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<br />Spurr and Garrey (1908), Ball (1908), and Bastin and Hill (1917) made the first comprehensive studies
<br />of the porphyries in parts of the Idaho Springs district dealt within their respective reports. Lovering and
<br />Goddard (1950) utilized these previous reports to aid their investigation of the whole Front Range mineral belt.
<br />More recently, Wells (1960) made a detailed petrographic study of the porphyries of the Idaho Springs and
<br />adjacent mining districts. With some modifications in terminology, Wells followed the classifications adopted
<br />by the earlier workers, and his classification is used in this report.
<br />Wells (1960) described 13 kinds of porphyries, of which 9 are exposed in the Idaho Springs district.
<br />These rocks are separable on the basis of color, texture of the groundmass, size, shape, and abundance of the
<br />phenocrysts, qualitative and approximate quantitative mineralogy, and the character of fractured surfaces.
<br />Because Wells gave complete petrographic descriptions of all porphyries exposed in the region, these rocks are
<br />not described in detail here. Their salient characteristics are summarized in table 2.
<br />The Tertiary igneous rocks were emplaced as listed in table 2, from oldest to youngest in ascending
<br />order. This sequence shown in table 2 agrees with that of Wells (1960, fig. 58), except that he reversed the
<br />emplacement order of trachytic granite porphyry and quartz bostonite porphyry; however, he noted (p. 229) that
<br />the intersecting relationships between these rocks do reverse locally. The sequence of intrusion was determined
<br />by crosscutting relations and faulting relations, some of which were observed in the Idaho Springs district. All
<br />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 formed. 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 alkalic 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 Greek. 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.
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