Geochemistry, isotopes, and morphology of coarse-grained pyrite from the Jinqingding gold deposit, Jiaodong Peninsula: Implications for episodic ore-forming fluid evolution

The Jinqingding gold deposit, a representative orebody in the Jiaodong gold province, hosts abundant coarse-grained pyrites that exhibit diverse morphologies, including cube (a), octahedron (o), tetragonal trisoctahedron (m), pentagonal dodecahedron (e), and combined forms (e.g., e + a). These pyrites display distinct morphological evolution with cores dominated by Pya/Pyo, mantles of Pyo/Pym, and rims dominated by Pye and Pye + a. Microstructurally, two pyrite types are identified: inclusion- and pore-rich pyrite (Pyi-j; i/j = morphology codes) and inclusion- and pore-poor pyrite (Pyi). Integrated morphological evolution, microstructural characterization, in-situ elements, and sulfur isotope analyses elucidate mineralization processes at the microscale.

Morphological evolution indicates a cooling trajectory from high to intermediate temperatures, coupled with increasing sulfur supersaturation. Trace elements (e.g., Au, Ag, Te, and Bi) are enriched in Pyi-j and formed by rapid pyrite crystallization and fluid immiscibility during pressure drops triggered by the “fault-valve” process. Gold primarily exists in the form of a solid solution (Au⁺), with enrichment processes that are controlled by Te and Bi rather than As. Sulfur isotopes (δ³⁴S: 7.8–22.2 ‰) display a core-to-rim decrease that is driven by oxidation (primary driver), Rayleigh fractionation and cooling during the “fault-valve” process. This study establishes that the “fault-valve” process governed episodic fluid pressure drops, promoting pyrite crystallization and metal deposition. The findings provide a microscale record of fluid evolution and highlight the critical roles of oxidation, temperature, and sulfur dynamics in gold mineralization.