Cu-rich porphyry magmas produced by fractional crystallization of oxidized fertile basaltic magmas (Sangnan, East Junggar, PR China)

Abstract

The concentration of Cu is crucial to the formation of porphyry Cu deposits. However, key petrologic evidence for the origin of Cu-rich magmas is scarce. Here, we describe newly discovered Cu-rich mafic porphyritic enclaves (CMPEs) with accumulated phenocrysts, porphyritic textures and disseminated sulfide minerals in a Carboniferous tonalite pluton in the Sangnan area, East Junggar. These CMPEs have diagnostic petrologic, geochronologic and geochemical signatures that indicate they are intruded globules of Cu-rich mafic magma. These signatures include microgranular elongated laths of zoned plagioclase, needle-shaped zircon, poikilitic K-feldspar, skeletal intergrowths of chalcopyrite and epidote, sharp contacts and different geochemical compositions than the host tonalite. They also display slight younger zircon U-Pb age but similar hornblende formation depths with their host tonalites. The host tonalites are calc-alkaline and adakitic, whereas the CMPEs are high-K calc-alkaline and dioritic and have the geochemical signature of normal arc rocks.The CMPEs contain groundmass magnetite in the carapace and Cu-Fe sulfide minerals in the core. Most Cu-Fe sulfide minerals are interstitial. The interstitial pyrite and chalcopyrite display late magmatic-hydrothermal textures, including intergrowths of chalcopyrite with magmatic oligoclase, K-feldspar and quartz, poikilitic chalcopyrite, skeletal and myrmekite-like intergrowths of chalcopyrite and epidote and sulfide-bearing miarolitic cavities. The interstitial sulfide minerals formed closely following the crystallization of the groundmass K-feldspar and quartz, and contemporary with albite and epidote. The CMPE magmas may be related to the formation of porphyry Cu deposits. The CMPE magmas are likely to have been produced by fractional crystallization of oxidized fertile basaltic magmas in a subcrustal chamber below the host tonalite chamber. The oxidized basaltic magmas were derived from metasomatised mantle wedges. The adakitic tonalite magmas are probably formed by the partial melting of mafic lower crust, that was triggered by the intrusion of the hot basaltic magmas.