Geological features and genesis of the Bayan Obo REE ore deposit, Inner Mongolia, China

Abstract

Geotectonically, the Bayan Obo deposit is located on the northern margin of the North China platform. When the Proterozoic Bayan Obo Group was formed, the area was probably a rift tectonic environment. Dolomite, the main wall rock (or host rock) of the orebody of the Bayan Obo REE deposit, shows a distinct stratified structure and is remarkably similar to REE-rich and Nb-rich carbonatite rocks elsewhere in the world in mineral constituents, petrochemical composition and incompatible element composition. K-feldspar rock, another host rock of the orebody, may have been formed as a result of volcanic exhalation. The Sm Nd whole rock (whole ore) isochron age of the Main orebody of the Bayan Obo deposit is 1.58 Ga, and the Sm Nd model age is 1.61 Ga, with ε Nd(t) being +6.1. The data suggest that the ore deposit was formed in the Middle Proterozoic, and that the ore-forming material came from the mantle. Middle Proterozoic seems to be the time when the Bayan Obo Group was deposited; the REE metallization occurred together with the deposition of the strata. The isochron age is close to the model age, implying that the ore-forming material was precipitated to form the ore deposit immediately after its separation from the mantle, without much contamination by crustal substances. Beside a Middle Proterozoic metallogenic age, Caledonian and Hercynian ages have also been recorded: for the former time interval, the Rb Sr isochron age of the Kuangou carbonatite vein determined by the authors is 433 Ma; for the latter time interval, the Th Pb age of aeschynite from the ore deposit and the Rb Sr isochron age of biotite granite from the southern part of the ore district were already determined by previous workers. These data indicate that the ore-forming process was polyphasic. In addition, δ 18O magnetite values of massive REE-Fe ore are −2.99 to +3.55%, and δ 18O magnetite values of riebeckite REE-Fe ore are +0.76 to +1.92%. These values, together with the S isotope data obtained by previous workers, also suggest that the ore-forming material for the ore deposit was derived from deep sources. The ore-forming process of the Bayan Obo deposit was, as indicated above, polyphasic, but the major metallization took place in the Middle Proterozoic. The ore-forming material was brought upward in the form of volcanic exhalations from the mantle, and then through sedimentation formed an ore deposit which was genetically of alkalic rock-carbonatite type. In the Caledonian time interval, with the intrusion of carbonatite veins, small amounts of ore-forming material might have been brought upward from depths, leading to extensive metasomatism. In the Hercynian time interval, with the intrusion of granite in the south, the ore deposit was transformed and the metasomatism further developed; nevertheless, little REE ore-forming material was brought into the orebody from depths or from the outside at that time.