Early Mesozoic Mo mineralization in the Qinling Orogen: An overview

As one of the most important Mo provinces in the world, the Qinling Orogen is host to mainly Late Mesozoic (Late Jurassic–Early Cretaceous) porphyry Mo systems. Recently, additional Early Mesozoic (mainly Triassic plus minor Early Jurassic) Mo deposits have been identified, which occur as fault-controlled quartz veins or as carbonatite veins and as such, they are fundamentally different from the commonly observed porphyry systems. Furthermore, Early Mesozoic porphyry systems are also present but they are different from those of Late Mesozoic age. Thus, Early Mesozoic Mo systems not only offer a new and comprehensive view of the Mo metallogenesis, but also provide unique insight into the tectonic evolution of the Qinling Orogen taking into consideration the role of mineral deposits as ideal probes of the geodynamic evolution. The known Early Mesozoic Mo deposits in Qinling can be classified into three types based on their geological and geochemical features: porphyry, carbonatite, and orogenic systems. Porphyry Mo systems, including porphyry, porphyry-skarn and intrusion-related veins, are spatially and genetically associated with granitic porphyries. They display typical porphyry type alterations and are formed by magmatic water. Carbonatites are considered as mantle-derived igneous rocks, and Mo mineralization is associated with magmatic fluids. Orogenic Mo systems occur as fault-controlled quartz lodes, and the ore-forming fluids are medium-high temperature, CO₂-rich metamorphic fluids. Spatially, Early Mesozoic porphyry Mo systems are concentrated in the western and middle part of the Qinling Orogen, whereas carbonatite veins and orogenic Mo systems are located in the northeastern part. Temporally, orogenic Mo deposits were the earliest (mainly between 220 and 250 Ma), followed by carbonatite veins (mainly between 205 and 225 Ma) and porphyry Mo systems (mainly between 190 and 205 Ma). With some exceptions, there is generally a trend of decreasing deposit ages toward the western part of the orogen. Detailed comparison reveals that the Early Mesozoic porphyry Mo systems in the Qinling Orogen are different from the Late Mesozoic ones in terms of their economic importance, spatial distribution, ore-causative intrusions, hydrothermal alteration and the ore-forming fluids. To explain the above geological and geochemical features, the Triassic Qinling is considered as an active continental margin caused by the northward subduction of the Mian–Lue oceanic plate. A transition from continental arc to back-arc basin is suggested for the formation of Triassic orogenic and carbonatite vein Mo deposits, whereas continental arc is host to the Early Mesozoic porphyry Mo systems.