Porphyry Cu mineralization processes of Xinqiao deposit, Tongling ore district: constraints from the geochronology and geochemistry of zircon, apatite, and rutile

Xinqiao is a large ore deposit that is well known for its stratabound massive sulfide and skarn ores. Recently, deep drilling has shown that Xinqiao also has typical porphyry mineralization and alteration features. The Cu grade is 0.3–0.6% and occurs as quartz-pyrite-chalcopyrite in potassic and phyllic alteration zones. The characteristics and genetic processes of the porphyry mineralization at Xinqiao have not previously been reported. Three intrusion types are recognized; pyroxene diorite, quartz diorite and quartz monzodiorite porphyry; Cu mineralization is associated with the quartz diorite and quartz monzodiorite porphyry. Zircon U-Pb dating shows that the three intrusions were all emplaced at about 140 Ma. Zircons from the quartz diorite and quartz monzodiorite porphyry show higher Ce⁴⁺/Ce³⁺ and Eu/Eu* (>0.5), 10,000(Eu/Eu*)/Y (>5), and (Ce/Nd)/Y (>0.01), than the pyroxene diorite, and apatites from the mineralized intrusions have slightly higher SO₃ (average 0.40) contents than ones from the pyroxene diorite (average 0.33). These results indicate that the fertile magmas were more oxidized, hydrous, and S-rich than the barren pyroxene diorite, consistent with their association with porphyry Cu mineralization. U-Pb dating of hydrothermal rutile from the porphyry mineralization and garnet from the skarn ore both yield ages of about 140 Ma, suggesting that the porphyry and skarn mineralization belong to same system. The geochemistry of hydrothermal rutile and apatite in the porphyry mineralization zone also indicates that Xinqiao ores were produced from a S-rich magmatic-hydrothermal fluid system. Our results highlight huge porphyry Cu mineralization potential beneath stratabound and skarn deposits in the Middle-Lower Yangtze River metallogenic belt.