Two-stage gold deposition in response to H2S loss from a single fluid in the Sizhuang deposit (Jiaodong, China)
Two-stage gold deposition in response to H2S loss from a single fluid in the Sizhuang deposit (Jiaodong, China)
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泗庄矿床（中国胶东）单一流体中 H2S 损失导致的两阶段金沉积
DOI：
10.1016/j.oregeorev.2020.103450
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发表时间：
2020-05
影响因子：
3.3
通讯作者：
Ma Wei-Dong
中科院分区：
文献类型：
--
作者：
Hu Huan-Long;Fan Hong-Rui;Liu Xuan;Cai Ya-Chun;Yang Kui-Feng;Ma Wei-Dong
胶东金矿床的物质来源、流体演化和成矿机制一直是人们争论的焦点。寺庄金存款矿床为研究这些问题提供了一个很好的对象。该存款中金主要在石英-金-黄铁矿（第二阶段）和石英-金-多金属（第三阶段）沉淀。第三阶段又可分为早期阶段（黄铜矿、方铅矿和少量银金矿、磁黄铁矿、黄铁矿、菱铁矿和闪锌矿）和晚期阶段（黄铜矿、磁铁矿、菱铁矿和少量磁黄铁矿）。第二阶段和第三阶段的黄铁矿含有一致的δ 34 S特征（分别为8.1 ‰-9.6 ‰和7.5 ‰-9.8 ‰）以及微量元素（例如Co：8.19 vs. 2.63 ppm，Ni：3.54 vs. 0.74 ppm，As：29.95 vs. 5.78 ppm，Bi：3.18 vs. 7.81 ppm）。这些一致的地球化学特征与前人的流体包裹体和C-H-O同位素研究相结合，证明了寺庄存款的成矿物质和第II、III阶段的成矿流体具有共同的来源。在第II和第III阶段，由于黄铁矿的沉淀（HS-被黄铁矿清除）和流体的不渗透性（H2S进入气相），成矿流体中的还原态硫物种可能已经丢失，导致成矿流体中总硫浓度（∑ Stotal）降低。不同∑ Stotaland恒压下的热力学计算（2000 bar）和温度（300 ° C）的变化表明，∑ Stotal的降低会导致磁黄铁矿的稳定场先扩大后缩小，磁铁矿的稳定场向酸性更强的区域扩展;硫物种以H2S为主酸性成矿流体中硼铁矿-黄铁矿相界面下的硼铁矿-黄铁矿相界面下的硼铁矿-黄铁矿相H_2S浓度的降低可有效地使金沉淀，使金的二硫化二溶解度降低2 - 3个数量级，而∑ Stotal仅降低约50倍。磁黄铁矿和磁铁矿的沉淀是由于在这一物理化学过程中稳定场的扩大，这与地质观测是一致的。研究认为，四庄存款中金的沉淀与矿物组合中磁黄铁矿和磁铁矿的产出耦合，可能是由单一成矿流体中H2S散失的两阶段金沉淀作用所致。
The material source, fluid evolution and mineralizing mechanism in the Jiaodong gold province have been hotly debated for several decades. The Sizhuang gold deposit provides an excellent object for studying these issues. Gold in this deposit mainly precipitated in the quartz–Au–pyrite (stage II) and quartz–Au–polymetallic (stage III) stages. Stage III can be further subdivided into the early stage III (chalcopyrite, galena with minor amounts of electrum, pyrrhotite, pyrite, siderite and sphalerite) and the late stage III (chalcopyrite, magnetite, siderite with a little pyrrhotite). Pyrite from stage II and stage III contains consistent δ34S signatures (8.1‰–9.6‰ and 7.5‰–9.8‰, respectively) as well as trace elements (e.g. Co: 8.19 vs. 2.63 ppm, Ni: 3.54 vs. 0.74 ppm, As: 29.95 vs. 5.78 ppm, and Bi: 3.18 vs. 7.81 ppm). These consistent geochemical features combined with previous fluid inclusion and C-H-O isotope studies argue for a common source for ore-forming materials and fluids of the stage II and III at Sizhuang deposit. During the stage II and III, reduced sulfur species may have been lost from ore-forming fluids in response to pyrite precipitation (HS-being scavenged by pyrite) and fluid immiscibility (H2S entering vapor phase), which led to the total sulfur concentration (∑Stotal) decrease in the ore-forming fluids. Thermodynamic calculations at different ∑Stotaland constant pressure (2000 bar) and temperature (300 °C) in Fe-Cu-O-S-H system reveal that ∑Stotaldecrease can lead to first enlargement and then shrinkage of pyrrhotite stability field with magnetite stability field being broadened to more acidic field; that sulfur species is dominated by H2S (>90 mol%) under the bornite–pyrite phase boundaries in the acidic ore-forming fluids; and that decreases in H2S concentration can efficiently cause gold precipitation by reducing gold bisulfide solubility up to 2–3 orders of magnitude while ∑Stotaljust decreases ~50 times. Pyrrhotite and magnetite precipitated due to enlarged stability field during this physicochemical process, which is in line with geologic observations. It is proposed that the two-stage gold deposition by H2S loss from a single ore-forming fluid could account for gold precipitation coupling with occurrence of pyrrhotite and magnetite in the mineral assemblage at Sizhuang deposit.
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