New insights on the formation of the Jingchong Cu-Co-Pb-Zn deposit, South China: Evidence from sphalerite mineralogy and muscovite ⁴⁰Ar-³⁹Ar dating

The northeastern Hunan Province hosts numerous hydrothermal Pb-Zn(-Cu-Co) polymetallic deposits. As a representative example, the Jingchong Cu-Co-Pb-Zn deposit is characterized by the lower Cu-Co and upper Pb-Zn mineralization, whereas little is known about Pb-Zn mineralization. In this study, three generations of sphalerite were identified based on their textural and geochemical differences. The Sph-I exhibits the oscillatory zoning that consists of reddish-brown Sph-Ia (poor in chalcopyrite inclusion) alternating with dark Sph-Ib with zoned chalcopyrite inclusion. Sph-II is composed of honey-brown Sph-IIa (abundant chalcopyrite droplets) and white clean Sph-IIb (rare chalcopyrite inclusion). The black Sph-III is characterized by nano-to submicron-sized chalcopyrite inclusions with typical "dusting" or "watermelon" texture and crosscuts all other sphalerite gen-erations in veinlets. The electron microprobe and laser ablation inductively coupled plasma mass spectrometry show that Sph-I has higher Fe and Mn contents, but lower Cd, Cu and Ag contents than Sph-II and Sph-III. Sphalerite geothermometry yields temperatures of 334-346 (+/- 58)degree celsius for Sph-I, 254-289 (+/- 60)degree celsius for Sph-II and 286 (+/- 55) degree celsius for Sph-III. The sulfur fugacity ranges from logfS(2) values of-9.03 to-8.26 for Sph-I to-11.77 to-10.63 for Sph-II and-10.82 for Sph-III. The combined textural features and chemical compositions indicate that the self-organized mechanism forms Sph-I, and that the coupled dissolution and precipitation re-actions triggered by the influx of Cu-elevated fluids are responsible for the formation of Sph-II and Sph-III. The associated pyrite and pyrrhotite inclusions in sphalerite are produced by the exsolution mechanism, while chalcopyrite inclusions are formed by co-precipitation due to local supersaturation at the interface of sphalerite with fluid. The Ar-40-Ar-39 dating of muscovite in the Jingchong deposit yields a mineralizing age of ca. 121.1 +/- 2 Ma, consistent with the ca. 130-120 Ma Pb-Zn mineralizing events in the northeastern Hunan Province. The sulfur isotopic values (-3.0 to +3.5 parts per thousand) of the Pb-Zn ores are similar to that of Cu-Co ores, indicating a magmatic sulfur origin. Together with the trace element affinity of sphalerite with magmatic-hydrothermal origin, it was proposed that the Jingchong Pb-Zn and Cu-Co mineralization were formed in the same magmatic-hydrothermal system. The placement of Pb-Zn orebodies at the upward zoning of Cu-Co orebodies could be attributed to the higher solubilities of Pb and Zn chloride complexes in hydrothermal fluids, relative to Cu chloride complex.