Textures of gold-bearing sulfides and gold precipitation mechanism, Wangu gold deposit, Jiangnan Orogen

Abstract

万古金矿床位于江南造山带中部，赋存于新元古界冷家溪群浅变质岩系中，受NNE-NE向长沙-平江断裂带和近EW向九岭-清水韧性剪切带联合控制，金资源量约85t。其主要矿石类型为毒砂-黄铁绢英岩型和石英-硫化物脉型，其次为构造角砾岩型。毒砂和黄铁矿为该矿床主要的载金矿物，分布广泛。金成矿作用可分为四个阶段，I为乳白色石英-绢云母-白钨矿阶段；Ⅱ为烟灰色石英-绢云母-毒砂-黄铁矿-金阶段；Ⅲ为烟灰色石英-绢云母-黄铁矿-毒砂-多金属硫化物-金阶段；Ⅳ为乳白色石英-方解石阶段。其中，Ⅱ、Ⅲ为成矿主阶段。根据成矿主阶段毒砂电子探针分析结果，Ⅱ阶段毒砂中As的含量在42.19%~44.84%之间，均值为43.42%（n=56），Ⅲ阶段毒砂中As的含量在40.08%~43.36%之间，均值为42.08%（n=19）。通过毒砂温度计相图估算出Ⅱ、Ⅲ阶段的形成温度和硫逸度分别为364±21℃、319±22℃和10^-9.7~10^-7、10^-11.5~10^-8.6。电子探针数据揭示的载金毒砂和黄铁矿中不可见金含量分别为0.01%~0.66%和0.01%~0.11%。黄铁矿Au-As元素投点分布于金溶解度曲线两侧，说明其内金主要以纳米级颗粒和固溶体金或晶格金的形式赋存；其中Ⅱ阶段黄铁矿纳米级金颗粒占比为73.33%，多于Ⅲ阶段黄铁矿（67.80%）。以上数据说明在水岩反应过程中，围岩中的含铁矿物与成矿流体中的H₂S发生反应，生成毒砂和黄铁矿。伴随着强烈的水岩反应，成矿温度和硫逸度降低，成矿Ⅱ阶段至Ⅲ阶段主要载金硫化物由毒砂转变为黄铁矿，强烈的硫化作用导致金-硫络合物失稳并释放金，金以置换的方式进入硫化物晶格或以显微-超显微金颗粒的形式沉淀，形成含金硫化物；即硫化作用是导致万古矿床不可见金沉淀的主导机制。;The Wangu gold deposit, with a proven gold resource of~85t, is located in the middle section of the Jiangnan Orogen and occurs in metamorphic rock series of the Neoproterozoic Lengjiaxi Group. It formed under the control of the NNE-NE-trending Changsha-Pingjiang fault zone and the EW-trending Jiuling-Qingshui ductile shear zone. The main ore types of the deposit include arsenopyrite-, pyrite-, sericite-, and quartz-altered slate and quartz-sulfide veins, followed by slate breccia within hydrothermal quartz. Arsenopyrite and pyrite are the main gold bearing minerals in the deposit, which are widely distributed in the deposit. Gold mineralization can be divided into four stages:I, the milky quartz-muscovite-scheelite; Ⅱ, the smoky gray quartz-muscovite-arsenopyrite-pyrite-gold; Ⅲ, the smoky gray quartz-muscovite-pyrite-arsenopyrite-polymetallic sulfide-gold; and Ⅳ, the milky quartz-calcite. Among them, the Ⅱ and Ⅲ are the main mineralization stages. Based on the results of Electron Probe Micro Analysis (EPMA) of arsenopyrite from the main mineralization stages, the contents of As in arsenopyrite at stage Ⅱ range from 42.19% to 44.84%, with an average of 43.42% (n=56). The contents of As in arsenopyrite at stage Ⅲ range from 40.08% to 43.36%, with an average of 42.08% (n=19). According to the phase diagram of arsenopyrite thermometer, the formation temperature of Apy-1 in the stage Ⅱ is estimated to be 364±21℃ with a sulfur fugacity varying within 10^-9.7~10^-7. The formation temperature of Apy-2 in the stage Ⅲ is 319±22℃, and its sulfur fugacity is 10^-11.5~10^-8.6. The contents of invisible gold in gold-bearing arsenopyrite and pyrite were 0.01%~0.66% and 0.01%~0.11%, respectively, revealed by EPMA data. The Au-As data of pyrite are plotted on both sides of the gold solubility curve, indicating that gold in pyrite mainly exists in the form of nano-scale particles and solid solution or lattice gold. The proportion of nanometer gold particles in Py-1 pyrite is 73.33%, more than that in Py-2 (67.80%). The above data indicate that in the process of water-rock reaction, the iron-bearing minerals in surrounding rocks react with H₂S in ore-forming fluid to form arsenopyrite and pyrite. Accompanied by a strong water-rock reaction, the main gold-bearing sulfides changed from arsenopyrite to pyrite during mineralization from stage Ⅱ to Ⅲ with the decrease of mineralization temperature and sulfur fugacity, i.e., strong sulfurization results in instability of the gold-sulfur complex and the release of gold. As a result, gold is deposited into the sulfide lattice by substitution or in the form of microscopic-ultra microscopic gold particles to form gold-bearing sulfide. Therefore, sulfidation is the main mechanism of the precipitation of invisible gold in sulfides at the Wangu gold deposit.