Prolonged basaltic magmatism and short-lived magmatic sulfide mineralization in orogenic belts

Abstract

Economically important Ni-Cu sulfide deposits occur in orogenic belts worldwide. However, the duration of timing and geological processes critical for generation of high-Mg basaltic magma and Ni-Cu sulfide mineralization during orogeny have not been well addressed. Here, we report a detailed geochronological study on the Huangshan-Jingerquan Ni-Cu metallogenic belt, NW China. The complexes were regarded as the results of a prolonged basaltic magmatism from 380 Ma to 270 Ma according to previous zircon U-Pb ages. Most of the zircons for the previous age dating were separated from mafic facies and only a few from ultramafic rocks. However, the Ni-Cu sulfide ore bodies are mostly hosted within the ultramafic facies. Thus, there are debates on tectonic setting of the magmatism associated with the Ni-Cu sulfide mineralization. In this study, the zircons separated from the sulfide-mineralized mafic/ultramafic facies of eleven complexes display U-Pb ages ranging from 285 to 280 Ma, indicating a short-lived high-Mg basaltic magmatism associated with the Ni-Cu mineralization. It is likely that the olivine-free gabbros formed in 380–300 Ma and 280–270 Ma occurred in subduction and post-collisional periods, respectively. Collision combined with regional strike-slip shearing at the latest Carboniferous induced slab break-off and asthenosphere upwelling in early Permian. Decompression melting of the upwelling asthenosphere was extensively intensified by addition of water released from the broken slab and generated voluminous high-Mg basaltic magma beneficial to the short-lived Ni-Cu sulfide mineralization in 285–280 Ma. The regional strike-slip shearing created fracture networks as pathways for magma ascending. The sulfides carried by the ascending basaltic magma deposited in the fractures and magma chambers in different depths and produced the sulfide-mineralized dykes and complexes, respectively.