Trace element geochemistry of the magnetite from the iron-rich supracrustal rocks of the Liaohe Group:Constraints on the genesis of Paleoproterozoic iron ores,a case study from the Zhoujia area
Iron ore is a kind of indispensable mineral resources for industrial development,and most iron ores occurred within the Precambrian Banded Iron Formation(BIF).The Paleoproterozoic was an important period for the formation of Superior type BIFs,however,this type BIFs are rare in the North China Craton.There are iron-rich supracrustal sequences with economically valuable massive and layered iron ores developed in the 2.2～2.1Ga Liaohe Group,which is extending along the Jiao-Liao-Ji belt.Studies on the genesis of the magnetite from the Liaohe iron-rich supracrustal sequences will provide new clues for the prospecting of Proterozoic iron deposits in the North China Craton.In this paper,studies on petrography and trace element geochemistry of the magnetite from the massive and layered iron ores,the infected and vein mineralized fine-grained gneiss and the weakly mineralized magnetite fine-grained gneiss exposed in the Zhoujia area were carried out to reveal their genesis,from which three types of magnetite are recognized in the regional supracrustal sequences.The first type magnetite came from the massive iron ore and infected mineralized fine-grained gneiss,which exhibited relatively higher Mg and Mn and lower Ti,V and Co contents.The second type magnetite is derived from the layered iron ore and weakly mineralized magnetite fine-grained gneiss.Contrast to the first type,they show lower Mg and Mn and higher Ti,V and Co contents.The third type magnetite is only from the vein mineralized fine-grained gneiss with Cr-Ni-V-Ti-Mn systematics unrelated to the other magnetite types.The similarities of trace element contents and their varying trends indicated that the formation of massive and layered iron ore is attributed to the continuous growth and enrichment of the magnetite from the infected mineralized and magnetite fine-grained gneiss,respectively.On the Cr-Ni,Ti-Ni/Cr and V-Ti diagrams,most of the studied magnetites are characterized by hydrothermal fluid originated geochemical affinities.Negative correlations between Ti and Ni of the magnetites from the weakly mineralized fine-grained gneiss suggested that they should be under gone a cooling process during their growth.Our study revealed that the magnetite from the Liaohe supracrustal sequences showed great difference from those of the Hamersley BIFs in composition,although the later were also subjected to metamorphic-hydrothermal processes.It can be inferred that the magnetite of the Liaohe iron-rich sequences should be generated from different temperature,oxygen fugacity and protolith from those of the Paleoproterozoic Superior type BIF.
MagnetiteGeochemistryPaleoproterozoicBIFLiaohe Group
万方数据
维普
