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<br />' 2.4 Geology <br />~~~ <br />The Gold Hill mining district is associated with the Colorado telluride belt which is the northeastern <br />' extension of the Front Range mineral belt. The telluride belt is part of a broad, north-trending area of <br />mineralization encompassing about 50 square miles. Host rock includes Pre-Cambrian granites, gneisses and <br />' schists which are bounded on the east by Paleozoic and Mesozoic sediments which form the foothills west <br />of Boulder. Early Tertiary dioritic and granitic dikes and stocks intrude the Precambrian rocks. The <br />telluraide ores are apparently related to small, dikes and intrusive breccias of biotite latite which are both <br />' spatially and temporally related to the ores. <br />T ocal C;eology <br />' A focused site investigation was conducted to assess conditions pertinent to the expansion of the <br />existing tailings impoundment facility. A discussion of the im~estigation and findings is presented in Section <br />' 2.6 of this Design Report. Surface and subsurface materials in the vicinity of the site consist of Quaternary- <br />age topsoil, and colluvial deposits covering the bedrock where bedrock is not outcropping. In areas other <br />' than those of outcropping rock, topsoil thicknesses range from less than one inch to more than one foot. <br />Although no samples were taken for analysis, the topsoil overlying the colluvium appears to consist of sands <br />and gravels with varying quantities of organic debris. <br />No attempt has been made [o differentiate possible variations in the colluvium that overlies the <br />bedrock in the areas where bedrock is not outcropping. The colluvium appears to consist predominately of <br />' gravelly and silty sands, with gravel and occasional boulders, which are derived, in part, from in-place <br />weathering of the underlying granitic bedrock. The insitu consistency of the colluvium appears to be <br />' medium dense although no specific tests were performed to assess consistency. <br />Bedrock within the area of the planned site redevelopment consists primarily of Boulder Creek <br />' • Granite. Granite of the Boulder Creek unit is described as moderately dark gray, coarse-grained, mottled <br />pink and black biotite granite. Relatively small outcroppings are scattered across the site. <br />' 2.5 Seismicity <br />The area surrounding the project site is of low seismic activity and the likelihood of a seismic event <br />' that would cause damage to the proposed impoundment expansion works is very low. The Uniform Building <br />Code (UBC}, indicates that the area in which the site is located is in Seismic Zone 1, which is the second <br />' lowest of six seismic risk zones. Emperical information published by Algermissen and others, 1990, <br />indicates that horizontal ground motions resulting from an earthquake near the site would not exceed 2.0% <br />of the acceleration due to gravity, for a seismic event with a 90% probability of not being exceeded in 50 <br />' years, and would not exceed 7% of the acceleration due to gravity for an event with a 90% probability of not <br />being exceeded in 250 years. <br />' 2.6 Focused Site Investigation and Geotechnical Laboratory Testing <br />4 <br />1 <br />