Ore geology and fluid inclusions of the Hucunnan deposit, Tongling, Eastern China: Implications for the separation of copper and molybdenum in skarn deposits

Abstract

The Hucunnan ore deposit is a representative skarn Cu–Mo deposit in the Tongling district, an important ore district of the renowned Middle–Lower Yangtze River metallogenic belt of China. The deposit shows distinct zonation of metals, with Cu mineralization distributed mainly in the exoskarn zone at shallow depths, and Mo mineralization occurring chiefly in the endoskarn zone located at deeper depths. Field evidence and petrographic observations indicate that the ore-forming processes can be divided into the skarn, quartz–molybdenite, quartz–chalcopyrite, and carbonate stages. Five types of fluid inclusions (FIs) are present in the deposit: solid-bearing (type 1), liquid-rich (type 2), vapor-rich (type 3), pure vapor (type 4), and CO2-bearing (type 5). The skarn stage contains mainly type 1 and 2, but also minor type 3 and 4 FIs; the FIs display homogenization temperatures of 434–570°C and salinities of 2.07–66.0wt.% NaCl equiv. The existence of hematite daughter minerals in the type 1 inclusions, together with the presence of magnetite in skarn, implies that the skarn-stage fluid was oxidizing. The skarnization of the granodiorite porphyry is commonly accompanied by potassic alteration, suggesting that the fluids were rich in alkali. Similar to the skarn stage, the quartz–molybdenite stage contains type 1 and 2, and minor type 3 and 4 FIs, which yield homogenization temperatures of 280–458°C and salinities of 1.40–54.2wt.% NaCl equiv. The presence of sulfide instead of hematite daughter minerals in the type 1 inclusions in quartz–molybdenite veins associated with sericitization indicates that the fluids of the quartz–molybdenite stage were more reducing and more acidic than the fluids in the skarn stage. The decrease in oxygen fugacity and increase in acidity could have resulted from magnetite crystallization and the consumption of alkali cations and OH−, respectively, during the skarn stage. The quartz–chalcopyrite stage contains all types of FIs, which show homogenization temperatures of 203–392°C and salinities of 1.22–46.6wt.% NaCl equiv. Observations of hematite-bearing type 1 FIs in quartz–chalcopyrite veins containing anhydrite associated with biotitization suggest that the fluids of the quartz–chalcopyrite stage were oxidizing and alkali-rich, probably on account of the inflow of meteoric water and boiling in an open system. In the carbonate stage, only type 2 FIs are present; these FIs yield the lowest homogenization temperatures of 156–276°C and the lowest salinities of 1.05–12.3wt.% NaCl equiv. Microthermometry and H–O isotope data indicate that the ore-forming fluids were dominated by magmatic water in the early stages (skarn and quartz–molybdenite stages), and that the magmatic water gradually mixed with circulating meteoric water during the late stages (quartz–chalcopyrite and carbonate stages). The coexistence of saline and vapor-rich FIs as internal trails or clusters within individual crystals, with similar homogenization temperatures but contrasting salinities and homogenization modes (to the liquid and vapor, respectively), in the first three stages strongly suggests that three episodes of fluid boiling occurred in these stages, as further supported by the hydrogen isotopic compositions of the fluids, which are lower than those of magmatic water. Based on the above data, we conclude that temporal changes in redox conditions, acidity, and temperature in the mineralizing fluids resulted in the temporal separation of Cu and Mo by selective sulfide precipitation in the Hucunnan skarn deposit. The competition among metals (e.g., Mo and Cu) for sulfur in magmatic fluids, along with vapor–brine immiscibility (fluid boiling), are major factors that contributed to the spatial separation of Cu and Mo in the deposit.