Granite zircon U-Pb geochronology and geochemistry and the geological significance of the Saibo copper deposit in the western Tianshan Mountains, Xinjiang Province, China

Abstract

The discovery of the Saibo copper deposit was a significant breakthrough in prospecting in the western Tianshan Mountains, Xinjiang Province. This deposit has great significance for studying the transboundary metallogenic belt because the deposit is associated with granitoids. Zircon dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yield 206Pb/238U ages of 385.9 ± 1.3 Ma and 387.2 ± 1.4 Ma for the granodiorite and granite porphyries from the deposit, respectively, indicating that the granitoids are the products of Middle Devonian magmatism. In terms of their geochemistry, these porphyries have high SiO2 contents (between 62.60% and 67.47%) and low TiO2 contents (between 0.32% and 0.52%), with aluminum saturation indexes (ASI) between 0.76 and 0.90; these characteristics indicate a metaluminous medium-potassium (K) calc-alkaline I-type granite that is enriched in large-ion lithophile elements such as K and Sr, as well as in incompatible elements such as Th and U and light rare earth elements (LREE); is relatively depleted in high-field strength elements such as Na, Ta, P, and Ti; and lacks an obvious Eu anomaly. The zircon Lu-Hf isotope ratios are relatively high, with 176Hf/177Hf values varying between 0.28253 and 0.28272 and εHf (t) that varies between −0.4 and +6.5 (mostly positive). These features demonstrate that the granites were the products of co-magmatic evolution originating from the depleted mantle and the partially melting of juvenile crustal material, which occurred due to the North Tianshan Ocean subduction towards the southern Yili block that led to magma ascent from the depleted mantle.