Sodium-rich volcanic rocks and their relationships with iron deposits in the Aqishan–Yamansu belt of Eastern Tianshan, NW China

Abstract

The volcanic rocks hosting the iron deposits in the Aqishan–Yamansu metallogenic belt are sodium-rich. The geochronology, petrography, and geochemistry of minerals and sodium-rich rocks as well as the relationship between these rocks and the iron deposits are studied. Geochemically, the ore-hosting volcanic rocks are sodium-rich (the averages of Na2O and Na2O/K2O are 4.31 ​wt.% and 8.56, respectively) and belong to the calc-alkaline series. They are enriched in LREEs and LILEs (Ba, U, K, and Sr), but depleted in HFSEs (Nb, Ta, and Ti). SHRIMP zircon U–Pb dating of the crystal tuff in the Aqishan Formation and the dacite in the Tugutu Bulak Formation yields ages of 337.5 ​± ​2.3 ​Ma (n ​= ​15, MSWD ​= ​0.85) and 313.0 ​± ​3.3 ​Ma (n ​= ​13, MSWD ​= ​0.74), respectively, indicating that the sodium-rich volcanic rocks formed from the early–late Carboniferous. Electron microprobe data from plagioclases demonstrate that albites and/or oligoclases were formed in the basic–intermediate–acid volcanic rocks. Two stages of albitization are identified, and the latter is likely attributed to the dissolution of iron in the Aqishan–Yamansu belt. The sodium-rich volcanic rocks probably formed by the interaction between volcanic lava and seawater after volcanoes erupted on the seafloor; meanwhile, the albites formed by element substitution in a low-metamorphic environment. The spatiotemporal coupling relationship between sodium-rich volcanic rocks and iron deposits in the Aqishan–Yamansu belt is favorable. Iron dissolved from the dark minerals of basic–intermediate volcanic rocks through sodium metasomatism is one of the material sources for the iron deposits.