Abstract Mississippi valley-type (MVT) ore deposits are epigenetic carbonate-hosted Pb-Zn deposits, that are formed by fluid expulsion from sedimentary sulfide successions. The sulfides were generated by thermochemical sulfate reduction (TSR) of the evaporitic sulfates dissolved in fluids. The initiation, processes and principles for TSR occurring in MVT ore deposition and the respective influence of major minerogenetic metal ions such as Pb2+ and Zn2+, as well as Fe2+, Sr2+, Ba2+ has not been clearly resolved. To evaluate the TSR activity of metal cations in MVT minerogenetic systems, a series of 300 °C to 450 °C gold-tube hydrous-pyrolysis experiments were separately conducted with FeSO4, PbSO4, ZnSO4, BaSO4 or SrSO4 and with n-octadecane (n-C18) as the hydrocarbon substrate. Based on the yields (from gas chromatography (GC) analysis) and carbon isotopic compositions (determined by GC-irMS) of the gases produced in the hydrous-pyrolysis gold-tube experiments, the TSR reactivity of the minerogenetic metal sulfates was ranked FeSO4 > ZnSO4 > Sr/BaSO4 > PbSO4. TSR occurred easily in the FeSO4 experiments at 300 °C, but hardly at all in PbSO4 experiments at 450 °C. Hence, S2− for the formation of the gelenite (PbS), sphalerite (ZnS) and pyrite (FeS2) in the MVT ore deposits appears to be related to Fe2+, which could initiate the TSR easily to produce reduced sulfur. The following two potential routes would provide good support for the TSR of FeSO4: (1) Hydrolysis of Fe2+, or the formation of a ferrous hydroxide-sulfate-hydrate complex, that increases the H+ concentration, resulting in the formation of HSO4− that initiates TSR; and (2) The oxidation of Fe2+ to Fe3+, which on subsequent hydrolysis (or the formation of iron-hydroxide-sulfate-hydrate complex), would greatly increase the concentrations of H+ and HSO4−, reduce the pH of the brine fluids and maintain acidic conditions favorable to TSR. However, the precipitation of pyrite greatly consumes S2−, limiting the concentration of H2S and thereby affecting the rate of TSR. In short, TSR is difficult to simulate using Zn2+ and Pb2+ sulfates, but easy with sulfates containing Fe2+ and Mg2+. This suggests that the occurrence of the FeSO4 and MgSO4 is critical in the formation of the large-scale MVT ore deposits, where they react as acid buffering agents, decreasing and maintaining a lower pH in the brine fluids and accelerating and sustaining TSR with higher concentrations of HSO4−. H2S/S2− would then be produced continually and participate as FeS2, ZnS and PbS in the large-scale MVT ore deposits.

中文翻译：

---

密西西比河谷型成矿金属硫酸盐在热化学硫酸盐还原中的含水热解研究

摘要 密西西比河谷型（MVT）矿床是后生碳酸盐岩铅锌矿床，是由沉积硫化物系列的流体排出形成的。硫化物是通过溶解在流体中的蒸发硫酸盐的热化学硫酸盐还原 (TSR) 产生的。MVT矿床中TSR的发生、过程和原理以及主要成矿金属离子如Pb2+和Zn2+以及Fe2+、Sr2+、Ba2+的各自影响尚未明确。为了评估 MVT 成矿系统中金属阳离子的 TSR 活性，分别使用 FeSO4、PbSO4、ZnSO4、BaSO4 或 SrSO4 和正十八烷（n -C18) 作为烃底物。根据水热解金管实验中产生的气体的产率（来自气相色谱 (GC) 分析）和碳同位素组成（由 GC-irMS 测定），成矿金属硫酸盐的 TSR 反应性排序为 FeSO4 > ZnSO4 > Sr/BaSO4 > PbSO4。在 300 °C 的 FeSO4 实验中很容易发生 TSR，但在 450 °C 的 PbSO4 实验中几乎不发生 TSR。因此，在 MVT 矿床中 S2- 用于形成胶凝矿 (PbS)、闪锌矿 (ZnS) 和黄铁矿 (FeS2) 似乎与 Fe2+ 相关，这可以很容易地引发 TSR 以产生还原硫。以下两条潜在路线将为 FeSO4 的 TSR 提供​​良好的支持： (1) Fe2+ 的水解，或氢氧化亚铁-硫酸盐-水合物复合物的形成，这会增加 H+ 浓度，导致 HSO4− 的形成，引发 TSR；(2) Fe2+ 氧化为 Fe3+，随后水解（或形成铁-氢氧化物-硫酸盐-水合物复合物），将大大增加 H+ 和 HSO4- 的浓度，降低盐水流体的 pH 值并保持有利于 TSR 的酸性条件。然而，黄铁矿的沉淀大量消耗 S2−，限制了 H2S 的浓度，从而影响了 TSR 的速率。简而言之，使用 Zn2+ 和 Pb2+ 硫酸盐很难模拟 TSR，但使用含有 Fe2+ 和 Mg2+ 的硫酸盐很容易模拟。这表明 FeSO4 和 MgSO4 的出现对于大规模 MVT 矿床的形成至关重要，它们在那里作为酸缓冲剂反应，降低和保持盐水流体中较低的 pH 值，并用更高浓度的 HSO4− 加速和维持 TSR。H2S/S2− 将不断产生并作为 FeS2、ZnS 和 PbS 参与大型 MVT 矿床。