中条山篦子沟铜矿两期成矿过程的矿物-流体地球化学示踪

Special sedimentary rocks as redox barrier may lead to the precipitation of ore-forming fluids, and play a crucial role in the formation of sediment-hosted stratiform copper (SSC-type) deposits. Difficulties in systematic studies of the original mineralization of the early Precambrian SSC-type deposits commonly arise from overprints of the subsequent metamorphism. The Bizigou copper deposit in the Zhongtiaoshan area was formed in ~2.0 Ga, which is one of the oldest SSC-type deposits in the world. The preliminary study of the applicant demonstrated that some banded ores formed in primary diagenetic stage are well-preserved. More importantly, primary fluid inclusions were discovered in some euhedral quartz grains within the banded sulfides, which make it possible to investigate the original mineralization of the early Precambrian SSC-type deposits directly. Both the banded ores of the diagenetic stage and the thick-vein-type ores of the metamorphism stage will be investigated with emphasis on systematic study of fluid inclusions. In situ element-isotope investigations will be carried out on representative sulfides and tourmaline in Bizigou. The results will shed light on the source of ore-forming materials, the origin and evolution of ore-forming fluids. By tracing the ore-forming processes of the Bizigou Cu deposit, new constraints on the primary mineralization of the early Precambrian SSC-type deposits will be proposed.

特殊的沉积岩组合诱发成矿流体氧化还原环境突变是导致沉积岩型层状铜矿床（SSC型）形成的关键。早前寒武纪SSC型铜矿普遍经历了后期变质叠加改造，给人们认识该类矿床的原生成矿过程带来了诸多困难。中条山篦子沟铜矿形成于～20亿年，是世界上最古老的SSC型铜矿床之一。申请者通过初步研究，在该矿床中发现了保存非常完好的原生成岩期纹层状铜矿化，并在纹层状黄铜矿内发现了含原生流体包裹体的自形石英颗粒，这为示踪早前寒武纪SSC型铜矿的原生成矿过程提供了独特研究窗口。本项目拟选取篦子沟铜矿成岩期纹层状矿化和变质期粗脉状矿化为研究对象，在详细岩相学研究基础上，对两期矿化开展系统的流体包裹体地球化学研究，并对典型矿物（黄铁矿、黄铜矿、电气石等）开展原位微区元素和同位素（硫和硼）特征研究，进而查明两期铜矿化的成矿物质来源、成矿流体来源与演化特征，示踪其成矿过程，为揭示早前寒武纪SSC型铜矿的成矿机制提供新的约束。

由于早前寒武纪SSC型铜矿普遍遭受了后期变质改造，关于其原生成矿过程信息的研究一直存在较大局限。鉴于此，本项目选取形成于～20亿年的世界上最古老SSC型铜矿床之一的中条山篦子沟铜矿作为研究对象，对该矿床中保存非常完好的成岩期原生纹层状铜矿化和后期变质成矿阶段的粗大脉状铜矿化进行了详细的矿床地质、流体包裹体、稳定同位素和典型矿物原位微区同位素分析，研究显示：1）早期铜矿化的主要成矿流体具有高盐度（20-40 wt.% NaCl）和中低温度（115-260 ℃）特征，流体体系为NaCl-CaCl2-H2O，具盆地卤水性质。晚期铜矿化流体具高盐度（35-55 wt.% NaCl）、中高温（230-470 ℃）和富CO2的特征，显示出变质流体的性质；2）碳酸盐和有机碳的碳同位素分析显示，成矿流体可能与黑色片岩发生了C同位素交换，暗示黑色片岩可能作为还原性地球化学障参与了矿石的沉淀过程；3）硫化物S同位素分析表明，早期成矿过程与溶解蒸发岩活动有关，而晚期矿化过程中的硫可能来源于岩浆-热液。综合各方面研究，篦子沟铜矿的原生铜矿化可能是由溶解蒸发岩形成的高盐度氧化性含铜卤水在运移至上部还原性黑色页岩层时，导致热液体系氧化还原性质突变所引起的。晚期铜的沉淀则与富CO2的变质成矿流体发生大规模不混溶导致流体物理化学条件突变有关。本研究为探究早前寒武纪SSC型矿床的形成机制提供了参考。