Primary magmatic biotite in granitoid intrusions associated with Late Mesozoic Jinchanggouliang-Erdaogou lode gold deposits, North China Craton: Linkage to gold mineralization

Abstract

The Late Mesozoic Jinchanggouliang and Erdaogou lode gold deposits in the central segment of the northern North China Craton are mainly hosted in Late Archaean metamorphic rocks of the Jianping Formation and Jurassic volcanic-sedimentary rocks. These deposits are spatially and temporally associated with a series of I-type granitoid intrusions: i.e., the 128–136 Ma Duimiangou granodiorite complex with a massive margin and a porphyritic center at Jinchanggouliang, the 161.4 ± 1.1 Ma Loushang quartz diorite intrusion and numerous syenite porphyry dykes (160.9 ± 1.0 Ma) at Erdaogou, and the ~ 227–217 Ma Xitaizi monzogranite batholith. Compositions of primary magmatic biotite in these granitoid intrusions have been used to investigate the nature of granitoid magmas and the evolution of magmatic processes and to identify possible linkages between gold mineralization and granitoid magmatism. All primary magmatic biotite grains are Mg-rich in composition (XMg = Mg/(Mg + Fe) = 0.41–0.69). The Cl contents of biotite (0.09–0.42 wt%) in the Loushang quartz diorite and the Duimiangou massive granodiorite correlate negatively with XMg and are consistent with the “Mg-Cl avoidance” rule. The Cl contents of biotite (0.01–0.14 wt%) in the Duimiangou porphyritic granodiorite, syenite porphyry and Xitaizi monzogranite, on the other hand, remain relatively constant and may be affected by multiple variables such as Cl-rich fluids and subtle changes in temperature. Halogen fugacity of the parental magma calculated from biotite shows that the Duimiangou granodiorite complex at Jinchanggouliang and syenite porphyry dykes at Erdaogou have the log(fHF/fHCl) ranges of −0.71 to −0.27 and −2.16 to −1.08, respectively, and have similar log(fH2O)/(fHCl) values from 3.53 to 4.23. Oxygen fugacities (logfO2) calculated from biotite equilibria for the Duimiangou granodiorite complex and syenite porphyry dykes are −13.7 to −7.2, straddling on the nickel-nickel oxide and hematite-magnetite buffers. The Xitaizi monzogranite and the Loushang quartz diorite have lower log(fO2) values from −14.9 to −12.4, between the nickel-nickel oxide and hematite-magnetite buffers. The Duimiangou granodiorite complex and syenite porphyry dykes with high oxygen fugacities are similar to oxidized magmas associated with porphyry Cu-Mo-Au deposits. These results suggest that multiple episodes of I-type granitoid magmas in the study area were rich in Cl and high in oxygen fugacity, especially those associated with the Duimiangou granodiorite complex and syenite porphyry dykes. The I-type melts represented by the Duimiangou granodiorite complex and syenite porphyry dykes as well as their related Cl-rich, oxidizing fluids were likely efficient in dissolving, transporting and precipitating gold for the formation of the Jinchanggouliang and Erdaogou lode gold deposits.