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BMRI -San Luis Pit Backfi• CT Tess • The rock designated for backfilling the West Pit come from the unmineralized and unaltered hangingwall rocks that are currently being mined (Jones, 1994). The backfill ~ will predominantly comprise two geologic units, the Santa Fe Formation (interbedded silts, sands and gravels of Tertiary agej and Precambrian Pink Gneiss. There may be minor amounts of Tertiary fault gouge from the dislocation surface that sepaaates the hanging wall from the mineralized footwall of the mined area. All gold mineralization and the hydrothermal alteration associated with it are local'ize~d!in the footwall rocks, Precambrian biotite-amphibole quartzo-feldspathic gneiss lpet is distinct from the Pink Gneiss both visually and structurally (i.e., by the Tertiary fault gouge). Detailed discussion of the geology, mineralogy and alteration of the rock units is presented in Jones (1994), submitted to the Division in May, 1994. The Santa Fe Formation comprises Tertiary bolson-fill, alluvial sediments deirived from the tectonic uplands of the Rio Grande Rift. The interbedded sequences of silts, sands and gravels indicate depositional environments of the Santa Fe Formation that are dominated by moderate- to high-energy fluvial systems that have transported sediments over many kilometers from their source areas. The significance of these depositional environments for the Santa Fe is that the Santa Fe Formation does not include locally derived fragments from the mineralized, footwall rocks. Regionally, the Santa Fe Formation is known to include caliche layers that impart a significant acid neutralization potential. Throughout the Rio Grande Rift, the Santa Fe Formation is the most important regional ground-water supply aquifer, typified by high carbonate alkalinity. Thus, on a geologic basis, impacts of the Santa Fe Formation on ground water, if any, would be expected to mitigate potential risks, if any, associated with leaching of other rock types. Data reported to the Division in July 1995 demonstrated that leaching of the Santa Fe Formation samples would produce an alkaline water with very low metals concentrations. Any such effluent would have no adverse impact on beneficial uses of water near the San Luis property. Table 1 presents test results for static testing of ten samples of gneiss. The static acid- base accounting data show that although the ratio of ANP:APP is less than 3:1 in nine of the ten samples, there is considerable ANP in each of the ten. Based on these data, it will be the relative reactivities of the acid-generating and the acid neutralizing phases that determine the nature of an effluent that could be generated during weathering. The appropriate method for evaluating weathering of waste rock is a kinetic test. Table 1 Key Results of Static ABA Tests, Pink Gneiss Waste Samples O1 02 03 04 05 06 07 08 09 10 Sulfur (%) 5.43 1.79 1.58 0.56 1.85 0.49 0.49 0.82 1.17 0.43 AGP 170 55.9 49.4 17.5 57.8 15,3 ]5.3 25.6 36.6 13.4 (tons CaCO3 / kton) ANP 8.8 36.9 8.8 14.9 6.3 12.4 57.7 54.0 43.0 10.0 (tons CaCO3 / kton) Geocbimica, Inc. 2 95021/1•Feb-96