Ore genesis of the Jinchanggouliang Au deposit in the northern North China Craton: Constraints from vein quartz and fluid inclusions

Abstract

The Jinchanggouliang gold deposit, containing exceeding 40 tons Au, is a quintessential large-scale deposit formed during the Yanshanian mineralization period in the northern margin of the North China Craton. However, previous studies have been highly controversial regarding the properties and evolution of ore-forming fluids, as well as the ore-forming conditions. This study employs H-O isotope analysis, fluid inclusion petrographic observation and microthermometry, in-situ LA-ICP-MS analysis of quartz and individual fluid inclusions to constrain the origin and properties of the ore-forming fluid, the fluid evolution process, and the Au precipitation mechanism. The mineralization process at Jinchanggouliang is divided into three stages: milky quartz (Q₁, stage I), quartz (Q₂) + pyrite (stage II), quartz (Q₃) + polymetallic sulfides (stage III). Stage III represents the principal mineralizing phase within the paragenetic sequence of the hydrothermal system. The primary fluid inclusion types are divided into three categories: two-phase aqueous inclusions (type I), two-phase CO₂-bearing aqueous inclusions (type II), and three-phase solid-bearing inclusions (type III). Microthermometry results indicate that the homogenization temperature progressively decreases from stage I to stage III, with significant salinity fluctuations. The ore-forming fluid is categorized within the H₂O-NaCl system, displaying features indicative of high-temperature conditions and a variable salinity range (1.7–37.8 wt% NaCl equiv.). Fluid boiling occurs at stage III. In-situ LA-ICP-MS analysis of fluid inclusions, combined with stable H-O isotope studies of quartz, delineates that the fluid is primarily sourced from magmatic water, with a contribution of meteoric water during the main metallogenic stage. The Rb/Cs ratio of fluid inclusions remains relatively stable across all stages, with values oscillating within a narrow range of 1–10, indicating a single source for the fluid system. As the progression from the stage I to III, the Ti content in quartz gradually decreases, while the Al and Sb contents increase. The correlation between Al and Li, as well as between Al and the sum of Li, Na and K, gradually increases, indicating a transition from single substitution to compensated substitution in quartz from the pre-ore to the main-ore stage. The formation conditions of quartz in each stage, calculated using TitaniQ, and fluid inclusion P-T formulas, are approximately 1.43 kbar at 548 °C for stage I, 0.73 kbar at 441 °C for stage II, and 0.14 kbar at 341 °C for stage III. As the fluid progressed towards the main mineralization stage, the lithostatic pressure transitioned to hydrostatic pressure. Fluid boiling in stage III is attributed to rapid pressure reduction. The escape of acidic volatiles and the addition of meteoric water gradually increase the pH. These physicochemical perturbations triggered extensive metal precipitation and mineralization, including Au. According to the experimental data and ore body occurrence, Jinchanggouliang gold deposit should be classified as the magmatic-hydrothermal vein type, which have a certain prospecting potential for porphyry and epithermal mineralization.