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the larger bodies of quartz diorite are dark gray, medium grained, and equigranular, whereas the margins are
<br />• typically well foliated. The massive rocks contain as much as 70 percent combined hornblende, clinopyroxene,
<br />and biotite, as much as 40 percent plagioclase (andesine), and generally less than 15 percent quartz. Accessory
<br />minerals-sphene, apatite, allanite, pyrite, magnetite, and zircon-form as much as 10 percent of the rock. Some
<br />specimens of the well-foliated rocks contain as much as 15 percent of untwinned orthoclase.
<br />BIOTITE-MUSCOVITE GRANITE
<br />Biotite-muscovite granite forms small sills, phacoliths, and a few thin dikes. Only two bodies of
<br />biotite-muscovite granite are large enough to be shown on plate 2, but some small bodies are shown on many of
<br />the mine maps. The rock is light tan or gray, fine to medium grained, equigranular to subporphyritic, and is
<br />characterized by abundant tabular crystals of feldspar that are as much as 1 cm in length. Near the margins of
<br />some bodies most of the tabular feldspar crystals and biotite books are oriented about parallel to the contacts.
<br />The biotite-muscovite granite contains approximately 30 percent quartz, 60 percent feldspar, and less than 10
<br />percent biotite and muscovite. Microcline predominates over plagioclase, which is mainly oligoclase.
<br />PEGMATITIC ROCKS
<br />Several types of pegmatitic rocks are exposed, but not all are readily distinguishable from the coarser
<br />parts of the granite gneiss. With one exception the two rock types are mineralogically similar, containing
<br />abundant quartz and microcline, subordinate amounts of plagioclase feldspar, and locally abundant biotite and
<br />magnetite. Some pegmatite dikes cut bodies of granodiorite or quartz diorite at various angles; they probably
<br />formed late in the cooling history of these rocks. Other dikes are similarly associated with biotite-muscovite
<br />granite. Still others cut some of the youngest Precambrian structural features of the district.
<br />TERTIARY INTRUSIVE ROCKS
<br />• The Idaho Springs district contains an intricate network of porphyry dikes and irregular plutons of
<br />early Tertiary age (pi. 1). These rocks constitute part of a belt of porphyries that extends northeastward across
<br />the Front Range. Lovering and Goddard (1950, p. 47) inferred that the porphyries of the eastern part of the
<br />Front Range are early Tertiary in age. The basis of their inference were (1) the presence of interbedded
<br />volcanic rocks in the Upper Cretaceous and Lower Tertiary (Paleocene) Denver and Middle Park Formations,
<br />and (2) the relation of porphyry intrusions in different parts of the Front Range to the chronology of Laramide
<br />orogenic movements. This age is close to approximate absolute age of 60 million years determined on uraninite
<br />from metalliferous veins of the region (Faul, 1954, p. 263), for the veins formed during the waning stage of
<br />igneous activity.
<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
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