Contrasting Tectonomagmatic Conditions for Coexisting Iron Oxide-Apatite Deposits and Porphyry and Skarn Cu ± Au Deposits in the Middle-Lower Yangtze River Metallogenic Belt, China

Abstract Porphyry Cu ± Mo ± Au and iron oxide-apatite (IOA) deposits rarely occur in spatial and temporal proximity in Phanerozoic arc-related settings, and the formation of these mineral deposit types in an evolving arc setting remains poorly understood. Specifically, the roles of magma composition and the tectonic regime remain the subject of some debate. Here, we systematically estimated the P-T-fO2 conditions and H2O-S-Cl contents for dioritic to granodioritic source magmas for porphyry and skarn Cu ± Au (150–135 Ma) and IOA deposits (~130 Ma) that formed in transpressional and transtensional settings in the Middle-Lower Yangtze River metallogenic belt, China. Our estimates show that, compared to IOA deposits, the porphyry- and skarn-related magmas were relatively felsic, cooler, and more hydrous. These geochemical features are consistent with the tectonic transition from subduction to slab rollback of the paleo-Pacific plate in the East Asia continental margin at <135 Ma and concomitant crustal extension and steepening of the regional geothermal gradient. Apatite data reveal that the silicate melts associated with the porphyry and skarn Cu ± Au and IOA deposits had comparable predegassed S concentrations (~0.13 ± 0.06 wt % vs. ~0.16 ± 0.09 wt % on average), but that IOA-related melts contained higher predegassed Cl/H2O ratios (~0.11 ± 0.03 vs. ~0.04 ± 0.03 for porphyry- and skarn-related magmas) that decreased by one order of magnitude after magmatic degassing. Magmatic fO2 estimated using zircon and amphibole, reported in log units relative to the fayalite-magnetite-quartz (FMQ) redox buffer, gradually increased during cooling of the porphyry- and skarn-related magmas (ΔFMQ +0.7 to +2.5) at 950° to 800°C and decreased to ΔFMQ +1 at 700°C owing to fractionation of Fe2+-rich minerals and subsequent S degassing, respectively. In contrast, the magmatic fO2 values for the IOA-related source magmas varied significantly from ΔFMQ –1.5 to ΔFMQ +2.5 but generally show an increasing trend with cooling from 970° to 700°C that probably resulted from variable degrees of evaporite assimilation, fractionation of Fe2+-rich minerals, and Cl degassing. These results are consistent with the hypothesis that Cl enrichment of the IOA-related source magmas played a determinant role in their formation. We propose that the porphyry and skarn Cu ± Au deposits in the Middle-Lower Yangtze River metallogenic belt formed in a transpressional setting in response to paleo-Pacific flat-slab subduction that favored storage and evolution of S-rich hydrous ore-forming magmas at variable crustal levels. A subsequent extensional setting formed due to slab rollback, leading to rapid degassing of Cl-rich IOA-related magmas. For the latter scenario, assimilation of evaporite by mafic to intermediate magmas would lead to an enrichment of Cl in the predegassed magmas and subsequent exsolution of hypersaline magmatic-hydrothermal fluid enriched in Fe as FeCl2. This Fe-rich ore fluid efficiently transported Fe to the apical parts of the magma bodies and overlying extensional normal faults where IOA mineralization was localized. The concomitant loss of S, H2O, and Cu with Cl by volcanic outgassing may have inhibited sulfide mineralization at lower temperatures.