白秧坪银、铜多金属矿集区成矿元素共生分异机制研究

Research on paragenesis-separation mechanism is now a fascinating area in the field of geochemistry of ore deposit at home and abroad. The distribution of epithermal base metal deposits in the Lanping basin, Yunnan province, SW China is controlled by Cenozoic thrust–nappe systems. The formed environment of these deposits is different from that of sediment–hosted base metal deposits elsewhere in the world. The Baiyangping Ag–Cu–Pb–Zn-Co deposits are representative of the epithermal base metal deposits in the Lanping basin. The Baiyangping deposits are enrich in Ag, Cu, Pb, Zn and Co, which are characterized by the uneven distribution and the superimposed enrichment. The Baiyangping Ag–Cu–Pb–Zn-Co deposits, with unique element association, are the very perfect site to study the paragenesis-separation mechanism. However, the paragenesis-separation mechanism in these deposits is still poorly understood. These deposits are taken as the research object. This proposal plans to carry out systematic in-depth studies on the key scientific problem, whether or not these deposits in the Baiyangping area were formed by diverse ore forming fluid systems or different metallogenic conditions in the same ore forming fluid system. Systematical studies on the precisely metallogenic geochronology, the material sources, ore-forming fluids, and metallogenic physico-chemical conditions, etc., will be conducted to better understand on the paragenesis-separation mechanism in this area..　　Based on these studies, it is anticipated that significant progress will be made toward understanding the paragenesis-separation mechanism for the migration and deposition of ore forming materials in the Baiyangping Ag–Cu–Pb–Zn-Co deposits. This study provides a scientific basis for in-depth understanding of the character and mechanism of paragenesis-separation mechanism, and develops ore forming theories of these deposits with unique elements association. Also, this study provides guidelines for regional prospecting.

元素的共生与分异机制，是国内外矿床地球化学研究的热点之一。白秧坪矿集区是云南兰坪盆地中继金顶超大型铅锌矿床之后，我国新近发现的又一超大型银、铜多金属矿集区。以沉积岩容矿的Pb–Zn–Ag组合矿床国内外较常见，对其研究也较深入。兰坪盆地中以沉积岩容矿的白秧坪矿集区成矿元素多样，银、铜、铅、锌、钴均达工业品位。其元素组合特征国内外罕见，是研究元素共生与分异机制的理想场所，但研究基础薄弱。本申请以该矿集区为解剖对象，针对矿集区中的矿床是由不同成矿流体系统所形成还是由同一成矿流体系统在不同成矿条件下形成这一关键问题，通过成矿精确年代学、成矿流体成因和控矿物理化学条件等方面的系统研究，揭示该区Ag–Cu–Pb–Zn–Co等元素共生与分异的机制。这些研究对建立特殊元素组合矿床的成矿理论和指导进一步的找矿勘查均具有重要意义。

兰坪盆地位于"三江"造山带的南段，夹持于金沙江断裂带与澜沧江断裂带之间。盆地内发育大量以沉积岩为容矿围岩的银铅锌多金属矿床，形成于大陆碰撞造山背景下，在西南地区乃至全国都具有代表性和典型性。白秧坪银铜多金属矿集区，位于盆地北段，在兰坪盆地银铅锌多金属大规模成矿作用中很具代表性，由东矿带（上三叠统碳酸盐岩建造内的Pb-Zn-Ag-Cu矿床）和西矿带（下白垩统碎屑岩建造内的Ag-Cu-Co-Pb-Zn矿床）两部分组成。.流体包裹体研究表明，白秧坪银铜多金属矿集区的成矿流体为低温（120~200℃）、高盐度（w（NaCleq）峰值范围为12.0~24.0wt%）流体。稀有气体同位素的研究显示，白秧坪矿集区白秧坪、八宝山、核桃箐Cu多金属矿床硫化物流体包裹体3He/4He比值变化范围较小，介于0.01~0.14 Ra之间，平均为0.07 Ra，小于0.1 Ra，位于壳源流体的范围之内，基本不含幔源He，流体包裹体3He/4He反映出流体来源为壳源。40Ar/36Ar值在298~382之间，平均323，与大气40Ar/36Ar值（295.5）相当。放射性成因的40Ar*的含量为0.19% ~ 20.40%，平均为5.94%。放射成因40Ar*来自地壳岩石，大气40Ar的贡献可达79.60% ~ 99.81%，平均为94.06%，表明大气Ar对成矿流体有着极其重要影响，成矿流体中Ar主要来自大气Ar，结合He同位素特征，表明，白秧坪矿集区成矿流体主要为循环的大气降水。利用方解石Sm-Nd定年技术，精确测定白秧坪矿集区白秧坪Cu-Co-Ag矿段成矿年龄为27.4±1.8Ma，吴底厂Pb-Zn-Ag矿段成矿时代为30.1±1.9Ma，在误差范围内两矿段的时代基本一致，约为25.6-32.0Ma，相当于三江造山带陆内大规模走滑、逆冲推覆的晚碰撞期。.基于成矿时代、大地构造背景、控矿因素和成矿流体耦合关系，确定受逆冲推覆构造控制的白秧坪矿集区，主要形成于陆陆碰撞的晚碰撞期-构造转换期。大气成因的盆地卤水在压扭/张扭应力作用下快速排泄，通过与盆地地层相互作用并淋滤地层中的成矿元素而形成富含Ag-Cu-Pb-Zn-Co的含矿流体。含矿流体通过主逆冲断裂或平移断层垂向沟通，汇聚于逆冲构造派生的次级构造部位，发育多金属流体成矿系统，形成不同样式的Ag-Cu-Pb-Zn-Co矿床。