Geology, Geochemistry And Ore-forming Process Of The Tieshanmiao-type Iron Deposits In The Henan Province, North China Craton

The Wuyang Tieshanmiao-type iron deposits, part of the Wuyang iron deposits, is located at the eastern part of the North China Craton(NCC), represent metamorphosed banded iron formation(BIF) hosted in the Neoarchean Taihua Group. The shape of the orebodies ranges from bedded, and lenticular, with a lateral stratigraphic extension. The marble of the deposits is very developed, and is the hanging wall and footwall of the ore bodies. The Tieshanmiao-type iron deposits have two types of ores: one is banded pyroxene- magnetite quartzite(BMQ), and the other is disseminated magnetite pyroxenite(DMP). The BMQ is similar to the BIF in the Anshan-Benxi and eastern Hebei areas. However, DMP is markedly different from the BMQ, and is mainly composed of pyroxene and magnetite, with only minor or no quartz. In this paper, we mainly compare the DMP, BMQ and the marble in geology and geochemistry with each other, and attempt to provide more information about iron Ore-formation process in The Tieshanmiao-type iron deposits.Pyroxene of the DMP is all clinopyroxene, whereas that from the BMQ includes both clinopyroxene and orthopyroxene. Electron microprobe data show that the clinopyroxene compositions in the DMP are low in Al2O3 and Ti O2, which are similar to those from the BMQ and altered marble and pyroxenite. However, the compositions are different from those in the metamorphosed ultramafic rocks. The DMP and BMQ also show similar major element contents, with dominant Si O2, total Fe2O3, Ca O, Mg O but low contents of Al2O3, Ti O2, Mn O, Na2 O, K2 O, indicating a similar source through submarine chemical precipitation with little input from terrestrial or volcanic materials. The BMQ, DMP and magnetite separates from these rocks exhibit seawater- like signatures of REE patterns with LREE depletion, positive La, Gd and Y anomalies and high Y/Ho ratios, indicating that seawater participated in the formation of the iron ores. Combined with strong positive Eu anomalies, we infer that the deposition of the BMQ and DMP was mainly controlled by the mixing of seawater with hydrothermal fluids. The lack of negative Ce anomalies of the DMP, BMQ and magnetite separates indicate an anoxic marine environment.The ore-bearing wall rocks of the Tieshanmiao-type iron deposits are mainly carbonate rocks. Black and thin layers/bands of DMP also occur in the gray marble, or the DMP occurs as alternating layers with the white impure marble, indicating coeval formation. Pyroxene is common in the altered marble. In addition, pyroxene from the DMP, BMQ, pyroxenite and altered marble possess similar geochemical characteristics, such as the low Al2O3 and Ti O2 contents, positive correlation between Mg O and Si O2, negative correlation between total Fe O and Si O2, total Fe O and Mg O. The marble, DMP and BMQ share many similarities including low ∑REE contents, strong positive Eu anomalies, and obviously positive Y anomalies, also suggesting that the marble is closely related to the ore-forming process.In conclusion, the geological and geochemical characteristics of the DMP and BMQ indicate that they possess similar material source and genetic mechanism. Therefore, we propose that the Tieshanmiao-type iron deposits represent metamorphosed quartz-carbonate iron-bearing formations.