Constraints and insights into the tectonics of the Bayan Obo deposit from the shallow velocity structure

Abstract

The Bayan Obo deposit in Inner Mongolia is a crucial source of rare earth elements-niobium-iron, with significant economic importance for China. The ore-bearing dolomite is believed to have originated from mantle-derived magma, indicating high prospectivity in the deeper sections. However, the deep distribution of the ore-bearing dolomite remains poorly understood. Here, we develop a 3D S-wave velocity ([Formula: see text]) structure down to 2.5 km depth beneath the Bayan Obo using ambient noise tomography techniques, which used 25 days of continuous waveform data recorded by 312 short-period seismometers. Combined with previous geological, geochemical, and geophysical studies, our results reveal the possible 3D spatial distribution of the ore-bearing dolomite beneath the Bayan Obo. The results indicate that the [Formula: see text] structures beneath the main, east, and west pits change from relatively high-velocity (high-V) zones (with [Formula: see text] of approximately 2.8–3.2 km/s) at shallow depths (depth to approximately 1.5–1.8 km) to significant low-velocity (low-V) zones (with [Formula: see text] < 2.6 km/s) at deeper depths (depth below approximately 1.8 km). We suggest that the content of ore-bearing dolomite may decrease significantly at deeper depths (depth below approximately 1.8 km) beneath these pits, which possibly indicates a diminished prospecting potential there. Our results also indicate a significant high-V zone (with [Formula: see text] > 3.4 km/s) at depths of approximately 1–2.5 km between the main and west pits. Its lithology remains unclear and is proposed to be a key scientific issue in the future to improve our understanding of the ore-controlling structure associated with the tectonic events in the Bayan Obo deposit.