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

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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 Imm 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. The quartz gneiss is light colored, fine to medium grained, and typically has a glassy luster. The gneissic structure of the rock results from slight differences in grain size in the laminae parallel to the rock layers. The rock contains as much as 80 percent quartz and some feldspar. Dark minerals, such as biotite and magnetite, are either sparse or absent. AMPIIIBOLITE AND ASSOCIATED CALC-SILICATE GNEISS Amphibolite commonly is exposed along the contact between the microcline gneiss and the biotite gneiss in the form of lenses locally more than 100 feet thick. Amphibolite also forms smaller layers and lenses in the microcline gneiss and, less commonly, in biotite gneiss and granite gneiss. The amphibolite is a dark-gray to black, fine- to medium-grained rock that contains homblende and andesine in various proportions and small amounts of quartz. Biotite and pyroxene are common, though rarely are both present in the same specimen. Some varieties of amphibolite are massive and nearly structureless, others are gneissic and laminated. The gneissic structure is produced by alternating hornblende- and plagioclase-rich layers and by the planar orientation of homblende. Calc-silicate gneiss locally forms irregular masses or crosscutting veinlike structures in amphibolite. The contacts are ragged and the calc-silicate gneiss appears to be an alteration product of the amphibolite. • The calc-silicate gneiss is mottled, light to dark colored, and fine to coarse grained. It contains calcium-rich garnet, quartz, abundant epidote, and some homblende, plagioclase, and, locally, clinopyroxene.