Magma mixing origin for the Aolunhua porphyry related to Mo–Cu mineralization, eastern Central Asian Orogenic Belt

Abstract

Many Cu–Mo–Au deposits are considered to be related to adakitic porphyries formed in non-arc settings, e.g., in collisional orogenic zones and intra-plate environments, but their genesis is still under discussion. The Aolunhua porphyry complex and its related Mo–Cu deposit from the eastern Central Asian Orogenic Belt (CAOB) provide important insights into this issue. The porphyries are characterized by high Sr (496–705ppm) and Sr/Y and La/Yb ratios similar to those of typical adakitic rocks, and low ISr ratios (0.7049–0.7052) and positive εNd(t) (+0.5 to +1.4) and εHf(t) (+3.5 to +9.8) values. These features, along with the occurrence of mafic microgranular enclaves (MMEs), compositional and textural disequilibrium of plagioclase phenocrysts and relatively high Mg# values (45–52), indicate that they were derived from mixing of felsic magma from partial melting of a juvenile arc-type lower crust and mafic magma from a lithospheric mantle previously metasomatized by subduction zone fluids/melts. High Sr/Y and La/Yb ratios are indicative of contribution from enriched mantle-derived materials (with high LILEs; e.g., Sr, La), which were strengthened by subsequent fractionation of ferromagnesian phases such as pyroxene and hornblende. MMEs hosted by the ore-bearing porphyry have zircon U–Pb ages of ca. 132Ma, similar to those of the host rocks. The enclaves have elevated Mg# (56–63), LILEs (e.g., Sr=660–891ppm), LREE (LaN=68–150, (La/Sm)N=3.0–4.0, (La/Yb)N=12.0–19.6) and ratios of radiogenic isotopes of Nd- and Hf (εNd=+0.7 to +1.6; εHf=+3.3 to +10.9), suggesting that their parental magmas were derived from the metasomatized mantle source. The Mo–Cu mineralization was probably related to the high water content, high oxygen and sulfur fugacity of hybrid magma. Formation of the adakitic porphyries and related Mo–Cu deposits of the eastern CAOB could be related to the Early Cretaceous lithospheric extension, caused by the subduction of the Paleo-Pacific plate and its induced reactivation of juvenile arc-type lower crust.