Contribution of magma mixing to the formation of porphyry-skarn mineralization in a post-collisional setting: The Machangqing Cu-Mo-(Au) deposit, Sanjiang tectonic belt, SW China

Abstract

The Machangqing Cu-Mo-(Au) deposit in the central of Jinshajiang–Red River fold belt is a Cenozoic porphyry-skarn deposit in the western Yangtze Craton, southwestern China. Igneous rocks at Machangqing mainly consist of mafic and felsic rocks, including mafic dykes and barren syenite porphyry, as well as mineralized monzonite and granite porphyries with mafic microgranular enclaves (MMEs) in the ore-bearing porphyries. In this study, we compiled new and published geochemical and isotopic data for these intrusions, their MMEs and ore sulfides from the Machangqing deposit. Both mineralized and barren felsic porphyries, MMEs and mafic dyke were formed during the late Eocene (33–37 Ma), and have shoshonitic affinities (K2O + Na2O > 6 wt%, K2O/Na2O > 1 and Sr/Y > 40) ratios. They are enriched in LILE and LREE, but depleted in HFSE. Both mineralized and barren felsic porphyries are characterized by high SiO2 (64.67–72.81 wt%) and Al2O3 (13.43–16.10 wt%), but low Cr (average 37.63 ppm) and Ni (average 24.64 ppm) contents and low Mg# values (11–66). They have low whole–rock εNd(t) (–6.5 to –3.3) and relatively high (87Sr/86Sr)i (0.7061–0.7076), which fall inside the range of coeval mafic rocks in western Yunnan. The geochronological, geochemical and isotopic evidence suggest that the felsic magmas were formed by partial melting of the juvenile lower crust that had been injected by shoshonitic mafic magma as documented by the MMEs. The mafic dykes have high Mg# (71–76), Cr (average 492 ppm), Ni (average 195 ppm) contents, and were likely derived from partial melting of upper mantle sources metasomatized by paleo-subduction processes. The MMEs at Machangqing were produced by the mingling of metasomatized mantle-derived mafic melts with the juvenile Neoproterozoic thickened crust-derived felsic melts. The MMEs have spheroidal shapes and contain acicular apatite, indicating that the MMEs represent globules of coeval mafic melt injected into and partly assimilated by the felsic magmas. The ore sulfides have similar trace elemental and Pb isotopic features (206Pb/204Pb = 18.562–18.659; 207Pb/204Pb = 15.628–15.664; 208Pb/204Pb = 38.894–38.986) to the coeval mafic dykes, and their S-isotopic compositions (-2.24 to + 3.5‰) resemble typical mantle-source sulfides, suggesting a metallogenetic link with the coeval mafic magmas/melts and MMEs. Magmas of both the ore-bearing and barren porphyries are relatively oxidized in the FMQ to MH range (zircon Ce4+/Ce3+ = 87–950, log(fO2) = –22 to −5), although the ore-bearing ones are more oxidized. Petrological modelling suggests that the mineralized porphyries are derived from magma mixing dominated by partial melts of metasomatically-enriched mafic mantle, while the barren porphyries are products of magma mixing dominated by a juvenile lower crust component. We propose that magma mixing between lower crustal material with injected mafic mantle-derived magma has provided sufficient metals, sulfur and water to form the Machangqing Cu-Mo-(Au) deposit. The hydrous mafic magmas may have also remobilized the residual sulfides remaining in the continental lithospheric mantle (CLM), thus adding to the metal budget of these fertile magmas.