SRK has been involved with characterizing the wastes and ore associated with REE and Niobium (Nb) projects for ML/ARD potential for projects in the USA and Canada. These deposits are often relatively low in sulphides, and therefore may have a different set of concerns to those of acidic drainage and leaching of metals with high mobility at low pH, which may be of concern for sulphidic base metal projects.
Alkaline igneous rocks including carbonatites and pegmatites are typically the host rocks to REE/Nb deposits. The REEs and Nb are currently not regulated in Canada or the USA in mine effluent; however, the body of literature on toxicity of some of these elements is increasing and regulation may come. REE- and Nb-bearing minerals typically contain thorium and uranium, which may be present in a wide array of minerals including oxides, phosphates, carbonates and silicates. Parameters that are regulated that are likely to be enriched in these deposits include fluorine, uranium, and several radionuclides which occur naturally and may be enriched further by mineral processing. In addition, leaching under neutral to alkaline pH conditions may enhance the mobility of elements such as arsenic, antimony, selenium, and molybdenum which may be present at low levels in waste rock and ore if trace sulphide minerals are present. Leaching of uranium from ore minerals may be problematic, particularly due to enhanced mobility of uranium in the presence of carbonate. Low levels of uranium-238, thorium-232, and their radioactive decay products may be present in waste rock, with higher activities in ore and processing wastes. Management of solid-phase wastes for radiation protection may be necessary, and at alkaline pH, uranium and radium radionuclides may be mobile in the aqueous environment.
