伊朗Mehdiabad超大型铅锌矿床的巨量重晶石成因与金属富集机制

The Mehdiabad deposit in Iran is the largest Mississippi Valley-type (MVT) Pb-Zn deposit in the world. Its metal enrichment mechanism becomes a remarkable scientific interest. Moreover, this deposit has a huge amount of barite, but the genetic relationship between the barite and the metal enrichment is unclear. . This proposal is planning to carry out geologic and geochemical studies to understand the metal enrichment mechanism and the genesis of the barite in the Mehdiabad deposit. Firstly, this proposal is planning to understand the crucial ore controls by virtue of further surface observation and logging of drill cores, with emphases on investigating the relations between fault, breccia, and the Pb-Zn mineralization. Secondly, this proposal is planning to uncover the fluid features and process of forming the barite and the Pb and Zn sulfides, to trace the sources of the fluids and sulfur, and to discuss the metal precipitation mechanism. The methods of fluid inclusion and mineral geochemistry will be employed, including quantitative elemental analysis of in individual fluid inclusion, analyses of noble gases and halogens in fluid inclusions, S-O-Sr isotopic systematics for barite, and in situ sulfur isotopic analysis of sulfide. Finally, these results together will be used to interpret how the huge amounts of barite and the Pb and Zn sulfides were accumulated in the Mehdiabad deposit. As a good example, this proposed study can help understand the ore formation process for a giant MVT deposit in a continent-continent collisional setting. This proposal may also obtain new insights into the metal enrichment mechanism and the source(s) of reduced sulfur for MVT deposits.

伊朗Mehdiabad矿床是目前世界上最大的密西西比河谷型（MVT）铅锌矿床，巨量金属如何富集，引人瞩目。同时，矿床还发育超大型矿床规模的重晶石，其成因也是理解矿床金属富集机制的关键。. 本申请拟在已有工作基础上，首先通过进一步的地表观察和钻孔编录，以区内主断裂及角砾岩与铅锌成矿关系为研究重点，揭示控矿地质要素。然后针对不同期次重晶石和铅锌硫化物，通过流体包裹体和矿物地球化学研究，特别是单个流体包裹体成分、包裹体稀有气体同位素-卤素组分、重晶石S-O-Sr同位素、原位S同位素分析等关键技术，查明重晶石和铅锌硫化物的成矿流体成分、性质及在成矿过程中的变化，示踪成矿流体和硫的来源，探讨矿质沉淀机制。最后综合分析，阐明矿床巨量重晶石成因与金属富集机制。该研究有助于理解陆-陆碰撞环境超大型MVT矿床的形成机制，有望在探索MVT矿床金属富集机制及硫的来源方面获得创新认识。

伊朗Mehdiabad矿床是目前世界上最大的密西西比河谷型（MVT）铅锌矿床，巨量金属如何富集，引人瞩目。矿床同时发育超大型矿床规模的重晶石，巨量重晶石的成因及与铅锌成矿的关系，有待破解，也是理解矿床金属富集机制的关键。本项目通过野外地质调查、详细岩相学和同位素地球化学工作，目标是：1）厘定矿床控矿地质要素，2）揭示矿床多期重晶石的成因，查明重晶石和铅锌硫化物的成因关系，3）阐明巨量金属的富集机制。.研究表明，矿床内Black Hill同沉积正断层上盘为早白垩世Taft组灰岩及同沉积角砾岩，其发生了菱铁矿化和重晶石化，断层控制了矿前菱铁矿化和重晶石化的范围。后来的铅锌硫化物总体呈层状特点，发育在Black Hill断层上盘的Taft组地层中，硫化物交代早期的菱铁矿和重晶石，其中，角砾岩化部位的矿化更为强烈。因此，同沉积断层、同沉积角砾岩、早期菱铁矿化、早期重晶石化均是重要的控矿地质因素。.在Mehdiabad矿床，至少识别3期矿前重晶石，三期重晶石的S-O同位素组成与早白垩世海水的接近，δ34S和δ18O均成线性关系，斜率从早到晚分别为5.36、1.32、0.33，三期重晶石的87Sr/86Sr介于0.707327和0.708961之间，与碳酸盐岩围岩的Sr同位素组成基本一致，第三期略高。因此，矿床巨量重晶石的形成不是热液活动的结果，而是海相富甲烷环境下大量自生重晶石沉淀的结果。早到晚，重晶石形成环境中甲烷含量逐渐增多，外来粘土或碎屑物质逐渐增多。.划分矿床的铅锌硫化物矿化为2阶段3类。第1阶段（S1）主要由黄铜矿、方铅矿、少量闪锌矿和黄铁矿组成，典型矿化以脉状形式产出，第2阶段包括交代重晶石式和菱铁矿式矿化（S2-R）和开放空间充填式的胶状矿化（S2-F），均以闪锌矿为主，含少量方铅矿，其中S2-R闪锌矿比S2-F闪锌矿沉淀略早。S1闪锌矿较S2闪锌矿低Fe和Ge、高In，与其富Cu有关。3类闪锌矿的δ34S值均为负值，集中在-10‰到-30‰之间，反映细菌硫酸盐还原作用（BSR）提供了还原硫，因此，矿前重晶石发生了的BSR作用产生还原硫，是Mehdiabad矿床铅锌富集的最重要机制。

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