Origin of the Eocene porphyries and mafic microgranular enclaves from the Beiya porphyry Au polymetallic deposit, western Yunnan, China: Implications for magma mixing/mingling and mineralization

Abstract

The origin of magmas with high H2O, S and metals that are linked to porphyry economic mineralization in continental collisional belts is controversial. In this paper, we studied the host porphyries and associated microgranular enclaves (MMEs) including gabbroic enclaves and dioritic enclaves from the large Beiya porphyry Au deposit in Jinshajiang-Ailaoshan mineralization belt, which provide first hand evidence of crust–mantle hybridization in the formation of fertile magma for porphyry deposit. Zircon LA-ICPMS U–Pb ages indicate coeval formation for the MMEs and the felsic porphyries in Eocene time (~36Ma). The MMEs show spheroidal shapes, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclases, which suggest that the MMEs are globules of a more mafic magma that was injected into and mingled with the host felsic magma. One gabbroic enclave sample exhibits high MgO (5.6wt.%), high TiO2 (1.7wt.%), enriched in large-ion lithophile element (LILE) and depleted in high field strength element (HFSE), with negative Nb, Ta and Ti anomalies. This indicates that the gabbroic enclave is similar to those of the coeval potassic mafic rocks in the Western Yunnan, which originated from the enriched mantle metasomatized by the slab-derived fluids. The dioritic enclave exhibit both potassic and adakitic-like features, and are characterized by high contents of MgO (4.4–4.9wt.%), K2O (6.4–7.4wt.%), and compatible trace elements (e.g. Ni: 115–142ppm; Cr: 214–291ppm), as well as by high Sr/Y ratios. The dioritic enclave samples yield εHf (t) values (−1.9 to +3.6) similar to those of the host porphyries. Petrographic, elemental, and isotopic evidence suggests that the Beiya dioritic enclave were formed by mixing between potassic mafic and adakitc-like melts, derived from metasomatized lithospheric mantle and lower crust, respectively. In contrast, the Beiya host porphyries are characterized by high SiO2 (66.9–71.9wt.%) and Al2O3 (13.8–15.9wt.%) contents, high Sr/Y ratios (35–79), low compatible element contents (e.g. Ni=0.9–7.2ppm; Cr =1.0–8.2ppm), low Mg# values (0.19–0.35), positive LILE anomalies, marked negative high HFSE anomalies, negative εNd(t) values (−6 to −2), and high (87Sr/86Sr)i values (0.7071–0.7081). These features indicate that the host porphyries were mainly formed by partial melting of subduction-modified lower crust with a certain assimilation of mafic melts represented by MMEs. MMEs hosted in the Beiya porphyries, formed from a hydrous, mafic magma although they formed in a non-arc setting. High magmatic water content is probably related to an enriched mantle metasomatized by fluids from a Proterozoic subducted oceanic slab. High Cu (341–626ppm) and Au contents of the MMEs suggest they crystallized from a metal-rich mafic melt. Complex assemblages of pyrite–chalcopyrite–apatite–rutile with hornblende are observed in the least altered MME. We suggest that the mafic melt probably supplied part of water, and metal to the Beiya porphyry Au system.