Isotope systematics and fluid inclusion studies of the Qiyugou breccia pipe-hosted gold deposit, Qinling Orogen, Henan province, China: Implications for ore genesis

Abstract

The Qiyugou gold deposits, Henan Province, are hosted in breccia pipes within the Xiong'er terrane (Qinling Orogen), on the southern margin of the North China Craton. In these deposits three paragenetic assemblages have been recognized: an early K-feldspar–epidote–quartz–pyrite; a middle quartz-polymetallic sulfide; and a late quartz–carbonate ± adularia. In this paper we review and interpret fluid inclusion and stable and radiogenic isotopic data of host rocks and ores. Fluid inclusions in quartz and calcite include water-rich, CO 2-rich, and daughter crystal-bearing. The CO 2-rich and daughter mineral-bearing fluid inclusions are common in the early-stage quartz and absent in the late-stage quartz and calcite which only contain water-rich fluid inclusions. Accordingly, the early-stage ore-fluids are magmatic in origin and characterized by high-temperature (> 350 °C), high-salinity (> 30 wt.% NaCl equiv.), and are CO 2-rich. Fluid-boiling in the middle ore stage resulted in CO 2-release, decreasing oxygen fugacity and rapid precipitation of ore materials. The late stage fluids, have low-temperature, low-salinity, are CO 2-poor and lack daughter minerals. These fluids are probably sourced from meteoric water. H–O–C isotope systematics confirm that, the ore–fluid system evolved from magmatic to meteoric. The carbon and lead isotope ratios indicate that the Meso-Neoproterozoic sequence south of the Xiong'er terrane was the likely source of the ore-forming materials. The Qiyugou breccia-pipes and their associated gold ores were emplaced during an extensional regime following a transition from collision to rifting tectonics, linked to collision and subduction of the Yangtze plate beneath the North China Craton. Geochronological studies show that Mesozoic magmatism in the region occurred between the Triassic and Cretaceous. The Triassic to Jurassic magmas were mostly derived from partial melting of ancient crust, whereas the Cretaceous magmas show juvenile signatures, indicating mantle-derived components. Processes of fragmentation of lithospheric roots, crustal and lithospheric thinning, extension and rifting were probably associated with the subducting Pacific (Izanagi) plate. These crust–mantle geodynamic processes were responsible for the development of anorogenic granitic melts that interacted with the Meso-Neoproterozoic volatile-rich sedimentary successions, producing a flow of gas-rich hydrothermal fluids that resulted in the emplacement of the Qiyugou auriferous breccia pipes. We conclude that the Qiyugou gold deposits are intrusion-related explosive breccia pipe-type that evolved from hypothermal through mesothermal to epithermal.