A Consideration of Viable Mineral Phases in Modeling Pit Lake Chemogenesis

AbstractMine permitting requires an analysis of reasonably forseeable impacts. In pit lake models, precipitates may be invoked which reduce dissolved aqueous concentrations, and thereby perceived groundwater impacts in the event of a throughflow pit lake after mine closure. The phases selected by modelers are generally based on geochemical theory rather than empirical data, and there is little information on the phases that actually precipitate in USA pit lakes. This paper focuses on the mineralogy of the former acidic Liberty pit pond mineral assemblage, predominantly amorphous ferric hydroxide and gypsum, while the former pH-neutral Cortez pit pond sediment contained barite, with biogenic calcite replacing the vascular cavities of horsetail with Ba, Mg, Mn and Zn in solid solution. Floc collected from the once-acidic, now pH neutral Lone Tree pit lake profile demonstrated trona dissolution with precipitation of an amorphous Fe, Al and Ca solid, with attendant As, Mn and Zn. Based on these data, appropriate authigenic precipitates in predictive pit lake studies should generally be constrained to amorphous, poorly crystalline solids, with reliance on exotic mineral phases appearing unwarranted.