The 109 porphyry Cu deposit in the western Tianshan orogenic belt, NW China: An example of Cu mineralization in a reduced magmatic-hydrothermal system in an extensional setting

Abstract

The 109 porphyry Cu deposit in the western Tianshan orogenic belt, NW China, is hosted in albite porphyry. The albite porphyry is metaluminous to peraluminous and alkaline in terms of composition, and has an affinity of A-type granite, and is considered to have formed in an extensional tectonic setting. Zircon grains from the albite porphyry yielded a weighted mean 238U/206Pb age of 300 ± 4 Ma (95% confidence) by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The age can be considered to be the crystallization age of the 109 albite porphyry. The zircon grains have Ce4+/Ce3+ ranging from 10.8 to 50.5. The ore minerals mainly include chalcopyrite, bornite and chalcocite, and the absence of magnetite, hematite and sulfates. The oxygen fugacity (fO2) was estimated to be below the quartz-fayalite-magnetite (QFM) + 2 buffer using the Ti-in-zircon thermometer. We therefore assumed that the 109 Cu deposit formed in a reduced magmatic-hydrothermal system, which is similar to the coeval Cu deposits elsewhere in the western Tianshan orogenic belt, indicating that they may have the same the reduced ore system is likely attribute to an extensional tectonic setting during the late Carboniferous in the western Tianshan orogenic belt. Sulfides in the ores display quench texture or exsolution texture. The rim of each sulfide grains always have δ34S higher than the cores, which is attributed to a fast decrease of temperature and degassing of sulfur from ore-forming fluid. The sulfur degassing may lead to the progressive increase of the fO2 of the ore-forming fluids. The ore-forming fluids with high fO2 can contain more sulfur leading to the formation of the 109 Cu deposits