Magmatic and metallogenic processes of the Zegulangbei deposit in the Jiama ore district: Insights from zircon and apatite geochemistry

Explanation: On account of the positive correlation between T Ti and Ti content of zircon, the crystallization temperature of zircon is an effective proxy for the trace element variations of zircon as a function of temperature. The Hf content in zircon increases with cooling of melt, indicating that it is an effective indicator for the crystallization of zircon. • Zircon and apatite geochemistry can be used to reveal the magmatic and metallogenic processes. • The input of mafic magma brings lots of heat, Cu and S content to the ore-bearing magma of Zegulangbei, and increases the oxygen fugacity. • The magma rich in F, Cl and S, high oxygen fugacity and input of mafic magmas were key factors in the formation of Zegulangbei deposit. Jiama is an exceptionally large colliding porphyry copper polymetallic system in the post-collisional Gangdese porphyry copper belt, Tibet. The Zegulangbei deposit is a recent discovery located within the Jiama system, but the magmatic and metallogenic processes that formed it are unclear. This research aims to enhance the understanding of the physicochemical conditions of magmatic and metallogenic processes within Zegulangbei using zircon and apatite geochemistry and zircon Hf isotopes. From oldest to youngest, host rocks at Zegulangbei include monzogranite porphyry (MGP), I-quartz diorite porphyry (I-QDP), granodiorite porphyry (GDP), diorite, II-quartz diorite porphyry (II-QDP), III-quartz diorite porphyry (III-QDP), and diorite porphyry (DP). Numerous mafic microgranular enclaves (MME) are observed in the MGP and GDP. Apatites in MGP, I-QDP, GDP, II-QDP and DP samples are enriched in LREEs with negative Eu anomalies, whereas those in the MMEs and diorite samples show no obvious differences between LREEs and HREEs. The MGP, I-QDP, GDP, II-QDP, and DP samples give low Ti-in-zircon thermometry (T Ti ) values ranging from 601 °C to 818 °C and high apatite saturation temperatures (ASTs) ranging from 839 °C to 1034 °C, indicating that apatite formed early during crystallization. In comparison with MGP, I-QDP, GDP, diorite, II-QDP, III-QDP and DP, the MMEs have different zircon geochemical compositions, with high amounts of Hf, Ti, Th, U, Nb, high T Ti temperatures, high Th/U, and low Ce/Sm, Yb/Gd, and initial Hf isotope ratios (εHf(t)) ranging from −0.3 to 5.2 distributed between the GDP (MMEs host magma) and the Miocene mantle-derived ultrapotassic rocks in the Gangdese. These findings suggest that the MMEs may be mixed products of the mafic magma from enriched lithospheric mantle and adakite-like magma formed by partial melting of the juvenile mafic lower crust. The Eu N /Eu N * ratios of zircon suggest that the ore-forming magma had high oxygen fugacity, mainly above the hematite-magnetite (HM) buffer. The volatile contents in apatite show that the magma related to MMEs had a high S content and high oxygen fugacity, while the ore-bearing magma (GDP and II-QDP) was F-, Cl- and S-rich and moderately oxidized. The input of mafic magma brought increased heat, Cu, and S to the ore-bearing magma of Zegulangbei and increased its oxygen fugacity. This enrichment in F, Cl and S; the high oxygen fugacity; and the input of hot mafic magma were therefore key factors in the formation of the Zegulangbei deposit.