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

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• trend northeast, whereas, in the Old Settler mine (southwest comer of the area) the main east-trending fault cuts a fault that trends north-east. Reversals in the sequence have been noted in adjacent areas also. (See Sims and others (1963, p. 19-20) ; Harrison and Wells (1956, fig. 9).) The distribution and attitudes of the Laramide faults were influenced somewhat by the Precambrian structure, as may be seen by comparing plate 2 and figure 3. In the central part of the district, most faults strike northeast to east-northeast and dip steeply north-west, subparallel to the layering of the Precambrian rocks. Here, east-trending faults are sparse. Southeast of the Idaho Springs anticlinal axis, where the rocks strike west to northwest and dip north, faults are far less abundant and the fault pattern is less well defined. The rocks in the northern part of the district are nearly flat lying, and most of the faults cut the layering of rocks at wide angles. Faults of the east-, east-northeast-, and northeast-trending sets are abundant here. In the southern part of the district, where the Precambrian rocks progressively flatten to the west, the veins show a parallel flattening apparently controlled by the flat dip of the country rock. The east-trending faults are sinuous, ranging in strike from N. 700 W. to N. 70° E. A few faults in the northern part of the district dip steeply south, but most of them dip north at medium to steep angles and a few dip north at angles as low as 25°. Offset on the faults is dominantly left lateral-the north wall having moved a few feet west relative to the south wall. East-northeast- and northeast-trending faults are difficult to differentiate according to strike, for some faults, such as the Lord Byron-Stanley, swing from N. 80°E. to about N. 40° E. Locally, however, a distinction between these sets of faults can be seen (fig. 3). Faults of both sets are nearly vertical in the central part of the district, but, northwestward, the dips flatten to as low as 30° to the north. Displacements have been observed in many mines and are consistently right lateral. The largest apparent displacement was observed in the Bald Eagle mine, where the Precambrian rocks on the northwest side of the fault have been shifted about 80 feet northeast relative to those on the southeast side. Most of the is slickenside striae are subhorizontal, which is evidence in support of the inferred dominant strike-slip movements. Downdip slickenside striae commonly cross the subhorizontal striae, and the latest formed sulfides commonly thicken in pods on the steeper parts of veins. This would indicate that slight normal downdip movement on many faults took place at a late stage of mineralization. The east-, east-northeast-, and northeast-trending Laramide faults show evidence of repeated movement. Relatively early movements preceded mineralization and produced the channelways for the pre- forming fluid. Later movements accompanied and followed mineralization, for sulfide and gangue minerals are commonly brecciated and cemented by later sulfide and gangue minerals. In many places postmineralization gouge coats the vein walls. A few nearly vertical north-striking faults that are grouped near the center of the district (fig. 3 ) probably formed last, for they cut several northeast- and east-northeast-trending faults. Evidently, the north- trending faults also postdate much of the sulfide mineralization, for they are characteristically lined with white clay gouge that contains pulverized sulfides that where probably dragged from the older faults. ORIGIN OF THE FAULTS The extensive through-going northwest-striking faults contrast sharply with faults that trend east, east- northeast, and northeast; they probably were formed much earlier and under a different stress system. The stress system is not understood, but the great extent of the northwest-striking faults (Lovering and Goddard, 1950, pl. 1), indicate that the stresses must have affected much of the Front Range. Although the northwest-trending faults show evidence of repeated movements, their inception was probably during Precambrian time (Tweto and Sims, 1963). Some flat-dipping faults that strike east to northeast, and some faults that strike north or north- northeast in parts of the Front Range - possibly including the Idaho Springs district - may also have formed in • Precambrian time. Evidence for this interpretation was summarized by Sims and others (1963, p. 16-17), but we did not find such evidence in the Idaho Springs district where the north-trending faults cut some of the other 17