Eocene arc magmatism and related Cu-Au (Mo) mineralization in the Shangalon-Kyungalon district, Wuntho-Popa Arc, northern Myanmar

Abstract

The Shangalon-Kyungalon Cu-Au (Mo) ore district is located on the south-eastern margin of the Kanzachaung batholith in the Wuntho-Popa Arc, Myanmar. Porphyry and epithermal Cu-Au mineralization are associated with the Late-Eocene magmatic rocks. In this study, zircon U-Pb dating of gabbroic diorite, diorite porphyry, tonalite porphyry, and granodiorite yielded ages of 38.0 ± 0.35 Ma, 37.8 ± 0.35 Ma, 37.4 ± 0.27 Ma, and 36.9 ± 0.26 Ma, respectively. These rocks show sub-alkaline characteristics, LREE-enriched patterns with weakly positive/negative Eu anomalies, enrichment of LILEs and depletion of HFSEs. They have obviously higher initial Sr ratios and lower εNd(t) and εHf(t) values (Isr = 0.7066; εNd(t) = −0.5 to −4.8; εHf(t) = 0.7–9.7) than the mid-Cretaceous Kanzachaung batholith (Isr = 0.7046; εNd(t) = 2.4–4.5; εHf(t) = 10.9–15.1), indicating crustal materials were most likely added to the arc magma source. In particular, the gabbroic diorite has higher initial Sr, more enriched Nd-Hf isotopic compositions (Isr = 0.7081, εNd(t) = −4.6 to −4.8, εHf(t) = 0.7–2.3) and lower SiO2 content relative to the other magmatic rocks studied (Isr = 0.7068–0.7059; εNd(t) = −0.5 to −2.8, εHf(t) = 5.0–9.7). The results suggest that the gabbroic diorite was derived from melting of a mantle wedge, metasomatized by subducted oceanic sediment. Underplating on juvenile lower crust followed, accompanied by MASH processes, which led to increased εNd(t) (and SiO2 content) and decreased initial Sr ratios in later formed rocks. Moreover, the adakite-like features (Sr/Y = 19.3–53.4, Y = 7.54–18.8,) suggest fractional crystallization of hornblende in a hydrous magma, potentially leading to formation of the Shangalon-Kyungalon porphyry-epithermal Cu-Au system. In summary, the Late-Eocene magmas in Shangalon-Kyungalon formed from a mantle source metasomatized by subducted sediments during Neo-Tethyan subduction beneath Western Burma terrane, with the involvement of juvenile crustal materials.