湘东北地区钴铜多金属成矿作用研究

Abundant copper polymetallic and W-Sn polymetallic ore deposits occur within the Qinhang Metallogenic Belt, South China, whereas few primary Co deposits are discovered. For the last decade, two Cu-Co polymetallic deposits have been newly discovered within the northeastern Hunan Province at the northern segment of the Qinhang Metallogenic Belt. They include Jingchong Co-Cu polymetallic ore deposit and Hengdong Co deposit which are hosted within the Devonian strata and Neoproterozoic Lengjiaxi Group, respectively. Spatially, the two Cu-Co deposits are distributed within the altered fracture zone in the outer contact zone of Yanshanian Lianyunshan pluton, and regionally are controlled by the NE-trending Changsha-Pingjiang deep fault zone. The geological characteristics obviously distinguish them from the typical stratabound Co-Cu ore deposits in the world. Here, we focus on the Jingchong Co-Cu polymetallic ore deposit and Hengdong Co deposit, and genetic mineralogy, fluid inclusion, isotope geochemistry (e.g. Cu, S-Pb, He-Ar, and H-O isotopes), and isotope geochronology (e.g. monazite and zircon U-Pb dating, as well as mica Ar-Ar dating) would be employed. By contrast with the Co-Cu ore deposits from the world, this project is aimed to ① determine the source of ore-forming material; ② trace the nature of the ore-forming fluids and their evolution so as to delineate the migration-enrichment mechanism of Co and Cu metals; and ③ constrain the ore genesis of the Co-Cu polymetallic mineralization. This study would not only give a new insight to the Co-Cu metallogenic theory, but also provide a useful guide for the exploration of the Co-Cu ore deposits in the Qinhang Metallogenic Belts.

华南钦杭成矿带分布有丰富的铜多金属和钨锡多金属矿产，但鲜有原生钴矿的报道。近年来，在钦杭成矿带北段湖南省东北部（简称湘东北）相继发现了井冲钴铜多金属矿床和横洞钴矿床，这两个矿床均产在燕山期连云山岩体外接触带地层中的蚀变构造岩带内，受长沙-平江深大断裂带严格控制，成矿地质特征明显不同于国内外典型层控型钴铜矿床。本项目拟选择井冲和横洞两个矿床为主要解剖对象，采用成因矿物学、流体包裹体、同位素地球化学（Cu、S-Pb、He-Ar、H-O等同位素）手段，结合同位素定年（独居石和锆石U-Pb法、云母Ar-Ar法），以期①揭示钴铜多金属成矿物质来源；②查明钴铜多金属成矿流体组成、性质与演化特征，阐明成矿元素迁移-富集机制；③确定湘东北钴铜多金属矿床的成因类型。此研究在丰富钴铜多金属成矿理论和在钦杭成矿带开展钴铜紧缺矿种资源的找矿勘查等方面重要理论和实际意义。

近年来，湘东北长沙-平江断裂带的钴资源找矿勘查取得了较大进展，相继发现了井冲铜钴多金属矿床（中型）、横洞钴铜矿床（中型）和大岩金钴矿化点等热液脉型钴独立/共生矿床（点）。本项目重点揭示了该类型钴铜多金属矿的成矿物质/流体来源及富集机制等。通过系统剖析湘东北钴铜多金属的成矿特征和控矿因素，识别出四个阶段热液成矿作用，从早到晚为：石英+绿泥石+黄铁矿阶段、石英+绿泥石+黄铁矿+黄铜矿阶段、石英+含钴黄铁矿+毒砂+辉砷钴矿+黄铜矿+镁菱铁矿阶段、石英+方解石±黄铁矿±黄铜矿+方铅矿+闪锌矿阶段，其中，第三阶段是钴的重要形成阶段。采用EPMA、LA-ICPMS、TIMA等方法开展了富钴矿物黄铁矿、毒砂、辉砷钴矿的精细矿物学研究，结果表明含钴矿物具有复杂的结构，特别是增生或交代作用强烈。黄铁矿明显可划分为两类：中-粗粒状黄铁矿（PyI、PyII、PyIV）和细粒黄铁矿（PyIII）：前者Co（0.16%~1.06%）和As（0.05%~0.28%）的含量较低，后者具有变化且高的Co（0.19%~13.48%）和As（0.99%~7.42%）含量。结合黄铁矿的结构和成分演变，认为溶解-再沉淀作用是富钴矿物形成的主要机制。绿泥石温度计和流体包裹体分析结果表明钴铜多金属的成矿温度范围为150~320 ℃，流体不混溶作用是含矿流体发生沉淀的机制。综合S、Pb、He-Ar、SIMS O同位素结果，认为湘东北钴铜多金属成矿物质/流体可能来源于地壳重熔成因的S型连云山花岗质岩与古老结晶基底的混合，成因类型为与岩浆热液作用有关的热液脉型。横洞矿区和大岩钴金矿点的蚀变碎裂岩型矿石中白云母的Ar-Ar坪年龄为130~125 Ma，结合区域同时期构造事件，认为湘东北钴铜多金属成矿作用与古太平洋板块的俯冲回撤或洋脊俯冲过程中板片窗的开启导致的长沙—平江断裂带的活化作用有关。最后，提出细粒黄铁矿+毒砂矿物组合可作为长沙—平江钴矿带寻找高品位钴矿的直接矿物标志，并结合含钴矿物的类型、分布与丰度，提出了初步的选冶建议。