Pyrite textures and compositions from the Zhuangzi Au deposit, southeastern North China Craton: implication for ore-forming processes

The Zhuangzi Au deposit in the world-class Jiaodong gold province hosts visible natural gold, and pyrite as the main ore mineral, making it an excellent subject for deciphering the complex hydrothermal processes and mechanisms of gold precipitation. Three types of zoned pyrite crystals were distinguished based on textural and geochemical results from EPMA, SIMS sulfur isotopic analyses and NanoSIMS mapping. Py0 has irregular shapes and abundant silicate inclusions and was contemporaneous with the earliest pyrite-sericite-quartz alteration. It has low concentrations of As (0-0.3 wt.%), Au and Cu. Py1 precipitated with stage I mineralization shows oscillatory zoning with the bright bands having high As (0.4-3.9 wt.%), Au and Cu contents, whereas the dark bands have low contents of As (0-0.4 wt.%), Au and Cu. The oscillatory zoning represents pressure fluctuations and repeated local fluid phase separation around the pyrite crystal. The concentration of invisible gold in Py1 is directly proportional to the arsenic concentration. Py1 is partially replaced by Py2 which occurs with arsenopyrite, chalcopyrite and native gold in stage II. The replacement was likely the result of pseudomorphic dissolution-reprecipitation triggered by a new pulse of Au-rich hydrothermal fluids. The delta S-34 values for the three types of pyrite are broadly similar ranging from + 7.1 to + 8.8aEuro degrees, suggesting a common sulfur source. Fluid inclusion microthermometry suggests that extensive phase separation was responsible for the gold deposition during stage II mineralization. Uranium-Pb dating of monazite constrains the age of mineralization to ca. 119 Ma coincident with a short compressional event around 120 Ma linked to an abrupt change in the drift direction of the subducting Pacific plate.

位于世界级胶东金矿省的庄子金矿床含有可见的自然金，黄铁矿为主要矿石矿物，这使其成为解读复杂的热液过程和金沉淀机制的绝佳研究对象。根据电子探针分析（EPMA）、二次离子质谱（SIMS）硫同位素分析以及纳米二次离子质谱成像的结构和地球化学结果，区分出了三种具有分带特征的黄铁矿晶体。Py0形状不规则，含有大量硅酸盐包裹体，与最早的黄铁矿 - 绢云母 - 石英蚀变同时形成。它的砷（0 - 0.3 wt.%）、金和铜含量较低。与第一阶段成矿作用同时沉淀的Py1显示出振荡环带结构，亮带具有较高的砷（0.4 - 3.9 wt.%）、金和铜含量，而暗带的砷（0 - 0.4 wt.%）、金和铜含量较低。这种振荡环带代表了黄铁矿晶体周围的压力波动以及局部流体相的反复分离。Py1中不可见金的浓度与砷浓度成正比。Py1部分被Py2所替代，Py2与毒砂、黄铜矿和自然金一起出现在第二阶段。这种替代可能是由富含金的热液流体的新脉冲引发的假象溶解 - 再沉淀的结果。三种黄铁矿的δ³⁴S值大致相似，范围从 + 7.1到 + 8.8‰，表明它们具有共同的硫源。流体包裹体显微测温表明，广泛的相分离是第二阶段成矿过程中金沉淀的原因。独居石的铀 - 铅定年将成矿年龄限定在约119 Ma，这与120 Ma左右的一次短暂挤压事件相吻合，该挤压事件与太平洋板块俯冲方向的突然改变有关。