Characterization of tailings, sediments, and vegetation and their impact on metal(loid) mobility in the Cobalt Mining Camp, Ontario

Abstract

This study investigates the mineralogical, geochemical, and vegetation-related controls on metal(loid) mobility from historical mine wastes in the Cobalt Mining Camp in Ontario. The research focuses on arsenic (As) and cobalt (Co) mobility as these elements are potentially harmful to the environment and because Co is an economically important critical metal in Canada. Comprehensive mine waste characterization is necessary prior to excavating tailings for remediation or reprocessing purposes. Mineralogical and geochemical analyses were performed on tailings from four deposits and pond sediments in Cobalt. The mass of readily soluble metal(loid)s in the tailings and sediments was evaluated using a shake flask experiment. Horsetails (Equisetum sp.) growing on the tailings were collected for chemical characterization and backscattered electron imaging. The tailings and sediments collected in 2021 contain As and Co concentrations (1,200 to 20,000 ppm As; 534 to 8,900 ppm Co) that exceed Canadian environmental quality guidelines for sediment (5.9 ppm As) and soil (50 ppm Co). The benthic sediments contain higher concentrations of most metal(loid)s relative to the nearby tailings. Arsenic and Co are hosted in primary minerals, alteration phases, and phases formed during mineral processing. The proportion of oxidized and reduced phases varies between the different depositional environments. Metal(loid)s that dissolved during the shake flask experiment include As (1,370 to 325,000 µg/L As) and Co (30 to 126,000 µg/L Co), both of which exceeded Canadian water quality guidelines for the protection of aquatic life (5 µg/L As; 1 µg/L Co). During the experiment, sulfide dissolution generated acid which facilitated leaching of metal(loid)s from solid phases. The horsetails contain elevated metal(loid) concentrations in the roots of the plant relative to the aboveground mass. Arsenic is sequestered by an oxidized Fe-bearing plaque on the horsetail roots. This study demonstrates that metal(loid) mobility in Cobalt is controlled by geochemical conditions in the mine waste, solid phase hosts, and vegetation growing on the tailings. Changes to redox conditions could mobilize metal(loid)s from tailings and sediments in Cobalt. These results can be used to inform long-term management decisions regarding the unremediated mine waste in the Cobalt Mining Camp.