Study on Composition of Inclusions in Minerals and Simulation Experiment on Hydrothermal Metasomatic Process of the Bayan Obo Iron Deposit

Abstract On the basis of the mechanism of formation of mineral inclusions, it may be assumed that a certain relation exists between the compositions of fluid inclusions in various minerals formed at the same stage of hydrothermal activity. In order to study the genetic relationships between different minerals in the Bayan Obo iron deposit, the compositions (K+, Na +, Ca+, Mg+, F+, Cl+, CO2−2, ∑SO2−4 and pH) of inclusions in fluorite (23), hematite (13), magnetite (3), sodium pyroxene (2) and dolomite (5) from the main mine and the eastern mine were determined by using the vacuum decrepitation and leaching methods, and cluster analyses of the data on the compostions were made. The Q‐mode cluster analysis indicates that some iron oxide minerals in the deposit are related to dolomite of sedimentary origin, while others are related to fluorite and sodium pyroxene — products of hydrothermal activity. The R‐mode cluster analysis shows that the components of the leaching solution may be divided into two groups: one includes CO2−2, Mg2+ and H+ (pH), which are obviously associated with dolomite; the other comprises Na+, K+, Ca+, F+, Cl+ and SO2−4, which may possibly represent the composition of hydrothermal solutions.The reaction of the Na‐F‐Cl solution (pH 4.72) with hematite dolomite at 300°C and 5 × 107 Pa and under alternately “static and dynamic” conditions produced large amounts of hematite and fluorite and small amounts of smectite and Na (Fe) silicates, and the hematite‐fluorite assemblage accords with the actual geological conditions in the deposit. From a comparison between the compositions of “static” and “dynamic” solution samples, it may be known that the flow reaction facilitates the migration of Fe, F, Ca and other components as well as Na‐metasomatism (Na and Si are fixed in a solid phase).The study of the compositions of mineral inclusions and simulation experiments on hydrothermal metasomatism have provided new evidence for the hypothesis of metamorphosed‐sedimentary and hydrothermal‐remoulding origin of the Bayan Obo deposit, and pointed out emphatically that hydrothermal metasomatism plays an important role in the formation of the mineral deposit, particularly in the main and the east mine.