MINERAL CONTRIBUTIONS TO PIT LAKE ARSENIC AT THE JAMESTOWN MINE, CALIFORNIA

The Harvard ore body at the Jamestown gold mine, located along the Melones Fault Zone in the southern Mother Lode Gold District, California, was mined in an open pit operation from 1987-1994. Dewatering produced a hydrologic cone of depression; recovery toward the premining ground water configuration has produced a pit lake presently 90 m deep. Water sources include ground water, springs and rain/runoff. The lake shows monomictic stratification with respect to temperature and dissolved oxygen. Major ion concentrations, dominated by Ca-Mg-sulfate-bicarbonate, are controlled by equilibrium with calcite and magnesite. Sulfate concentrations have increased with time and currently are not limited by equilibrium with a solid phase. Arsenic concentrations, measured approximately quarterly March 1998 to November 2000, also are not presently limited by equilibrium with a solid phase, and pH conditions (nearly constant at approximately pH 8) do not promote arsenic sorption.

Arsenic occurs in arsenian pyrite along the ore zone and in the fault zone hanging wall, and in niccolite, gersdorffite, and cobaltite in altered serpentinites of the fault zone foot wall. Pyrite weathering products which accumulate on the pit walls during summer include goethite, jarosite, magnesiocopiapite and hexahydrite. These secondary minerals incorporate arsenic at concentrations from less than 100 to 1200 ppm, either as sorbed complexes (goethite) or as coprecipitated solid solutions (sulfate minerals).

Several scenarios for primary and secondary mineral contributions to pit lake arsenic concentrations were assessed. Lake volume and relative surface areas of hanging wall, ore zone and foot wall rocks used in the evaluation were calculated from a 3d model generated using Arcview software. Arsenian pyrite along fractures extending away from the pit may be a primary source of arsenic to the pit lake. Temporary seasonal increases in arsenic concentration may result form erosional transport and dissolution of arsenic-bearing weathering products of pyrite during early winter rains.