特提斯喜马拉雅带东段哲古错含金(砷)细粒石英闪长岩的发现及其意义

Discovery of the Zhegucuo Au(As)-bearing fine granular quartz diorite in the eastern Tethyan Himalayan Belt and its significance.

在特提斯喜马拉雅带东段哲古错中低温热液型锑-金矿床内首次发现了含金(砷)细粒高镁石英闪长岩,它们呈岩脉或岩枝状侵入于上三叠统—下侏罗统黑色页岩中。矿区内及外围有许多辉长岩脉及少量橄榄二辉岩小岩体产出。含金(砷)细粒高镁石英闪长岩主要为细粒结构,其次为斑状结构,具有与高镁埃达克岩相似的地球化学特征,表现为:SiO_2≥56%,高Al_2O_3(＞15%)、高MgO(Mg~#值为48～59),高Cr(75×10^(-6)～130×10^(-6))、高Ni(54×10^(-6)～74×10^(-6)),富集LILE,亏损HREE及HFSE,(La/Yb)_N＞12,Eu弱负异常(δEu=0．81～0．91),Sr含量较高且变化大(209．28×10^(-6)～685．14×10^(-6),平均381×10^(-6))、低Y和Yb,Y＜16×10^(-6),Yb＜1．7×10^(-6),具有平坦的HREE配分型式,(Ho/Yb)_N≈1,Sr/Y值较高且变化大(14．58～47．28,平均24)。Nd-Sr-Pb同位素组成特征为:高I_(Sr)(0．709726～0．711203),ε_(Nd)(t)弱负(-3．08～-1．15),Pb同位素组成变化幅度不大,^(206)Pb/^(204)Pb=18．638～18．672,^(207)Pb/^(204)Pb=15．694～15．702,^(208)Pb/^(204)Pb=38．983～39．016。含金(砷)细粒高镁石英闪长岩的K-Ar年龄为42．54±0．94Ma和43．21±1．14Ma,其岩浆可能是由地球化学性质类似于哲古错辉长岩的底侵玄武质下地壳部分熔融形成的,此底侵玄武质岩浆源自轻度富集EMⅡ物质的大陆下岩石圈地幔,于晚侏罗世底侵到下地壳。距今65Ma前,印度板块与欧亚大陆碰撞,地壳逐渐加厚。约43Ma前,地壳加厚到40km以上,同时发生了构造事件,诱发玄武质下地壳熔融形成含金(砷)细粒高镁石英闪长岩岩浆,此岩浆与构造活动耦合上升到地壳浅部成岩成矿。细粒高镁石英闪长岩是对金成矿有利的岩石,并且赋存有斑岩型金(砷)矿(化)体,指示特提斯喜马拉雅东段可能存在斑岩型金矿带;同时,含金(砷)细粒高镁石英闪长岩的发现对特提斯喜马拉雅带构造-岩浆作用及地球动力学研究也有意义。

This paper reports newly discovered Au（ As ） -bearing fine granular high-Mg quartz diorites in the Zhegncuo hydrothermal Sb-Au deposit, in the eastern Tethyan Himalayan Belt. The rocks have been mostly altered and disseminated sulfide mineralized, and intruded in the Upper Triassic-Lower Jurassic black shale as veins or apophyses. Many gabbroid veins and few olivine websterite bodies are distributed in the Zhegncuo area. The geochemical characteristics of the Au（ As ） -bearing fine granular high-Mg quartz diorites are similar to high-Mg adakite, such as SiO2 1〉56%, high Al2O3 content （ 〉 15% ）, high Mg^#（48 -59）, high Cr （75 × 10^-6 - 130 × 10^-6） and Ni content （54 × 10^-6- 74 × 10^-6）, rich in large ion lithophile elements （LILE）, relatively depleted in high strength field elements （HFSE） and HREE, （La/Yb）N 〉 12, weakly negative Eu anomaly （δEu = 0.81 -0.91 ）, relatively high and variable Srcontent（209.28× 10^-6-685. 14 × 10^-6）, low Y and Yb content, Y〈 16 × 10^-6, Yb 〈 1.7 × 10^-6, fiat HREE pattern, （Ho/Yb） N ≈ 1, relatively high and variable Sr/Y （ 14.58 - 47.28 ）. The Nd-Sr-Pb isotopic characteristics of the Au（As） -bearing fine granular high-Mg quartz diorites are high Isr （0. 709726 - 0.711203 ）, end （t） = - 3.08 - - 1. 15, 206 pb/204 Pb = 18. 638 - 18. 672, ^206Pb/^204Pb = 15. 694 - 15. 702, ^208Pb/^204Pb = 38. 983 - 39. 016. Two K-Ar isotopic ages of 42.54 ＋ 0.94Ma and 43.21 ＋ 1.14Ma for Au（ As ）-bearing fine granular high-Mg quartz diorites have been obtained. The rocks were probably formed by the melting of underplated basaltic lower crust having the same geochemical characteristics as gabbroid occurred in the Zhegocuo area, and source of which was the continental mantle with weakly enriched in EMII matter. In Late Jurassic, a basaltic magma underplating took place in the eastern Tethyan Himalayan Belt. In Eocene （ ≈43Ma）, the thickness of crust in the studied area had been thickened more than 40km due to the collision between India and Asia continents （≈65Ma）. A tectonic event induced melting of the underplated basaltic lower crust in that time, by which the magma of Au（As）-bearing fine granular high-Mg quartz diorites was produced. Then the magma moved up and intruded shallow crust, along with the formation of the Au （As） -bearing fine granular high-Mg quartz diorites and the porphyry Au （As） deposit. The Au （As）-bearing fine granular high-Mg quartz diorites are favorable to Au mineralization. It is approximately determined that the rocks contain porphyry-type Au （ As ） mineralized bodies. The discovery indicates that a porphyry- type Au （As） mineralized zone probably existed in the eastern Tethyan Himalayan Belt. It is also of significance for understanding tectonics and geodynamics of the Tethyan Himalayan Belt.