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2.0 HYDROGEOLOGY AND GEOCHEbIISTRY This section provides a brief discussion of the geologic and hydrologic settings for the Carlton Tunnel. 2.1 Geology The Carlton Tunnel is partially located within a mineralized volcanic diatreme that is extensively fractured and contains historic underground mine workings. The diatreme covers approximately 7 square miles (4,480 acres) at the surface. The diatreme extends to a depth greater than the Carlton Tunnel, which is approximately 3,100 feet below ground surface (bgs) in the center portion of the diatreme. Several different types of volcanic rock comprise the diatreme. The predominant rock types in the diatreme include phonolite and brecciated phonolite. Numerous phonolite and lamprophyre dikes cross-cut the diatreme and several brecciated lamprophyre pipes also exist. Precambrian-aged Pikes Peak granite and Cripple Creek granite surround the diatreme. These granites can host ore deposits in the Cripple Creek and Victor area, but to a lesser degree than the diatreme. The Carlton Tunnel was excavated through the granite to the diatreme. 2.2 Geochemistry and IVlineralogy The primary minerals within the diatreme are calcium, potassium, and sodium feldspars, biotite and muscovite micas, pyroxene minerals (e.g., augite), amphiboles (e.g., hornblende), calcite, dolomite, and quartz. The st(rrounding granites are relatively coarse grained, fractured masses consisting of predominantly plagioclase, potassium feldspar, micas, and quartz. These minerals comprise the bulk of the rock types in [he district. The minerals that form the ore and trace mineral concentrations include pyrite, sphalerite, tetrahedrite, sylvite, calaverite, chalcopyrite, and fluorite. Of these minerals, sphalerite is the major primary mineral that contains zinc. Zinc also is capable of substituting for other metals in the mineral phases and may be present in secondary minerals. Historical geochemical investigations have also reported the presence of efflorescent salts that • form as surface precipitates on underground mine walls. Lindgren and Ransome (1906) describe Cripp(c Creek & Pctor Ga(r( 5(ining Comprtny $hepUerA Miller, fnc. iID/AAIO.VD~P-ORlVE1IOOJETRrponUnnrhnaU.JOr 2 November l2, l999