Abstract

An investigation of the structure and hydrology of the West Elk Mine region reveals that variable local development of faults above an igneous cupola are parallel to systematic joints and in situ stresses, which in turn control groundwater storage and flow in the region. Six faults were found superimposed on a regional systematic joint set. Fault displacement and development decrease away from a magnetic anomaly interpreted as a pinnacle-shaped pluton. Measurements of displacement along the strike- and dip-lengths of faults reveal variations in shape, size, and structural architecture that correlate with the degree of fault zone development and structural complexity. A progression is found from single-fracture faults to more distributed, then more localized deformation, with increasing displacement toward the igneous intrusion.

Increasing rates of groundwater discharge also correlate with increasing fault displacement and development. Pump tests show immediate communication 50 m away from the most developed fault, but no response from the nearest neighboring fault (600 m away). The combined factors of very low matrix permeability (<10−3 darcy), large groundwater storage volumes, variable and sometimes high groundwater discharge rates, and the absence of hydraulic communication between adjacent fault zones indicate the groundwater system is variably compartmentalized by differences in fault architecture.

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