新疆吐拉苏盆地京希-伊尔曼德金矿地质和地球化学特征研究

Geology and geochemistry of Jingxi-Yelmand gold deposit in Tulasu basin,North Tianshan,Xinjiang

京希-伊尔曼德金矿位于新疆北天山吐拉苏盆地的西北缘,赋存于泥盆纪-早石炭世火山-沉积地层底部的凝灰岩、凝灰质砂岩中,围岩经历了绢云母化、黄铁矿化、多期硅化和角砾化、碳酸盐化和重晶石化,金矿化与硅化围岩紧密伴生。矿体呈透镜状、层状和似层状,产状与围岩基本一致,主要由热液角砾岩型矿石组成,其热液演化期由四个阶段组成:I:硅化及绢云母化——在围岩凝灰岩和凝灰质砂岩中形成大量浸染状石英、绢云母和少量黄铁矿;II:角砾化及硅化——形成含金热液角砾岩a,角砾为早期蚀变围岩,胶结物为烟灰色玉髓状石英、黄铁矿、毒砂和少量金矿物;III:角砾化及硅化——形成含金热液角砾岩b,角砾为热液角砾岩a和蚀变围岩,胶结物为细粒石英、黄铁矿、毒砂和少量金矿物;IV:方解石-重晶石阶段——形成大量粗大的方解石-重晶石脉。京希-伊尔曼德金矿成矿流体本身富集V、Cr、Ni、Cu、Sb,且其中的Mn、Co、Zn、Bi以及大离子亲石元素LILE主要来自火山岩围岩。从成矿早期到晚期,成矿流体轻稀土元素逐渐富集、氧化性增强。水-岩体系氢、氧同位素组成模拟计算表明,京希-伊尔曼德金矿成矿流体主要为与区内火山岩再平衡的岩浆水,其中金浓度为1×10-6～2×10-6,形成该矿需要约1×108～0.5×108t岩浆热液,蚀变围岩和矿石中黄铁矿富集轻稀土元素。角砾化作用及其伴随的氧逸度升高是导致金沉淀的主要机制。

Jingxi-Yelmand gold deposit is located in the northwest of Tulasu basin,North Tianshan(Xinjiang,NW China),which is hosted in tuff and tuff sandstone in Devonian to Early Carboniferous volcanic-sedimentary strata.Silicification,sericitization,pyritization,brecciation and carbonation are found in the host rocks,in which silicification is closely related to gold mineralization.Orebody in Jingxi-Yelmand gold deposit,mainly composed of hydrothermal breccias and strongly silicified host rocks,is lenticular,stratoid and stratified,parallel to the volcanic-sedimentary strata.The ore-forming process consists of four stages:Silicification and sericitization(stage I),forming abundant disseminated quartz,sericite and pyrite in host rocks;Brecciation and silicification(stage II),made the formation of hydrothermal breccia-a,composed of altered host-rock fragments,which were cemented by chalcedonic quartz,pyrite,arsenopyrite and some auriferous minerals;Brecciation and silicification(stage III),formed hydrothermal breccia-b composed of altered rock and breccia-a fragments cemented by fine-grained quartz,pyrite,arsenopyrite and auriferous minerals.The hydrothermal process is terminated by forming coarse-grained calcite-barite veins(stage IV).Geochemistry study suggests that the original ore-forming fluid enrich in V,Cr,Ni,Cu and Sb,and leach Mn,Co,Zn,Bi and LILE from wall-rocks.Ore-forming fluid is magmatic water reequilibrated with host volcanic-sedimentary rocks,and becomes more oxidized from early to late stage.Model calculation suggests that gold concentration in ore-forming fluid is about 1 × 10-6 ～ 2 × 10-6,and about 1 × 108 to 0.5 × 108 ton magmatic hydrothermal fluid is needed for forming Jingxi-Yelmand gold deposit.Brecciation and the related increase of fO2 value might be the major factors causing gold precipitation.