Genesis of Makeng-type Fe-polymetallic deposits in SE China: New constraints by geochronological and isotopic data from the Dapai–Makeng metallogenic system

The southwestern Fujian depression belt (SFDB) is an economically important Mesozoic Fe metallogenic belt in South China and is renowned for its Makeng-type Fe deposits, in which stratified skarn Fe orebodies generally occur in or near the contact zone between late Paleozoic carbonate sequences and Mesozoic granites. However, the genesis and geodynamic setting of these deposits remain unclear because the characteristics of the widely distributed Pb–Zn–Cu and Mo orebodies in these deposits and the temporal, spatial, and genetic relationships between magmatism and mineralization are poorly defined. The Dapai Fe polymetallic deposit in the SFDB is a typical example of Makeng-type Fe deposits but also has regional significance, whereby the stratified skarn Fe orebodies have overprinted the stratabound Pb–Zn–Cu mineralization followed by final fissure-filling by vein-disseminated Mo mineralization. A detailed geological investigation suggests that episodic magmatic–hydrothermal events were involved in the formation process of the Dapai Fe polymetallic deposit.Pyrite and sphalerite from the Pb–Zn–Cu orebodies yield an Rb–Sr isochron age of 175.5 ± 3.3 Ma, which is regarded as the timing of Pb–Zn–Cu mineralization. Zircon grains from Fe-mineralized granodiorite porphyry and Mo-mineralized monzogranite yield weighted-mean ²⁰⁶Pb/²³⁸U ages of 146.3 ± 0.9 Ma and 131.7 ± 0.4 Ma, interpreted as the timings of Fe and Mo mineralization, respectively. Six zircons from granodiorite also yield a ²⁰⁶Pb/²³⁸U model age cluster of ~184 Ma, which coincides reasonably with the timing of Pb–Zn–Cu mineralization and implies the existence of an unidentified ore-related intrusion in the Dapai deposit. Five further zircons from the porphyritic granodiorite yield an age cluster of ~150 Ma, consistent with the timing of Fe mineralization. Galena, pyrite, and sphalerite from the Dapai and Makeng deposits have similar S–Pb isotopic compositions and suggest a magmatic source. Combining our results with published isotopic data for the SFDB, we suggest that the Pb–Zn–Cu mineralization in this area was derived from crustal magmas that mixed with mantle-derived magma prior to emplacement. The δ⁵⁶Fe and δ⁵⁷Fe values of magnetite from Dapai and Makeng are both slightly lower than those of the ore-related granites, suggesting that Fe in the initial fluid in both deposits was derived mainly from coeval granitic rocks. The Fe isotopic variation between intrusions and skarn Fe orebodies is interpreted as resulting from mass fractionation that occurred during fluid exsolution from melt. Contents of Re in molybdenite from published data for the SFDB indicate crust–mantle mixed sources of Mo and Re. The Makeng-type Fe polymetallic deposits formed as a result of three magmatic–hydrothermal episodes, generating Pb–Zn–Cu mineralization at 185–160 Ma, Fe–Mo mineralization at 150–140 Ma, and Mo–Fe mineralization at 135–130 Ma. The different metal associations formed during multiple stages of magmatism caused by ongoing subduction and rollback and/or retreat of the paleo-Pacific Plate.