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2010-07-27_PERMIT FILE - M2009076 (23)
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2010-07-27_PERMIT FILE - M2009076 (23)
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Last modified
8/24/2016 4:16:50 PM
Creation date
7/29/2010 2:59:30 PM
Metadata
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Template:
DRMS Permit Index
Permit No
M2009076
IBM Index Class Name
PERMIT FILE
Doc Date
7/27/2010
Doc Name
Adequacy Response- Ex. T- EPP
From
Venture Resources
To
DRMS
Email Name
ACS
Media Type
D
Archive
No
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GRANITE GNEISS AXD PEGMATITE <br />• Granite gneiss (or granite gneiss and pegmatite) is exposed throughout the district in layers and lenses <br />that range in thickness from less than an inch to several hundred feet. It is generally associated with biotite <br />gneiss, and in many places relatively equal amounts of these two rock types are intimately mixed in thin <br />alternating layers that form migunatite. Granite gneiss, some of which is also associated with microcline gneiss, <br />is most abundant, in the southwest corner of the district. Northeastward, layers of biotite gneiss are more <br />abundant, and layers of granite gneiss are less abundant and discontinuous. <br />The granite gneiss is light colored and contains sparse to abundant wisps, laminae, and layers of biotite <br />gneiss. Excluding the layers of biotite gneiss, most of the unit is nearly devoid of dark minerals and has the <br />composition of a true granite; it contains abundant quartz and microcline and subordinate amounts of sodic <br />plagioclase. The rock is typically fine to medium grained, and the gneissic structure is produced mainly by <br />layers of slightly different grain size as well as by conformable inclusions of biotite gneiss. Locally, the rock is <br />pegmatitic and contains feldspar crystals as much as 3 feet across. The coarse feldspar is white to pink and <br />contains graphic intergrowths of quartz. <br />MICROCLINE GNEISS <br />N icrocline gneiss (or microcline-quartz-plagioclase-biotite gneiss) is exposed in thin discontinuous <br />layers near Idaho Springs and in a major layer that extends northward and eastward far beyond the limits of the <br />mapped area. This extensive microcline gneiss layer wedges near the south margin of the district (pl. 2). The <br />contacts between microcline gneiss and biotite gneiss are typically sharp, traceable for long distances, and <br />provide the best structural "markers" in the district. <br />The microcline gneiss is a fine- to medium-grained light-gray or light-tan rock that is characteristically <br />thinly laminated and well foliated. Laminae are typically Imm or less thick and are in alternating layers that are <br />• characterized by abundant or sparse biotite. Biotite-rich layers rarely exceed I inch in thickness. The rock <br />contains 25-50 percent quartz, 30-55 percent oligoclase, as much as 35 percent microcline, and typically less <br />than 10 percent biotite. <br />The microcline gneiss contains many small conformable layers and lenses of amphibolite and also <br />some lenses of biotite gneiss and granite gneiss that are large enough to show on plate 2. <br />QUARTZ GNEISS <br />Several thin layers of quartz gneiss are exposed along the southeast side of the area, directly north and <br />south of Idaho Springs. The layers are rarely more than about 15 feet thick, but they may be traced as far as 1 <br />mile along their strike. <br />The quartz gneiss is light colored, fine to medium grained, and typically has a glassy luster. The <br />gneissic stricture of the rock results from slight differences in grain size in the laminae parallel to the rock <br />layers. The rock contains as much as RO percent quartz and some feldspar. Dark minerals, such as biotite and <br />magnetite, are either sparse or absent. <br />AMPHIBOLITE AND ASSOCIATED CALC-SILICATE GNEISS <br />Amplnibolite commonly is exposed along the contact between the microcline gneiss and the biotite <br />gneiss in the form of lenses locally more than 100 feet thick. Amphibolite also forms smaller layers and lenses <br />in the microcline gneiss and, less commonly, in biotite gneiss and granite gneiss. <br />The amphibolite is a dark-gray to black, fine- to medium-grained rock that contains hornblende and <br />andesine in various proportions and small amounts of quartz. Biotite and pyroxene are common, though rarely <br />are both present in the same specimen. Some varieties of amphibolite are massive and nearly structureless, <br />• others are gneissic and laminated. The gneissic stricture is produced by alternating hornblende- and <br />plagioclase-rich layers and by the planar orientation of hornblende. <br />Page 13 of 31
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