Mehdiabad is the world’s largest Mississippi Valley-type (MVT) Zn-Pb deposit (394 million tonnes [Mt] of metal ore at 4.2% Zn, 1.6% Pb) and contains significant barite resources (>40 Mt). Such large accumulations of barite are not common in carbonate-hosted Zn-Pb deposits. Therefore, the origin of the barite and its association with the Zn-Pb mineralization is of significant interest for further investigation.Field work and petrographic studies indicate that the Zn-Pb-Ba orebodies in the Mehdiabad deposit are hosted by Lower Cretaceous carbonate units of the Taft and Abkuh Formations. Fine- to coarse-grained barite with lesser siderite formed in three stages (S1, S2, and S4), along with a quartz-sulfide stage (S3) with minor quartz, sphalerite, galena, chalcopyrite, and pyrite, and the main Zn-Pb sulfide stage (S5) with massive sphalerite and galena.The barites have δ³⁴S values from 17.7 to 20.6‰, δ¹⁸O values from 13.2 to 16.8‰, Δ³³S_V-CDT values from –0.001 to 0.036‰, and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.707327 ± 0.000008 to 0.708593 ± 0.000008 (V-CDT = Vienna-Canyon Diablo Troilite). The siderites have δ¹³C_V-PDB values from –3.8 to –2.7‰, and δ¹⁸O_V-SMOW values from 18.2 to 20.9‰ (V-PDB = Vienna-Pee Dee Belemnite, V-SMOW = Vienna-standard mean ocean water). These geochemical data, and the barite morphology, point to a diagenetic origin for all stages of barite. We suggest that S1 and S2 barite precipitated from pore fluids at the sulfate-methane transition zone in a methane-diffusion-limited environment with increasing methane content. S4 barite precipitated when the methane- and barium-bearing cold-seep fluid migrated to the shallow carbonate sediments and formed a methane-in-excess setting. For the three stages, the SO42- in barite came from the residual SO42- in pore fluids undergoing sulfate-driven anaerobic oxidation of methane, and the Ba²⁺ came from dissolved biogenic barite and terrigenous materials in the Taft and Sangestan Formations.Primary fluid inclusions trapped in S3 quartz have salinities of 5.6 to 8.1 wt % NaCl equiv and homogenization temperatures of 143.8° to 166.1°C. The quartz has δ¹⁸O_V-SMOW values ranging from 9.8 to 22.5‰ and δ³⁰Si values from –1.3 to –0.9‰. These data indicate hydrothermal fluid flow occurred between the diagenetic S2 and S4 events. Secondary fluid inclusions with salinities of 17.70 to 19.13 wt % NaCl equiv and homogenization temperatures of 123.0° to 134.0°C are found in the S3 quartz, too. They might represent the hydrothermal event formed by basinal brines in S5.According to the ore textures and the comparison of the sulfur isotopes between S5 Zn-Pb sulfides and the digenetic barites, the barite provided a host and a sulfur source for the later Zn-Pb mineralization. The relationship between barite and the Zn-Pb mineralization indicates that significant accumulations of sulfates may be a critical exploration target for this kind of giant deposit.

中文翻译：

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伊朗梅赫迪亚巴德巨型锌铅钡矿床重晶石特征及成因

梅迪亚巴德 (Mehdiabad) 是世界上最大的密西西比河谷型 (MVT) 锌铅矿床（金属矿石 3.94 亿吨，锌含量为 4.2%，铅含量为 1.6%），蕴藏着丰富的重晶石资源（>40 公吨）。如此大量的重晶石堆积在碳酸盐岩锌铅矿床中并不常见。因此，重晶石的成因及其与 Zn-Pb 矿化的关联对于进一步研究具有重要意义。现场工作和岩相学研究表明，Mehdiabad 矿床中的 Zn-Pb-Ba 矿体由下白垩统碳酸盐岩单元赋存。塔夫脱和阿布库赫地层。细粒至粗粒重晶石，含少量菱铁矿，分三个阶段（S1、S2 和 S4）形成，以及石英硫化物阶段（S3），含少量石英、闪锌矿、方铅矿、黄铜矿和黄铁矿，³⁴ S 值从 17.7 到 20.6‰，δ ¹⁸ O 值从 13.2 到 16.8‰，Δ ³³ S _V-CDT值从 –0.001 到 0.036‰，初始⁸⁷ Sr/ ⁸⁶ Sr 比率从 0.707327 ± 0.000008 到 0.708593 ± 0.000008 （ V-CDT = 维也纳峡谷 Diablo Troilite）。菱铁矿的 δ ¹³ C _V-PDB值为 –3.8 至 –2.7‰，δ ¹⁸ O _V-SMOW值从 18.2 到 20.9‰（V-PDB = Vienna-Pee Dee Belemnite，V-SMOW = 维也纳标准平均海水）。这些地球化学数据和重晶石形态表明了重晶石所有阶段的成岩起源。我们认为，在甲烷扩散受限的环境中，随着甲烷含量的增加，S1和S2重晶石从硫酸盐-甲烷过渡带的孔隙流体中沉淀出来。当含甲烷和钡的冷泉流体迁移到浅层碳酸盐沉积物并形成甲烷过量环境时，S4重晶石沉淀。在这三个阶段中，重晶石中的SO42-来自硫酸盐驱动的甲烷厌氧氧化过程中孔隙流体中残留的SO42-，而Ba ²⁺来自塔夫脱和桑格斯坦地层中溶解的生物重晶石和陆源物质。S3 石英中捕获的原生流体包裹体的盐度为 5.6 至 8.1 wt% 氯化钠当量，均质温度为 143.8° 至 166.1°C。石英的 δ ¹⁸ O _V-SMOW值范围为 9.8 至 22.5‰，δ ³⁰Si 值范围为 –1.3 至 –0.9‰。这些数据表明热液流体流动发生在成岩 S2 和 S4 事件之间。在 S3 石英中也发现了次生流体包裹体，其盐度为 17.70 至 19.13 wt% NaCl 当量，均质温度为 123.0°C 至 134.0°C。它们可能代表了S5盆地卤水形成的热液事件。根据矿石结构以及S5 Zn-Pb硫化物与成岩重晶石硫同位素的比较，重晶石为后来的Zn-Pb硫化物提供了宿主和硫源。铅矿化。重晶石与锌铅矿化的关系表明，大量硫酸盐富集可能是此类巨型矿床的重要勘探目标。