陕西八卦庙金矿床NE向黄铁矿-石英成矿期成矿物质来源和成矿机制探讨:原位硫同位素证据  

Ore-forming material source and metallogenic mechanism of NE-oriented pyrite-quartz veins in the Baguamiao gold deposit,Shaanxi Province:evidence fromin situ S isotope

作　　者：蔡光耀 安芳[1] 袁轶 刘芳 刘炜 张京渤 CAI Guangyao;AN Fang;YUAN Yi;LIU Fang;LIU Wei;ZHANG Jingbo(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi’an,Shaanxi 710069,China;Shaanxi Fengxian Sifang Gold Mining Co.,Ltd.,Baoji,Shaanxi 721705,China;Longshou Mine,Jinchuan Group Co.,Ltd.,Jinchan,Gansu 737100,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]陕西凤县四方金矿有限责任公司,陕西宝鸡721705 [3]金川集团股份有限公司龙首矿,甘肃金昌737100

出　　处：《地质学报》2021年第5期1561-1572,共12页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号41421001)资助的成果。

摘　　要：八卦庙金矿床位于秦岭造山带凤太铅锌多金属矿田北部,是陕西省规模较大的金矿床之一,已探明金储量约106 t。赋矿围岩是上泥盆统星红铺组浅变质泥质碎屑岩和碳酸盐岩,其成矿过程可划分为3期,分别是(1)顺层磁黄铁矿-石英成矿期;(2)NE向黄铁矿-石英成矿期;(3)裂隙硫化物-方解石成矿期。目前关于NE向黄铁矿-石英成矿期成矿物质来源和成矿机制仍存在争议。本文通过矿相学观察,将NE向黄铁矿-石英成矿期划分为自形磁黄铁矿-黄铁矿-粗粒石英阶段(I)、他形黄铁矿-银金矿-细粒石英阶段(II)、他形磁黄铁矿-自然金-方解石阶段(III)和黑云母阶段(IV),并在此基础上采用激光剥蚀-多接收等离子体质谱(LA-MC-ICP-MS)分析方法对千枚岩围岩中以及不同阶段形成的硫化物进行了原位S同位素测试,结果显示围岩中原生磁黄铁矿的δ34S值集中于11.6‰~13.0‰之间,介于前人报道的原生黄铁矿δ34S值变化范围(3.3‰~16.0‰);阶段I形成的黄铁矿的δ34S值为8.4‰~10.1‰,磁黄铁矿为7.6‰~8.0‰;阶段II黄铁矿的δ34S值相对较高,为14.0‰~15.9‰;阶段III磁黄铁矿的δ34S值介于6.4‰~8.3‰之间。八卦庙金矿NE向黄铁矿-石英成矿期总体相对富集重硫同位素、离散程度较大,各阶段硫化物的硫同位素值介于矿集区花岗岩和围岩硫之间,显示其具有岩浆硫与地层硫混合的特征。结合阶段I到阶段III矿物组合的变化(黄铁矿+磁黄铁矿→黄铁矿→磁黄铁矿),推断阶段II δ34S值增高是水岩反应引起的硫化作用所致,而阶段III硫同位素值降低可能与岩浆水的加入密切相关。结合成矿物理化学条件,推断该成矿期成矿热液中金主要以Au(HS)2-的形式迁移,阶段I到阶段II由水岩反应引起的硫化作用是导致他形黄铁矿-银金矿-细粒石英阶段(II)金矿化的主要原因,而岩浆水的混入可能是导致他形磁黄铁矿-自然金-方解石阶段(IThe Baguamiao gold deposit discovered in the early 1980 s is located in the northern part of Pb-Zn multi-metal ore-field in the Fengtai basin, Qinling orogen. It is one of the largest gold deposits in Shaanxi Province, with proven Au reserves of about 106 t. Its dominant country rock is low-grade metamorphosed muddy clastic rock and carbonate rocks of the Upper Devonian Xinghongpu Formation. The ore-forming process of the Baguamiao gold deposit was composed of three main ore-forming periods, including(a) strata-bounded pyrrhotite-quartz vein period,(b) NE-oriented pyrite-quartz vein period, and(c) fracture-hosted sulfide-calcite vein period. The NE-oriented pyrite-quartz vein period were further divided into four ore-forming stages based on mineralogical characteristics, including euhedral pyrrhotite-pyrite-coarse-grained quartz stage(Ⅰ), anhedral pyrite-electrum-fine-grained quartz stage(Ⅱ), anhedral pyrrhotite-native gold-calcite stage(Ⅲ), and biotite stage(Ⅳ). In-situ S isotope analyses of sulfide in country rock and NE-oriented pyrite-quartz veins were carried out using the LA-MC-ICP-MS, in order to constrain the ore-forming material source and mineralization mechanism of NE-oriented pyrite-quartz veins. The results show that the δ34S values of pyrrhotite in phyllite without alteration are concentrated in 11.6‰~13.0‰, which lie between the δ34S data of original pyrite from 3.3‰ to 16.0‰ in previous publication. δ34S data of stage I euhedral pyrite are from 8.4‰ to 10.1‰, and those for euhedral pyrrhotite are from 7.6‰ to 8.0‰;δ34S values of stage Ⅱ anhedral pyrite are much higher, from 14.0‰ to 15.9‰;while those of stage Ⅲ pyrrhotite are from 6.4‰ to 8.3‰. The δ34S values of sulfide in the NE-oriented pyrite-quartz veins have large dispersion and enriched heavy sulfur isotope, and the values of sulfide in different stages are between that of granite in ore concentrated area and the wall rock, indicating that the sulfur source is generally characterized by the mixing of

关 键 词：八卦庙金矿床 成矿期次 原位S同位素 成矿物质来源 成矿机制 

分 类 号：P618.51[天文地球—矿床学]