斑岩型钼矿床的形成机制与地球化学过程

斑岩型钼矿床是世界钼矿床中最重要的种类,其中90%以上的钼矿床都和斑岩有关。斑岩型钼矿床主要分布于环太平洋成矿带和特提斯成矿带上,主要与板块俯冲过程有关,可以分为斑岩铜钼矿床、高氟型斑岩钼矿床和低氟型斑岩钼矿床。我们通过对全球斑岩型钼矿床的时空分布与钼元素地球化学性质分析,认为斑岩型钼矿床的物质来源是钼元素通过表生地球化学作用进行初始富集后形成的富钼沉积物。新元古代晚期(750～542Ma)大气氧再次升高之后,富钼的黑色页岩等才大量出现,因此斑岩型钼矿床主要形成于500Ma之后。富钼黑色页岩等沉积物在板块俯冲过程中脱水,形成富含Mo和Re的变质流体,同时两者发生分异。这种变质流体交代上覆地幔楔使Mo和Re留存在其中。随着俯冲洋壳的部分熔融,形成富Cu(Au)的岩浆,穿过富含Re(Mo)的上覆地幔楔,形成斑岩型铜钼矿床,因此这类矿床的辉钼矿Re含量更高。而随后出现的板块后撤,使软流圈上涌,板片上大量多硅白云母分解,形成了富含F的岩浆,穿过富含Mo的上覆地幔楔,进而形成高氟型斑岩矿床。低氟型钼矿床很可能与俯冲关系较小,富钼沉积物通过造山过程被深埋,在适当的条件下形成低氟型斑岩钼矿床。

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.