Comparative mineralogical study of base metal mine tailings, with various sulfide contents, subjected to laboratory column oxidation and field lysimeter tests, Copper Cliff, Ontario
Creators: name: Shaw, Shannon C.
Publisher: name: University of British Columbia
Year: 2009

description: Since 1993, Inco Ltd. has been investigating the possiblity of using a flotation-derived low-sulfide tailings as a means of providing a potentially inactive cover and dam construction material for their Copper Cliff tailings area. The investigation involves open-air field lysimeter and laboratory oxidation column tests of a low-sulfide tailings product produced by the Clarabelle mill; as well as, the evaluation of the concurrent alteration of two tailings products, a main tailings (1.0 wt. % S), and a total tailings product (2.5 wt. % S), to provide comparisons of oxidation rates and the geochemical evolutions that accompany the sulfide-mineral oxidation in the different sulfide-bearing tailings. A pyrrhotite-rich tailings (approximately 14 wt. % S) was also subjected to oxidation in the laboratory columns for the same period of time and was also examined for comparison. This project was undertaken to identify the solid phases that are the primary sources of potential or known contaminants, and solid phases that provide potentially acid-neutralizing capacity to the tailings, as well as to identify secondary precipitates that serve to control the pore-water concentrations of dissolved ions in the various tailings types. The analytical methods employed to achieve these objectives include powder x-ray diffractometry, optical and scanning electron microscopy, energy dispersion spectroscopy and multi-element x-ray mapping techniques, electron probe microanalysis, and Debye-Scherrer x-ray film methods. In these tailings, the sulfide mineral of primary concern with respect to acid generation is pyrrhotite. The oxidation of pyrrhotite is marked by replacement with iron oxyhydroxides, including goethite and lepidocrocite, native sulfur, ferric iron sulfates, and covellite. Nickeliferous pyrrhotite is the primary source of pore-water nickel, with minor contributions from the oxidation of pentlandite and nickeliferous slag particles. Secondary goethite detected in the saturated zone of the tailings contains more nickel than the goethite from the unsaturated zone of the tailings and is the primary "sink" for dissolved nickel. Pentlandite has oxidized to various degrees in the tailings and is demarcated by replacement with iron oxyhydroxides. The oxidation of chalcopyrite is also evident contributing to pore-water concentrations of dissolved copper. Alteration is most commonly seen as dissolution and subsequent precipitation of secondary covellite, as well as replacement by iron oxyhydroxides. Slag particles, although volumetrically of less importance than the sulfides, may be a source of metal contamination including Ni, Cu, Co, and Cr. The oxidation of slag particles in the tailings is evident and is most intense in the total tailings resulting in the formation of secondary covellite and iron oxyhydroxides. Other secondary phases detected in the tailings include gypsum, jarosite, a venniculite-type clay mineral, and montmorillonite. The venniculite, and most likely the montmorillonite, are products of biotite alteration which poses the greatest potential for acid neutralization. Plagioclase is another source of neutralization potential in the tailings and shows some evidence of dissolution. The different tailings types show varying and progressive degrees of oxidation correlative with their specific sulfur contents. The degree of oxidation is determined by the relative extent to which the sulfides have reacted as well as the maximum depth to which oxidation is evident in the tailings. The low sulfur tailings (0.4 wt. % S), show the least degree of oxidation, the main tailings (1.0 wt. % S), show alteration intermediate between the low sulfur tailings and the total tailings (2.5 wt. % S), which have reacted the most. Goethite, gypsum, and jarosite, which are present in abundance in the pyrrhotite-rich tailings (14 wt. % S), have formed as secondary cements which, to all appearances, have impeded the oxidation occuring in the pyrrhotite-rich tailings column. The column tests indicate much higher degrees of oxidation and sharper demarcation boundaries between the oxidized and the unoxidized tailings. More pronounced differences among the three chemically different tailings are seen in the column samples than in the field lysimeters, and pyrrhotite in the tailings from the columns also show "leached" textures not seen in the field lysimeters.