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• GEOLOGIC HISTORY SUMMARIZED
<br />In Precambrian time a thick sequence of sedimentary rocks was deeply depressed into the earth's crust;
<br />it recrystallized at high temperatures and pressures and was intensely deformed. The rocks were deformed
<br />plastically into large, mainly open folds whose axes trend north to northeast. Concurrently, small bodies of
<br />granodiorite, quartz diorite, and quartz gabbro were emplaced, recrystallized, and slightly deformed after they
<br />solidified. Small bodies of biotite-muscovite granite probably were emplaced late in this early deformation -
<br />apparently early enough to form phacoliths in folds of this deformation but too late to be deformed or
<br />recrystallized.
<br />After an interval of deep erosion, the rocks were deformed again. This younger Precambrian
<br />deformation was largely restricted to the narrow Idaho Springs-Ralston shear zone which extends at least 20
<br />miles northeast and a few miles southwest of the Idaho Springs district. Within this zone, which is about 2 miles
<br />wide in the Idaho Springs district, pervasive granulation and folding were superposed on the previously folded
<br />rocks.
<br />Probably in late Precambrian time the through-going northwest-trending Idaho Springs fault, the J. L.
<br />Emerson-Gem fault system, and some smaller faults formed. A few relatively flat-lying faults and some steep
<br />north-trending faults may also have had their inception at this time, although their main movements came much
<br />later.
<br />The Front Range was arched during the Paleozoic, as indicated by the sedimentary record on its flanks
<br />(Lovering and Goddard, 1950, p. 29, 30) but this arching did not leave a recognizable record in the Idaho
<br />Springs district.
<br />During the Laramide orogeny in Late Cretaceous and early Tertiary time, the Front Range highland
<br />• was arched and uplifted (Lovering and Goddard, 1950, p. 57-60). In the Idaho Springs district this arching was
<br />accompanied by the formation of the regional joint system and was accompanied or followed by the successive
<br />emplacement of nine varieties of porphyritic igneous rock - first, primarily in irregular plutons, thick dikes, and
<br />lenticular masses, and later, primarily in long narrow dikes. After most of the igneous activity but before the
<br />emplacement of the latest variety, a network of faults formed that trends east, east-northeast, and northeast. The
<br />north-west-trending faults were then reactivated. Openings along the faults were filled with ore and gangue
<br />minerals, and, as indicated by the brecciation of minerals in many veins, fault movements continued during
<br />mineralization. This arching, jointing, and faulting evidently took place under compressive stresses that were
<br />oriented east-northeast. Late in the stage of mineralization, a few north-trending faults formed, and renewed
<br />movements occurred on some northwest-trending faults. The movement patterns indicate that this late faulting
<br />occurred under tensional stress oriented east-northeast.
<br />Since the Laramide disturbance the region has undergone deep erosion. The steep valley walls, narrow
<br />canyons, and fast-moving streams are indications that erosion continues today. The coarse alluvium in Clear
<br />Creek Canyon may, in part, be outwash from late Pleistocene valley glaciers that did not extend into the district.
<br />The continuous downhill movement of loose material is locally indicated by the colluviul creep debris sheets,
<br />though these, in part, may have formed in late Pleistocene time (Harrison and Wells, 1959, p. 26).
<br />(USGS Bulletin 1208, Economic Geology of the Idaho SjLnnas District - Clear Creek and Gilpin
<br />Counties, Colorado, Moench, Robert H. and Drake, Avery Ala, Jr., 1966).
<br />6.4.20(8)(d)
<br />The only major, identifiable fault know to exist in the area is the Idaho Springs Fault which is illustrated in USGS
<br />Bulletin 1208, Plate 2, Generalized Geologic Map and Sections of the Idaho Springs District, Colorado attached to
<br />this Exhibit T.
<br />This fault is located 1,500' to the south of the proposed operation and does not impact the underlying geology.
<br />. Detailed above in 6.4.20(8)(b) is a description of the vein and fracture systems that underlie this area and provide a
<br />hydraulic drainage connection to the Big Five Tunnel.
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