Trace elements and melt inclusion in zircon within the Qunji porphyry Cu deposit: Application to the metallogenic potential of the reduced magma-hydrothermal system

Abstract

The Qunji porphyry copper (Cu) deposit, located within the Awulale metallogenic belt (AMB) in the western Tianshan orogenic belt, exhibits distinct geochemical and mineralogical characteristics that have not been fully understood. Despite its contribution to our understanding of Cu mineralization mechanisms in reductive metallogenic systems, the oxygen fugacity and ore-forming potential of this deposit are lower than those of typical porphyry Cu deposits. Therefore, there is a need to further investigate the unique features of this deposit. The primary objective of this study was to analyze minerals and trace elements in zircon to quantify the oxidation states of the magma and investigate the presence of reduced porphyry-type magmas and ore-forming fluids in the western AMB. Through this research, we aimed to provide insights into the metallogenic mechanisms and contribute to the understanding of similar deposits in reductive environments. The zircon grains from the Qunji albite porphyry have a Ce4+/Ce3+ ratio of 37.0, indicating that the deposit likely formed in a reduced magmatic–hydrothermal system, which is common in the western Tianshan orogenic belt. Our analytical methods included the use of a cathodoluminescence scanning electron microscope and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) to analyze trace elements in situ. The results revealed a distinct enrichment of heavy rare earth elements and a relative deficit of light rare earth elements, with a positive Eu anomaly. The zircon crystallization environment was determined to be a continental crust environment. Additionally, the Qunji albite porphyry exhibited an A-type granite geochemical affinity and was formed within an extensional environment following tectonic collisions. The oxygen fugacity of the Qunji albite porphyry was notably low, which is a significant characteristic distinguishing it from typical porphyry Cu deposits. Consequently, the Qunji Cu deposit qualifies as a reduced porphyry Cu deposit with relatively low ore-forming potential.

Keywords

• zircon
• fo2
• melt inclusion in zircon
• reduced ore system
• metallogenic potential