UBC Theses and Dissertations
Investigating metal attenuation processes in mixed sulfide carbonate bearing waste rock Laurenzi, Laura
The objective of this study was to identify the trace metal/secondary mineral phase associations in a heterogeneous waste rock dump that contains carbonate bearing lithologies and a mix of metal sulfides. The identification of attenuation processes can be used to better predict the drainage chemistry from waste rock at this site and/or other sites with similar waste rock. This study also provides the opportunity to investigate metal attenuation at the largest scale of complexity and compare these observations to those made from the smaller scale tests conducted for this site and is useful for understanding scalability of the smaller scale tests. This study shows that in carbonate bearing waste rock the predominant processes that attenuate copper (Cu) and zinc (Zn) are precipitation of hydroxycarbonate and hydroxysulfate phases and sorption onto iron oxides. Arsenic (As) and molybdenum (Mo) are associated with iron oxides, although for Mo this association was observed in only a few samples. Lead (Pb) was observed in association with iron oxides. Wulfenite observed in a few samples provides an additional attenuation process for Mo and Pb. The stability of the phases and potential for remobilization of these metals can also be suggested from this study. The hydroxycarbonate/hydroxysulfate phases are the least stable phases identified and can dissolve at pH<5. Iron oxides are considered a stable phase, as such, the As, Cu, Pb and Zn associated with these phases may also be quite stable. Sorption of Mo is limited at neutral pH but wulfenite is a stable phase that is not expected to dissolve once formed. Geochemical modelling of seepages from the dump show that iron oxides are supersaturated and wulfenite and gypsum are at equilibrium. Two mixed Cu:Zn hydroxycarbonate phases and hydrozincite were added to the geochemical database and are supersaturated, while malachite and smithsonite were generally undersaturated. Brochantite and antlerite were also generally undersaturated, but the observations made in this study show that copper hydroxysulfates and mixed copper/zinc hydroxysulfates are precipitating. In mixed sulfide/carbonate bearing waste rock mixed of Cu:Zn hydroxycarbonate and hydroxysulfate phases may require consideration for adequate prediction of Cu and Zn concentrations in drainage.
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