Primary Controls on Arc Magma Fertility in the Gangdese Belt, Tibet and Implications for Post-Subduction Porphyry Cu Mineralization

Xilian Chen

East China University of Technology/Seoul National University

xilianchen@outlook.com

Copper is a critical metal for the global low-carbon transition. While most global Cu resources are hosted in PCDs associated with subduction-related arc magmas or post-subduction adakitic magmas, these magmas exhibit significant heterogeneity in their metal fertility. A previous hypothesis suggested a genetic linkage between these settings, where metals sequestered in sulfide-bearing cumulates during barren arc stages are subsequently remelted to form post-subduction deposits. However, the fundamental factors controlling magma fertility and direct evidence for such a link remain ambiguous. The Jurassic Gangdese arc in southern Tibet provides an ideal natural laboratory to assess this, exhibiting a pronounced fertility contrast between the mineralized Xietongmen and barren Dazi arc igneous suites. By integrating zircon U–Pb geochronology, trace-element geochemistry, and whole-rock platinum-group element (PGE) data, the primary controls on metal fertility are characterized and the genetic link between these two systems is re-evaluated. Accordingly, whilst these suites are contemporaneous, they underwent distinct magmatic evolutionary histories. Notably, the Xietongmen suite, which originated in a frontal arc setting, fractionated a significant amount of amphibole during the andesitic to dacitic stages, whereas the Dazi suite formed in a distal arc or back-arc environment and assimilated greater amounts of crustal material. PGE systematics demonstrate that both suites reached magmatic sulfide saturation, a process further quantified by Monte Carlo simulations showing that Pd depletion in both suites results from the fractionation of similarly limited amounts of magmatic sulfide. Although both suites originated from oxidized magmas, zircon geochemistry suggests that the fertile Xietongmen magma was significantly more hydrous and exsolved fluids at deeper crustal levels compared to the barren Dazi magma. Therefore, we propose that magmatic water content and/or crustal storage depth, rather than oxidation state or sulfide saturation history, are the major factors controlling the metal fertility of arc magmas in the Gangdese arc. The similarly limited amounts of sulfide segregation between fertile and barren arc magmatic suites also imply that the genetic link between subduction-related and post-collisional deposits via the traditional sulfide remelting model may not be suitable for this region.