2009-09-25_PERMIT FILE - M2009076 (31)

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• GRANITIC ROCKS Granitic rock units are intrusive igneous rocks and were emplaced in the following order: Granodiorite, quartz diorite, and biotite-muscovite granite. The granodiorite and quartz diorite occur as small, nearly conformable bodies, and the biotite-muscovite granite forms small sills, phacoliths, and a few thin crossdressing dikes. GRANODIORTTE Several small bodies of granodiorite crop out in the southeastern part of the district. The granodiorite exposed here is fine grained and schistose and is similar to the granodiorite found near the borders of larger bodies in the Chicago Creek area (Harrison and Wells, 1959, p. 12). The rock is dark gray and composed of about IS percent quartz, 40 percent oligociase-andesine, 15 percent microcline, 20 percent biotite, and 10 percent accessory minerals-sphene, magnetite, apatite, allanite, and epidote. QUARTZ DIORITE Quartz diorite forms several small nearly conformable bodies on the southeast side of the district. The rock is similar in geologic occurrence and texture to the granodiorite but is darker in color. The central parts of the larger bodies of quartz diorite are dark gray, medium grained, and equigranular, whereas the margins are typically well foliated. The massive rocks contain as much as 70 percent combined homblende, clinopyroxene, and biotite, as much as 40 percent plagioclase (andesine), and generally less than 15 percent quartz. Accessory 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. 11