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INTRODUCTION
The Jamestown mine is located in the Jamestown mining district in western
Tuolumne County, California (see Fig. 1). This district is one of many
located on or near the Melones fault zone, a major regional suture in
the Sierra Nevada foothills. The districts along the Melones fault comprise
the Mother Lode gold belt (Clark, 1970).
The Harvard pit is the largest of several open pits mined at the Jamestown
site by Sonora Mining Corporation between 1986 and 1994 (Fig. 2; Algood,
1990). It is at the site of an historical mine named the Harvard that
produced about 100,000 troy ounces of gold, mainly between 1906 and 1916
(Julihn and Horton, 1940).
Sonora Mining mined and processed about 17,000,000 short tons of ore,
with an overall stripping ratio of about 4.5:1, yielding about 660,000
troy ounces of gold (Nelson and Leicht, 1994). Most of this material came
from the Harvard pit, which attained dimensions of about 2700 ft (830
m) in length, 1500 ft (460 m) in width, and 600 ft (185 m) in depth. The
bottom of the pit is at an elevation of 870 ft (265 m). Since mining operations
ceased in mid-1994, the
open pit has been filling with water. As of November, 2000, lake level
had reached an elevation of about 1170 ft (357 m).
Water quality
monitoring data gathered after mine closure showed rising levels of arsenic,
sulfate, and other components in the lake, with particularly notable increases
accompanying a period of rapid filling in 1995 (County of Tuolumne, 1998).
The largest potential source for arsenic in the vicinity of the Harvard
pit is arsenian pyrite, the most abundant sulfide mineral related to gold
mineralization. A previous study of weathering of arsenian pyrite in similarly
mineralized rocks at the Clio mine, in the nearby Jacksonville mining
district, showed that arsenic released by weathering of arsenian pyrite
is effectively attenuated by adsorption on goethite or coprecipitation
with jarosite, depending upon the buffering capacity of the pyrite-bearing
rock (Savage and others, 2000). Although jarosite would be expected to
dissolve in water having the composition of the developing pit lake, iron
oxyhydroxide species (ferrihydrite and goethite) would be stable, and
strong partitioning of arsenic onto suspended particles or bottom sediments
containing these iron phases would be expected. Arsenic release to the
lake would not be expected until stratification develops, producing a
reducing, non-circulating hypolimnion in which the iron phases would be
destroyed by dissolution.
The fact that arsenic concentrations increased rapidly before the pit
lake was deep enough to stratify shows that arsenic may not be attenuated
in the ways that the earlier Clio mine area study indicated, and suggested
that our understanding of release and transport of arsenic in this environment
is incomplete. Therefore, in 1997 we decided to study the chemical evolution
of the Harvard pit lake as part of a project on environmental impacts
of gold mining in the Sierra Nevada, and in early 1998 we developed a
cooperative study with several of the investigators in the Stanford University
Department of Geological and Environmental Sciences who had done the Clio
study. The U.S. Geological Survey portion of the project has been funded
by the Mineral Resources Program.
It is anticipated that a better understanding of the release and transport
of arsenic into the Harvard pit lake and its accumulation there will contribute
to more accurate predictions of arsenic release from weathering of sulfide-bearing
rocks exposed by mining or other activities or events, and to better forecasts
of pit lake evolution in this and similar environments, leading to more
effective monitoring and mitigation strategies.
An accurate
predictive model is needed for the Harvard pit lake to forecast trends
in metal concentrations, particularly arsenic, and also concentrations
of major cations and anions. As the lake approaches pre-mining groundwater
levels the lake water could move down the hydrologic gradient to the southeast
into domestic wells, and could also affect the surface water of
Woods Creek (see Figures 1-3).
This report presents data for water samples collected from March, 1998
through September, 1999. Selected preliminary data for the pit lake for
the 1998 calendar year have been reported (Savage and others, 2000).
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