The formation of the Dabaoshan porphyry molybdenum deposit induced by slab rollback

Abstract

Nanling is the largest W–Sn mineralization belt in the world, the formation of which remains obscure. In contrast to most other deposits in the Nanling region, Dabaoshan is a polymetallic deposit, located in north Guangdong province, southeastern China. Porphyry Mo deposit was found in 2008 in the north part of Dabaoshan ore district. Here we report zircon and molybdenite ages and geochemistry results of zircon and apatite. Zircon U–Pb LA–ICP-MS dating shows that the porphyry Mo deposit formed at 167.0±2.5Ma (2σ), which is identical to the molybdenite Re–Os age for the ore deposit (166±1Ma) within error. These ages are marginally older than the major W–Sn mineralization event in the Nanling region (160±5Ma). Zircon grains associated with the Dabaoshan porphyry Mo deposit have high Ce(IV)/Ce(III) values (356–1300), which indicate high oxygen fugacity, likely associated with plate subduction. Apatite from the Dabaoshan porphyry has high and varied F with low Cl concentrations, suggesting that it formed in a F-enriched environment with high F/Cl components in the magma source. This is consistent with abundant high-F granites in the Nanling region. Chlorine is highly mobile at the early stage of plate subduction. In contrast, F is mainly hosted by minerals that are fairly stable at shallow depths, e.g., apatite, phengite, such that is much less mobile than Cl before phengite decomposition. Therefore, the F/Cl ratio increases with increasing distance from the subduction zone. Compared to the Dexing porphyry deposit to the northeast, the Dabaoshan porphyry has lower Ce(IV)/Ce(III) and high F/Cl. It is also about 5Ma younger than the Dexing porphyry Cu deposits. All these phenomena can be plausibly interpreted by slab rollback of the obliquely subducted Pacific plate in the Jurassic. We propose that the subducting slab reached the Dabaoshan region before ~167Ma, through a “flat” subduction regime, resulted in high oxygen fugacity in the magmas, which is followed by abrupt slab rollback. The onset of slab rollback is the most favorable event that leads to decomposition of phengite in the subducting slab because of abruptly elevated temperatures, releases F and consequently increases F/Cl in magmas. The Ti-in-zircon temperatures of the mineralized porphyry range from 608 to 641°C, with an average of about 630±15°C, whereas average temperature of the barren granodiorite nearby is 700±9°C. Considering that the Mo porphyry deposit appears as an egg shell surrounding the granodiorite and the geochemical behavior of Mo, we propose that the mineralization was related to the assimilation of metamorphosed sedimentary rocks by the granodiorite.