Petrogenesis and mineralization of the Huangshandong mafic-ultramafic intrusion in eastern Tianshan, NW China

The Huangshandong deposit, located in the Central Asian Orogenic Belt (CAOB), is the largest Ni-Cu sulfide deposit in the eastern Tianshan and Beishan regions of China. The deposit is hosted in a postorogenic intrusion that comprises diorite, gabbrodiorite, bojite, olivine gabbro, gabbronorite, troctolite, and lherzolite. Here, we report the Permian zircon U-Pb crystallization ages of 278.5 +/- 2.1 Ma, 278.1 +/- 1.9 Ma, and 279.6 +/- 1.9 Ma for the olivine gabbro, bojite, and gabbrodiorite, respectively, as well as geological, petrological, and geochemical data for the various lithofacies of the deposit. As a result of the edge cooling effect, the intrusion cooled rapidly from the outside inwards as follows: crystallizing diorite -> gabbrodiorite -> gabbro -> lherzolite -> gabbronorite. In addition, further tectonic activity resulted in the injection of a more primitive magma in place, which crystallized the lherzolite in the bottom and middle transitional zones of the intrusion. The crystallization sequence of the intrusion is generally as follows: olivine -> orthopyroxene + labradorite -> clinopyroxene + labradorite -> amphibole + andesine. At the beginning of mineral crystallization, the metalliferous magma was segregated due to sulfide immiscibility and separated by gravity. With gabbronorite crystallization, the immiscible liquid sank to the base of the magma chamber and crystallized as Cu- and Ni-rich sulfides. Tholeiitic and calc-alkaline rocks are characterized by relative enrichment in large-ion lithophile elements and depletion in high-field-strength elements, and values of Th/Ta (2.25-5.94), (Nb/La)(N) (1.88-4.39), and Th/Nb (0.29-0.82) indicate that the mantle source was composed of a dehydrated subducted plate or mantle wedge. In addition, values of Nb/U (2.64-10.68) and Ce/Pb (1.33-3.53) suggest that the magma was contaminated by the continental crust during its ascent along pre-existing structures in the Carboniferous strata.