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Baseline Hydrologic Monitoring Plan — Hansen Project 18 <br />The Hansen Andesite /Latite Member is 0 to 45 feet thick and consists of a gray to purple gray <br />andesite or latite. It is a dense, gray, localized deposit and vuggy in places where phenocrysts have <br />been altered to clay. The unit may represent a volcanic flow. <br />According to Rampart (1979), the Tallahassee Creek Ash -Fall Member varies from 0 to 10 feet in <br />thickness and consists of gray to green claystone. In places, the claystone is a bentonite or <br />kaolinite derived from the alteration of an ash fall. This member may represent an ash fall during <br />the last volcanic event in Tallahassee Creek Conglomerate time. <br />• Thirtynine Mile Andesite. The Thirtynine Mile Andesite is 0 to 800 feet thick. This unit consists <br />of maroon to purple andesitic breccias, flow breccias, welded tuffs, and some interbedded <br />tuffaceous waterlain sediments and unconsolidated basal agglomerates. <br />• Antero Formation. The Antero Formation is 0 to 200 feet thick, consisting mostly of whitish, <br />massive andesite (WWE, 1979). <br />• East Gulch Tuff, Gribbles Park, and Thorne Ranch Tuff. The East Gulch Tuff is commonly <br />grouped with the Gribbles Park Tuff and Thorne Ranch Tuff, because these units are difficult to <br />distinguish in hand sample. The unit is 200 to 800 feet thick and consists of pink to reddish brown <br />pyroclastic material containing sanidine, plagioclase, quartz, and biotite (WWE, 1979). <br />2.3.2 Ore Deposits <br />The uranium ore in the Tallahassee Creek district is primarily hosted in the Oligocene Tallahassee Creek <br />Conglomerate and the Eocene Echo Park Formation. These formations represent fluvial and/or or alluvial <br />deposits, laid down in paleochannels. The paleochannel sediments were deposited on older Precambrian <br />granites. <br />The source of uranium is believed to be primarily from the Wall Mountain tuffaceous volcanic unit, units <br />of the Thirtynine Mile volcanic center, and Precambrian granite and granodiorite that are locally depleted <br />of uranium but not thought to be a sufficient source (Dickinson, 1981). Further localized sources include <br />the upper ash fall member of the Tallahassee Creek Conglomerate (Chapin, 1983). Uranium was dissolved <br />from the Wall Mountain Tuff by groundwater leaching and then deposited as uraninite in favorable zones <br />in the Echo Park Alluvium and Tallahassee Creek Conglomerate (Dickinson, 1981). Uranium was <br />deposited in the absence of oxygenating ground waters and in the presence of reducing carbonaceous <br />material and bacteria. Uranium deposits stayed intact where structurally shielded from oxygenating <br />groundwater (Chapin, 1983). <br />4153A.140129 Whetstone Associates <br />