Two-Stage Superimposed Gold Mineralization in the Xiejiagou Gold Deposit, Shandong Province: Insights from Fluid Inclusions, H-O-S Isotopes, and Trace Elements

Abstract

The Xiejiagou gold deposit located in the Zhaoyuan-Laizhou gold belt is composed of altered-rock-type gold mineralization and superposed auriferous quartz veins, showing unique two-stage gold mineralization. Oxygen and hydrogen isotopic analyses yielded the following results: δ18OH2O = 0.8‰ to 4.4‰ and δD = −106‰ to −85‰ for altered-rock-type mineralization, and δ18OH2O = 3.6‰ to 5.6‰ and δD = −98‰ to −89‰ for auriferous quartz-veins. Combined studies on Co/Ni, Sb/Bi and As/Ag ratios of pyrites, it can be inferred that the ore-forming fluids were dominated by magmatic water mixed with very little meteoric water. The fractured altered rocks in the ore-hosting fault zones are characterized by mylonitization, cataclastic lithification, and structural lenses, reflecting a compressional (closed) ore-forming system. In contrast, the occurrence of auriferous quartz veins in fissures of altered-rock-type orebodies and the fact that altered-rock-type ores commonly occur as breccias cemented by auriferous quartz veins that reflect an extensional (open) ore-forming system for the vein mineralization. The increase in δ34S values from stage I (5.8‰–7.2‰) to stage II (6.6‰–9.0‰) indicate that the altered-rock-type mineralization was the result of intense water–rock interaction, while the occurrence of immiscible inclusions in auriferous quartz veins demonstrates that fluid immiscibility contributes significantly to gold deposition in the vein-type mineralization. Fluid-inclusion microthermometric data indicate that the fluids for the altered-rock-type mineralization are characterized by moderate-to-high temperature (262–368 °C), and low-to-moderate salinity (4.3–10.8 wt.% NaCl equivalent). In contrast, halite-bearing inclusions are found in auriferous quartz veins, and its fluids are characterized by moderate-to-high temperature (290–376 °C) and moderate-to-high salinity (5.1–41.9 wt.% NaCl equivalent). From early stages (I and II) to the late stage (III), homogenization temperature and high temperature element (W, Sn, and Mo) concentrations in pyrite first decrease and then increase, δ34S values and metallization-related element (Au, Ag, and Bi) concentrations in pyrite first increase and then decrease. Therefore, it can be inferred that the two distinct types of gold mineralization in the Xiejiagou gold deposit may be two separate mineralization events. The presence of magnetite in the auriferous veins suggests an increase in oxidation state during the vein mineralization. Importantly, the Xiejiagou gold deposit preserves two types of mineralization in a single deposit and uniquely records a metallogenic transition from a compressional, reduced environment to an extensional, oxidized environment, as a result of a regional stress field transition that occurred in the Zhaoyuan-Laizhou gold belt at ca. 120 Ma.