A tectono-genetic model for porphyry–skarn–stratabound Cu–Au–Mo–Fe and magnetite–apatite deposits along the Middle–Lower Yangtze River Valley, Eastern China

The Middle–Lower Yangtze River Valley metallogenic belt (YRB), situated along the northern margin of the Yangtze craton, is characterized by porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits in the areas of uplift and magnetite–apatite deposits in Cretaceous fault basins. Following detailed field investigations and a review of published data, we recognize two episodes of magmatism and mineralization in the YRB: 1) 156–137 Ma high-K calc-alkaline granitoids associated with 148–135 Ma porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits and 2) 135–123 Ma shoshonitic series, associated with 134.9–122.9 Ma magnetite–apatite deposits. A-type granitoids and associated alkaline volcanic have a small age range from 126.5 to 124.8 Ma and are temporally, spatially and genetically associated with the second episode. The geodynamic history of the YRB did not experience the Paleozoic to Mesozoic lithospheric thickening that took place in the North China craton. This process is inferred to be linked to partial melting of the delaminated lower crust at high pressures, resulting in the development of C-type adakitic rocks. The petrochemical and Sr/Nd isotopic data show that both the shoshonitic series and A-type granitoids are quite different from adakites, with only some of the K-calc-alkaline granitoids having adakitic signatures. Previous ore genesis models were established based on an assumed relationship with adakites and a continuous tectono-thermal evolution from 150 to 100 Ma.All data obtained for the Middle–Lower Yangtze River region consistently show that the Tan–Lu regional strike-slip fault zone, initiated at 233 ± 6 to 225 ± 6 Ma from the collision between the North China and Yangtze cratons and was reactivated at ca. 160 Ma. The Tan–Lu fault was caused by the oblique subduction of the Izanagi plate, which along the YRB the low-angle subducted slab and the overlying crust was disrupted or broken due to the disharmonious movement of the two blocks. The high-K calc-alkaline granitoids magmas were derived from melting of the subducted slab, with some input of crustal material. These magmas were emplaced at the intersections between NE- and EW-trending faults and formed porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits between 156 and 137 Ma. After 135 Ma the subducted plate changed its direction of motion to northeast, now running parallel to the Eurasian continental margin, and leading to large-scale continental extension. The shoshonitic series and subsequent A-type granitoids magmatism and the development of magnetite–apatite ores in the YRB, took place in both fault basins and NE-trending rifts between 135 and 124 Ma.