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• 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
<br />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,
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