Molybdenite Re–Os, titanite and garnet U–Pb dating of the Magushan skarn Cu–Mo deposit, Xuancheng district, Middle–Lower Yangtze River Metallogenic Belt

摘要

The Magushan skarn Cu–Mo deposit is a representative example of the skarn mineralization occurring within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt of eastern China. The precise age of an ore deposit is important for understanding the timing of mineralization relative to other geological events in a region and to fully place the formation of a mineral deposit within the geological context of other processes that occur within the study area. Here, we present new molybdenite Re–Os and titanite and andradite garnet U–Pb ages for the Magushan deposit and use these data to outline possible approaches for identifying genetic relationships in geologically complex areas. The spatial and paragenetic relationships between the intrusions, alteration, and mineralization within the study area indicates that the formation of the Magushan deposit is genetically associated with the porphyritic granodiorite. However, this is not always the case, as some areas contain complexly zoned plutons with multiple phases of intrusion or mineralization may be distal from or may not have any clear spatial relationship to a pluton. This means that it may not be possible to determine whether the mineralization formed as a result of single or multiple magmatic/hydrothermal events. As such, the approaches presented in this study provide an approach that allows the identification of any geochronological relationships between mineralization and intrusive events in areas more complex than the study area. Previously published zircon U–Pb data for the mineralization-related porphyritic granodiorite in this area yielded an age of 134.2?±?1.2?Ma (MSWD?=?1.4) whereas the Re–Os dating of molybdenite from the study area yielded an isochron age of 137.7?±?2.5?Ma (MSWD?=?0.43). The timing of the mineralizing event in the study area was further examined by the dating of magmatic accessory titanite and skarn-related andradite garnet, yielding U–Pb ages of 136.3?±?2.5?Ma (MSWD?=?3.2) and 135.9?±?2.7?Ma (MSWD?=?2.5), respectively. The dating of magmatic and hydrothermal activity within the Magushan area yields ages around 136?Ma, strongly suggesting that the mineralization in this area formed as a result of the emplacement of the intrusion. The dates presented in this study also provide the first indication of the timing of mineralization within the Xuancheng district. providing evidence of a close genetic relationship between the formation of the mineralization within the Xuancheng district and the Early Cretaceous magmatism that occurred in this area. This in turn suggests that other Early Cretaceous intrusive rocks within this region are likely to be associated with mineralization and should be considered highly prospective for future mineral exploration. This study also indicates that the dating of garnet and titanite can also provide reliable geochronological data and evidence of the timing of mineralization and magmatism, respectively, in areas lacking other dateable minerals (e.g., molybdenite) or where the relationship between mineralization and magmatism is unclear, for example in areas with multiple stages of magmatism, with complexly zoned plutons, and with distal skarn mineralization.