Key factors controlling the late Mesozoic diverse porphyry Mo, Cu, and Au mineralization in the Qinling Orogenic Belt, Central China

Abstract The Qinling Orogenic Belt of Central China hosts one of the world-class porphyry Mo belts (proven Mo resource of 8.43 Mt), several porphyry-skarn Cu and Au deposits, and abundant coeval barren granitoid intrusions. The mineralized porphyry systems of the Late Jurassic to Early Cretaceous periods are the major contributors to the metal resources that are distributed in the eastern segment of the Qinling Orogenic Belt. The key factors controlling the fertility and diversity of the porphyry are not well understood, such as the magma source, magmatic water content, magma oxidation state, sulfur and halogen values, and the nature of the magma chamber, among other aspects. The fertile porphyries of Mo and Au deposits are characterized by enriched Sr-Nd-Hf isotopes, relatively low δ18O values of 4.58‰–8.65‰, abundant inherited zircons from the Neoarchean to Paleozoic periods in age, and high apatite MnO contents. These conditions indicate mixed magma sources from partial melting of the Neoarchean to Paleoproterozoic lower crust, subducted continental crust of the Yangtze Block, and the metasomatized lithospheric mantle. The Cu (-Mo) ore-forming porphyries have depleted isotopic features and a distinct magma source derived from partial melting of the Triassic juvenile lower crust with significant contributions of mantle-derived mafic magma. Lithospheric architecture and magma source are considered to be the primary controlling factors for the diverse Cu (-Mo), Mo (-W), and Au mineralization in the Qinling Orogenic Belt. Chlorine and fluorine contents also show contrasts between Cu-related and Mo-related porphyries, with the former being Cl-rich, which is favorable for extraction and transportation of Cu, and the latter is F-rich, which is crucial for porphyry Mo mineralization by lowering viscosity. This process promotes crystal fractionation and partitioning of Mo into fluid. The fertile porphyries in the Cu (-Mo) deposits have adakite-like Sr/Y ratios and high magmatic water content involving early amphibole fractionation and the suppression of plagioclase fractionation. The less-evolved porphyry Mo (-W) magma shares geochemical features similar to those of the porphyry Cu (-Mo) and Au magmas, whereas the highly evolved porphyry Mo (-W) magma shows moderately high water content that cannot suppress plagioclase fractionation. The diverse porphyry Mo (-W) and Au mineralization with comparable magma sources is controlled by the higher magmatic water content and oxidation state of the porphyry Au magma. The barren suites show overlapping Sr-Nd-Hf-O isotopes, whole-rock Fe2O3/FeO ratios, zircon ΔFMQ and Ce4+/Ce3+ values, and apatite SO3 contents with Mo-, Au-, or Cu-fertile porphyries. The flat roofs of the barren batholiths that inhibit fluid focused into a small volume could account for the barren porphyries, whereas the highly evolved late phases with favorable geometry are conducive to porphyry mineralization potential in the region.