CHEMICAL ANALYSIS AND AQUEOUS SPECIATION OF SURFACE WATERS AT THE HISTORIC LIGHTS CREEK MINERAL DISTRICT, PLUMAS COUNTY, CALIFORNIA

The Lights Creek Mineral District (District) at the northern end of the Walker Lane Mineral Belt in Plumas County, California, is a site of historic mining for copper, gold, and silver, and a site of current exploration for copper. Waste rock and mine tailings occur along the banks of Lights Creek and water drains into Lights Creek from abandoned adits. Effects of historic mining on surface water quality were examined using field and laboratory analyses of samples collected from November 2008 to May 2009. Field data included temperature, pH, Eh, dissolved oxygen, and electrical conductivity. Commercial laboratory analyses provided data for major and trace element concentrations in filtered and unfiltered samples. Equilibrium aqueous speciation models were generated to evaluate controls on water chemistry. Measured pH and concentrations of dissolved metals are typical of unaffected surface water except for water emerging from adits. Although adit water has near neutral pH, dissolved oxygen is low and sulfate and heavy metal concentrations are elevated compared to background (upstream) samples and samples from Lights Creek. Arsenic, antimony, and lead exceed EPA maximum drinking water limits in adit water. Model solutions are typically supersaturated with respect to aluminum oxy-hydroxides and aluminosilicates. Equilibrium speciation using redox constraints based on both dissolved oxygen and Eh yields oxidized dissolved metals. However, model adit water exhibits strong supersaturation with respect to manganese and selenium minerals indicating that aqueous manganese and selenium occur as metastable reduced species. Copper in speciated water occurs predominantly as cupric carbonate complexes, and adit solutions are saturated with respect to malachite, which is widely observed in the District. Analyses and models for adit water are interpreted to indicate that the discharge had interacted with sulfide minerals (which are observed in core samples) producing elevated sulfate and metal concentrations. Reactions with aluminosilicates and oxygen of atmospheric origin neutralized pH and led to measureable dissolved oxygen contents, supersaturations with respect to aluminum oxy-hydroxides, and metastable aqueous manganese and selenium.