AQUEOUS SPECIATION MODELING OF SURFACE WATERS IN THE LIGHTS CREEK MINERAL DISTRICT, PLUMAS COUNTY, CALIFORNIA

Filtered and unfiltered samples of waters at the historic Lights Creek Mineral District, Plumas County, California were analyzed for a complete suite of major and trace elements. Field redox measurements (for dissolved oxygen and Eh) were in disagreement for waters exiting mine adits, indicating redox disequilibrium as a result of the evolving oxidation states of the dissolved metals leaving the more reducing subsurface environments. Aqueous speciation modeling based on the assumption of chemical equilibrium of the solution demonstrated that samples were supersaturated with a variety of aluminum oxy‑hydroxides and aluminosilicate minerals, a condition commonly observed for low temperature surface waters. No solutions were close to saturation with respect to sulfide minerals, including pyrite, indicating that sulfur was oxidized by the time the water samples were collected. Using the log oxygen fugacity redox constraint for aqueous speciation calculations, samples exhibited large supersaturations with respect to oxidized manganese minerals. Dissolved manganese was apparently reduced in the solutions and out of equilibrium with respect to dissolved oxygen. Water samples were shown to be saturated in none of these minerals using field Eh as the redox constraint in aqueous speciation calculations. The waters were also strongly supersaturated in Sb₂O₅. The majority of copper in adit waters occurred as neutral copper carbonate complexes, and solutions were calculated to be supersaturated with malachite (Cu₂(OH)₂CO₃), which is widely observed as a secondary mineral at the mine site. Aqueous speciation modeling aids interpretation of the evolving mine water as it exits the subsurface environment and is exposed to atmospheric conditions.