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

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Temporary storm water management controls using silt fences and/or straw bales will be used during all • phases of excavation. See Tailings Impoundment Design Drawings attached as Appendix C, and Stormwater management plan attached as Appendix D. 3.2.1 Geological Data A. Unconsolidated materials in this area consists of topsoil ranging in thickness of 0"-24" B. Consolidated materials: Refer to 3.2.1 (C) Regional and local geological structure for a detailed description. C. Regional and local geological structure (excerpt from USGS Bulletin 1208, Economic Geology of the Idaho Springs District - Clear Creek and Gilpin Counties Colorado Moench, Robert H. and Drake, Avery Ala, Jr., 1966): GENERAL GEOLOGY The Idaho Springs district is underlain dominantly by gneissic, granitic, and pegmatitic rocks of Precambrian age (pl. 2), which constitute part of the core of the Front Range. These rocks are intruded by numerous small porphyritic dikes and irregular plutons of early Tertiary age and are cut by numerous faults. Some faults possibly originated in Precambrian time, but most formed near the close of the emplacement period of the early Tertiary magma sequence. Physical character and structure of the Precambrian and Tertiary rocks had a marked influence on the formation of the fault patterns. Accordingly, a brief description of the rock types and the structure of these rocks is given in the pages that follow. A more comprehensive report on the Precambrian rocks has been published . separately (Moench, 1964). The petrography and structure of the Tertiary intrusive rocks in this district and adjoining ones were reported in detail by Wells (1960). PRECAMBRIAN ROCKS The Precambrian rocks are a generally conformable succession of interlayered gneissic, granitic, and pegmatitic units. The gneissic rocks, which are dominantly metamorphosed sedimentary rocks, are the oldest and by far the most widespread and abundant rocks in the district. These rocks have been invaded by three varieties of granitic rock (from oldest to youngest) : granodiorite, quartz diorite, and biotite-muscovite granite. Several types of small pegmatite bodies are interlayered with and locally cut the gneissic and granitic rocks. The major gneissic rock units are interlayered biotite gneisses, granite gneiss, and microcline-quartz- plagioclase-biotite gneiss, which, for convenience, in most of the text are called, respectively, biotite gneiss, granite gneiss, and microcline gneiss. The biotite gneiss and granite gneiss are intermixed in layers that range from a fraction of an inch to several hundred feet in thickness. The thicker layers can be mapped individually at 1 : 6,000, but on plate 2 these two rock types are combined so that the major units shown are microcline gneiss and a mixture of biotite gneiss and granite gneiss. Within the latter mixed unit, granite gneiss increases in abundance southwestward across the district, apparently at the expense of the biotite gneiss. Thinly layered rocks that consist of roughly equal proportions of biotite gneiss and granite gneiss are termed migmatite. Small bodies and layers of amphibolite, calc-silicate gneiss, and quartz gneiss are associated with the major units; however, they are not shown on plate 2. The biotite gneiss and associated minor rocks were assigned to the Idaho Springs Formation by Ball (1906) and by Lovering and Goddard (1950, p. 19-20) ; the microcline gneiss near Idaho Springs was mapped by Lovering and Goddard (1950, pl. 2) as quartz monzonite gneiss and gneiss pegmatite. The granite gneiss and associated pegmatite were mapped by Harrison and Wells (1956, p. 50-53) in the adjoining Freeland-Lamartine district. 11