Ore Geology Reviews | 2019
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
Abstract The 109 porphyry Cu deposit is located in the Western Tianshan Orogenic Belt, NW China. This study characterizes the age and geochemical affinity of its intrusive phase and its tectonic setting. The 109 albite porphyry is metaluminous to peraluminous, alkaline, and has A-type granite geochemical affinity, indicating that this porphyry formed in an extensional tectonic setting. The Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) zircon U-Pb age indicate that the 109 albite porphyry was emplaced 300±4 Ma (95% confidence). The zircon grains from the 109 albite porphyry have Ce 4+ /Ce 3+ ratios ranging from 10.8 to 50.5. A reduced magmatic-hydrothermal system is evidenced by (1) the presence of chalcopyrite, bornite and chalcocite and absence of magnetite, haematite, and sulfates, (2) a low Ce 4+ /Ce 3+ value ( f O 2 ) below the quartz-fayalite-magnetite (QFM)+2 buffer recorded by the Ti-in-zircon thermometer (T) and logƒO 2 . The reduced porphyry-related magmas and ore characteristics in the 109 porphyry Cu deposit are of similar age to those in the Western Tianshan Orogenic Belt, Xinjiang, suggesting that reduced magmatism and related ore deposits within this area may have a similar genesis. As the magma formed in the extensional setting has low a water content, leading to low oxygen fugacity, formation of these reduced ore systems may be attributed to the extensional tectonic setting during the late Carboniferous in the Western Tianshan, Xinjiang. Sulfides in the 109 porphyry Cu deposit usually occur as quench textures (exsolution texture). The in-situ sulfur isotopic variations in the samples shows that the rims of sulfide always have higher δ 34 S values than those of the respective cores. These phenomena are attributed to degassing of sulfur in the ore-forming fluid. Due to sulfur degassing, the f O 2 of ore fluids gradually increased, recorded by the within-grain sulfur isotopic variations. The elevation of f O 2 induced the sulfide content in ore fluid to increase and thus to form the economic Cu deposit.