Complementary Textural, Trace Element, and Isotopic Analyses of Sulfides Constrain Ore-Forming Processes for the Slate-Hosted Yuhengtang Au Deposit, South China
Complementary Textural, Trace Element, and Isotopic Analyses of Sulfides Constrain Ore-Forming Processes for the Slate-Hosted Yuhengtang Au Deposit, South China
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华南板岩型余亨塘金矿床硫化物的组构、微量元素及同位素互补分析对其成矿过程的制约
DOI：
10.5382/econgeo.4847
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发表时间：
2021-12
期刊：
影响因子：
5.8
通讯作者：
Wei Li;N. Cook;Guiqing Xie;J. Mao;C. Ciobanu;B. Fu
中科院分区：
文献类型：
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作者：
Wei Li;N. Cook;Guiqing Xie;J. Mao;C. Ciobanu;B. Fu
玉横塘金矿是江南造山带中具有代表性的板岩型Au存款，Au储量约55 t，平均品位约3.9 g/t。金矿化以细脉和浸染状矿石为特征，矿石中含有自然金、含金黄铁矿和毒砂。玉横塘存款的共生作用可分为三个阶段。前矿期1为新元古代板溪群板岩中黄铁矿的平行层理。主矿石阶段2代表Au矿化阶段，可区分两种不同的矿化类型：石英细脉中可见的Au-毒砂-黄铁矿和蚀变板岩中浸染的含金毒砂-黄铁矿。后矿阶段3由石英-黄铁矿-方解石-铁白云石脉组成。在这项研究中，我们集成了电子显微探针，激光烧蚀电感耦合等离子体质谱（LA-ICP-MS）和高分辨率离子探针（SHRIMP）分析文件纹理，同位素和组成之间的变化纹理复杂的黄铁矿和毒砂组合细脉和浸染状矿石。此外，黄铁矿的LA-ICP-MS硫同位素填图突出了微量元素和硫同位素在粒度尺度上的协变行为，从而使控制热液Au矿床硫同位素分馏的因素受到约束。沉积成因的黄铁矿（第1阶段），其Au含量可忽略不计（<1.6 ppm），但富含δ 34 S（15.6-25.8‰）。第2阶段细脉矿化的黄铁矿和毒砂均表现出多孔结构和溶解-再沉淀结构，Au浓度较低（分别<4和<78 ppm），δ 34 S变化较大（分别为-2.7 ~14.7 ‰和-10.3 ~12.1 ‰）。而浸染型矿化的黄铁矿和毒砂则分别具有振荡环带结构和均匀的背散射电子像，其不可见Au含量较高（分别达90和263 ppm）和δ 34 S值范围较窄（0 ~ 5.3‰）。这些数据表明，岩浆-热液流体贡献了玉衡塘Au存款的大部分Au和S的预算。细脉型矿化和浸染型矿化在结构、微量元素含量、黄铁矿和毒砂δ 34 S特征等方面的差异反映了Au沉淀机制的不同和成矿过程物理化学参数的演化，特别是fO 2和流体-岩石相互作用强度的不同。第3阶段黄铁矿在BSE图像上表现出均匀性，但δ 34 S值变化较大（1.2-31.4‰），进一步突出了物理化学条件（即，压力）对硫化物的δ 34 S特征的影响。LA-ICP-MS硫和微量元素的耦合填图结果表明，第2阶段的一些带状黄铁矿颗粒是由富Au、轻δ 34 S（2.4‰）的热液边缘过度生长到贫Au、重δ 34 S（18.1-18.5‰）的沉积岩心中而形成的。所有的结果都支持多个沉积机制内的动态矿物系统负责Au浓度和定义特定的纹理，成分和硫同位素特征的硫化物共存的静脉/细脉和浸染型矿化。新数据突出了基于成矿过程的矿床成因解释，并强调了进行补充性原位矿物学分析的重要性，以阐明成矿流体的来源和演化，并正确解释热液Au系统的结构。
Yuhengtang is a representative slate-hosted Au deposit in the Jiangnan orogenic belt, South China, with a reserve of ~55 t Au and an average grade of ~3.9 g/t. Gold mineralization is characterized by veinlet and disseminated ores comprising native gold, auriferous pyrite, and arsenopyrite. Paragenesis of the Yuhengtang deposit can be divided into three stages. Pre-ore stage 1 is composed of bedding-parallel layers of pyrite in slate of the Neoproterozoic Banxi Group. Main ore stage 2 represents the Au mineralization stage, and two distinct types of mineralization can be distinguished: visible Au-arsenopyrite-pyrite in quartz veinlets and auriferous arsenopyrite-pyrite disseminated within altered slate. Post-ore stage 3 consists of quartz-pyrite-calcite-ankerite veins. In this study, we integrate electron microprobe, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and high-resolution ion microprobe (SHRIMP) analyses to document textural, isotopic, and compositional variation among texturally complex pyrite and arsenopyrite assemblages in veinlet and disseminated ores. Additionally, LA-ICP-MS sulfur isotope mapping of pyrite highlights the covariation behavior between trace elements and sulfur isotopes at the grain scale, thus allowing the factors controlling sulfur isotope fractionation in hydrothermal Au deposits to be constrained. Pyrite, of sedimentary origin (stage 1), hosts negligible Au (<1.6 ppm) but is enriched in δ34S (15.6–25.8‰). Pyrite and arsenopyrite from stage 2 veinlet mineralization both display porous and dissolution-reprecipitation textures, have low Au concentrations (<4 and <78 ppm, respectively), and show a large variation in δ34S (–2.7 to 14.7‰ and –10.3 to 12.1‰, respectively). Pyrite and arsenopyrite from disseminated mineralization are, in contrast, characterized by oscillatory zoning textures and homogeneous appearance in backscattered electron (BSE) images, respectively, and are obvious by their relatively high contents of invisible Au (up to 90 and 263 ppm, respectively) and restricted range of δ34S values (0–5.3‰). These data suggest that magmatic-hydrothermal fluids contribute most of the Au and S budget in the Yuhengtang Au deposit. The major differences between veinlet and disseminated mineralization in terms of texture, trace element concentrations, and δ34S signatures of pyrite and arsenopyrite reflect contrasting mechanisms of Au precipitation and an evolution of physicochemical parameters of the ore-forming processes, particularly fO2 and the intensity of fluid-rock interaction. Pyrite from stage 3 appears homogeneous in BSE images yet displays a wide variation in δ34S values (1.2–31.4‰), further highlighting the controlling role played by physicochemical condition (i.e., pressure) on the δ34S signature of sulfides. Results of the coupled LA-ICP-MS sulfur and trace element mapping reveal that some zoned pyrite grains from stage 2 formed via overgrowth of Au-rich, light δ34S (2.4‰) hydrothermal rims onto Au-poor, heavy δ34S (18.1–18.5‰) sedimentary cores. All results support that multiple depositional mechanisms within a dynamic mineral system were responsible for Au concentration and define the specific textural, compositional, and sulfur isotope signatures of sulfides in coexisting vein/veinlet and disseminated mineralization. The new data highlight the ore-forming processes-based interpretation for ore genesis and underpin the importance of performing complementary in situ mineralogical analyses to elucidate the source and evolution of ore-forming fluids and enable correct interpretation of the architecture of the hydrothermal Au system.