2009-09-25_PERMIT FILE - M2009076 (31)

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• older folds, the younger folds trend N. 55° E. in a remarkably consistent pattern. Only a few folds exceed 100 feet in width, but even the small ones may be traced for long distances. Their plunge is extremely variable; it ranges from nearly horizontal just south of Idaho Springs to steeply northeast in many places to the north. The folds range in shape from structural terraces having nearly flat crests and steep northwest limbs to sharpcrested chevron folds and, locally, to nearly isoclinal folds. East-facing monoclines are locally present on the southeast limb of the Idaho Springs anticline. These shapes depend on the position of the minor folds on the Idaho Springs anticline and their origin may be explained by differential movements in which the northwest sides have been raised relative to the southeast sides (Moench and others, 1962). Some of the large younger Precanibrian folds are shown in sections on plates 2 and 3. Plate 3 was drawn from the map of the Big Five tunnel (Moench and Drake, 1966, fig. 6) and the geologic map of the district (Moench, 1964, pl. 1). The largest younger folds in the Big Five tunnel are exposed. in biotite gneiss between the Hudson( ?) and Shafter veins (pl. 3). On the tunnel level these folds are sharp-crested chevrons that have intricately crinkled opposing limbs. This is the most common shape of the younger folds in the district. The microcline gneiss, in contrast with the biotite gneiss, has been sheared on northwest-dipping fracture surfaces but has not been folded. This relationship has been observed at several places at the surface where the younger folds in the biotite gneiss were traced to the contact with the microcline gneiss but were not traced farther beyond. Small folds and lineations of many types are related to the larger younger Precambrian folds. Small crinkles that are parallel to the younger fold axes are abundant in the folded biotite gneiss, and biotite and sillimanite have recrystallized locally along the younger fold axes. In addition, slickenside striae, rodding, and, locally, small folds and crinkles, consistently bear about N. 25° W. or S. 250 E. on the limbs of the younger folds that bear N. 55° E. The slickensides and rodding that bear N. 250 W. and S. 25°E. as well as the small drag folds that bear N. 55° E. were formed largely by slippage of successively higher beds toward the larger anticlines. In many outcrops the lineations of both the older and the younger fold systems are well preserved. • The younger folds clearly bend and deflect the older lineations. FAULTS Faults are abundant and closely spaced in the Idaho Springs district and form a complex intersecting pattern. With few exceptions the faults were mineralized with sulfide and gangue minerals in early Tertiary time, forming the veins shown on plate 1. To eliminate distortions of the pattern due to topographic relief, the major veins and faults were projected to a horizontal plane of 9,000 feet (fig. 3). The faults of the Idaho Springs district formed in two different periods of fracturing. During the first period, probably in Precambrian time (Tweto and Sims, 1963, p. 1001), two prominent and a few lesser northwest-trending faults were produced. The second period of fracturing produced an anastomosing pattern of north-east, east-northeast, and east-striking faults. These faults cut all but the youngest of the Tertiary intrusive rocks, most of which were intruded along joints (not along earlier faults), and probably formed in early Tertiary time. A few east- to northeast-striking faults having unusually flat dips, however, may have formed in response to the major Precambrian (?) faulting. In addition, a few short north-trending faults of small displacement appear to have formed later than the second major stage of faulting. Evidence from other parts of the Front Range, however, indicates that some of the north-trending faults may have had their inception during Precambrian time. Most of the faults are inconspicuous in outcrops, and it is very likely that many more exist than are shown on plate 1. Those that had well-developed gossans were prospected in the early stages of mining in the district and are now marked by rows of prospect pits and shafts. Many faults are narrow, gouge-lined slip surfaces that are subparallel to the layering of the rocks and are not obvious in the outcrops. If it were not for the sulfide ores they contain, the abundance and extent of the faults probably would not have been recognized. Most of the faults dip northward at medium to steep angles, have dominant, strike-slip displacements, • and are right or left lateral. These terms refer to the apparent relative movements of the two walls when viewed in plan; right lateral indicates clockwise movement, and left lateral indicates counterclockwise movement. 15