An Early Cretaceous carbonate replacement origin for the Xinqiao stratabound massive sulfide deposit, Middle-Lower Yangtze Metallogenic Belt, China

Abstract

Stratabound massive sulfide deposits are widespread along the Middle-Lower Yangtze Metallogenic Belt (MLYMB) and serve as an important copper producer in China. Two contrasting genetic models have been proposed, interpreting the stratabound massive sulfide deposits as a Carboniferous SEDEX protore overprinted by Cretaceous magmatic-hydrothermal system or an Early Cretaceous carbonate replacement deposit. These two contrasting models have been applied to the Xinqiao stratabound Cu-Au sulfide deposit, which is dominated by massive sulfide ores hosted in marine carbonates of the Carboniferous Chuanshan and Huanglong Formations, with minor Cu-Au skarn ores localized in the contact zone between the Cretaceous diorite Jitou stock and the Carboniferous carbonate rocks. New SIMS zircon U-Pb dating suggests that the Jitou stock formed at 138.5±1.1Ma (2σ, MSWD=0.6). Pyrite Re-Os dating yields an imprecise date of 142±47Ma (2σ, MSWD=7.8). The geochronological data thus constrain the mineralization of the Xinqiao deposit at Early Cretaceous.Fluid inclusions in prograde skarn diopside have homogenization temperatures of 450–600°C and calculated salinities of 13–58wt.% NaCl equiv. Quartz from the stratabound ores and pyrite-quartz vein networks beneath the stratabound ores have homogenization temperatures of 290–360 and 200–300°C, with calculated salinities of 5–12 and 2–10wt.% NaCl equiv., respectively. Quartz from the skarn ores and veins beneath the stratabound ores have δ18O values of 12.32±0.55 (2 SD, n=22) and 15.57±1.92‰ (2 SD, n=60), respectively, corresponding to calculated δ18O values of 6.22±1.59 (2σ) and 6.81±2.76‰ (2σ) for the equilibrated ore-forming fluids. The fluid inclusion and oxygen isotope data thus support a magmatic-hydrothermal origin rather than a SEDEX system for the stratabound ores, with the hydrothermal fluids most likely being derived from the Jitou stock or associated concealed intrusion. Results from this study have broad implications for the genesis and exploration of other stratabound massive sulfide deposits along the MLYMB.