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

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In contrast to neighboring areas, granitic rocks are sparse in the Idaho Springs district. The few bodies • of granitic rocks large enough to show on plate 2 are small and appear to be satellite to the larger plutons or batholiths that crop out to the southwest, west, and north of the district. The granodiorite is similar to the Boulder Creek Granite (Lovering and Goddard, 1950, p. 25-27) ; the biotite-muscovite granite is similar to the Silver Plume Granite from the type locality at Silver Plume, Colo., about 16 miles southwest of Idaho Springs (Ball, 1906). Lithologic names rather than geographic formational names are used here because the stratigraphy of the Precambrian metasedimentary rocks and correlations of the intrusive rocks are not fully established. GNEISSIC ROCKS The gneissic rocks are divided into three major lithologic units: microcline gneiss (or microcline- quartz-plagioclase-biotite gneiss), biotite gneiss (or interelayered biotite gneisses) , and granite gneiss (or granite gneiss and pegmatite). The biotite gneiss and granite gneiss are grouped on plate 2, and together they are conformably interlayered with units of microcline gneiss. If these units are assumed not to be overturned, a stratigraphic succession can be recognized as shown in section B-B' of plate 2. The lowermost unit, a thick layer of mixed biotite gneiss and granite gneiss, forms the core of the Idaho Springs anticline on the southeast side of the district. This lowermost unit is overlain by a thin and discontinuous layer of microcline gneiss. The microcline gneiss, in turn, is overlain by a thick layer of mixed biotite gneiss and granite gneiss. A thick unit of microcline gneiss is higher in the stratigraphic succession, and it is overlain by a thick layer of mixed biotite gneiss and granite gneiss - the uppermost unit in the district. In addition to these major units, small bodies of amphibolite, calc-silicate gneiss, and quartz gneiss are exposed, but these small outcrops are not shown on plate 2. All the gneissic rocks are described here without regard to their apparent stratigraphic position. • BIOTITE GNEISS Two main varieties of biotite gneiss are recognized: biotite-quartz-plagioclase gneiss and sillimanitic biotite-quartz-gneiss, both of which are locally garnetiferous. These rocks alternate with one another in layers that range from about an inch to several feet in thickness and probably represent the original bedding. In outcrops the biotite gneiss is marked by its dark-gray color, pronounced layering, and tendency to split parallel to the layering. Conformable layers and lenses of granite gneiss, present in most exposures, emphasize the layered appearance of the unit. Typical biotite-quartz-plagioclase gneiss is fine grained, equigranular, light to dark gray, and is faintly to intensely foliated. The gneiss typically contains quartz and plagioclase in nearly equal amounts and 19-35 percent biotite. The feldspar composition generally ranges from oligoclase to andesine. Foliation is produced by a parallel alignment of biotite and locally by segregation of minerals into light and dark layers. The sillimanitic biotite-quartz gneiss is light to dark gray and is well foliated; it is flecked with pock and smears of white fibrous sillimanite and. has a marked schistose structure. The rock contains abundant quartz, about 20 percent biotite, as much as 30 percent sillimanite (but generally much less), and, commonly, some microcline and albite-oligoclase. Small amounts of muscovite can be found in most specimens, and garnet (almandine-spessartite) is locally abundant. GRANITE GNEISS AXD PEGMATITE Granite gneiss (or granite gneiss and pegmatite) is exposed throughout the district in layers and lenses that range in thickness from less than an inch to several hundred feet. It is generally associated with biotite gneiss, and in many places relatively equal amounts of these two rock types are intimately mixed in thin alternating layers that form migmatite. Granite gneiss, some of which is also associated with microcline gneiss, is most abundant, in the southwest corner of the district. Northeastward, layers of biotite gneiss are more • abundant, and layers of granite gneiss are less abundant and discontinuous. 6