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locally, small folds and crinkles, consistently bear about N. 25° W. or S. 25° E. on the limbs of the younger
<br />• folds that bear N. 55° E. The slickensides and rodding that bear N. 25° W. and S. 25°E. as well as the small drag
<br />folds that bear N. 55° E. were formed largely by slippage of successively higher beds toward the larger
<br />anticlines. In many outcrops the lineations of both the older and the younger fold systems are well preserved.
<br />The younger folds clearly bend and deflect the older lineations.
<br />FAULTS
<br />Faults are abundant and closely spaced in the Idaho Springs district and form a complex intersecting
<br />pattern. With few exceptions the faults were mineralized with sulfide and gangue minerals in early Tertiary
<br />time, forming the veins shown on plate 1. To eliminate distortions of the pattern due to topographic relief, the
<br />major veins and faults were projected to a horizontal plane of 9,000 feet (fig. 3).
<br />The faults of the Idaho Springs district formed in two different periods of fracturing. During the first
<br />period, probably in Precambrian time (Tweto and Sims, 1963, p. 1001), two prominent and a few lesser
<br />northwest-trending faults were produced. The second period of fracturing produced an anastomosing pattern of
<br />north-east, east-northeast, and east-striking faults. These faults cut all but the youngest of the Tertiary intrusive
<br />rocks, most of which were intruded along joints (not along earlier faults), and probably formed in early Tertiary
<br />time. A few east- to northeast-striking faults having unusually flat dips, however, may have formed in response
<br />to the major Precambrian (?) faulting. In addition, a few short north-trending faults of small displacement
<br />appear to have formed later than the second major stage of faulting. Evidence from other parts of the Front
<br />Range, however, indicates that some of the north-trending faults may have had their inception during
<br />Precambrian time.
<br />Most of the faults are inconspicuous in outcrops, and it is very likely that many more exist than are
<br />• shown on plate 1. Those that had well-developed gossans were prospected in the early stages of mining in the
<br />district and are now marked by rows of prospect pits and shafts. Many faults are narrow, gouge-lined slip
<br />surfaces that are subparallel to the layering of the rocks and are not obvious in the outcrops. If it were not for
<br />the sulfide ores they contain, the abundance and extent of the faults probably would not have been recognized.
<br />Most of the faults dip northward at medium to steep angles, have dominant, strike-slip displacements,
<br />and are right or left lateral. These terms refer to the apparent relative movements of the two walls when viewed
<br />in plan; right lateral indicates clockwise movement, and left lateral indicates counterclockwise movement.
<br />Many faults, however, show evidence of repeated movements, some of which were quite different from the
<br />dominant relative displacements, and some show evidence of small dip-slip movement.
<br />The faults of the Idaho Springs district may be grouped according to strike into five principal sets,
<br />which strike, respectively, north-west, northeast, east-northeast, east, and north. These sets show consistent
<br />movement patterns, and some of the sets have other distinguishing characteristics. As stated, the northwest-
<br />trending faults are inferred to have formed in Precambrian time and are called Precambrian (?) faults. The other
<br />faults formed largely in early Tertiary time and are called Laramide faults.
<br />FAULTS OF PRECAMBRIAN(?) AGE
<br />Two major faults-the Idaho Springs fault and the J. L. Emerson-Gem fault system-as well as a few
<br />lesser faults strike north-west. These faults are similar in trend and in many of their other characteristics to the
<br />faults known as "breccia reefs" or "breccia dikes" (Lovering and Goddard, 1950, pls. 1 , 2; Lovering and
<br />Tweto, 1953, p. 30). Previously, Lovering and Goddard (1950, p. 79-82) postulated that fractures of this set
<br />formed during the Laramide orogeny, though they recognized that some may have followed pre-existing zones
<br />of weakness. However, data summarized by Tweto and Sims (1963, p. 1001) suggest that these fractures formed
<br />in Precambrian time and were reactivated during the Laramide orogeny.
<br />The Idaho Springs fault strikes N. 50°-60° W. and dips steeply north. Regional mapping by Sims
<br />(1964) in the Central City quadrangle has shown that the Idaho Springs fault extends a few miles northwest of
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