Small-Scale Porphyry Cu (Au) Systems in Collisional Orogens: A Case Study of the Xifanping Deposit with Implications for Mineralization Potential in Western Yangtze Craton, SW China

The Xifanping Cu–(Au) deposit, a small-scale porphyry system in the central Jinshajiang–Red River tectonic belt (JSRR), formed in a Cenozoic collisional setting. This study integrates zircon U–Pb geochronology, Lu–Hf isotopes, whole-rock geochemistry, and zircon trace element analyses of ore-bearing and barren porphyries, combined with regional comparisons, to constrain magma sources, metallogenic controls, and genetic processes. Ore-bearing biotite quartz monzonite porphyries were emplaced at 32.15 ± 0.43 Ma and 32.49 ± 0.57 Ma, post-dating barren quartz monzonite porphyry (33.15 ± 0.51 Ma). These ages are consistent with molybdenite Re–Os ages (32.1 ± 1.6 Ma), indicating near-synchronous magmatism and mineralization. Both porphyry types belong to the shoshonitic, peraluminous series, enriched in LILE, depleted in HFSE, enriched in LREE, and lacking significant Eu anomalies. Their εHf (t) values (–2.94 to +3.68) and crustal model ages (TDM2 = 0.88–1.30 Ga) indicate derivation from Neoproterozoic subduction-modified lower crust. Ore-bearing porphyries, however, exhibit higher zircon Ce4+/Ce3+ ratios (average = 584 vs. 228 for barren porphyries) and elevated hydrous mineral contents (>10 vol.% amphibole + biotite), indicating more oxidized and water-rich parental magmas. Compared with large-scale porphyry systems (e.g., Dexing, northern Chile), the absence of adakitic signatures and only moderate oxidation limited the scale of mineralization. Overall, the Xifanping deposit formed through partial melting of Neoproterozoic subduction-modified lower crust in a post-collisional extensional regime: at ~33.2 Ma, melting of metasomatized ancient lower crust generated barren porphyries; at ~32 Ma, further evolution and differentiation of this lower crust magmas led to the extraction and enrichment of ore-forming materials from the thicken lower crust, producing hydrated, oxidized, ore-bearing magmas that intruded at shallow levels to form base and precious metal mineralizations. These results underscore the distinctive metallogenic characteristics of small-scale porphyry systems in collisional settings and provide new insights into how source composition and magma oxidation state constrain mineralization potential.