川滇黔地区MVT型铅锌矿床成矿物质来源及成因研究

The contiguous area of the Yunnan-Sichuan-Guizhou provinces is well-known for the abundant Mississippi Valley-type Pb-Zn ore mineral resource. Extensive investigations on the metallogeny of the ore deposits have been carried out in the last decades, however, the lack of experimental results on the abundances, status, and lability of ore metals in the basement and sedimentary rocks, as well as the igneous rocks, and Pb isotope fractionation during water/rock alteration, in particular, significant hinders the endeavor to delineate the ore metal source(s), and thus the metallogeny of the ore province. In this proposed project, sequential leaching experiments will be carried out to test the potential of the rock units including the basement, sedimentary rocks, and igneous rocks like the Emeishan basalts as ore metal source(s), and illustrate the Pb isotope fractionation features during water/rock interaction. Three representative high grade and large tonnage deposits, namely the Huize, Maoping, and Lehong deposits, as the integral parts of the large hydrothermal ore-forming system in the ore province, will be investigated comprehensively. Ages of ore-formation of the deposits will be dated by using ID-TIMS sphalerite Rr-Sr method with pretreatment of crushing and leaching technique to remove the possible effects of fluid inclusions, and sphalerite Ar40/Ar39 using progressive crushing and stepwise heating techniques. In-situ analyses of S, Sr, and Pb isotope compositions of sphalerite and galena, and C-O isotope compositions of hydrothermal dolomite and calcite, along with systematic examination of the fluid geochemical characteristics of fluid inclusions in gangue minerals and sphalerite will be carried out to delineate the source(s) of ore fluids and ore metals, paths of fluid migration, evolution and mineralization and, therefore, to constraint the metallogeny and controls on the emplacement of the high grade and large tonnage ore deposits or ore bodies.

川滇黔是我国重要的MVT型铅锌多金属矿集区，其成矿物质来源和成矿机制一直备受关注。区内基底和赋矿围岩中成矿元素的赋存形式、在水-岩作用过程中的活化、迁移能力和Pb同位素分馏特征等实验数据的匮乏, 严重制约着成矿物质来源及成矿机理研究的深入。本项目拟以会泽、毛坪、乐红三个高品位大型-超大型矿床为研究对象，通过矿床地质和矿床地球化学的综合研究，揭示典型矿床的成矿流体特征及成矿规律；将通过分级淋滤模拟实验, 评估相关地层(岩石)提供成矿物质的潜力，探讨水-岩作用过程中Pb同位素分馏特征，为限定成矿物质来源提供实验支撑；在流体包裹体地球化学研究基础上，通过闪锌矿击碎-酸淋ID-TIMS法Rb-Sr等时线及Ar-Ar法精确定年、碳酸盐矿物C-O同位素和硫化物原位S-Sr-Pb同位素组成的综合研究，揭示川滇黔铅锌多金属成矿区成矿物质来源，反演成矿流体的运移路径, 阐述该矿集区大而富铅锌矿床的形成机制。

川滇黔接壤地区是我国最重要的铅锌矿集区，也是我国重要的铅锌产地，铅锌矿床主要为MVT型矿床，并具有巨大的资源潜力。前人对该矿集区内矿床的成矿机制已经进行了大量的研究，然而由于成矿时代的不确定性和矿床地质特征的复杂性，区内MVT矿床的成矿金属元素和成矿流体来源及演化问题一直未得到很好的解决。为此，本项目以区内会泽、毛坪、乐红三个代表性超大型-大型铅锌矿床为主要研究对象，开展了矿石、基底岩石、赋矿地层的铅同位素组成分析和闪锌矿、方解石、白云石流体包裹体研究。矿石铅同位素原位分析结果表明，会泽铅锌矿床方铅矿的铅同位素组成在空间上非常均一，而毛坪和乐红铅锌矿床以及区内其他MVT矿床方铅矿和闪锌矿的Pb同位素组成均存在自深部向地表富集放射性成因铅的变化趋势。初步的淋滤实验表明，水/岩作用过程中进入流体相的铅与全岩样品铅同位素组成上存在显著的差异。基底昆阳群碎屑岩10%HCl浸出液较全岩相对贫放射性成因铅，而黑色页岩浸出液则显著富放射性成因铅。表明碎屑岩中地球化学性质稳定的碎屑矿物（副矿物）相对富放射性成因铅，而黑色页岩的胶结物及有机质相对富放射性成因铅。对比分析表明，会泽、毛坪、乐红铅锌矿成矿金属物质主要源于基底碎屑岩（可能也包括基底中元古代岩浆岩类），而毛坪、乐红等铅锌矿床可能还包括少量来自相对富放射性成因铅的围岩地层的贡献。流体包裹体数据显示成矿流体的温度从矿化早期至晚期具有降低的趋势，而成矿流体温度与盐度的解耦指示低温、高盐度盆地卤水体系中存在高温、低盐度流体（可能富成矿金属元素）的加入。另外，单个闪锌矿颗粒的流体包裹体研究也表明闪锌矿沉淀过程中存在高温流体和低温卤水的混合。总的来说，川滇黔铅锌矿集区成矿金属元素主要源于基底碎屑岩，在含矿流体沿通道向上部运移过程中可能汲取一部分赋矿地层（沉积盖层）的成矿金属元素。川滇黔地区MVT成矿作用可能与印支期古特提洋闭合导致前陆盆地抬升、深循环的含矿热液长距离迁移并在构造有利部位与浅部循环的盆地卤水混合导致金属硫化物沉淀有关。成矿金属元素主要源于基底碎屑岩的确定也为深部找矿指明了方向。基底由于赋矿地层包括中-晚元古界昆阳群至三叠系的众多地层，区域深部断裂构造、不整合面、含水层等均可能蕴藏MVT矿体，在区域深部找矿勘查实践应予以充分重视。