Abstract
Globally, porphyry deposits constitute the principal source of copper, accounting for more than half of global reserves. Elevated oxygen fugacity (ΔFMQ) and volatile contents are widely recognized as critical parameters governing their formation, yet the factors that establish these conditions in post-collisional settings remain poorly constrained. To address this issue, we investigated magmatic rocks of different ages and petrogenetic origins from Nanmling in the southern Gangdese belt. Integrated geochemical data from apatite, zircon, and whole-rock analyses demonstrate that variations in crustal thickness exert a first-order control on ΔFMQ, water content, and sulfur content in the lower crust, while ultrapotassic magmas may locally modify the sulfur budget in restricted domains. Crustal thickness in the Gangdese belt increased from ∼35 km in the Late Triassic to >60 km by the Late Eocene. In the early Eocene, when the belt entered a post-collisional stage, porphyry mineralization was negligible, consistent with the low water content of the magmas. In contrast, during the Neogene, the central Gangdese belt reached peak values in crustal thickness and ΔFMQ, while magmas exhibited high water contents and elevated sulfur contents. This interval coincided with the widespread development of porphyry Cu–Mo systems. We conclude that crustal thickness exerts a primary control on porphyry Cu fertility in post-collisional settings, whereas the absence of deposits in the western Gangdese belt may reflect early eruption of massive ultrapotassic magmas that depleted the crustal sulfur budget.