GENERAL48752 - Laserfiche WebLink

Home Browse Search

rn~tl developed on this si-.e for a minim" ~f'four years 'of milling operations at the a~iticipated rate of milling activity. The topography of this site is suitable for the construction and maintenance of a hillside tailings pond, with gentle open slopes. The subsurface conditions are ideal for constructing a stable embankment and safely containing the tailings that will be i~roduced. The entire area is one of low seismic activity, and there is little risk to the tailings pond embankment from earthquake activity in this location. The terrain above the proposed tailings pond that could provide a precipitation catchment area is limited, and there are no streams above or directly below this site. Abundant waste rock from previous mining operations is readily available for use in constructing the impoundment dam, which will be sized and segregated as a result of the white Cloud and Wynona mine dump sorting operations. This site is readily accessible during adverse winter weather conditions, and its close proximity to the millsite will make security and safety monitoring much easier. There are no homesite~ located below the proposed tailings pond, and the private acc~~^= *oad that traverses the ~~~perty below the site will be -.•~osed to the public during •~ life of the operation. Finally. by supple- menting the e;.:~ting trees and shrubs, the tailings pond and embankment area will be well screened from public view. The proposed tailings pond site is situated on 2.125 acres of land that is encompassed by the boundaries of ~ the Oscar and Hercules patented lode mining claims. A sma!1 portion of intervening land is covered by the JoAnne unE~atented lode mining claim. .~ Topographically, the proposed tailings impoundment area can be characterized as gently sloping terrain, The present slope gradient of the ground that has been selected varies from 16 percent on the northeastern edge of the site to 14 percent on the southwestern edge. The absence of even an intermittent stream channel through this area makes this site a suitable ,\1 one for the construction of a hillside tailings dam. ~10(~ c ~u~~ A geological investigation of the site was conducted to determine the subsurface and surficial conditions of the tailings disposal area, This examination disclosed that the entire area is underlain by Precambrian Boulder Creek granite. Typical Boulder Creek granite is a moderately dark gray, coarse-grained, pink and b1acY, mottled biotite granite. Zt forms several conspicuous small outcrops in the area studied for foundation suitability. The granite bedrock present herel~ will provide a very competent and relatively impervious foundation for the tailings dam. Although the proposed tailings pond is situated on portions of three lode mining claims, there are no old mine workings beneath this area. Both the Oscar and Hercules Lodes were located on very tight, narrow veins, which proved uneconomical; and the only mine workings in this area are a few shallow prospect pits and trenches that probably date from the 1880's. A geologic examination of the surficial soil overlying the bedrock in this location revealed a thin mantle of soil between 3 and 6 inches in thickness. This dark grey, very gravelly, sandy loam was derived from the weathering of the underlying Boulder Creek granite, and it contains no transported material. Geologically, this is a very stable environment, with n J evidence of faulting or significant zones of weakness in th I bedrock, and a shallow soil cover with no pattern of dis similar or compressible soils. A very important factor in the selection of this location for tailings retention was the hydrological investigation, which focused on the precipitation, ground water, and potential water runoff inflow. Based upon histo~~.r_al streamflow rec- ords, the U.S. Geological Survey has calculated that 42 percent of the annual precipitation in Boulder County's mountain region flows to the plains each year. Because of the -11-