Metasomatic alteration of volcanic-sedimentary terrane in the Early Devonian: Implication for the formation of the Mengxi porphyry copper deposit, eastern Junggar, Central Asian Orogenic Belt

The Mengxi porphyry copper deposit, situated in the eastern Junggar, Central Asian Orogenic Belt, is not a typical porphyry copper deposit, and the origin of the deposit has been a topical debate for a decade. The ore bodies of the Mengxi porphyry copper deposit mainly developed in the volcanic-sedimentary terrane (Huangcaopo Group) near a granite porphyry, with strong wall-rock alteration as well as vertical zonation and fracture-related vein fillings. The result of zircon U-Pb dating indicates that the granite porphyry was emplaced at 432.6 +/- 6.7Ma. The granite porphyry has relatively low initial Sr-87/Sr-86 ratios (0.70465-0.70537), high whole-rock ?Nd(t) values (-2.2 to 6.6), and similar geochemical affinities of adakites and Andean island arc volcanic rocks, indicating that the Mengxi granite porphyry was derived from the partial melting of the upper mantle modified by slab-derived fluids, with negligible assimilation of crust. Pyrites from the porphyry and volcanic-sedimentary terrane have a similar S-34 and Pb isotopic compositions, indicating that different pyrites may have similar source. Based on the detailed field investigation, observed mineral assemblages, and crosscutting relationships, massive fine anhedral chalcopyrite and bornite disseminated in the Huangcaopo Formation, chalcopyrite as inclusion in pyrite veins in the contact zone, and the amount of Cu-bearing minerals increased from terrane to veins have been recognized. All these observations reveal that the ore-forming materials of the Mengxi copper deposit were mainly derived from the metasomatic alteration of the Huangcaopo Group triggered by the extensive Early Devonian tectonic-magmatic events.