Environmental Characterization of Waste Rock and Tailings from a Carbonatite- Hosted Niobium and Rare Earth Element Deposit

Abstract

Rare earth elements (REEs) have become essential in green energy and high-tech applications and the development of new REE mines is expected in the coming years. There is currently little published research on the behaviour of REE-bearing mine wastes in the environment. The objective of this thesis is to evaluate the mobility of elements of environmental concern from mine wastes derived from REE-enriched carbonatite deposits. One of the primary sources of REEs are carbonatite intrusions. The St. Lawrence Columbium Mine in Oka, Quebec is the only carbonatite deposit in Canada with untreated niobium (Nb)- and REE-enriched mine wastes. This former Nb mine contains waste rock and tailings that have been weathering in situ for several decades. Drainage from these wastes could carry potentially harmful elements into the environment. This site provides a unique opportunity to determine the long-term mobility of these elements from carbonatitic mine wastes. In 2017 and 2018 waste rock and tailings were collected from the mine site. Field observations indicate that most waste rock is calcite-rich sövite, which has deeply weathered in some areas forming piles of loose grains. Also present are areas of secondary mineralization including crusts of manganese oxides and sulphide-rich lenses weathered to iron oxides. Mine wastes collected from the site were characterized using chemical and mineralogical analysis. Analyses show that mine wastes have elevated levels of REEs, Nb, uranium (U), thorium (Th), and fluorine (F). EPMA and LA-ICP-MS analyses indicate that the main hosts of these elements are pyrochlore, apatite, biotite, calcite and various REE-carbonates. Shake flask tests were used to predict which elements would be easily mobilized from particular rock types. The results show that Nb and Th are relatively immobile, while some REEs, U and F are likely to be released into drainage derived from the mine wastes. Most of the U and F is leached from sövite. However, the rare sulphide-rich lenses are the most important sources of easily leached REEs. Manganese oxides will sequester a portion of the REEs. The results of this study will help guide waste management strategies during the development of similar carbonatite-hosted REE deposits.