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.