Controls on the differential enrichment of metal assemblages in post-collisional porphyry mineralization systems, Jinshajiang-Ailaoshan metallogenic belt, SW China

Abstract

Post-collisional porphyry deposits are among the main global sources of copper (Cu), gold (Au), and molybdenum (Mo). However, the controls on metallogenic assemblages in post-collisional porphyry deposits remain poorly constrained. The Jinshajiang-Ailaoshan metallogenic belt hosts a suite of Eocene-Oligocene post-collisional porphyry Cu, Au-Cu, and Au deposits, offering an ideal natural laboratory to elucidate the factors controlling the differential enrichment of metal assemblages in post-collisional porphyry mineralization systems. To identify the primary factors controlling the differential enrichment of metallogenic assemblages, over 2500 published whole-rock and mineral geochemical data from mineralization-related porphyries in post-collisional porphyry deposits of the Jinshajiang-Ailaoshan metallogenic belt were compiled and combined with geophysical and thermodynamic modeling results. The results indicate that the parental magmas of mineralization-related porphyries in the Jinshajiang-Ailaoshan metallogenic belt were generated by variable mixing proportions of juvenile lower-crust melts and underlying metasomatized subcontinental lithospheric mantle (SCLM) melts. Our results demonstrate that the degree of magma differentiation, oxygen fugacity, water contents, and crustal thickness progressively decrease as the metallogenic assemblage transitions from Cu to Au-Cu and finally to Au. Magma undergoes prolonged evolution in lower crustal reservoirs (2.19 kbar) beneath thick crust (60–63 km), exhibiting higher degrees of magma differentiation, oxygen fugacity (∆FMQ = 1.79–2.76), and water contents (3.89 wt% H₂O), thereby promoting the formation of post-collisional porphyry Cu deposits. However, higher pressure and water contents promote early sulfide saturation, which in turn causes Au depletion through the precipitation of sulfides. Additionally, high oxygen fugacity (∆FMQ > ∼1.00) suppresses Au dissolution in the magma. In contrast, shallow magma reservoirs (< 1.00 kbar) in thinner crust (33–36 km) undergo brief evolution and are characterized by lower degrees of magma differentiation, oxygen fugacity (∆FMQ = 0.78), and water contents (2.91 wt% H₂O). Meanwhile, lower pressures and water contents delayed sulfide saturation, favoring Au enrichment and creating conditions conducive to forming post-collisional porphyry Au deposits. Moreover, moderate degrees of magma differentiation, pressure (1.01–1.64 kbar), water contents (3.29–3.60 wt% H₂O), and oxygen fugacity (∆FMQ = 1.22–1.98) in medium-thick crust (42–51 km) collectively promote the simultaneous enrichment of Au and Cu in the magma, providing sufficient metal for the formation of post-collisional porphyry Au-Cu deposits. During fluid exsolution, the salinity of the exsolved fluid decreases with decreasing magma emplacement depth, thereby reducing Cu extraction but enhancing Au extraction from the melt, as Au is primarily transported as bisulfide rather than chloride complexes. Therefore, we propose that the degree of magma differentiation, oxygen fugacity, water contents, and crustal thickness constitute four key factors controlling metallogenic assemblages in post-collisional porphyry mineralization systems. These four factors provide critical insights for exploring post-collisional porphyry deposits with different metal assemblages in collisional orogenic belts.