Geochronology and petrogeochemistry of Miocene porphyries from the Beimulang deposit, western Gangdese copper belt

Abstract

The Beimulang deposit, a newly discovered large porphyry copper deposit, containing 1.3 Mt @ 0.38% Cu, prominently features mineralized orebodies and hydrothermal alteration in the monzogranite porphyry. Zircon U-Pb dating reveals ages of 14.4–14.0 Ma for the monzogranite porphyry and 11.2–11.0 Ma for the granite porphyry. A consistent Re-Os age of 14.1 ± 0.4 Ma from molybdenite suggests a possible link between mineralization and the monzogranite porphyry. The Beimulang Miocene porphyries are rich in SiO2 (70.01–72.81 wt%) and Sr (217–864 ppm) and have high Sr/Y ratios (26–139), which were originated from partial melting of the subduction-modified Tibetan juvenile lower crust. The Beimulang Miocene porphyries show high log w(Fe2O3/FeO) values (>0) in whole rock and high δEu values (mostly > 0.3), high ΔFMQ (4.32–6.43) in zircons, suggesting a high oxidation state. The Beimulang Miocene porphyries show high V/Sc ratios in whole rock and high 10,000 × (δEu)/Y, high (Ce/Nd)/Y, low Dy/Yb ratios (i.e., mostly > 13, > 4, > 0.02 and < 0.3, respectively) in zircons, indicating they are notably hydrated. The granite porphyry has lower εNd (t) (0.70849 – 0.70996) values and higher (87Sr/86Sr)i (-9.3 – -7.8) in whole rock, lower εHf (t) (-5.25 – -2.70) and higher U/Yb (6.0–18.3), δ18O (6.42–7.57‰) in zircon than the monzogranite porphyry (εNd(t) = -5.52 – -4.45; (87Sr/86Sr)i = 0.70744–0.70818; εHf (t) = -3.24 – -0.92; U/Yb = 2.62–8.96; δ18O = 6.10–7.53‰), indicating that the granite porphyry contains fewer juvenile materials compared to the monzogranite porphyry. The high-Mg dioritic rocks in the Beimulang-Zhunuo area reveal significant correlations with monzogranite porphyry rather than granite porphyry in terms of Sr, Sr/Y, Dy/Yb, δEu and SiO2 contents. Taking into account the contemporaneous presence of high-Mg dioritic rocks with the monzogranite porphyry, which are not contemporaneous with the granite porphyry at Beimulang, it can be inferred that these diorite magmas played a significant role in forming the Miocene monzogranite porphyry rather than granite porphyry. Accordingly, the low fertility of the granite porphyry is possibly due to the limited contribution of juvenile melts and lack of the dioritic magmas.