Mantle Plume and Metallization-Controlling in Northwest Hebei An-Ag-Polymetallic Enriched Region
试论冀西北金银多金属矿产富集区地幔热柱及其成矿制约

An enormous thermal dome structure, precisely called mantle plume, has been revealed at the An-Ag-polymetallic enriched area in Northwest Hebei by comprehensive studies of various informations such as geology, geophysics, and geochemistry (see Fig. 1, 2 ). The plume, about 200km in diameter, various strata and structures around it, has oversteped two different tectonic units respectively called inner Mongolian axis and Yanshan platform fold belt. From its centre to periphery, it is characterized by the following evolutions of geological processes: (1) Mesozoic fractures from ductile shear zone to brittle-ductile shear nappe structure zone and to ductile-brittle fracture zone; (2) Mesozoic ultrametamorphism, such as migmatization, granitization and rheologic gneiss, from intensive to weak; (3)Mesozoic magma activities from mantle source magma, resulting alkali basalts and alkalineper-alkaline rocks, to crust-mantle mixture source magma resulting continental rift type of volcanic rocks extensively distributed; (4) Mesozoic known large and superlarge deposits from gold deposits, forming Chongli-Chicheng gold-bearing shear zone, to Agpolymetal deposits, associated with the semicircular ductile-brittle shear nappe structures, that is, a time and spatial distribution pattern of An-Ag-polymetallic deposits characterized by"gold inner and silver outer".Based on the preceding discussions, the paper considers that the An-Ag-polymetallic enriched area in Northwest Hebei may be the ultimate evolution products of the plume in Mesozoic era, and that in the whole evolutional process, massively upwelling of mantle source substances and crustmantle interaction may act as a large enrichment system of An-Ag-polymetal, the enormous dome structure being the trans-ferring system for ore materials, and hypabyssal structures, magmatism and thermal fluid's action making the locating system of ore deposits. Therefore, the three systems of enrichment, trans-ferring, and locating of ore-forming substances may form a crust-mantle metallogenic system or model of the mantle plume.