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the Idaho Springs district, but the extent of the fault southeastward is not known. The Precambrian rocks on
<br />opposite walls of the fault apparently have been displaced 500-600 feet by left-lateral movements (pl. 2), but
<br />several Tertiary porphyritic dikes cross the fault without displacement, and one dike of quartz-monzonite
<br />porphyry follows the course of the fault for more than a mile. The Cornucopia vein, which has a left-lateral
<br />displacement of about 100 feet, may branch southeastward from the Idaho Springs fault. The Idaho Springs
<br />fault shows little other evidence of branching or "horsetailing." The fault is a broken zone, about 50 feet wide,
<br />that has been well cemented with silica, some of which is chalcedonic, and that has been sparsely pyritized.
<br />Locally, the silicified breccia stands as low walls on the surface, much like the "breccia reefs" described by
<br />Lovering and Goddard (1950, p. 79).
<br />The Idaho Springs fault shows evidence of repeated movements. Locally, it reopened in early Tertiary
<br />time and was filled by sulfide and gangue minerals, such as those at the Waltham mine (pl. 1). Still later, minor
<br />northwest-trending faults near the Idaho Springs fault displaced the early Tertiary veins, as in the Dover,
<br />Lawrence L., and Syracuse mines. These relatively young fractures (faults) are typically gouge-lined and show
<br />small right-lateral displacements, opposite to the dominant left-lateral movements on the Idaho Springs fault.
<br />The J. L. Emerson-Gem fault system strikes N. 50°-80° W. and dips steeply northward. Regional
<br />mapping by P. K. Sims (oral commun., 1960) has shown that the fault system continues north-west of the Idaho
<br />Springs district, and mapping by Lovering and Goddard (1950, pl. 2) suggests that it may join the Floyd Hill
<br />fault several miles to the southeast. In the Idaho Springs district the J. L. Emerson-Gem fault system contains
<br />several interconnected branching faults (fig. 3) whose total displacement is not known. Except for mines that
<br />mark its course, the fault system is inconspicuous at the surface, and it does not appreciably displace the
<br />Precambrian rocks. In mine workings the fault system was found to be similar to the Idaho Springs fault,
<br />containing as much as 30 feet of broken, silicified, and slightly pyritized rock. It was, however, much more
<br />extensively reopened and mineralized than the Idaho Springs fault, for it contains some of the most productive
<br />veins of the district.
<br />FAULTS OF LARAMIDE AGE
<br />Closely spaced Laramide faults form an interconnecting network of fractures that strike east, east-
<br />northeast, and northeast. The faults cut all the early Tertiary intrusive rock types except the biotite-quartz latite-
<br />the youngest intrusive rock. These faults probably originated about contemporaneously, for they show
<br />consistent movement patterns that can be related to a simple compressional (or rotational) stress system. This
<br />interpretation is reinforced by the fact that the apparent order of formation of particular sets of faults is reversed
<br />from place to place. East-trending faults in the north-east part of the district, in general, are cut by faults that,
<br />trend northeast, whereas, in the Old Settler mine (southwest corner of the area) the main east-trending fault cuts
<br />a fault that trends north-east. Reversals in the sequence have been noted in adjacent areas also. (See Sims and
<br />others (1963, p. 19-20) ; Harrison and Wells (1956, fig. 9) .)
<br />The distribution and attitudes of the Laramide faults were influenced somewhat by the Precambrian
<br />structure, as may be seen by comparing plate 2 and figure 3. In the central part of the district, most faults strike
<br />northeast to east-northeast and dip steeply north-west, subparallel to the layering of the Precambrian rocks.
<br />Here, east-trending faults are sparse. Southeast of the Idaho Springs anticlinal axis, where the rocks strike west
<br />to northwest and dip north, faults are far less abundant and the fault pattern is less well defined. The rocks in the
<br />northern part of the district are nearly flat lying, and most of the faults cut the layering of rocks at wide angles.
<br />Faults of the east-, east-northeast-, and northeast-trending sets are abundant here. In the southern part of the
<br />district, where the Precambrian rocks progressively flatten to the west, the veins show a parallel flattening
<br />apparently controlled by the flat dip of the country rock.
<br />The east-trending faults are sinuous, ranging in strike from N. 700 W. to N. 70° E. A few faults in the
<br />northern part of the district dip steeply south, but most of them dip north at medium to steep angles and a few
<br />dip north at angles as low as 25°. Offset on the faults is dominantly left lateral-the north wall having moved a
<br />few feet west relative to the south wall.
<br />East-northeast- and northeast-trending faults are difficult to differentiate according to strike, for some
<br />faults, such as the Lord Byron-Stanley, swing from N. 80°E. to about N. 40° E. Locally, however, a distinction
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