The Sodium-rich Marine Volcanic Rocks And Iron Mineralization In The Aqishan-Yamansu Metallogenic Belt,Xinjiang

The marine volcanic-hosted iron oxide deposits have been highly concerned and researched in recent years due to their high grade and large tonnage.Aqishan-Yamansu metallogenic belt is the main concentration area of marine volcanic-hosted iron deposits in eastern Tianshan,Xinjiang.In this paper,the sodium-rich marine volcanic rocks and two typical iron deposits(namely Yamansu and Hongyuntan)are studied to reveal the mineralization relationship between them in the belt.The volcanic rocks comprise basalt,andesite,dacite,rhyolite,and a few trachyte and trachyte andesite in the Aqishan-Yamansu belt but are predominantly andesites.SHRIMP zircon U-Pb dating of the volcanics yield coeval ages of337.5Ma?327.4Ma.Electron microprobe data of plagioclases demonstrate that albites and/or oligoclases were formed during the crystallization of the volcanic rocks.Two stages of albitization are identified and the late one may closely associate the dissolution of iron.The ore-hosting volcanic rocks are sodium-rich(the averages of Na₂O and Na₂O/K₂O are 4.52%and 9.13,respectively)and calc-alkaline series.The Aqishan-Yamansu belt was a fore-arc basin in the early Carboniferous,which was built during the southward subduction of the Paleo-Kangguer oceanic slab beneath the Middle Tianshan block.The sodium-rich volcanic rocks were probably formed by the interaction between volcanic lava and seawater after volcano eruption on the seafloor.The ore-forming process for Yamansu iron deposit can be divided into two stages.Our dating of pyrite samples which associated with magnetite yields an Re-Os isochron age of 320.3±9.1Ma,and the LA-ICP-MS garnet U-Pb dating yields coeval age of 324.2±9.7 Ma,which are consistent with the age(327.4±3.1Ma)of volcanic rocks in the Yamansu district.The LA-ICP-MS trace elements data on magnetite indicate that Yamansu was a magmatic-hydrothermal iron deposit:the magnetite crystals in the breccia and disseminated ores have high values of Ti,V,Cr,Ni and Co,whereas the magnetite crystals in the massive,banded,and veined ores have high contents of Si,Al,Mn,Mg,Ca.The studies on fluid inclusions suggest that the ore-forming fluids evolved from high temperature and high salinity to low temperatures and low salinities.We can identify two principal ore-forming stages in the Hongyuntan district.Pyrite associated with magnetite has a Re-Os isochron age of 324±31Ma,and the garnet U-Pb dating yields coeval age of 331.0±5.3Ma.These new ages are consistent with the ages(333Ma?324Ma)of the volcanic rocks from the Yamansu Formation.The fluid inclusions and LA-ICP-MS data on magnetite show that the temperatures(T_h(°C))and salinities(wt%(Na Cleqv))of the ore-forming fluids,and the contents of trace elements(Ti,V,Cr,Ni,Ga,Sn and Mn,Si,Zn)fluctuate in the Hongyuntan deposit.These characteristics may be due to the replenishment of energy and materials by multi-stage volcanic eruption.The iron orebodies in the ore districts from Aqishan-Yamansu belt are mostly developed into stratoid,banded,and conformably contact with the Carboniferous sodium-rich volcanic rock strata.Surveyed volcanic stratigraphic section suggest that these deposits were formed as a result of volcanic activities but have no clear spatial association with specific igneous intrusions.Consequently,there is a certain favorable spatiotemporal coupling relationship between the sodium-rich volcanic rocks and the iron deposits in the Aqishan-Yamansu metallogenic belt.The iron materials in these deposits were originated the iron-rich basaltic magma evolved from mantle wedge metasomatized by slab-derived fluids.The released Fe through sodium replacement is one of the material sources of the iron ore deposits in the Aqishan-Yamansu belt.