Origin and geodynamic setting of adakitic intrusions in the Bairong porphyry Cu–Mo deposit, central Gangdese Belt, Tibet

Abstract

Situated in the heart of the Gangdese metallogenic province in southern Tibet, the Bairong Cu-Mo porphyry deposit anchors itself within the core sector of one of Asia’s most prolific porphyry Cu domains. Three distinct stages of Miocene granitic intrusions have been identified in this deposit: Pre–mineralized porphyritic monzogranite (PMG; 16.9‒12.7 Ma), syn-mineralized monzogranite porphyry (MGP; 13.9‒12.1 Ma), post–mineralized dacite porphyry (11.8–11.6 Ma). The magma from all three stages is characterized by high oxygen fugacity and H₂O-rich. However, the differences in the sources of magma and ore-forming materials between ore-bearing and barren intrusions, as well as the genesis of adakite features in the Bairong Cu-Mo deposit, are not well understood. In this study, we conducted systematic geochemical and Sr-Nd-Pb-Hf-O isotopic analysis on the ore-bearing monzogranite (MZG), mineralized mafic enclaves (MMEs) within the MZG and ore-barren tonalite-porphyrite (TP) from the Bairong deposit. The ore-bearing MZG and MMEs exhibit remarkably consistent Sr–Nd–Pb–Hf–O isotopic compositions. In contrast, the ore-barren TP displays distinctly different isotopic signatures, characterized by significantly lower ε_Nd(t) values (as low as –8.65) and markedly higher ⁸⁷Sr/⁸⁶Sr(t) ratios (up to 0.707973). The ore-bearing MZG and its MMEs also show notably lower δ¹⁸O values (down to 5.2 ‰) and lower Pb isotopic ratios, with ²⁰⁶Pb/²⁰⁴Pb(t), ²⁰⁷Pb/²⁰⁴Pb(t), and ²⁰⁸Pb/²⁰⁴Pb(t) reaching minimum values of 18.4968, 15.6647, and 38.8553, respectively. These isotopic compositions indicate that both the MMEs and their host granitoids originated through hybridization between mantle-derived mafic melts and crustal-derived felsic magmas. Notably, the ore-bearing MZG and MMEs exhibit a greater mantle contribution than the ore-barren TP. The lithospheric mantle source of the Bairong MMEs and their host rocks experienced metasomatism, with the metasomatic agents likely related to subducted components from the Tethyan Ocean. The injection of mantle-derived magmas contributed additional water and ore-forming metal elements to the mineralization process. The adakitic features of the Bairong magmas likely formed during magmatic differentiation (e.g., amphibole fractionation) rather than being directly inherited from their primary magma source.