Unraveling the role of mantle and crustal processes in Mo–Cu enrichment: Insights from the large Tongchanggou deposit, eastern Tibet

Porphyry-type molybdenum (Mo)–copper (Cu) deposits are major sources of Mo and significant contributors to Cu, making them a focal point of research in the field of mineral deposits. However, the origin of Mo and Cu in these deposits—whether they derive from a shared magma or distinct sources—remains an unresolved question. In this study, we present geochronological and geochemical data from mafic microgranular enclaves (MMEs), along with in-situ elemental compositions of plagioclase from the ore-forming granodiorite porphyry in the large Tongchanggou Mo–Cu deposit, integrating these with existing data to address this issue. Zircon U–Pb geochronology shows that the mafic magma represented by MMEs formed at ca. 86.4 Ma, which closely matches the ages of the ore-forming granodiorite porphyry (84.6–87.6 Ma) and Mo–Cu orebodies (85.2–86.8 Ma). Enriched radiogenic isotope compositions (( 87 Sr/ 86 Sr) i = 0.70647–0.70695; εNd(t) = −5.18 to −3.79; εHf(t) = −2.02 to 3.99), along with potassic alkaline signatures characterized by elevated biotite, K 2 O, and alkali contents, suggest that the mafic magma was derived from a Neoproterozoic enriched lithospheric mantle beneath the western Yangtze Block. In contrast, the adakitic signatures and more evolved radiogenic isotopes (( 87 Sr/ 86 Sr) i = 0.70626–0.70704; εNd(t) = −4.12 to −6.33; εHf(t) = −8.10 to −1.30) indicate that the granodiorite porphyry likely originated from partial melting of a thickened mafic lower crust. Moreover, the cyclic variations in plagioclase An values (i.e., anorthite content in plagioclase) from core to mantle in the granodiorite porphyry, along with the covariant trends observed in FeO, CaO, and SiO 2 contents, strongly suggest that the ore-forming porphyry underwent repeated replenishment by mafic magma. By integrating the contrasting metal contents of granodiorite porphyry (high Mo: 5.81–242 ppm, low Cu: 84.3–493 ppm) and MMEs (high Cu: 252–1020 ppm, low Mo: 0.15–4.93 ppm), the proposed magmatic model suggests that the Mo-rich granitic magma, sourced from the lower crust, was repeatedly recharged by Cu-rich mafic magma derived from an enriched mantle. The above age spectrum and genetic models indicate that the Late Cretaceous extension produced Cu-rich basaltic magma, which heated the lower crust to form Mo-rich granitic magma, with the subsequent mafic magma recharge into granitic magma chamber playing a crucial role in the formation of the Tongchanggou Mo–Cu deposit. These findings emphasize the distinct roles of mantle and crustal processes in controlling magma composition and fertility, with their interactions ultimately governing the metal endowment of associated mineralization systems. • Mafic magma, represented by MMEs, formed around 86 Ma in the Tongchanggou Mo–Cu deposit. • Mafic magma originated from a Neoproterozoic enriched mantle in the western Yangtze Block. • Ore-forming porphyry from thickened lower crust underwent repeated mafic magma replenishment. • Injection of Cu-rich mafic magma into Mo-rich granitic magma controls Mo–Cu deposit formation.