2009-04-20_GENERAL DOCUMENTS - M2009076 (6)

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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 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 (L.overing 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 Laramide faults and lack any indication of previous movement. The flat-dipping faults differ in no major respects, other than angle of dip, from the other east-, east- northeast-, and northeast-striking faults. Further, many steep faults flatten in depth, and many flat faults steepen; this indicates that the apparently flat and apparently steep breaks do not differ genetically. For these reasons the flat-dipping and the north-trending faults are grouped with the Laramide faults in this report. Most of the Laramide faults appear to have formed after the emplacement of the quartz bostonite porphyry but before the biotite-quartz latite. In places outside the Idaho Springs district, the east-, east- northeast-, and northeast-striking faults formed in a definite sequence, but the fact that this sequence reverses locally suggests that the three sets of faults all formed over a short period of time. This apparently short period of time and the consistent movement patterns of the fault sets indicate that they are a conjugate shear system. The east-trending faults show left-lateral offset, the east-northeast-, and northeast-trending faults show right- lateral offset, and all three sets may have formed by shear stress related to compressive stress that was oriented east-northeast. The few north-trending faults formed slightly later than the main network of Laramide faults, and their origin is less certain. These faults are similar in orientation but are en echelon to the Precambrian (?) Dory Hill fault of the Central City district. (See Sims and others, 1963, fig. 6.) Conceivably, these faults may have formed by a reactivation of the Dory Hill fault during the Laramide orogeny. • 15