Age and origin of the H9 member from the Bayan Obo Group: Implications for the genesis of the giant Bayan Obo Fe-Nb-REE deposit, China

Abstract

The Bayan Obo deposit hosted in a set of metamorphic sedimentary strata contains the world’s largest rare earth reserve. However, the genesis of its ore-bearing dolomites remains controversial. The temporal and spatial relationships and compositions of the surrounding strata could provide indirect genetic indications for ore-bearing dolomites, particularly the overlying H9 member. Our data suggest that the compositions of H9 member are complex, consisting of quartz sandstone, biotite shist, metamorphic volcanic rocks and alkaline rocks. The origin and genesis of different lithologies in the H9 member are different. Based on the age distribution of detrital zircons, the basement rocks from the Bayan Obo region are presumed to be the material sources for the quartz sandstone in the H9 member. The biotite schist is the alteration products of wall rocks reacting with the fluids extracted from the ore-bearing dolomite. Metamorphic volcanic rocks and alkaline rocks are also distinguished in the H9 member. Th-Pb dating of zircons from the H9 volcanic rocks indicates its formation age is around 1.4 Ga, which is earlier than the mineralization age of the ore body, and the H9 member is also affected by the Caledonian tectonic compression recorded by the age of 427 Ma. According to the temporal and spatial distribution of H9 member and ore-bearing dolomites, the dolomites are considered to be magmatic origin. H9 member acts as the caprocks of the ore-bearing dolomite, and the contribution of its material composition and entrapment play an important role in the formation of the Bayan Obo deposit.