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Geochemistry of Arsenic, Manganese, and Iron in the Dexter Pit Lake, Tuscarora, Nevada
AdvisorTempel, Regina N.
Geological Sciences and Engineering
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The Dexter pit lake is a mine pit lake located in north-eastern Nevada, located in the Tuscarora volcanic field. The pit lake formed in 1990 following cessation of open-pit gold and silver mining operations. This pit lake has been the subject of previous geological study, which served to outline local hydrogeology and aqueous geochemical behavior of numerous conservative elements in pit lake waters. The previous studies additionally identified arsenic (As), manganese (Mn), and iron (Fe) as elements that displayed non-conservative behavior in the pit lake. The purpose of the current research is to characterize and quantify the physical and chemical processes that control the aqueous concentrations of these elements in Dexter pit lake waters. The quantitative focus of this study is driven by the paucity of current pit lake research that specifically and quantitatively addresses controls on aqueous As. The methods used in this research included the statistical techniques of both correlation and principal components analysis; the geochemical modeling programs EQ3/6 and Visual MINTEQ were also utilized to simulate fluid mixing, water-rock reactions, and surface adsorption. These methods were employed to determine to what extent these geochemical processes control the cycling of As, Mn and Fe in the Dexter pit lake. Results of statistical analyses indicate that the elements As, Mn, and F may be affected by similar physiochemical processes; and that Dexter pit lake may influence the dissolved As concentration in down-gradient groundwater. Results of geochemical modeling imply that the redox driven dissolution of As, Mn, and Fe in both wallrock and sediment is necessary to account for observed dissolved concentrations in lake waters. Geochemical modeling also shows that upon turnover events, the oxide minerals manganite (MnOOH) and goethite (FeOOH) are precipitated, and that these minerals control the solubility of Mn and Fe. Adsorption modeling shows that As is subsequently adsorbed to these oxide phases, but that likely only ~10% of As present in lake waters is adsorbed. The conclusions of this research are that As, Mn, and Fe are cycled through the Dexter pit lake system from solid phases (Mn and Fe) and adsorbed species (As) to aqueous species. Additionally, this chemical cycling is likely caused by chemical changes that are linked to the physical limnology of the lake. The research also indicates that although adsorption of As is occurring, this process is not likely a major factor in pit lakes of similar geologic character to Dexter pit lake throughout the Great Basin. The geologic characteristics which lead to similar conditions include precious metal deposit type, of which the Dexter pit lake is low-sulfidation epithermal, and major wallrock mineralogy, which in the case of the Dexter pit lake is a significant carbonate wallrock component.