Genesis of the Datuanshan stratabound skarn Cu(–Mo) deposit, Middle–Lower Yangtze Valley, Eastern China: constraints from geology, Re–Os geochronology, mineralogy, and sulfur isotopes

Abstract

Stratabound deposits are the most abundant and economically significant ore type in the Middle–Lower Yangtze River Valley, one of the most important metallogenic belts in China. The Datuanshan deposit is one of the largest and most representative stratabound Cu(–Mo) deposits in the Tongling district of the Middle–Lower Yangtze River metallogenic belt. All the orebodies of the Datuanshan deposit occur around Mesozoic quartz monzodiorite and are tabular or semi-tabular bodies along bedding-parallel faults within upper Permian to Lower Triassic strata. However, discordant and crosscutting relationships (e.g., the host rocks crosscut by skarn– and quartz–sulfide veins, with alteration halos around the veins) have also been found, especially along the skarn–host contact and orebody–host contact, indicating that skarnitization and mineralization postdated the deposition of the host sediments. The skarn consists mainly of prograde garnet and pyroxene and retrograde alteration assemblages of amphibole, epidote, and chlorite, as well as quartz and sulfides. Electron microprobe analyses show that the garnets and pyroxenes are grossular–andradite and hedenbergite–diopside series, respectively, and all samples plot in the field of typical skarn copper deposits worldwide. Molybdenite samples from stratiform copper ores yield Re–Os model ages of 138.2–139.9 Ma with a weighted mean age of 139.2 ± 0.9 Ma. This is reasonably consistent with the ages of the stratiform Mo ores (138.0–140.8 Ma) and genetically related quartz monzodiorite (135.2–139.3 Ma) in the Datuanshan deposit, indicating that the stratiform Cu and Mo mineralization was contemporaneous with emplacement of the quartz monzodiorite magmas in the Early Cretaceous. Fifteen δ34S values for sulfides range from −1.8 to +4.7 ‰, with a mean of 0.5 ‰, indicating that the sulfur was derived mainly from a magmatic source. Moreover, the sulfur isotope values of the ores are consistent with those of Mesozoic intermediate–acid intrusions but are different from those of sediments in the Shizishan orefield. Based on these lines of evidence, we conclude that the Datuanshan stratabound Cu(–Mo) deposit is the result of replacement related to Mesozoic magmatic rocks and is not a product of submarine exhalative sedimentary processes.