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

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The kinds of intrusive bodies formed by the porphyries changed as time passed. Several of the older is porphyries-the albite granodiorite, light-colored granodiorite, and alkalic syenite-form irregular plutons and thick dikes in the northeast corner of the district, whereas the younger porphyries tend to form thin dikes throughout the district. The complex patterns of intersecting dikes in the northeast part of the district (pl. 1) indicate that composite plutons exist at some depth. Quartz monzonite prophyry, which is intermediate in age, forms one large lens and several small concordant lenses on the south end of Belleview Mountain as well as many dikes throughout the district. Biotite-quartz latite, the youngest porphyry recognized, tends to form small lenticular bodies south of Clear Creek, many of which mere emplaced along preexisting veins. Except for the dikes that have been intruded along the Idaho Springs fault and the biotite-quartz latite dikes that follow veins, the porphyries appear to have been intruded along joints, not along faults. Characteristically, the country rock on either side of a dike has been separated but not offset, except locally where a dike has guided a later fault. QUATERNARY DEPOSITS The Quaternary deposits are composed of alluvium, colluvial creep debris, and talus. Talus is common on the steep slopes below cliffs. Colluvial creep debris is widespread but was mapped only where it completely covers broad areas. The debris sheets rarely exceed 10 feet in thickness, but they effectively cover large areas of bedrock. These debris sheets are composed of a heterogeneous mixture of angular rock fragments and fine- grained material, some of which has moved downhill a considerable distance. Ridges of creep debris as high as 20 feet are common in many gullies that are flanked by debris sheets. The ridges are probably the result of pressures created by the persistent downhill creep on both sides of the gullies. The creep debris sheets may have formed partly in late Pleistocene time because of the more intense frost conditions that prevailed then (Harrison and Wells, 1959, p. 26). Alluvium covers the floor of Clear Creek Canyon, parts of the valleys of Trail Creek and Spring Gulch, and, locally, the terraces that are well above Clear Creek. The alluvium at the present drainage levels consists of fine to coarse gravels, some of which is locally derived and some of which is derived from several miles upstream. Ball (1908, p. 83-84) noted three sets of terraces near Idaho Springs; these are cut in bedrock at about 160 feet, 55 feet, and 25 feet respectively above Clear Creek. The two higher terraces are capped by about 20 feet of gravel, and the lower is capped by about 5 feet of gravel. The terrace gravels are fine to coarse and contain well-rounded boulders and cobbles. Some of the gravels on the terraces or on the present valley floors may have been deposited in Pleistocene time by melt waters from valley glaciers, which are known to have existed in the headwaters of Clear Creek and some of its tributaries. There is no evidence of glaciation in the Idaho Springs district other than the gravel deposits. STRUCTURE The structural framework of the Idaho Springs district is outlined by the major units of conformable gneisses, which are folded along northeast-trending axes (pl. 2 ). During Precambrian time these rocks, which now strike generally northeast, were deformed at least twice, and may have been faulted. The first deformation was pervasive plastic folding that took place at considerable depth at high temperatures and pressures and was accompanied by intense recrystallization and the emplacement of many small bodies of granitic rocks. The second deformation was characterized by intense granulation as well as by folding in a relatively narrow zone. I t apparently took place at a somewhat shallower depth throughout a 2-mile-wide zone, termed the Idaho Springs-Ralston shear zone, which extends at least 20 miles northeastward to the margin of the Front Range (Tweto and Sims, 1963). During the Laramide orogeny the rocks were jointed, intruded by a sequence of porphyritic igneous rocks, and cut by an anastomosing network of faults. The Precambrian structure of the district was described in detail by Moench (1964), and the joint patterns and Precambrian structure of a larger area were described by Moench, Harrison, and Sims (1962) and by Harrison and Moench (1961). These facets of the geology of the district are summarized here. 10