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

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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 altemating 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. The granite gneiss is light colored and contains sparse to abundant wisps, laminae, and layers of biotite gneiss. Excluding the layers of biotite gneiss, most of the unit is nearly devoid of dark minerals and has the • composition of a true granite; it contains abundant quartz and microcline and subordinate amounts of sodic plagioclase. The rock is typically fine to medium grained, and the gneissic structure is produced mainly by layers of slightly different grain size as well as by conformable inclusions of biotite gneiss. Locally, the rock is pegmatitic and contains feldspar crystals as much as 3 feet across. The coarse feldspar is white to pink and contains graphic intergrowths of quartz. MICROCLINE GNEISS Microcline gneiss (or microcline-quartz-plagioclase-biotite gneiss) is exposed in thin discontinuous layers near Idaho Springs and in a major layer that extends northward and eastward far beyond the limits of the mapped area. This extensive microcline gneiss layer wedges near the south margin of the district (pl. 2). The contacts between microcline gneiss and biotite gneiss are typically sharp, traceable for long distances, and provide the best structural "markers" in the district. The microcline gneiss is a fine- to medium-grained light-gray or light-tan rock that is characteristically thinly laminated and well foliated. Laminae are typically lmm or less thick and are in alternating layers that are characterized by abundant or sparse biotite. Biotite-rich layers rarely exceed 1 inch in thickness. The rock contains 25-50 percent quartz, 30-55 percent oligoclase, as much as 35 percent microcline, and typically less than 10 percent biotite. The microcline gneiss contains many small conformable layers and lenses of amphibolite and also some lenses of biotite gneiss and granite gneiss that are large enough to show on plate 2. QUARTZ GNEISS Several thin layers of quartz gneiss are exposed along the southeast side of the area, directly north and south of Idaho Springs. The layers are rarely more than about 15 feet thick, but they may be traced as far as 1 . mile along their strike.