Geochemical Evolution of a High Arsenic, Alkaline Pit-Lake in the Mother Lode Gold District, California

The Harvard orebody at the Jamestown gold mine, located along the Melones fault zone in the southern Mother Lode gold district, California, was milled in an open-pit operation from 1987 to 1994. Dewatering during mining produced a hydrologic cone of depression; recovery, toward the: premilling ground-water configuration produced a monomictic hit lake with alkaline Ca-Mg-HCO3-SO4-type pit water, concentrations of As lip to 1,200 mu g/L, and total dissolved solids (TDS) up to 2,000 mg/L. In this study; hit-wall rocks seen; mapped and chemically analyzed to provide a contest for evaluating observed variability in the composition of the pit-lake waters ill relationship to seasonal weather patterns. All integrated hydrogeochemical model of pit-lake evolution based oil observations elf hit-lake volume, water composition (samples collected between 1995-2000, 2004), and processes occurring oil pit walls was developed ill three: stages Using the: Computer Code PHREEQC. Stage 1. takes account of seasonally variable water fluxes from precipitation, evaporation, springs, and ground water, as swell as lake stratification and mixing processes. Stage 2 adds CO2 fluxes and wall-rock interactions, allot stage 3 assesses the predictive capability of the model. Two major geologic units in fault contact comprise the pit walls. The hanging wall is composed of interlayered state, metavolcanic and metavolcaniclastic rocks, and schists; the footwall rocks are chlorite-actinolite and tale-tremolite schists generated by metasomatism of greenschist-facies mafic awl ultramafic igneous rocks. Alteration in the ore zone provides evidence for mineralizing fluids that introduced CO2, S, and K2O, and redistributed SiO2. Arsenian pyrite associated with the alteration weathers to produce goethite and jarosite oil hit walls and joints, as well as copiapite and hexahydrite efflorescences that accumulate on wall-rock faces during dry California summers. All of those pyrite: weathering products incorporate: arsenic at concentrations from < 100 up to 1,200 ppm. In the pit lake, pH and TDS reach seasonal highs in the summer epilimnion: pH is lowest ill the summer hypolimnion. Arsenic and bicarbonate covary ill the hypolimnion, rising as stratification proceeds and declining during winter rains. The computational model suggests that water fluxes alone de not account for this seasonal variability. Loss of CO2 to the atmosphere, interaction with pit walls including washoff, of efflorescent salts during the first flush and seasonal rainfall, and arsenic sorption appear to Contribute to the observed pit-lake characteristics.