Porphyry deposits of the Cu-Mo-Au-Re metal spectrum mainly occur in arc settings, but only some segments of the same arc host significant metal resources. The factors controlling the variable metal endowment in magmatic arcs remain unclear. Here, we conducted zircon U-Pb age, trace element, and Hg isotope studies on the ore-bearing (i.e., fertile) and coeval barren granitic rocks from the Upper Triassic Yidun arc, eastern Tibetan Plateau. The results show that the barren granites from the northern Yidun arc display normal arc magma features, and have low oxygen fugacities (ΔFMQ= -3.7 to -0.5), low water contents. Their negative Δ¹⁹⁹Hg values (-0.20 to 0.02‰) indicate that they were mainly derived from continental basement rocks. The fertile granites from the southern Yidun arc exhibit adakitic geochemical affinity (i.e., high Sr/Y and La/Yb ratios), high oxygen fugacities (ΔFMQ = 0.2 to 2.7), and high water contents. Their positive Δ¹⁹⁹Hg values (-0.07 to 0.23‰) indicate an oceanic source of the Hg and suggest that they were derived from an enriched mantle source modified by oxidizing, subduction-related fluids/melts. The contrasting characteristics of fertile and barren granites indicate that magma sources likely have a critical control on the metallogenic potential of arc magmas, with slab-derived fluids imprinting high fO₂ and volatile contents for the formation of productive intrusions in arc settings. Arc magmas derived from oxidized and water-riched magma sources have a predisposition to form porphyry Cu deposits, and should be regarded as priority targets for porphyry deposit exploration.