西藏甲玛铜多金属矿床似埃达克岩的成岩成矿作用
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摘要
西藏甲玛铜多金属矿床以夕卡岩-角岩型为主,与驱龙斑岩铜矿床毗邻,是冈底斯成矿带东段两颗璀璨的明珠,最具代表性的世界级特大型多金属矿床。矿床内主要出露下白垩统林布宗组沙板岩和上侏罗统多底沟组灰岩,矿化发生于岩体与上侏罗统多底沟组接触带附近,以及和岩浆热液导通的多底沟组和林布宗组界线的层间破碎带,矿体整体呈上陡下缓似层状,矿化中心显示NNE向的雁列型排布。矿床围岩蚀变发育,以热接触蚀变和热交代蚀变为主,具相似的水平蚀变分带和垂向蚀变分带。
     矿床中酸性斑岩的类型主要有花岗斑岩、花岗闪长斑岩、二长花岗斑岩、(石英)闪长玢岩,其中花岗斑岩主要分布于矿床南端,以岩珠状整体呈近EW向主动侵位围岩褶皱核部,矿物以填隙状的白云母为标志,显示强过铝质S型花岗岩特征,属于高钾钙碱性岩石系列;花岗闪长斑岩、二长花岗斑岩、(石英)闪长玢岩主要分布于铜山-则古郎地区,以岩脉(枝)近SN向雁列型被动侵位围岩,岩石中普遍发育岩浆混合现象,显示偏铝质I型花岗岩特征,属于中-高钾钙碱性C型埃达克岩。
     甲玛似埃达克岩表现埃达克岩-钙碱性岩的演化系列,整体表现为Fe、Mg矿物的分异。随着岩浆酸度的增加,主元素只有w(K2O)含量增高,为正常岩浆演化序列。在演化过程中,甲玛似埃达克岩岩浆与基性岩浆发生了三次不同程度的岩浆混合作用,对应矿床的三期Cu、Mo成矿作用过程。
     甲玛似埃达克岩的形成于亚洲-印度板块的“后碰撞”造山过程中,是该时期构造-岩浆-成矿活动的集中体现。其源区显示深部岛弧岩浆岩变质为角闪岩相后部分重熔,并受到富集地幔(EMⅡ)的混合作用的特征。甲玛似埃达克岩的形成过程可以描述如下:在14Ma前后,随着软流圈的大规模上涌,冈底斯地壳快速减薄,形成近南北向的正断层及裂陷盆地系统(墨竹工卡-错那裂谷)。在底侵玄武质岩浆房(可能来自EMⅡ)的加热作用下,导致更为上层的地壳部分熔融(角闪岩相的岛弧岩浆),形成似埃达克岩岩浆。伴随着底侵玄武质岩浆的侵位,似埃达克质岩浆与基性岩浆发生不同程度的岩浆混合作用,从而导致矿质的萃取与富集,随着混合岩浆结晶分异的持续进行,矿质不断从岩浆中分馏出进入流体系统,并沿近NS向的张性构造破碎带中沉淀、成矿。
JiaMa copper polymetallic deposits of Tibet which is a hornfels-skarn type deposit locates at the eastern of Gangdese metallogenic belt. It is adjacent to the QuLong porphyry copper deposit. The stratums of deposit are mainly Limbuzong group of the lower Cretaceous and duodigou group of the upper Jurassic. The Ore body is located between the two stratums as like-layered. The mineralized center have NNE trend arranged in echelon-type The wall rock alteration of deposit is developed, whit the similar horizontal and vertical alteration zoning
     The main types of medium-acid porphyry in Jiama deposit are granite porphyry, granodiorite porphyry, monzogranite porphyry and (quartz) diorite porphyry. The granite porphyry arranged in EW are mainly distributed in the southern. The rock is charactered as muscovite, classified as strongly peraluminous S-type granite, also as high-K calc-alkaline series rocks. The other porphyry arranged in NS as echelon-type, classified as metaluminous I-type granite, also as medium-high-K calc-alkaline and C-type adakites.
     Jiama adakites show the evolutionary trend of from adakites to calc-alkaline rock, that is the fractionation of Fe, Mg mineral. With the crystallization differentiation, all the main elements content reduce except the w(K2O), consistent with the normal magma evolution sequence. In the process of evolution ,there are three magma mingling processes between JiaMa adakites and basic magma, corresponding to the three Cu、Mo mineralization.
     The formation of the Jama adakite-like rock is under the "post-collision" orogeny between the Asian and the Indian plates, which is the concentrated expression of the tectonics, the magmatism and the mineralization during that period. The source shows that the ignerous and metamorphic rock in the deep arc is amphibolite face through partial remelting , and mixed by the enriched mantle (EMⅡ).The formation process can be described like this : around 14Ma, with the large-scale asthenospheric upwelling and the rapid thinning of the crust Gangdese, the nearly N-S normal faults and the rift basin system formed. Under the heating of the underplating basaltic magma chamber, the upper crust (the island-arc magma of amphibolite face partially melted, and finally there resulted in the formation of the adakite-like rock. With the emplacement of underplating basaltic magma, there were different degrees of magma mixing between the adakite-like rock and the basic magma, which leading to the extraction and enrichment of the mineral. With the the mixed magma crystallization and differentiation continuing, the mineral separated from the magma affluxed into the fuild system, and precipitated and mineralized in the nearly N-S tensile frature zones .
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