2012-02-28_REVISION - M1980244 (100)

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ECOSA Evaluation Adrian Brown 2. SETTING 2.1 Topography The ECOSA is located on the southern flank of Grassy Valley, and has a northern aspect. The site on which it is to be constructed has a natural slope of 14.5 °. The upper portion of the slope comprises mine backfill placed in the northern extent of the East Cresson Mine. 2.2 Soil The ECOSA site has a surface layer of clayey silty soil, typically 12 inches or less thick. 2.3 Colluvium The soil is underlain by an approximately 20 to 60 feet thick mantle of colluvium, ranging in texture from boulder clay to silty sandy gravel. This material has been selectively mined and used for clay liners for the Valley Leach Facilities ( "VLFs "). 2.4 Bedrock ECOSA is located in the northeast quadrant of the Cripple Creek Mining District ( "District "). The District is located in the Cripple Creek Diatreme ( "Diatreme "), a Tertiary-age volcanic caldera filled with phonolites and hydrothermal breccia, surrounded by Precambrian granite, granodiorite, schist, and gneiss (Plate 3). The Diatreme is a breccia -filled volcanic pipe system that was formed by volcanic activity at three eruptive centers. The volcanic inclusion extends to great depth, with a subsurface shape identified by drilling and geophysical evaluation as shown in Plate 4. To the northeast there is a shallow overflow volcanic apron, which is largely barren of gold ore and in general un- mined. The ECOSA is located in the northern extent of the shallow volcanic apron. It is underlain by up to 1,500 feet of Tertiary phonolites and breccias of the Cripple Creek Diatreme. Calcareous sediments have been identified in the volcanic rocks underlying the ECOSA footprint. Loughlin and Koschmann (1935) described the sedimentary rocks in the Cameron and School Section Mines and, in 1939, Koschmann collected samples from calcareous shales /siltstones and marls (limey sediments) in the area. Recent drilling has confirmed that the calcareous sediments cover much of the footprint of the ECOSA, with thicknesses that range from approximately 5 to 25 feet thick. Underlying the Tertiary volcanics in the ECOSA area are Precambrian granite, schist, and gneiss. These rocks form a relatively impermeable containment for the diatremal rocks. The upper surface of the Precambrian rocks slopes to the southwest under the ECOSA at approximately 8 °, directing subsurface flow of infiltrating precipitation towards the main portion of the Diatreme and eventually into the regional ground water system that is intersected by the historic Carlton Tunnel to ultimate discharge under permit at the historic Carlton Tunnel. 1385E.20120224 2