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

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。

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

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 f_O2 value might be the major factors causing gold precipitation.