Cretaceous Cu–Au, pyrite, and Fe-oxide–apatite deposits in the Ningwu basin, Lower Yangtze Area, Eastern China

Abstract

The Cretaceous Ningwu volcanic basin of the Middle and Lower Yangtze River Valley metallogenic belt of eastern China hosts numerous Fe-oxide–apatite, Cu–Au, and pyrite deposits. The mineralization in the Ningwu basin is associated with subvolcanic rocks, consisting of gabbro–diorite porphyry and/or pyroxene diorite. However, the mineralization is associated with subvolcanic and volcanic rock suite belonging to the Niangniangshan Formation in the Tongjing Cu–Au deposit, including nosean-bearing aegirine–augite syenites, quartz syenites, and quartz monzonites. The zoning displayed by the alteration and mineralization comprises: (1) an upper light-colored zone of argillic, carbonate, and pyrite alteration and silicification that is locally associated with pyrite and gold mineralization, (2) a central dark-colored zone of diopside, fluorapatite–magnetite, phlogopite, and garnet alteration associated with fluorapatite–magnetite mineralization, and (3) a lowermost light-colored zone of extensive albite alteration. The Cu–Au and pyrite orebodies are peripheral to the Fe-oxide–apatite deposits in this area and overlie the iron orebodies, including the Meishan Cu–Au deposit in the northern Ningwu basin and the pyrite deposits in the central Ningwu basin. The δ34S values of sulfides from the Fe-oxide–apatite, Cu–Au, and pyrite deposits in the Ningwu basin show large variation, with a mixed sulfur source, including magmatic sulfur and/or a mixture of sulfur derived from a magmatic component, country rock, and thermochemical reduction of sulfate at 200–300°C. The ore-forming fluids associated with iron mineralization were derived mainly from magmatic fluids, and the late-stage ore-forming fluids related to Cu–Au and pyrite mineralization may have formed by the introduction of cooler meteoric water to the system. The Fe-oxide–apatite, Cu–Au, and pyrite deposits of the Ningwu basin formed in an extensional environment and are associated with a large-scale magmatic–hydrothermal system.