GEOCHEMICAL MODELING AND PRINCIPAL COMPONENT ANALYSIS OF THE DEXTER PIT LAKE, TUSCARORA, ELKO COUNTY, NEVADA

The Dexter Pit Lake (DPL) is a 24 year old mine pit lake in the Tuscarora volcanic field in north-east Nevada. The lake partially fills the Dexter open pit mine which produced Au from 1987-1990. Previous study of DPL has shown that many elements (e.g. Cl, F, Mg, B, Li) behave conservatively in lake waters, but that As, Mn and Fe behave non-conservatively. Non-conservative behavior is likely linked to seasonal chemical and thermal stratification, which occurs annually from ~June-December, and is coupled with mineral precipitation and adsorption. In late fall stratification of the lake produces reducing conditions at depth, with respective Eh and pH values of of 200-250 mV and 7-8 in the hypolimnion. Reducing conditions likely lead to reductive dissolution of Mn/Fe (hydr)oxides which in turn release adsorbed As into hypolimnion waters. Upon turnover of the lake the hypolimnion and epilimnion are mixed, resulting in oxidative precipitation of (hydr)oxides and surface adsorption of As. The dissolved concentrations of As, Mn and Fe in the hypolimnion show respective decreases of 19 to 5 μg/L, 370 to 41 μg/L and 300 to 4.4 μg/L following mixing.

Water mixing calculations using EQ3/6 indicate that lake waters are predominantly ground water, with lesser input from precipitation and surface run off. End-members for water mixing calculations were local groundwater and regional precipitation. Modeled interaction between lake water and wall rock minerals indicates that the dissolution of orpiment, manganite, and hematite accounts for dissolved concentrations of As, Mn and Fe observed in the hypolimnion during stratification. Model calculations also show that birnessite and goethite precipitate during mixing events.

Principal component analysis (PCA) was applied to separate chemical components into groups that are influenced by the same physiochemical factors. PCA shows a grouping between As, Mn, and F, indicating that As and F may be derived from similar sources, likely volcanic rocks, and that dissolved concentrations of all three of these elements may be affected by adsorption. EQ3/6 modeling also supports an association between As and F, as the complex HAsO₃F^2- is predicted to make up a large proportion of aqueous As.