1995-10-02_REVISION - M1977493 (7)

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This Technical Attachment discusses the role of the Mosquito Fault in the hydrogeologic environment in the Arkansas Valley. The fault, which cuts across the upper Arkansas Valley near the Climax Mine just east of Highway 91, is considered a barrier to east -west bedrock ground water flow in the valley. In addition, the low permeability Precambrian bedrock present on the east side of the fault also limits east -west ground water flow. The following sections describe the hydrogeologic influence of the Mosquito Fault in the Arkansas Valley. Site Geology Overview The Hydrogeologic Influence of the Mosquito Fault • Arkansas Valley The Mosquito Fault is a major geologic feature at the Climax Mine site. The fault separates the mostly Paleozoic sedimentary formations on the west side of the fault from the Precambrian Silver Plume Granite and Idaho Springs Formation on the east. Geologic data indicate the fault vertical displacement may be as high as 14,000 feet (Tweto and Case, 1972) along a fault zone that has been documented to be over 700 feet wide (AIME, 1970). The consequence of this massive uplift is a extensive fractured rock, which over millions of years has been filled and sealed with fault gouge, breccia and clay minerals, and now delineates the Mosquito Fault line (Brumbaugh, 1977; AIME, 1970). Covering the bedrock formations and the fault zone are recent deposits of glacial and alluvial materials. On the east side of the fault the thickness of the glacial and alluvial material is estimated to vary from 10 to 20 feet in the upper valley (Zimmerman, 1983) to approximately 45 feet near the fault zone as determined by site data from Core Hole 703. These deposits are composed of mostly glacial till and clays, and are of low permeability relative to their counterpart to the west (Sacrison, 1984; Surface, 1980). West of the fault, glacial and alluvial materials of greater depth (between 40 to over 60 feet deep), and of higher permeability have been deposited in the valley. At the location of the Arkansas Valley monitoring well (west of the fault), 40 feet of gravel and sand with a large fraction of silt and clay overlay the Minturn Formation (Kumar and Associates, 1993).