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青海东昆仑阿斯哈金矿矿床地质特征及成因研究
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摘要
阿斯哈金矿位于东昆中隆起带东段,紧邻昆中岩石圈断裂,为印支早期形成的脉型金矿床。矿区内地层简单,仅出露有古元古界金水口群白沙河组。矿体受印支早期构造、岩浆活动控制。矿区内构造主要为断裂构造,近EW向断裂及不同等级的次级断裂分布全区,控制矿化形成和分布的断裂多为NNE向和NW向断裂,这两组断裂局部又是容矿构造。区内侵入岩分布广泛,主要为印支早期形成的花岗闪长岩、闪长岩岩基和岩株状产出的花岗岩。脉岩主要为云煌岩。
     矿区内发现规模不等的含金蚀变破碎带(矿脉)十余条,其中以Ⅰ、Ⅱ号矿脉最为重要,其次是Ⅳ号矿脉。Ⅰ号矿脉受NNE向左旋张扭性断裂控制,Ⅱ号矿脉受NW向右旋压扭性断裂控制。矿石类型主要为石英脉型,其次为构造蚀变岩型;矿石矿物主要有黄铁矿、黄铜矿、毒砂、自然金及方铅矿、辉铅铋矿、辉铋银铅矿等铅矿物;脉石矿物主要有石英、黑云母、绢云母、绿泥石、绿帘石、长石、方解石等。具有它形粒状结构、自形—半自形粒状结晶结构、碎裂结构、交代结构等;浸染状构造、团块状构造、脉状构造和网脉状构造。与矿化关系最为密切的近矿围岩蚀变主要为硅化、绢英岩化等。成矿过程由早到晚划分为石英-黄铁矿-毒砂阶段,石英-多金属硫化物阶段,石英-碳酸盐阶段三个阶段。
     作为阿斯哈金矿容矿围岩的闪长岩—花岗闪长岩和花岗岩的岩石地球化学研究分别显示出碰撞前(俯冲晚期)活动大陆边缘特征和后碰撞花岗岩特征。昆仑洋壳向北不断俯冲拼合导致底侵作用发生,使来自幔源的基性岩浆提供巨量热动力导致地壳物质熔融,在该带区域上形成了规模宏大的花岗质岩带。花岗质岩体Au含量低,不太可能为成矿提供主要矿质。云煌岩空间上与矿脉密切伴生,通过研究表明其可能来源于交代富集地幔,推测其形成的构造环境为陆内造山由挤压向伸展转换的相对松弛阶段,与矿床时间近一致,成因上联系密切。
     含金石英脉中流体包裹体主要有富CO_2三相、气液两相2种类型。成矿流体具有低的盐度(1.83~8.13wt%NaCl.eqv)、低密度(0.69~0.96g/cm3)、中等温度(155.3℃~425.6℃)。流体包裹体气相成分主要有CO_2,CH_4,N_2和H_2O,属H_2O-CO_2-N_2-CH_4体系。成矿Ⅰ阶段流体为低盐度、富CO_2的高温流体。成矿Ⅱ阶段富CO_2型和气液两相流体包裹体共存,发生了以CO_2逸失为特征的不混溶或沸腾,致使残余流体盐度升高。成矿Ⅲ阶段为气液两相包裹体。
     阿斯哈金矿含金石英脉δ~(18)O值为11.3‰~15.1‰, δD_(V-SMOW)值为-59.6‰~-117.7‰,δ~(18)O_(H_2O)值为2.7‰~9.2‰。氢氧同位素和流体包裹体激光拉曼光谱研究显示,阿斯哈金矿成矿流体应以含有幔源物质、富含CO_2的岩浆流体为主,在成矿晚阶段有大气水混入。矿床中黄铁矿样品δ~(34)S值的变化范围为+5.0‰~+7.4‰,平均值为+5.8‰,硫同位素特征以及煌斑岩与矿脉空间上的密切关系显示成矿物质主要来源于幔源岩浆,在上升运移的过程中萃取所流经围岩的成矿物质。
     通过对阿斯哈金矿的成因研究,认为成矿作用应发生在早印支期陆内造山由挤压构造体制由向伸展作用的转换时期,成旷时期应略晚于花岗质岩浆的侵位时间。阿斯哈金矿在矿床成因上与造山型金矿具有相似的基本特征,成矿构造背景为陆内造山环境,矿床成因类型为中成造山型金矿。
Asiha gold ore deposit is located in the eastern part of middle uplifted basementand granitic belt, adjacent to the central Kunlun fault. It is a vein-type gold depositsformed in the early Indosinian. The strata outcropped in the mining area is simple, justthe Palaeoproterozoic Jinshuikou group only.The ore bodies are controlled by earlyIndosinian tectonic and magmatic activities. Fault structure is the main structure in themining area,nearly EW trending faults and different grades of secondary faultswidely distributed in the area, while the NNE and NW trending faults are the mainlyore-bearing structures.Intrusive rocks are widely distributed, mainly for the earlyIndosinian granodiorite, diorite batholith and granite stock.Vein rocks mainly forminette.
