佳木斯地块早古生代岩浆-变质事件的成矿响应: 以鸡西苇子沟金矿床为例

Late Paleozoic and Mesozoic ore deposits were abundant and early Paleozoic deposits were rare in Northeast China. The regional metallogenic theory and the geological prospecting work would be promoted by strengthening the identification and research of early Paleozoic metallogenesis. Early Paleozoic magmatic-metamorphic events commonly exist and relevant deposits have been found in Jiamusi Block. The Jiamusi Block is expected to become another breakthrough point after the Duobaoshan ore-concentrated area to recognize the early Palaeozoic metallogenesis in Northeast China. The project is focused on the Weizigou gold deposit in Jixi area. Based on a detailed study of the regional geological setting and mineral deposit geology, the microthermometry and component analysis of metallogenic fluid inclusions were carried out to determine the physical and chemical conditions and the fluid properties in the different mineralization period and the genetic type of the deposit. To determine the magmatic-metamorphic-mineralization epoch, the isotopic dating analysis of the mineral-related geological bodies, such as intrusive rocks and metamorphic rocks, and ores were carried out. On basis of comprehensive research on the geological and geochemical and isotopic results, the ore-forming material source and metallogenic background could be found out, and then it could reveal the gold mineralization constraints on the early Paleozoic magma-metamorphic events. Finally, to reveal the early Paleozoic metallogenic dynamics background in Jiamusi Block, we would research the mineral deposits as the macroscopic indicators of geodynamic background by analyzing the type of ore deposits and metallogenic epoch and combining with the regional tectonic evolution.

我国东北地区具有晚古生代和中生代矿床多而早古生代矿床少的特点，加强早古生代成矿作用识别和研究将促进区域成矿理论的认识水平及地质找矿工作。佳木斯地块普遍存在早古生代岩浆-变质事件，并已发现相关矿床（点），因而有望成为继多宝山矿集区后，认识东北地区早古生代成矿作用的另一窗口地区。本项目以鸡西苇子沟金矿床为重点，在区域地质和矿床地质研究基础上，开展成矿流体包裹体显微测温及成分分析，判定成矿物理化学条件、流体性质及矿床成因类型；通过侵入岩、变质岩等成矿相关地质体和矿石的同位素测年结果，厘定岩浆-变质-成矿时代；综合地质研究和岩矿石的元素-同位素地球化学测试结果，示踪成矿物质来源，确定成矿构造背景，进而揭示早古生代岩浆-变质事件对金矿成矿作用的制约。最后，将所研究矿床作为地球动力学背景的宏观指示物，根据矿床类型和成矿时代，结合区域构造演化，实现揭示佳木斯地块早古生代成矿动力学背景的科学目标。

我国东北地区具有中生代矿床多而古生代矿床少的特点，加强古生代成矿作用识别和研究将提高区域成矿理论的认识水平，为下一步的地质找矿工作指明方向。本项目以鸡西苇子沟金矿床为重点，在详实的野外地质研究基础上，识别出该矿床成矿岩体并非前人认为的花岗质片麻岩，而是变辉长岩。通过对矿区花岗质片麻岩和变辉长岩锆石、磷灰石、独居石LA-ICP-MS U-Pb同位素定年以及黄铁矿Re-Os同位素定年，厘定了五期岩浆-变质-热液事件，即新元古代（~895Ma）花岗质片麻岩原岩岩浆侵入结晶、晚寒武世（~500Ma）花岗质片麻岩遭受变质、早二叠世（~292Ma）变辉长岩原岩岩浆侵入花岗质片麻岩、中二叠世（~258Ma）变辉长岩遭受变质和早侏罗世（~197Ma）岩浆热液叠加改造。鉴于苇子沟矿床金成矿作用与变辉长岩遭受角闪岩相变质作用密切相关，表明其成矿时代为二叠纪，即晚古生代，而非前人认为的早古生代。金属硫化物的δ34S值为4.70‰-12.83‰，而花岗质片麻岩和变辉长岩的δ34S值分别为7.44‰-8.44‰和4.37‰-10.54‰，这也说明硫的来源与花岗质片麻岩和变辉长岩密切相关。金属硫化物206Pb/204Pb为18.605-20.136，207Pb/204Pb为15.637-15.710，208Pb/204Pb为38.534-39.129，具有混合铅的特征。金属硫化物和氧化物原位LA-ICP-MS微量元素分析结果显示，与金成矿密切相关的磁铁矿（Mag-II）具有低Ti和高Zn含量特征，暗示其可能为热液成因；黄铁矿和磁黄铁矿的Co/Ni比值大于1，且具有低As含量特征，暗示其属于岩浆热液成因。以上研究表明，苇子沟矿床成因类型为IOCG型，并遭受后期岩浆热液的叠加改造，这是首次在佳木斯地块识别出IOCG类型矿床，具有矿床学、大地构造学的双重理论意义，以及指导找矿的实际意义。基于花岗质片麻岩和变辉长岩岩石地球化学特征及锆石Hf同位素测试结果，并结合区域构造演化研究，认为花岗质片麻岩~500Ma的变质事件形成于后碰撞伸展环境，是麻粒岩相变质作用高峰后地壳增厚崩塌的结果；变辉长岩属于拉斑玄武岩系列，形成于弧后环境。

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