Most known copper (Cu) skarns are associated with oxidized intrusions. In this contribution, we report a Cu skarn associated with a reduced monzodioritic intrusion at Huanren, northeastern China, which contains 0.41 Mt of Cu accompanied by economic concentrations of Zn, Pb, Fe, Mo, and Ag. Copper-polymetallic mineralization in the Huanren deposit is concentrated in skarns located between the contacts of the monzodiorite (SiO2 = 52−55 wt%) and the Cambrian carbonate rocks, with minor molybdenite-bearing veinlets/veins and dissimilated chalcopyrite mineralization hosted within the monzodiorite. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology indicates the monzodiorite crystallized at 125.4 ± 0.6 Ma (2σ). Isotope dilution (ID)-ICP-MS Re-Os molybdenite geochronology indicates mineralization at Huanren occurred at 125.3 ± 0.8 Ma (2σ). Whole-rock major- and trace-element and zircon Hf isotopic compositions suggest enriched (subduction metasomatized) lithospheric mantle−derived sources for the parental magma of the monzodiorite without significant crustal assimilation. Zircon trace-element and magmatic apatite major-element compositions indicate the reduced nature of the monzodiorite, as evidenced by low magmatic oxygen fugacity (fayalite-magnetite-quartz [FMQ] buffer = 1.09 ± 0.19) and negligible apatite SO3 contents (<0.05 wt%). A reduced magmatic-hydrothermal system at Huanren is also supported by the predominance of magmatic ilmenite over magnetite in the monzodiorite and by the presence of pyrrhotite and the absence of anhydrite and hematite in the ore. Chalcopyrite from the Huanren deposit has an average δ34S value of 4.34‰ ± 0.88‰ (1σ), which is clearly higher than values from most porphyry-skarn Cu ± Mo ± Au deposits. Accordingly, we suggest that interaction between (1) external oxidized fluids equilibrated with evaporites and (2) reduced Cl-bearing magmas and related exsolved fluids may have played a critical role in the formation of the Huanren Cu skarn by increasing the ability of fluids to scavenge Cu from the reduced magma and subsequently precipitate Cu in the carbonate rocks. This study defines a new type of Cu skarn and thereby opens new potential for Cu skarn exploration proximal to intrusive units previously deemed too reduced to be Cu fertile, especially in non-arc settings. Moreover, we conclude that the availability of Cl and S in magmatic-hydrothermal systems may be as critical as fO2 in facilitating the actual ore-forming event in Cu skarn systems.

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