     There are more than ten different scales of gold altered fracture (veins) found inthe mining area,1#vein and2#vein are the most important veins, followed by4#vein.1#vein was controlled by the NNE trending left lateral tension-torsional fault,while2#vein was controlled by the NW trending right lateral compression-torsional fault.Ore types can be classified into quartz vein type and structurally altered rocktype,maily of quartz vein type. Ore minerals are pyrite, chalcopyrite, arsenopyrite,galena, native gold, galenobismutite and gustavite, etc. Gangue minerals are quartz,sericite, plagioclase, calcite, chlorite and biotite, etc. Major ore textures are anhedralgranular texture, euhedral-subhedral granular crystal texture, cataclastic texture andmetasomatic texture,etc. Ore structures are mainly disseminated structure, massivestructure, vein structure and net-vein structure,etc. Alteration consists of sericitization, silicification, chloritization and carbonation,etc. Mineralization can be divided intoquartz pyrite arsenopyrite stage, quartz polymetallic sulfide stage and quartzcarbonate stage from early to late.
     As the ore-bearing wall rocks of Asiha gold deposit, diorite-granodiorite andgranite show the characteristics of pre-collisional(late stage of subduction) activecontinental margin and post-collisional granite through the rock geochemistry studies.The northward subduction and collage effect of Kunlun oceanic crust lead to theunderplating process that makes the mafic magma from the mantle provide hugeamount of thermal dynamic and leads the crustal material to melt,then formlarge-scale granitic rock band in this area. The granitic rocks content low grade ofgold,are not likely to provide the main mineral for the mineralization. The minette isclosely associated with veins in space and it may originate from the metasomaticenriched mantle.We speculate that its tectonic setting is the relative relaxation stage ofcompression converting to extension which is after the continent-continent collisionand orogenesis.This is nearly consistent with the deposit in time and closelyassociated with its genesis.
     Fluid inclusions of quartz crystals can be classified into aqueous two-phase andCO_2-rich three phase. Ore-forming fluids are characterized by low salinities,Low-densities and medium temperatures. The salinities of fluid inclusions range from1.83to8.13wt%NaCl.eqv,the densities of fluid inclusions range from0.69to0.87g/cm~3, and the homogenization temperatures of fluid inclusions range from155.3℃to425.6℃. The main gas-phase of fluid inclusions make up of CO_2,CH_4, N_2and H_2O, it belongs to the H_2O-CO_2-N_2-CH_4system. In Mineralization Stage Ⅰ, fluidinclusions characterize by low salinity, high-temperature, and CO2-rich. InMineralized stage Ⅱ, CO_2-rich fluid inclusions and gas-liquid two-phase inclusionscoexist, because of immiscibility or boiling characterized by CO_2escaping, whichleads to salinity increasing of the residual fluid. Fluid inclusions in mineralized stageⅢ are the gas-liquid two-phase inclusions.
     The variation range of δ~(18)O、δD_(V-SMOW) and δ~(18)O_(H_2O)of gold-bearing quartz veinfrom Asiha gold deposit is respectively11.3‰~15.1‰,-59.6‰~-117.7‰and 2.7‰~9.2‰. Hydrogen oxygen isotopes and laster Raman spectroscopy of fluidinclusion research shows that ore-forming fluid of Asiha gold deposit is mainly frommagmatic fluid which contains mantle material and rich in CO_2, organic water such asmeteoric water interfused at late ore-forming stage.The δ~(34)S values of pyrites varybetween+5.0‰and+7.4‰, average value is+5.8‰.Both the characteristic of sulfurisotope and the close relationship which lamprophyre dykes and ore vein have inspace show that the metallogenic material is mainly from deep mantle-derivedmagma,which extract metallogenic material from stratum and rocks at the process ofrise and migration.
     By the genesis study of Asiha gold deposit,the author considers that themineralization may occurs in the transition period from compression to extensiontectonic environment during the intracontinental orogenic movement in the earlyIndosinian, the mineralization phases should be a little later than the emplacementtime of granitoid magma. In the aspect of ore genesis, Asiha gold deposit’scharacteristic is similar to the orogenic gold deposits, the metallotectonic setting isintracontinental orogeny. The genetic type of ore deposit is mesozonal orogenic golddeposit.
引文
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