Genesis Study Of The Alkali Rock Series, Rare Earth Element And Magnetite Deposit In Bayan Obo, Inner Mongolia

Bayan Obo REE-Nb-Fe ore deposit is the world's largest deposit and world important producer for rear earth elements. Since discovery by Prof. Ding Daoheng in 1927, many geologists have carried out a great sum of research on it. Although great achievement has been made, its genesis is still unknown owning to mixture of minerals, combination of multiple-stage geologic structures and superposition of various metallogenesis, etc. Through studying on up-dated papers published worldwide, we summarized current problems related to the Bayan Obo deposit: genesis of ore-bearing dolomitite and K-rich slate, formation of iron ore, mineralization stages and ore-forming model. A systematic research, along with reconnaissance, was focused on the ore-bearing country rocks-dolomitite, K-rich slate and Na-rich rocks. Discovery of volcanic breccias and volcanic shatter breccias at southeast of the Main and East Orebodies suggests volcanic sedimentary formation of the dolomitic rock. The fact that Na-rich rocks are common at the Main and East Orebodies further indicates their igneous origin. On the basis of chemical analysis, the Na-rich rocks can be classified into volcanic eruptive albititite, albititite-arfvedsonite volcanic rock, arfvedsonitic hydrothermal dykes, arfvedsonite-bearing carbonatite dykes and arfvedsonatized dolomitite and so on. The analysis suggests that the K-rich slate should be K-feldspar siliceous rock crystallizing from post-volcano hydrothermal solution, only on the condition of which the K-feldpar can deposit. The paper concludes genetic relationship between the ore-bearing dolomitite, K-rich slate and Na-rich slate and believes that they are one alkali rock series of comagmatic origin. Extensive study was concentrated on the carbonatite dykes which are common at the ore area. Mineralogically, they can be divided into three types: Ca-rich, Mg-rich and Na-rich carbonatite dykes, which, along with the ore-bearing rocks, are all believed to be comagmatic products Combined with mineralogical characteristics of Fe minerals at Bayan Obo, the paper compares Fe2O3/FeO ratios of the Bayan Obo deposit with those of all kinds of iron deposits in China. The result reflects that the Bayan Obo deposit is featured by supergenetic crystallization sedimentation. Through comparison between the Bayan Obo deposit with the two world's famous REE deposits, Mountain Pass REE deposit, US, and Olympic Dam REE deposit, AU on the hosting rocks, ore-forming ages, and genetic models, the dissertation summarizes the similarities and differences between them, as well as the important relation between REE deposits and alkali carbonatite at continental margin rift. Based on our research, the paper proposes that the Bayan Obo ore deposit is one group of the alkali volcanic rocks resulting from mantle-driven magma's exsolution, which causes separation Na-rich silicate magma from carbonatite magma at the boundary of the mantle and crustal, when magma erupts at mid-Proterozoic continental margin rift. Magmatic exsolution and syngenetic hydrothermal replacement are main characteristics of mineralization of the deposit. Structural and magmatic hydrothermal events at the Caledonian and Hercynian imposed distinct reformation to the deposit and redistributed ore-forming material, and was responsible for change in the structure and texture of the deposit.