Abstract:

On the basis of the insitu melting model of granite genesis, the authors reencoded the geological, geochemical and mineralogical data concerning the Zhongchuan composite granite body (ZCGB) and found that the ZCGB was formed by repeated crustal melting in the earlier Mesozoic. Repeated crustal melting resulted in not only the formation of a homocentric structure and the evolution of chemical composition of the ZCGB, but also the mineralization of different oreforming elements. Relatively high goldbearing rockstrata involved in the earlier Mesozoic melting process of this area led to the enrichment of gold in hydrothermal stage and the formation of numerous gold and polymetal sulfide deposits in the covering strata over the Indosinian and the Early Yansanian granite layers. According to the NNE fracture system developed within the ZCGB and the remnants of hydrothermal activity within the fractures, a remelting event was very likely to occur in the Late Cretaceous to the Early Cenozoic in the lower part of the Early Mesozoic granite layer.  Repeated melting would increase oxygen fugacity f(O2) of magma system, leading U4+ to change into U6+ that would be not able to enter the lattice of silicates and thus be expelled out of the magma system during the hydrothermal stage, forming the “granitetype” uranium veins within the fracture in the old covering granite layers.

Key words: Zhongchuan area of Gansu, composite body, repeated remelting, mineralization