Geochemical evolution of ore-forming fluids and key controlling factors in super-large gold deposits: A case study of the North Sanshandao gold deposit, Jiaodong Peninsula, Eastern China

Precise characterization of ore-forming fluid evolution is crucial for understanding the mineralization mechanisms of large-scale gold deposits. The North Sanshandao gold deposit, China’s first offshore super-large gold resource (>562 t Au @ 4.35 g/t), serves as an exemplary case study for deciphering the formation processes of both super-large and Jiaodong-type gold deposits. Petrographic observations and crosscutting relationships define four distinct mineralization stages. Fluid inclusion (FI) studies reveal four FI types in quartz across these stages: (i) liquid (H₂O)-dominated two-phase (type 1), (ii) vapor (H₂O)-dominated two-phase (type 2), and (iii) CO₂-bearing (liquid and vapor) inclusions (types 3a and 3b). Stage I (254–375 °C, 3.53–10.74 wt% NaCl eq.) contains all four FI types, whereas stage II (195–313 °C, 2.06–10.37 wt% NaCl eq.) is marked by types 1, 2, and 3a. Stage III (196–336 °C, 5.05–9.47 wt% NaCl eq.) exhibits only types 1 and 3a. Notably, stages I–III display clear evidence of fluid immiscibility, indicative of a low-salinity, medium-temperature NaCl–H₂O–CO₂ hydrothermal system. In contrast, stage IV (134–184 °C, 3.53–9.34 wt% NaCl eq.) transitions to a simpler NaCl–H₂O system, with only type 1 inclusions present.

Stable isotope analysis reveals that the δ¹⁸O_H2O (5.7–10.0 ‰) and δD (90.9–78.4 ‰) values indicate a predominantly magmatic fluid source. However, stage IV records a significant influx of meteoric water (δ¹⁸O_H2O =  − 9.4 ‰, δD =  − 100.5 ‰). Pressure estimates for stages I–III range from 54.2–81.0 MPa (avg. 69.3 MPa; ∼5.5–8.3 km depth), 42.8–90.9 MPa (avg. 63.1 MPa; ∼4.4–9.3 km depth), and 54.7–87.6 MPa (avg. 60.9 MPa; ∼5.6–8.9 km depth), respectively. Pyrite noble gas isotopes provide further insights: ³He/⁴He ratios in auriferous pyrite from stages II and III range from 1.37 to 1.44 Ra (25–26 % mantle-derived He) and 0.07–1.29 Ra (1–23 % mantle-derived He), respectively, highlighting the mantle-derived fluids contribution to mineralization. Combined H-O-He-Ar isotopic data strongly support a unified magmatic origin for the ore-forming fluids across the Jiaodong gold deposits.