The mineralogy, geochemistry and microbiology of Cobalt-bearing mine tailings from the Cobalt Mining Camp in Northeastern Ontario, Canada

Abstract

Advancements in the field of biotechnology have proven bioleaching processes as an economic and environmentally safe form of mining. Most bioleaching studies to date however have been focused on Fe, Cu, and Au-sulfidic mine tailings, rather than metalloid-rich, neutral-pH tailings. This thesis will apply a multidisciplinary and multi-variate statistical approach to explore whether the neutral, Co and As-rich tailings material within the Cobalt mining camp can be efficiently bioleached. Tailings material within 30-cm depth profiles from three tailings sites (sites A, B and C) were characterized for their mineralogical, chemical and microbial community compositions, followed by the execution of bench-scale oxidative and reductive bioleaching experiments. Tailings material from sites A, B and C are composed primarily of quartz, albite, clinochlore, calcite and dolomite with minor safflorite, arsenopyrite, erythrite and annabergite. The material at site A contains on average more (sulf-)arsenides and higher concentrations of Fe than site B. Site C however is altogether geochemically and mineralogically dissimilar, with the presence of distinct reduced and oxidized zones. Variations in the Co+As+Sb+Zn (Co#), Fe (Fe#), and total S (S#) have been identified as geochemical markers for the presence of Fe, Coarsenides versus secondary Co, Ni, Zn-arsenates, e.g. tailings material with a high Co# and low Fe# tend to have a higher proportion of secondary arsenate minerals. In the tailings material of sites B and C, a lower average As valence coincides with a higher S#. Three distinct site-specific groupings are observed for 1) the Co vs Fe and S#’s and 2) the microbial communities. The Cobalt tailings are primarily composed of Actinobacteria and Proteobacteria and N, S, Fe, methane, and (possible) As-cycling bacteria. The tailings from sites B and C have a larger abundance of Fe- and S-cycling bacteria (e.g. Sulfurifustis and Thiobacillus), of which are more abundant at greater depths, whereas the tailings of site A have a higher proportion of potential As-cycling and -resistant genera (e.g. Methylocystis and Sphingomonas). The microbial communities appear to be highly correlated to depth, S#, Fe#, pH, and the average valence of As. The variation in the average valence of As correlates well with the abundances of N, S, Fe, and methane-cycling bacteria (e.g. Nitrospira sp., the order Thermodesulfovibrionia, and Methylocystis sp.). Aerobic and anaerobic bioleaching experiments were conducted on three samples (in duplicate) from each site, with six samples characterized by high and/or low Co, Fe and S#’s and three bulk, site-specific samples. The experiments used the tailings native consortia and had three methods/treatments. The first two were for chemolithotrophic and heterotrophic bacteria enrichment, with the third serving as a control or baseline experiment. The experiments ran for 18-weeks, with (1) biweekly measurements of pH, Eh, and total and dissolved metals, and (2) analysis of the microbial community composition through 16S rRNA DNA extraction at experiment completion. The highest abundance of As-reducing and -oxidizing genera (i.e. Delftia and Dechloromonas) were observed in the heterotroph-enrichments, which was additionally characterized as having the highest Co and As recovery (0.94 and 29.8 %, respectively). Samples that were composed of a higher proportion of Co, Ni, Zn-arsenates and Fe, Co-arsenides were observed to be enriched in Fe-reducing bacteria or As-reducing and - oxidizing genera, respectively. The enrichments were able to successfully shift the microbial communities to those involved in As-cycling. However, further work is required to determine what hindered the metal recovery process, i.e. future micro- and/or nano-scale studies may indicate that Co and As precipitated as Co-phosphates.