洋向北俯冲的产物；(3)南部玢岩由亏损地幔源区来源的新生地壳物质的部分熔融形成；(4)南部玢岩在II、III号矿体中是不含矿的成矿后玢岩,但在其他地段的含矿性则有待进一步的地质勘查工作确认。%The Xiongcun district, located in the southern margin of the Gangdese belt, is a large metallogenic district for copper along with gold and silver. So far, No.I, No.II, No.III deposits and some other copper/gold anomalies have been discovered within the district. The ore bodies are hosted in the Jurassic intrusive/extrusive rocks with an estimated reserve of more than 2.5 million tonnes (Mt) copper, >250 tonnes (t) gold, and >1100 t silver. Previous exploration programs targeting the Jurassic porphyry and geochemical anomalies had successfully leaded to several discoveries. Thus, the geochronology study for the porphyries in the Xiongcun district may provide valuable guidance for future exploration program. We carried out detailed field investigation, zircon SHRIMP U-Pb dating and rock geochemical study on the southern porphyry (hornblende quartz diorite porphyry with coarse-grained quartz phenocrysts) in the No.II deposit to constrain age of the southern porphyry. We hope our research can provide beneficial guidance for the arrangement of the exploration work, and achieve new understanding for the relationship between the magmatic evolution and metallogenesis. The conclusions of this study are:(1) Zircon SHRIMP U-Pb age of the south porphyry is 174.4±1.6 Ma, and the porphyry shows geochemical features of island arc magmas, such as weakly negative Eu anomaly, enrichment of LILEs (K, Rb, Sr and Ba) and LREE (La and Ce), and depletion of HFSE (Nb, Ta, Ti and P). So we suggest that the southern porphyry is the product of the northwards subduction of the Neo-Tethys oceanic plate in Middle Jurassic;(2) The southern porphyry is characterized by SiO2 ranging from 62.68%to 67.1%, MgO varying from 1.22% to 1.52%, Mg# ranging from 39.5 to 63.7 (average is 48.9) and εHf(t) of zircon varying from +12.50 to +16.26, which suggest that the magma was derived from partial melting of juvenile crust; (3) The barren southern porphyry intruded into No.II and No.III deposit, but it did not intrude into the No.I deposit. This field relationship suggests the metallogenic epoch of No.II and No.III deposit should be earlier than the southern porphyry (174.4±1.6 Ma), and metallogenic epoch of No.I deposit should be later than the southern porphyry. This understanding is consistent with the previous studies on the mineralization age of the No.I and No.II deposits;(4) The southern porphyry was not impacted by the mineralization in No.II and No.III deposits, however the mineralization potential of the southern porphyry in the other areas still need to be confirmed.