Characterization and dissolution of secondary weathering products from the gibralter mine site

Metal leaching and acid rock drainage is one of the most important environmental problems facing the mining industry today. Weathering of mined waste rock leads to the dissolution of minerals, releasing a variety of metals, some of which are retained and temporarily stored. Secondary weathering products formed in mine waste weathering environments can serve as a sink for a variety of metals. Literature suggests that up to 90% of metals released from weathering are retained in waste rock dumps. Mobilization of these stored metals can occur in response to environmental changes and waste rehandling, potentially resulting in detrimental effects for the receiving environment. This research examines the composition, and investigates the stability, of stored weathering products in waste rock at the Gibraltar mine, and assesses their role in controlling the future contribution to metal loading using a pedogenetic approach to waste rock and secondary weathering products. The results indicate that the dominant metals retained in the waste rock were oxidized Fe phases, with lesser amounts of Cu and Al. These phases were in the form of sulphates and oxyhydroxysulphates. Zones of accumulation were identifiable from morphological features (colour). Colour, imparted by Fe allowed for the identification of two distinct zones. Zones of Fe accumulation were associated with higher levels of Al, Mo, S, and Si, and zones of poor Fe accumulation were associated with higher levels of Cu and K. The chemically different zones within the rock dump were not correlated to distinct mineralogical or physical differences. Secondary phases seemed to become more stable with aging and advanced weathering, suggesting mineral rearrangement was occurring. This was confirmed by changes in the quantities of soluble and metastable weathering products in different areas of the rock dump. Dissolution of soluble secondary weathering products was shown to be a rapid process, with zones of Fe accumulation releasing greater quantities of metals than zones of low Fe accumulation. After the initial dissolution of soluble weathering products, metastable phases begin to contribute to the level of metals in solution. Dissolution of minerals and the release of metals was more effective with organic chelating acids than with HNO3,which was more effective than unbuffered water, suggesting long term waste management practices must consider changes in metal release as waste rock becomes more soil-like with time through the integration of the biotic component and the incorporation of greater quantities of soil organic matter.Land and Food Systems, Faculty ofGraduat