Pit Water-Pit Wall Interactions in Nevada Precious Metal Mines: Experimental Investigation of Four Representative Geologic Systems

ABSTRACT
Static leaching experiments were conducted on samples from
four Nevada open pit mines characteristic of deposit types in which
pit lakes will eventually form. Three samples from sedimentaryrock-
hosted deposits from two different mines were leached for 935
days, and two samples from volcanic-rock-hosted deposits were
leached for 415 days, each with groundwater obtained near the
respective pits. Our experimental systems indicate that in the large
bulk mineable sedimentary-rock-hosted gold deposits, concentrations
of elements such as Ba, Co, Cu, Mn, Rb, and Sr will generally
decrease in the pit water with time, and are unlikely to contribute to
poor water quality. Slight increases were observed in the As, Se, U
and Sb concentrations of simulated pit waters, but all were generally
quite low. Alkalinity, pH, and most major- and trace-element components
appear to stabilize rapidly in these systems (in 50 to 200
days); however, As took considerably longer to equilibrate, suggesting
that this, and possibly other elements mobile in neutral to basic
waters, may require long-term monitoring in the pit lakes. The volcanic-
rock-hosted systems sampled are a low sulfur, low base-metal
variety relatively common in Nevada, but not typical of volcanicrock-
hosted systems in general. Concentrations of As increased in
one of the systems (Boss Mine), so where As is elevated in this type
of deposit, it may pose an environmental concern. Increases in Sb
were also observed, but concentrations were well below levels of
concern. Se was not present in measurable amounts in the systems
considered here, but where deposits are highly enriched in this element,
it could be expected to behave similarly to As, and may
become elevated in the eventual pit waters, as could Hg.
Results suggest that, at shallow, oxidizing levels of the ultimate
pit lakes, water will be slightly basic and contain negligible concentrations
of most metals. However, our simulated inorganic geochemical
evolution suggests that elements such as As, mobile in
neutral to alkaline waters, may be elevated in some systems.
Evaporative concentration of the pit lake waters may exacerbate this
problem. Nevertheless, our results suggest that neither of the deposit
types studied should contribute to particularly poor water quality in
the ultimate pit lakes, unless trace metals are present in much greater
abundance than seen in the wall rock of the pits evaluated in our
leaching experiments, or other site-specific processes not considered
here contribute to poor water quality (e.g., high evaporation rates
combined with low water inflow volumes).