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

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minerals-sphene, apatite, allanite, pyrite, magnetite, and zircon-form as much as 10 percent of the rock. Some specimens of the well-foliated rocks contain as much as 15 percent of untwinned orthoclase. BIOTITE-MUSCOVITE GRANITE Biotite-muscovite granite forms small sills, phacoliths, and a few thin dikes. Only two bodies of biotite-muscovite granite are large enough to be shown on plate 2, but some small bodies are shown on many of the mine maps. The rock is light tan or gray, fine to medium grained, equigranular to subporphyritic, and is characterized by abundant tabular crystals of feldspar that are as much as 1 cm in length. Near the margins of some bodies most of the tabular feldspar crystals and biotite books are oriented about parallel to the contacts. The biotite-muscovite granite contains approximately 30 percent quartz, 60 percent feldspar, and less than 10 percent biotite and muscovite. Microcline predominates over plagioclase, which is mainly oligoclase. PEGMATITIC ROCKS Several types of pegmatitic rocks are exposed, but not all are readily distinguishable from the coarser parts of the granite gneiss. With one exception the two rock types are mineralogically similar, containing abundant quartz and microcline, subordinate amounts of plagioclase feldspar, and locally abundant biotite and magnetite. Some pegmatite dikes cut bodies of granodiorite or quartz diorite at various angles; they probably formed late in the cooling history of these rocks. Other dikes are similarly associated with biotite-muscovite granite. Still others cut some of the youngest Precambrian structural features of the district. TERTIARY INTRUSIVE ROCKS The Idaho Springs district contains an intricate network of porphyry dikes and irregular plutons of early Tertiary age (pl. 1). These rocks constitute part of a belt of porphyries that extends northeastward across the Front Range. Lovering and Goddard (1950, p. 47) inferred that the porphyries of the eastern part of the Front Range are early Tertiary in age. The basis of their inference were (1) the presence of interbedded volcanic rocks in the Upper Cretaceous and Lower Tertiary (Paleocene) Denver and Middle Park Formations, and (2) the relation of porphyry intrusions in different parts of the Front Range to the chronology of Laramide orogenic movements. This age is close to approximate absolute age of 60 million years determined on uraninite from metalliferous veins of the region (Faul, 1954, p. 263), for the veins formed during the waning stage of igneous activity. Spurr and Garrey (1908), Ball (1908), and Bastin and Hill (1917) made the first comprehensive studies of the porphyries in parts of the Idaho Springs district dealt within their respective reports. Lovering and Goddard (1950) utilized these previous reports to aid their investigation of the whole Front Range mineral belt. More recently, Wells (1960) made a detailed petrographic study of the porphyries of the Idaho Springs and adjacent mining districts. With some modifications in terminology, Wells followed the classifications adopted by the earlier workers, and his classification is used in this report. Wells (1960) described 13 kinds of porphyries, of which 9 are exposed in the Idaho Springs district. These rocks are separable on the basis of color, texture of the groundmass, size, shape, and abundance of the phenocrysts, qualitative and approximate quantitative mineralogy, and the character of fractured surfaces. Because Wells gave complete petrographic descriptions of all porphyries exposed in the region, these rocks are not described in detail here. Their salient characteristics are summarized in table 2. The Tertiary igneous rocks were emplaced as listed in table 2, from oldest to youngest in ascending order. This sequence shown in table 2 agrees with that of Wells (1960, fig. 58), except that he reversed the emplacement order of trachytic granite porphyry and quartz bostonite porphyry; however, he noted (p. 229) that the intersecting relationships between these rocks do reverse locally. The sequence of intrusion was determined by crosscutting relations and faulting relations, some of which were observed in the Idaho Springs district. All varieties of porphyry except the biotite-quartz latite, the youngest of the sequence, are cut by the metalliferous veins at many places and were emplaced before the veins formed. The biotite-quartz latite, on the other hand, • cuts metalliferous veins in many places. 9