Formation mechanism and geochemical process of porphyry molybdenum deposits

Porphyry Mo deposits are the most important type of Mo deposits, which account for over 90% of Mo reserves in the world. Porphyry Mo deposits can be classified as porphyry Cu-Mo deposits, high-fluorine porphyry Mo deposits and low-fluorine porphyry Mo deposits. They are mainly distributed on the convergent plate margins such as the circum-Pacific metallogenic belt, and closely related to plate subduction. Based on the statistical analysis of the spatial and temporal distribution of global porphyry Mo deposits and the geochemical properties of element Mo, we believe that the material source of porphyry Mo deposits is Mo-rich sediments formed by initial enrichment of Mo element through supergene geochemical processes. Since the Mo-rich black shale appeared in large quantities after the rise of atmospheric oxygen around 550Ma, the porphyry Mo deposits were mainly formed after 500Ma. During the process of plate subduction, sediments such as Mo-rich black shale dehydrated and form metamorphic fluids rich in Mo and Re. Then the metamorphic fluid metasomatized the overlying mantle wedge to retain Mo and Re. With the partial melting of the subduction oceanic crust, magma rich in Cu ( Au) was formed and passed through the overlying mantle wedge and led to the formation of porphyry Cu-Mo deposits. In the process of plate retreat, the upwelling of asthenospheric mantle would cause the decomposition of polysilicate muscovite. The magma then passed through the overlying mantle wedge rich in Mo, forming high fluorine porphyry deposits. However, the formation of low fluorine molybdenum deposits is likely to have little relationship with plate subduction. Sediments rich in molybdenum formed by high chemical weathering would accumulate in areas such as paleo back arc basin, and then deeply buried during orogeny, forming low fluorine porphyry molybdenum ore under appropriate conditions.