Petrogenesis and metallogenic potential of granodiorite porphyry in the Kalatag district, East Tianshan, NW China: Constraints from geochronology, mineral geochemistry, Sr–Nd–Hf–O–S isotopes and sulfide trace elements

Abstract

The Hongshi epithermal Cu deposit is located in the northern margin of the Dananhu arc in the East Tianshan Orogenic Belt, NW China. The Hongshi deposit is hosted by andesite and is famous for its lode type sulfide ores. Recently, the deep drill hole exploration shows a porphyry Cu mineralization and is predominantly hosted in granodiorite porphyry in the Hongshi. As for porphyry mineralization, the metallic minerals mainly include chalcopyrite, magnetite, and pyrite, with minor amounts of molybdenite. Ore minerals occur as dense dissemination and veins. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb dating of zircons showed that the ore-bearing granodiorite porphyry was emplaced at ca. 434 Ma, recording the early subduction of the Paleo-Tianshan oceanic plate. Major element geochemical characteristics show that granodiorite porphyry samples have A/CNK values of 1.04–1.10 and moderate Mg# values of 45–49, indicating an I-type granitoid attribute. Trace element geochemical analysis indicates that ore-bearing granodiorite porphyry samples exhibit obvious large ion lithophile elements (LILEs) enrichment, high field strength elements (HFSEs) depletion, together with mantle-like bulk Sr–Nd ((87Sr/86Sr)i = 0.706417–0.708601; εNd(t) = 6.3–7.0) and zircon Hf–O isotope signatures (εHf(t) = 15.5–21.7, δ18O = 3.8 ‰–4.7 ‰), indicating that they were generated from partial melting of metasomatized mantle by the subducted slab fluids. In situ S isotope results of pyrite (1.46 ‰–2.07 ‰) and chalcopyrite (0.79 ‰–1.93 ‰) imply that granodiorite porphyry magmas and volcanic rocks provided ore-forming components (S and metals) for porphyry Cu mineralization in the deep drill hole ZK7501. The ore-bearing granodiorite porphyry samples have high logfO2 values ranging from − 12.4 to − 10.7, corresponding to ΔFMQ values of 0.6–5.2 (FMQ is the fayalite-magnetite-quartz buffer). Zircons from granodiorite porphyry have high Ce4+/Ce3+ ratios ranging from 74 to 187 and high Ti-in-zircon temperatures (764 to 869 °C). Electron microprobe analyses (EPMA) composition results of plagioclase, apatite and amphibole reveal that granodiorite porphyry magma had systematically high oxygen fugacity, high water contents (7.6–8.4 wt%), and low Cl contents (0.08–0.27 wt%), further demonstrating that granodiorite porphyry may be beneficial to the porphyry Cu mineralization in the deep drill hole ZK7501. Pyrite in the deep drill hole ZK7501 is enriched in Co, Ni, Ag, Cu, As, and Pb, but poor in Mn, Ga, Ge, Cd, In, Sn. Mn, As, Tl, Pb and Sb in pyrite substituted Fe2+ by coupled mechanisms, while Ag and Cu can enter the mineral lattice in the form of solid solution or micro-inclusions, which was also demonstrated by LA–ICP–MS mapping and textural studies. Pyrite shows variable Co/Ni ratios ranging from 10 and 455, indicating that ore-forming fluid originated from a magmatic source. The involvement of highly oxidized magmas is fundamental for the formation of porphyry Cu mineralization in an arc tectonic setting. Additionally, water concentrations also play an important factor that controlled metal fertility of the Early Paleozoic porphyry Cu mineralization in the Kalatag district.