Early fluid exsolution suppressed sulfide saturation in Triassic arc volcanic rocks from the Gangdese belt: Implications for Cu depletion in arc magmas and mineralization potential

Abstract

The formation of porphyry–epithermal CuAu deposits depends critically on Cu and Au fertility of arc magmas. Arc magmas commonly exhibit decreasing Cu and Au contents during magmatic differentiation, which is believed to result from sulfide saturation. However, the Cu and Au contents of the arc magmas would also be impacted by fluid exsolution, particularly when magma differentiation occurs in thin arcs. Here we report the variations of chalcophile element contents, including Cu and platinum-group elements (PGEs), with magmatic differentiation in the Qushui–Changguo (QS–CG) arc volcanic rocks from the Gangdese magmatic belt, Tibet. Zircon ages for QS–CG arc volcanic rocks range from 235 to 232 Ma and represent the earliest magmatism correlated to the subduction of the Neo-Tethys Ocean. Magmatic zircons yield an oxygen fugacity of FMQ +1.0 ± 0.6, which is within the range of typical arc magmas worldwide. The crystallization pressures (2.5 kbar) and magmatic H₂O contents (5 wt%) estimated from amphibole and clinopyroxene compositions suggest the arc magmas were hydrous and differentiated at low pressures. The primitive rocks have Cu/Pd ratios within the mantle range and relatively high PGE contents, indicating that most chalcophile metals were transferred to the melt during partial melting to form the chalcophile fertile arc magma. The Cu contents decreased from >100 ppm in the primitive rocks to <30 ppm in the evolved rocks (e.g., ∼2 wt% MgO), which resembles the trend observed in global arc magmas. However, the Pt contents exhibit a slight reduction as the MgO decreases, which is in accordance with the crystallization of Pt-rich alloys. No decrease in Pd contents is observed in the samples, and Pd contents increase to ∼10 ppb in the most evolved samples. The absence of a dramatic decrease in PGE contents reveals that the magma did not reach sulfide saturation during differentiation, even in the highly evolved magma after magnetite crystallization. In contrast, the decoupling of Cu and PGE contents in the QS–CG arc magmas and decreasing Cu/Pd ratios are indicative of early aqueous volatile exsolution during magmatic differentiation. We suggest that early volatile exsolution in the arc magmas suppressed sulfide saturation during magmatic differentiation. As a result, early fluid exsolution may also be a key factor in reducing Cu contents in arc magmas in thin arcs, especially for magmas with high volatile contents that undergo differentiation at low pressures. Cu and Au would have been enriched in exsolved fluid prior to sulfide saturation. The Triassic metallogenic potential of the Gangdese belt should be re-evaluated based on recent discoveries. Our results indicate the Triassic magmatism could also be fertile, with the ability to form subduction-related porphyry CuAu deposits in the Gangdese metallogenetic belt.