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varieties of porphyry except the biotite-quartz latite, the youngest of the sequence, are cut by the metalliferous <br />• veins at many places and were emplaced before the veins fanned. The biotite-quartz latite, on the other hand, <br />cuts metalliferous veins in many places. <br />The kinds of intrusive bodies formed by the porphyries changed as time passed. Several of the older <br />porphyries-the albite granodiorite, light-colored granodiorite, and alkalis syenite-form irregular plutons and <br />thick dikes in the northeast corner of the district, whereas the younger porphyries tend to form thin dikes <br />throughout the district. The complex patterns of intersecting dikes in the northeast part of the district (pl. 1) <br />indicate that composite plutons exist at some depth. Quartz monzonite prophyry, which is intermediate in age, <br />forms one large lens and several small concordant lenses on the south end of Belleview Mountain as well as <br />many dikes throughout the district. Biotite-quartz latite, the youngest porphyry recognized, tends to form small <br />lenticular bodies south of Clear Creek, many of which mere emplaced along preexisting veins. <br />Except for the dikes that have been intruded along the Idaho Springs fault and the biotite-quartz latite <br />dikes that follow veins, the porphyries appear to have been intruded along joints, not along faults. <br />Characteristically, the country rock on either side of a dike has been separated but not offset, except locally <br />where a dike has guided a later fault. <br />QUATERNARY DEPOSITS <br />The Quaternary deposits are composed of alluvium, colluvial creep debris, and talus. Talus is common <br />on the steep slopes below cliffs. Colluvial creep debris is widespread but was mapped only where it completely <br />covers broad areas. The debris sheets rarely exceed 10 feet in thickness, but they effectively cover large areas of <br />bedrock. These debris sheets are composed of a heterogeneous mixture of angular rock fragments and fine- <br />grained material, some of which has moved downhill a considerable distance. Ridges of creep debris as high as <br />20 feet are common in many gullies that are flanked by debris sheets. The ridges are probably the result of <br />pressures created by the persistent downhill creep on both sides of the gullies. The creep debris sheets may <br />• have formed partly in late Pleistocene time because of the more intense frost conditions that prevailed then <br />(Harrison and Wells, 1959, p. 26). <br />Alluvium covers the floor of Clear Creek Canyon, parts of the valleys of Trail Creek and Spring <br />Gulch, and, locally, the terraces that are well above Clear Creek. The alluvium at the present drainage levels <br />consists of fine to coarse gravels, some of which is locally derived and some of which is derived from several <br />miles upstream. Ball (1908, p. 83-84) noted three sets of terraces near Idaho Springs; these are cut in bedrock at <br />about 160 feet, 55 feet, and 25 feet respectively above Clear Creek. The two higher terraces are capped by about <br />20 feet of gravel, and the lower is capped by about 5 feet of gravel. The terrace gravels are fine to coarse and <br />contain well-rounded boulders and cobbles. <br />Some of the gravels on the terraces or on the present valley floors may have been deposited in <br />Pleistocene time by melt waters from valley glaciers, which are known to have existed in the headwaters of <br />Clear Creek and some of its tributaries. There is no evidence of glaciation in the Idaho Springs district other <br />than the gravel deposits. <br />STRUCTURE <br />The structural framework of the Idaho Springs district is outlined by the major units of conformable <br />gneisses, which are folded along northeast-trending axes (pl. 2 ). During Precambrian time these rocks, which <br />now strike generally northeast, were deformed at least twice, and may have been faulted. The first deformation <br />was pervasive plastic folding that took place at considerable depth at high temperatures and pressures and was <br />accompanied by intense recrystallization and the emplacement of many small bodies of granitic rocks. The <br />second deformation was characterized by intense granulation as well as by folding in a relatively narrow zone. <br />I t apparently took place at a somewhat shallower depth throughout a 2-mile-wide zone, termed the Idaho <br />Springs-Ralston shear zone, which extends at least 20 miles northeastward to the margin of the Front Range <br />(Tweto and Sims, 1963). Dining the Laramide orogeny the rocks were jointed, intruded by a sequence of <br />. porphyritic igneous rocks, and cut by an anastomosing network of faults. <br />Page 16 of 31