Iron isotopic constraints on the genesis of Bayan Obo ore deposit, Inner Mongolia, China

Abstract

The giant polymetallic Bayan Obo REE-Nb-Fe ore deposit is the largest REE deposit in the world. Despite the fact that a great number of works have been done on the deposit, its origin remains controversial. Various genetic models have been proposed, including sedimentary origin, magmatic origin, hydrothermal origin or origin with multiple processes. Here the Fe isotope compositions of different types of rocks from the deposit and related geological formations, such as carbonatites, mafic dykes, and Mesoproterozoic sedimentary iron formation and carbonates, are systematically investigated. For the ore deposit, the average δ56Fe values are −0.03±0.16‰ (2SD, n=14), 0.01‰±0.24‰ (2SD, n=6), −0.07±0.24‰ (2SD, n=19) for bulk samples of fine-grained iron ores, gangue rocks and ore-hosting dolomite marble, respectively; and 0.01±0.14 (2SD, n=14), 0.08±0.18 (2SD, n=3), −0.21 for the magnetite, hematite and dolomite, the main Fe oxide and carbonate minerals in the deposit. The narrow range of the near-zero δ56Fe values of fine-grained iron ores and Fe oxide minerals are consistent with those of magmatic products such as igneous rocks and magmatic iron ores, but different from those of sedimentary or hydrothermal products like Precambrian sedimentary iron formations and hydrothermal iron ores reported previously. The slightly negative Fe isotope values of the ore-hosting dolomite marble are consistent with those of the carbonatite dykes in Bayan Obo area and the typical carbonatites worldwide, but different from those of Mesoproterozoic sedimentary carbonates. The small Fe isotope fractionations between magnetite and dolomite (Δ56Femagnetite-dolomite≈0.23‰), and between hematite and magnetite (Δ56Fehematite-magnetite≈0.07‰), indicate that the ore deposit experienced a very high temperature. Overall, the Fe isotope compositions are inconsistent with either sedimentary or hydrothermal origin, but support a magmatic origin for the Bayan Obo deposit.