甘肃枣子沟金矿床成矿过程分析——来自矿床地质特征、金的赋存状态及稳定同位素证据

摘要

The Zaozigou gold deposit is located in the Tongren-Xiahe-Minxian gold metallogneic belt in west Qinling orogen. The ore bodies are hosted in fine-grained clastic rocks intercalated with limestone of the Middle-Triassic Gulangti Formation and varied dikes. The gold ore bodies are strictly controlled by the NE, NW and SN trending tensional and shearing faults with high dipping angle. The ore-forming hydrothermal period is divided into four stages: pyrite-quartz, pyrite-arsenopyrite quartz, stibnite-quartz-calcite and quartz-calcite. Quartz, calcite and sericite are the main alteration minerals. Environmental scanning electron microscope and electron probe analyses show that the gold is micro-visible or invisible. Micro-visible gold is seen in the intercrystalline space or in the fissures of quartz and silicates. Invisible gold is captured in pyrite, arsenopyrite, stibnite and other sulfide mainly as nanoparticles. Gold concentration in sulphides of ore-forming stages is between 0. 003% and 0. 658% with an average of 0. 257%. All the characteristics above lead us to draw the conclusion that the Zaozigou deposit is a typical carlin-type. Hydrogen and oxygen isotope compositions show that ore-forming fluids came from meteoric water while sulfur isotope compositions show that the sulfur came from the sedimentary strata. The meteoric water went deep into the crust through the deep cutting faults and was heated and circulated to extract the metals from the wall rocks. Gold may be transported in the hydrothermal fluids in forms of AuH3SiO40、Au(HS)-2 , H2 Au(Sb, As)S20 and HAu(Sb,As)S3-. In the early stage, the breakup of the AuH3SiO40 complex resulted in the deposition of Au and Si, Fe, Na, Al etc; while in the later stages the breakup of Au(HS)2-, H2 Au (Sb, As) S20 and HAu (Sb, As) S3-complexes led to the simultaneous deposition of sulfides and gold nanoparticles in them.%枣子沟金矿位于同仁—夏河—岷县金成矿带,矿区赋矿地层为三叠纪中统古浪堤组下段细碎屑岩及灰岩,并发育大量闪长质脉岩.矿体既产于地层中,也出现在脉岩或其接触带中,但其产状均严格受NE、NW及近SN向3组断裂构造控制,控矿构造为高角度的张剪性及旋扭性断裂.热液成矿期可划分为黄铁矿-石英阶段,黄铁矿-毒砂-(闪锌矿-方铅矿-黄铜矿-辉银矿-绢云母-绿泥石-)石英阶段,辉锑矿-石英-方解石阶段及石英-方解石阶段.围岩蚀变类型主要为硅化、方解石化及绢云母化.环境扫面电镜及电子探针测试数据表明,金呈显微可见金存在于矿物裂隙和粒间隙中或以纳米不可见金捕获在载金矿物中.成矿期不同硫化物金的质量分数均高出检测限,其范围为0.003％～0.658％,平均值为0.257％.枣子沟金矿床具有卡林型金矿床的典型特征.氢氧同位素数据显示成矿流体主要来自大气水,硫同位素数据则表明硫主要来自沉积地层.其成矿过程可能为深切割断裂导通地下水,在深部被加热循环萃取围岩中成矿物质,并在浅表张性断裂中充填交代围岩,致使成矿物质沉淀富集成矿.金的迁移形式可能存在AuH3SiO04、Au(HS)2、H2Au(Sb,As)S02和HAu(Sb,As)S3ˉ等多种迁移方式.成矿早阶段可能以金硅络合物的解体为主,成矿晚阶段则可能是硫氢(锑)络合物发生解体,致使金与硫化物同时沉淀,以显微纳米金的形式包含在硫化物中.