| 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 sodiumrich（the averages of Na2 O and Na2 O/K2 O 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. |
