Origin of dioritic magma and its contribution to porphyry Cu–Au mineralization at Pulang in the Yidun arc, eastern Tibet

The giant Pulang porphyry Cu–Au deposit in the Yidun arc, eastern Tibet, formed due to westward subduction of the Garze–Litang oceanic plate in the Late Triassic. The deposit is hosted in an intrusive complex comprising primarily coarse-grained quartz diorite and cored quartz monzonite. Here, we investigate a suite of simultaneous (216.6 ± 1.9 Ma) diorite porphyries within the complex. The diorite porphyries are geochemically similar to mafic magmatic enclaves (MME) hosted in coarse-grained quartz diorite, and both are characterized by low SiO₂ (59.4–63.0 wt%) and high total alkali (Na₂O + K₂O = 7.0–9.2 wt%), K₂O (3.5–6.4 wt%), MgO (3.2–5.5 wt%), and compatible trace element (e.g., Cr = 72–149 ppm) concentrations. They are enriched in large-ion lithophile and light rare earth elements (LILE and LREE, respectively), but depleted in high field-strength and heavy rare earth elements (HFSE and HREE, respectively), and yield variably high (La/Yb)_N ratios (17–126, average 65) with weak to negligible Eu anomalies. Furthermore, they yield low (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7054–0.7067), weakly negative ε_Nd(t) (−2.8 to −0.8) values, and variable zircon ε_Hf(t) (−5.4 to +0.8) and δ¹⁸O (6.0‰–6.7‰) values. These geochemical features indicate that the diorite porphyry and MME formed through crustal assimilation of a magma produced during low-degree partial melting of metasomatized phlogopite-rich subcontinental lithospheric mantle. In contrast, the coarse-grained quartz diorite and quartz monzonite have relatively high concentrations of SiO₂ (61.1–65.3 wt%), K₂O (4.1–5.4 wt%), and total alkali (Na₂O + K₂O = 7.1–8.1 wt%), and low concentrations of MgO (generally <3.0 wt%) and compatible trace elements (e.g., Cr = 38–61 ppm). They yield high Sr/Y ratios (50–63) that indicate an adakitic affinity, and are enriched in LILE, depleted in HFSE, and yield lower (La/Yb)_N values (13–20, average 17) than the diorite porphyry and MME. They yield low (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7046–0.7066), negative ε_Nd(t) (−3.3 to −1.7) values, and zircon ε_Hf(t) and δ¹⁸O values of −2.9 to −0.1 and 5.7‰–6.5‰ respectively, suggesting that they represent high-K calc-alkaline to shoshonitic adakitic magmas that were sourced from subduction-modified juvenile lower crust. Observations of the newly identified diorite porphyry and previously reported MME suggest that input of such dioritic magma into the upper crustal porphyry magma chamber would have contributed not only the necessary metals (e.g., Cu and Au), sulfur, but also H₂O to the system, thus aiding in the generation of the giant Pulang porphyry Cu–Au deposit.