Geological and geochemical characteristics and geological implications of the Mengxi porphyry copper-molybdenum deposit in East Junggar region

Abstract：The Mengxi porphyry copper-molybdenum deposit is located in the Beitashan- Qiongheba copper, nickel, gold and iron ore district of the Junggar-Southern Mongolia metallogenic belt within the Paleo-Asian oceanic metallogenic domain. Based on an analysis of the geological background as well as geological and geochemical features of this region, the authors hold that the ore-bearing porphyry consists mainly of Middle-Late Silurian intermediate-acid rocks, with the wall rocks being Middle-Upper Ordovician Huangcaopo Group (O2-3hcq), and the ore deposit is an Early Paleozoic porphyry copper deposit instead of a Late Paleozoic porphyry copper deposit considered by previous researchers. The ore-forming process can be divided into three stages, i.e., the pyrite-quartz stage, the chalcopyrite-pyrite-quartz stage, and the carbonate rock-quartz stage. Metallic mineral ores include more than 70% pyrite, 15% chalcopyrite, and a small amount of magnetite and molybdenite. Secondary minerals are malachite, limonite, and blue copper ore. The mineral assemblages are mainly of medium-low temperature. Element geochemical features show that rocks of the copper-molybdenum deposit are Al-high, Na-high calc-alkaline granite relatively enriched in ion lithophile elements Sr, K, Rb, Ba, Th and depleted in high-field strength elements Nb, Ta, Hf, Ti. The discrimination diagrams of (Yb + Nb)-Rb and Y-Nb show that the rocks were formed in a volcanic arc. The (R1-R2) structural diagram suggests syn-collisional granite. It is thus concluded that the main intrusive rocks in the region resulted from the participation of volcanic arc crustal collision in the tectonic setting, and that the structure in the Mengxi porphyry copper-molybdenum deposit is a syn-collisional continental island arc.