2007-12-19_REVISION - M1988112 (13)

Home Browse Search

TECHNICAL MEMORANDUM To: Bill Lyle Date: January 3, 2004 Page 2 PRE-MINING CONDITIONS Geology A detailed presentation of the geologic history of gold mineralization at the San Luis Mine is available in Benson (1997), and is shown on the geologic map of Wallace and Soulliere (1996). Bedrock in the area is a mixture of felsic to biotite gneiss, with a distinct breccia zone, which hosts the local mineralization. A clay-rich fault gouge that crops out in the area of the West Pit overtopped the breccia. The Tertiary-aged Santa Fe Formation is found above the fault gouge and is composed of material eroded from the igneous rocks in the area. The youngest geologic deposit in the vicinity of the West Pit is the Quaternary alluvium deposited by the Rito Seco. Figure 2 shows a generalized stratigraphic section for the San Luis Mine area. A complex history of structural activity has bounded the rocks of the San Luis Mine on all sides by faults (Figure 3). The extent of apparent structural isolation of the West Pit was the basis of an earlier interpretation regarding the extent of the associated ground water system (Appendix G, M-88-112). Most geologic faults limit ground water flow across the fault, while occasionally enhancing ground water flow along the fault. As shown in Figure 3, the trace of the fault gouge (locally called the "green clay") outcrop (as the thrust fault) extended over the site limiting the amount of ground water flow and communication between the underlying fractured bedrock and the overlying Santa Fe Formation. Figure 4 shows a conceptualized cross-section of the local geology based on the structural map shown in Figure 3 and on the geologic map for the Ojito Peak quadrangle (Wallace and Soulliere, 1996). Geomorphic evolution of the Rito Seco has formed a layer of unconsolidated sediments as valley fill material. Much of the alluvial material derives from physical weathering of the bedrock and Santa Fe Formation. Borehole logs from exploration and monitoring well installation show that the Santa Fe Formation has been eroded completely on the east side of the West Pit with alluvium in direct contact with the fault gouge material and the fractured bedrock aquifer. The area of direct contact between the bedrock aquifer and the Rito Seco alluvium has implications for pre-mining ground water flow as will be presented in the next section. Hydrology and Geochemistry The ground water evaluation conducted in support of the mine permit application focused primarily on the bedrock aquifer. Due to the presence of the fault gouge zone, the bedrock aquifer had water table conditions north of the fault gouge outcrop and confined conditions immediately adjacent to the Rito Seco. Figure 5 was taken from the mine permit documentation and shows measured ground water levels in bedrock wells prior to mining. Of particular to note in the context of this evaluation is that south of the fault gouge p. Iprojectsl212-newmont(sanluis)LsmluisVrv28wtrgltylptemine westpit.doc