Formation of the giant Chalukou porphyry Mo deposit in northern Great Xing'an Range, NE China: Partial melting of the juvenile lower crust in intra-plate extensional environment

Abstract

Abstract The Chalukou porphyry Mo deposit (2.46 Mt @ 0.087% Mo), located in the northern Great Xing'an Range, NE China, is the largest Mo deposit discovered in China so far. The host rocks consist of aplite porphyry, granite porphyry and quartz porphyry, and are intruded into Lower Ordovician intermediate-felsic volcanic–sedimentary rocks and pre-ore monzogranite and are cut by post-ore feldspar porphyry, diorite porphyry and quartz monzonite porphyry. Here, we present the zircon U–Pb ages, whole-rock geochemistry, Sr–Nd isotopic and zircon Hf isotopic data for the pre-ore, syn-ore and post-ore intrusive rocks. The Chalukou ore-forming porphyries intruded during 147–148 Ma and have high-silica, alkali-rich, metaluminous to slightly peraluminous compositions and are oxidized. They are enriched in large ion lithophile elements (e.g. K, Rb, U and Th), light REE and depleted in high-field strength elements (e.g. Nb, P and Ti). Depletions in Eu, Ba, Sr, Nb, Ta, P and Ti suggest that they have experienced strong fractional crystallization of plagioclase, biotite, hornblende and accessory minerals. The pre-ore monzogranite (~ 172 Ma) also belongs to the high-K calc-alkaline series. Highly fractionated REE patterns ((La/Yb) N = 19.6–21.7), high values of Sr/Y (54–69) and La/Yb (29–32), are adakite-like geochemical features. The post-ore rocks (~ 141–128 Ma) have similar geochemical characteristics with ore-forming porphyries except that quartz monzonite porphyry shows no Ba–Sr negative anomaly. All intrusive rocks have relative low initial 87Sr/86Sr (0.705413–0.707889) and eNd (t) values (− 1.28 to + 0.92), positive eHf (t) values (+ 2.4 to + 10.1) and young two-stage Nd and Hf model ages (TDM2 (Nd) = 863–977 Ma, TDM2 (Hf) = 552–976 Ma). These geochemical and isotopic data are interpreted to demonstrate that the ore-forming porphyries formed by partial melting of the juvenile lower crust caused by underplating of mafic magmas in an intra-plate extensional setting. The pre-ore monzogranite formed by partial melting of thickened lower crust in a collisional setting caused by closure of Mongol–Okhotsk Ocean. The post-ore feldspar porphyry shares a similar magma source with ore-forming porphyry, but the quartz monzonite porphyry has a relatively deeper magma source region and has not experienced as much fractional crystallization. The transformation from middle Jurassic compression to late Jurassic extension created favorable conditions for the generation and emplacement of the ore-forming magma. The juvenile lower crust provided the main source of molybdenum for Chalukou deposit. Enrichment of Mo by fractional crystallization played an important role in concentrating Mo during formation of the Chalukou Mo deposit. The age (~ 147 Ma), high fluorine, and associated Pb–Zn deposits are all different from other major porphyry Mo deposits in NE China; Chalukou is a new mineral deposit type in the Great Xing'an Range.