Metallogenic mechanism of Ankou gold deposit in the Qixia-Penglai Gold Belt, Jiaodong Peninsula, China: Constraints from sericite Ar-Ar geochronology, H-O isotope, and in-situ trace element of pyrite

Abstract

Jiaodong Peninsula is home to the largest gold district in China, encompassing significant deposits within the Qixia-Penglai Gold Belt, renowned for the presence of visible gold. Nonetheless, the timing of deposit formation and the intricate processes of gold transport and precipitation mechanisms remain subjects of ongoing debate. To deepen the comprehension of these processes, sericite geochronological analysis, fluid inclusion micro-thermometry, H-O isotope students, and trace element pyrite geochemical analysis were undertaken at the Ankou deposit. ⁴⁰Ar/³⁹Ar dating of three representative deposits in the Ankou-Heilangou ore field limits the mineralization to a tight age range of 118–120 Ma. Our study identifies four distinct stages of ore-forming, namely the pyrite-quartz stage (Stage 1), the quartz-pyrite stage (Stage 2), the quartz-polymetallic sulfide stage (Stage 3), and the quartz-calcite stage (Stage 4). The fluid inclusion homogenization temperatures range from 227 to 372 °C, 217 to 334 °C, 168 to 306 °C, and 106 to 265 °C, respectively. Salinities recorded during these stages are predominantly below 10 wt% NaCl equivalent; H-O isotope analysis of the main metallogenic stage (Stages 2 and 3) yielded δ¹⁸O_H2O values ranging from +5.2 ‰ to +7.2 ‰ and +4.5 ‰ to +7.4 ‰, alongside δD_SMOW ranges from −94.6 ‰ to −83.0 ‰ and −95.8 ‰ to −87.7 ‰, respectively. Meanwhile, these results support a model of mineralization originating by incremental mixing of meteoric and magmatically-sourced fluids, with gold precipitation linked to fluid boiling and immiscibility reactions. From micro-textural observations, backscattered electron (BSE) imaging, and geochemical analysis, ten sub-generations of pyrite were identified. Stage 1 pyrites (Py1-1, Py1-2, and Py1-3) tracing records changes in As and Au concentrations, mineralogical capture, and hydrothermal alteration processes. Stage 2 pyrites (Py2-1, Py2-2, Py2-3a, and Py2-3b) indicate an obvious cored-mantle-edge structure, and the outer edge of the pyrite exhibits oscillatory BSE-bright and -dark reactions rims linked to similar changes in Au and As concentrations, consistent with pressure fluctuations and fluid immiscibility. Furthermore, Stage 3 pyrite (Py3-1, Py3-2, and Py3-3) exhibits comparable BSE-bright and dark zonation patterns, reflecting Au and As variability triggered by fluid pulsing. These findings reveal that the ore-forming fluids within the Ankou gold deposit exhibit characteristics identical to those found in mesothermal deposits. The ore-forming fluids are primarily derived from magmatic water, with a small amount of meteoric water added in the process of the main metallogenic stages. Furthermore, gold was probably transported at Ankou in the form of $Au{\left(HS\right)}_2^{-}$ complexes. Meanwhile, the addition of Au-As-rich fluids during the main ore stages increased the content of Au and As elements in the ore-forming fluids, and the fluid immiscibility reduced the solubility of $Au{\left(HS\right)}_2^{-}$, resulting in the deposition of gold. Consequently, the addition of Au-As-enriched fluids and fluid immiscibility emerge as the primary mechanisms driving gold mineralization at the Ankou deposit.