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PRECAMBRIAN ROCKS
<br />The Precambrian rocks are a generally conformable succession of interlayered gneissic, granitic, and
<br />pegmatitic units. The gneissic rocks, which are dominantly metamorphosed sedimentary rocks, are the oldest
<br />and by far the most widespread and abundant rocks in the district. These rocks have been invaded by three
<br />varieties of granitic rock (from oldest to youngest) : granodiorite, quartz diorite, and biotite-muscovite granite.
<br />Several types of small pegmatite bodies are interlayered with and locally cut the gneissic and granitic rocks.
<br />The major gneissic rock units are interlayered biotite gneisses, granite gneiss, and microcline-quartz-
<br />plagioclase-biotite gneiss, which, for convenience, in most of the text are called, respectively, biotite gneiss,
<br />granite gneiss, and microcline gneiss. The biotite gneiss and granite gneiss are intermixed in layers that range
<br />from a fraction of an inch to several hundred feet in thickness. The thicker layers can be mapped individually at
<br />1 : 6,000, but on plate 2 these two rock types are combined so that the major units shown are microcline gneiss
<br />and a mixture of biotite gneiss and granite gneiss. Within the latter mixed unit, granite gneiss increases in
<br />abundance southwestward across the district, apparently at the expense of the biotite gneiss. Thinly layered
<br />rocks that consist of roughly equal proportions of biotite gneiss and granite gneiss are termed migmatite. Small
<br />bodies and layers of amphibolite, calc-silicate gneiss, and quartz gneiss are associated with the major units;
<br />however, they are not shown on plate 2.
<br />The biotite gneiss and associated minor rocks were assigned to the Idaho Springs Formation by Ball
<br />(1906) and by Lovering and Goddard (1950, p. 19-20) ; the microcline gneiss near Idaho Springs was mapped
<br />by Lovering and Goddard (1950, pl. 2) as quartz monzonite gneiss and gneiss pegmatite. The granite gneiss and
<br />associated pegmatite were mapped by Harrison and Wells (1956, p. 50-53) in the adjoining Freeland-Lamartine
<br />district.
<br />In contrast to neighboring areas, granitic rocks are sparse in the Idaho Springs district. The few bodies
<br />. of granitic rocks large enough to show on plate 2 are small and appear to be satellite to the larger plutons or
<br />batholiths that crop out to the southwest, west, and north of the district. The granodiorite is similar to the
<br />Boulder Creek Granite (Lovering and Goddard, 1950, p. 25-27) ; the biotite-muscovite granite is similar to the
<br />Silver Plume Granite from the type locality at Silver Plume, Colo., about 16 miles southwest of Idaho Springs
<br />(Ball, 1906). Lithologic names rather than geographic formational names are used here because the stratigraphy
<br />of the Precambrian metasedimentary rocks and correlations of the intrusive rocks are not fully established.
<br />GNEISSIC ROCKS
<br />The gneissic rocks are divided into three major lithologic units: microcline gneiss (or microcline-
<br />quartz-plagioclase-biotite gneiss), biotite gneiss (or interelayered biotite gneisses) , and granite gneiss (or
<br />granite gneiss and pegmatite). The biotite gneiss and granite gneiss are grouped on plate 2, and together they
<br />are conformably interlayered with units of microcline gneiss. If these units are assumed not to be overturned, a
<br />stratigraphic succession can be recognized as shown in section B-B' of plate 2. The lowermost unit, a thick
<br />layer of mixed biotite gneiss and granite gneiss, forms the core of the Idaho Springs anticline on the southeast
<br />side of the district. This lowermost unit is overlain by a thin and discontinuous layer of microcline gneiss. The
<br />microcline gneiss, in turn, is overlain by a thick layer of mixed biotite gneiss and granite gneiss. A thick unit of
<br />microcline gneiss is higher in the stratigraphic succession, and it is overlain by a thick layer of mixed biotite
<br />gneiss and granite gneiss - the uppermost unit in the district.
<br />In addition to these major units, small bodies of amphibolite, calc-silicate gneiss, and quartz gneiss are
<br />exposed, but these small outcrops are not shown on plate 2. All the gneissic rocks are described here without
<br />regard to their apparent stratigraphic position.
<br />BIOTITE GNEISS
<br />Two main varieties of biotite gneiss are recognized: biotite-quartz-plagioclase gneiss and sillimanitic
<br />biotite-quartz-gneiss, both of which are locally garnetiferous. These rocks alternate with one another in layers
<br />. that range from about an inch to several feet in thickness and probably represent the original bedding. In
<br />outcrops the biotite gneiss is marked by its dark-gray color, pronounced layering, and tendency to split parallel
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