投稿时间：2022-03-28
修订日期：2023-02-10
网络发布日期：2023-09-06
中文摘要:八卦庙金矿床位于西秦岭造山带凤太多金属矿田北部,已探明金储量约106 t(平均金品位1.56 g/t),属于特大型金矿。金矿体受脆韧性剪切带控制,总体走向NWW,倾向北,赋矿地层为上泥盆统星红铺组下段。文章在成矿期次研究基础上,采用硫化物原位S-Pb同位素分析方法,对八卦庙金矿成矿过程和成矿物质来源进行了约束。基于详细的野外观察和矿相学研究,矿区识别出了2个金成矿期:顺层磁黄铁矿-石英成矿期和NE向黄铁矿-石英成矿期。顺层磁黄铁矿-石英成矿期可划分为自形黄铁矿-石英(Ⅰ)、磁黄铁矿-自然金-铁白云石(Ⅱ)、白云母-绿泥石-绿帘石-石英(Ⅲ)3个阶段;NE向黄铁矿-石英成矿期由自形磁黄铁矿-黄铁矿-粗粒石英(Ⅳ)、他形黄铁矿-细粒石英(Ⅴ)、他形磁黄铁矿-自然金-铁白云石(Ⅵ)和石英-黑云母(Ⅶ)4个阶段构成。顺层磁黄铁矿-石英成矿期硫化物的δ34S值集中在1.60‰~5.89‰和14.07‰~16.87‰两个区间;NE向黄铁矿-石英成矿期硫化物的δ34S值也集中在2.77‰~5.52‰和9.11‰~15.9‰两个区间,通过与围岩和区内岩浆岩对比,笔者认为2个成矿期成矿热液中硫的来源和演化具有相似之处,初始流体中的硫既有围岩硫的贡献,也有岩浆硫的加入,随着成矿作用和水-岩相互作用的进行,成矿流体中围岩硫的比例逐渐升高。顺层成矿期硫化物具有混合铅的铅同位素特征(206Pb/204Pb=18.041~18.436,207Pb/204Pb=15.364~15.724,208Pb/204Pb=38.138~38.754),指示区内中酸性岩浆和赋矿地层来源铅的混合。NE向成矿期硫化物的铅同位素(206Pb/204Pb=18.393~18.400,207Pb/204Pb=15.639~15.687,208Pb/204Pb=38.648~38.917)略低于围岩硫化物,指示成矿流体中的铅主要来源于地层,但有少量岩浆铅的混入。
Abstract:The Baguamiao gold deposit is in the northern part of Fengtai metal ore-concentrating district, West Qinling orogenic belt, with gold reserves of about 106 t@1.56 g/t. It is controlled by a brittle-ductile shear zone striking NWW, and hosted in the lower section of Upper Devonian Xinghongpu Formation. Its mineralization processes, and the sources of ore-forming materials were discussed by in situ S-Pb isotope analysis of sulfides in this study. Two ore-forming periods including strata-bounded pyrrhotite-quartz vein and NE-oriented pyrite-quartz vein periods were identified based on detailed field observations. The former can be divided into three stages:euhedral pyrite-quartz stage (Ⅰ), pyrrhotite-native gold-ankerite stage (Ⅱ), and biotite-chlorite-epidote-quartz stage (Ⅲ). The later includes four stages:euhedral pyrrhotite-pyrite-coarse-grained quartz stage (Ⅳ), anhedral pyrite-fine-grained quartz stage (Ⅴ), anhedral pyrrhotite-native gold-ankerite stage (Ⅵ), and quartz-biotite stage (Ⅶ). In situ S isotope results show that the δ34S values of sulfide in strata-bounded pyrrhotite-quartz veins cluster in two intervals of 1.60‰~5.89‰ and 14.07‰~16.87‰; The δ34S values of sulfide in the NE-oriented veins are also concentrated in 2.77‰~5.52‰ and 9.11‰~15.90‰. By comparison with the δ34S values of the wall-rocks and granitoid intrusions in the area, we figured that the initial sulfurs of the two periods ore-forming fluids are originated from both wall rock and contemporaneous magmatism, and the proportion of wall rock sulfur gradually increases with time. The lead isotopes of the sulfide in strata-bounded veins indicate that the lead was mainly from contemporaneous granitoid magmatism and wall-rocks(206Pb/204Pb=18.041~18.43, 207Pb/204Pb=15.364~15.724, 208Pb/204Pb=38.138~38.754), meanwhile, those of sulfides in the NE-oriented veins are very close to sulfides in the wall rocks(206Pb/204Pb=18.393~18.400, 207Pb/204Pb=15.639~15.687, 208Pb/204Pb=38.648~38.917), indicating that the lead mainly comes from the wall-rocks, with small contributions of magmatism.
keywords:geochemistry paragenesis in situ S-Pb isotope source of ore-forming material ore-forming history Baguamiao gold deposit
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基金项目:本文得到国家级大学生创新创业训练计划项目(编号:202110697096)、国家自然科学基金项目(编号:42130102)和西北大学地质学系科研基金项目的联合资助
引用文本：
徐一帆,贺文彬,安芳,蔡光耀.2023.西秦岭八卦庙金矿床原位S-Pb同位素组成对成矿物质来源和成矿过程的指示意义[J].矿床地质,42(4):773~790XU YiFan,HE WenBin,AN Fang,CAI GuangYao.2023.In situ S-Pb isotopes study of Baguamiao gold deposit, western Qinling: Constraints on its ore-forming material sources and metallogenic process[J].Mineral Deposits42(4):773~790
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