Geochronology, redox-state and origin of the ore-hosting porphyry in the Tongkuangyu Cu deposit, North China Craton: Implications for metallogenesis and tectonic evolution

Abstract

Tongkuangyu Cu deposit, located at the southern edge of the North China Craton, is hosted by a suite of sedimentary-magmatic rocks. Genesis of this giant ore deposit has been debated for over half a century. New data from geochronological, geochemical and isotopic analyses on the ore-hosting monzogranitic porphyries are presented. Seventy four zircon U-Pb dates by laser ablation ICPMS suggest that the porphyries were emplaced at ca. 2180Ma. Geochemically, they show transitional features between typical Archean TTG and sanukitoid, e.g. displaying sodium enrichment with elevated K contents, and exhibiting strong REE fractionation with elevated HREE, Mg, Cr contents. Ti-in-zircon thermometer reveals a crystallization temperature of 675±16°C. Cerium anomalies in zircons indicated that the porphyry magmas have low oxygen fugacity of 0.5 log unit below the fayalite-magnetite-quartz buffer (ΔFMQ −0.5). Zircons from the porphyries have ɛHf(t) values of −4.9 to 2.5 with two-stage depleted mantle model ages of ∼2.8Ga, which point to a magma source resembles the newly-discovered 2.7Ga TTGs and diorite in this region. Zircons from the porphyries are slightly more enriched in 18O than that of mantle zircons, with δ18O values of 5.2–5.9‰ (averaging 5.6±0.1‰). Geochemical modeling suggests that the porphyry magmas were likely sourced from melting of the 2.7Ga diorite leaving behind hornblende and biotite as residues, with minor contribution (<20%) from juvenile, mantle-derived materials. The reducing nature of the porphyry magmas makes them incapable of sequestering sufficient Cu and S, thus rendering them unsuitable for porphyry-style copper mineralization. By tentative inference, we suggest that magmatic sulfide in the porphyries may have acted as a reducing agent to precipitate copper from the ore-forming fluids. Combining the above evidence and previous interpretations, we propose a tectonic evolution model envisaging an oceanic subduction at ∼2.7Ga, followed by a continental collision at ∼2.5Ga, and ended up with an extension at ∼2.2Ga.