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  

In situ S-Pb isotopes study of Baguamiao gold deposit,western Qinling:Con⁃straints on its ore-forming material sources and metallogenic process

作　　者：徐一帆 贺文彬 安芳[1] 蔡光耀 XU YiFan;HE WenBin;AN Fang;CAI GuangYao(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi’an 710069,Shaanxi,China;Longshou Mine,Jinchuan Group Co.,Ltd.,Jinchang 737100,Gansu,China)

机构地区：[1]大陆动力学国家重点实验室西北大学地质学系,陕西西安710069 [2]金川集团股份有限公司龙首矿,甘肃金昌737100

出　　处：《矿床地质》2023年第4期773-790,共18页Mineral Deposits

基　　金：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);西北大学地质学系科研基金项目的联合资助。

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/^(204)Pb=18.041~18.436,^(207)Pb/^(204)Pb=15.364~15.724,^(208)Pb/^(204)Pb=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/^(204)Pb=18.393~18.400,^(207)Pb/^(204)Pb=15.639~15.687,^(208)Pb/^(204)Pb=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。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-finegrained quartz stage(Ⅴ),anhedral pyrrhotite-native gold-ankerite stage(Ⅵ),and quartz-biotite stage(Ⅶ).In situ S isotope results show that theδ34S values of sulfide in strata-bounded pyrrhotite-quartz veins cluster in two intervals of 1.60‰~5.89‰and 14.07‰~16.87‰;Theδ34S values of sulfide in the NE-oriented veins are also concentrated in 2.77‰~5.52‰and 9.11‰~15.90‰.By comparison with theδ34S 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/^(204)Pb=18.041~18.43,^(207)Pb/^(204)Pb=15.364~15.724,^(208)Pb/^(204)Pb=38.138~38.754),meanwhile,those of sulfides in the NE-oriented veins are very close to sulfides in the wall rocks(206Pb/^(204)Pb=18.393~18.400,^(207)Pb/^(204)Pb=15.639~15.687,^(208)Pb/^(204)Pb=38.648~38.917),indicating that the lead mainly comes

关 键 词：地球化学 成矿期次 原位S-Pb同位素 成矿物质来源 成矿过程 八卦庙金矿 

分 类 号：P618.51[天文地球—矿床学]