Geochronology and geochemistry of the granitoids and ore – forming age in the Xiaoyao tungsten polymetallic skarn deposit in the Jiangnan Massif tungsten belt, China: Implications for their petrogenesis, geodynamic setting, and mineralization

Abstract

The Xiaoyao tungsten polymetallic skarn deposit in the eastern Jiangnan Massif of Yangtze Block is located at the contact between a granodiorite pluton and Sinian–Cambrian limestone. The intrusions in the tungsten-rich district comprise the Xiaoyao and other granodiorite plutons and granite porphyry dikes. The age determinations by LA–ICP–MS U–Pb dating of zircons indicate that the granodiorite formed at 149.4±1.1Ma, whereas the granite porphyry was emplaced at 133.2±0.7Ma. Re–Os dating of molybdenite from the skarn orebodies yielded a weighted average age of 148.7±2.3Ma (n=5). These ages indicate that the tungsten mineralization is temporally related to the granodiorite. The granodiorites are metaluminous (A/CNK=0.86–0.98) and in the high-K calc-alkaline series. They contain hornblende and have a negative correlation between P2O5 and SiO2, indicating that they are typical I-type intrusions. The granite porphyries exhibit high alkali contents (Na2O+K2O=7.97–9.53%), elevated FeOT/(FeOT+MgO) ratios (0.83–0.94), high concentrations of Zr, Nb, Ce, and Y, and high Zr saturation temperatures (average of 812°C); thus, they are geochemically similar to A-type intrusions. The initial 87Sr/86Sr and εNd(t) values range respectively from 0.7074 to 0.7083 and from −7.9 to −1.3 for the granodiorite, and from 0.7008 to 0.7083 and from −6.3 to −4.7 for the granite porphyry. In addition, two-stage Nd model ages (T2DM) of 1.0–1.6Ga for the granodiorite and 1.3–1.4Ga for the granite porphyry indicate that the Proterozoic crustal rocks of the Shangxi Group could have contributed to the Xiaoyao magmas. The rhenium contents of the molybdenite grains vary from 32 to 136ppm, suggesting that the molybdenum was derived mainly from a mixture of mantle and crustal sources. Based on the new geochemical data and regional geological investigations, we propose that the Late Jurassic mineralization-related I-type granodiorite was derived from the Neoproterozoic Shangxi metamorphic rocks with some additional input of mantle material. The magmatism was triggered by asthenospheric upwelling induced by slab tearing during oblique subduction of the paleo-Pacific plate beneath the South China Block.