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TECHNICAL MEMORANDUM <br />To: Bill Lyle <br />Date: January 3, 2004 <br />Page 2 <br />PRE-MINING CONDITIONS <br />Geology <br />A detailed presentation of the geologic history of gold mineralization at the San Luis Mine <br />' is available in Benson (1997), and is shown on the geologic map of Wallace and Soulliere <br />(1996). Bedrock in the area is a mixture of felsic to biotite gneiss, with a distinct breccia <br />' zone, which hosts the local mineralization. A clay-rich fault gouge that crops out in the <br />area of the West Pit overtopped the breccia. The Tertiary-aged Santa Fe Formation is <br />found above the fault gouge and is composed of material eroded from the igneous rocks in <br />the area. The youngest geologic deposit in the vicinity of the West Pit is the Quaternary <br />alluvium deposited by the Rito Seco. Figure 2 shows a generalized stratigraphic section <br />for the San Luis Mine area. <br />A complex history of structural activity has bounded the rocks of the San Luis Mine on all <br />sides by faults (Figure 3). The extent of apparent structural isolation of the West Pit was <br />the basis of an earlier interpretation regarding the extent of the associated ground water <br />system (Appendix G, M-88-112). Most geologic faults limit ground water flow across the <br />fault, while occasionally enhancing ground water flow along the fault. As shown in Figure <br />' 3, [he trace of the fault gouge (locally called the "green clay") outcrop (as the thrust fault) <br />extended over the site limiting the amount of ground water flow and communication <br />between the underlying fractured bedrock and the overlying Santa Fe Formation. Figure 4 <br />' shows a conceptualized cross-section of the local geology based on the structural map <br />shown in Figure 3 and on the geologic map for the Ojito Peak quadrangle (Wallace and <br />Soulliere, 1996). <br />' Geomorphic evolution of the Rito Seco has Formed a layer of unconsolidated sediments as <br />valley fill material. Much of the alluvial material derives from physical weathering of the <br />' bedrock and Santa Fe Formation. Borehole logs from exploration and monitoring well <br />installation show that the Santa Fe Formation has been eroded completely on the east side <br />of the West Pit with alluvium in direct contact with the fault gouge material and the <br />' fractured bedrock aquifer. The area of direct contact between the bedrock aquifer and the <br />Rito Seco alluvium has implications for pre-mining ground water flow as will be presented <br />in the next section. <br />Hydrology and Geochemistry <br />' The ground water evaluation conducted in support of the mine permit application focused <br />primarily on the bedrock aquifer. Due to the presence of the fault gouge zone, the bedrock <br />aquifer had water table conditions north of the fault gouge outcrop and confined conditions <br />' immediately adjacent to the Rito Seco. Figure S was taken from the mine permit <br />documentation and shows measured ground water levels in bedrock wells prior to mining. <br />Of particular to note in the context of this evaluation is that south of the fault gouge <br />p:N jeoM3/2,neo-rmmgsynluiawanLiiNr-2Anvylnynemine nesgicMa <br />