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1950 BUREAU OF MINES REPORT <br />Igneous intrusive. Two types of igneous intrusives <br />occur in the sediments of the Aspen district. Only one, <br />a quartz porphyry'" occurs in Smuggler mountain. It is <br />a sheet nearly conforming to bedding at or near the base <br />of the Weber [Belden). In the Zaugg crosscut it is 30 <br />ft. thick and occurs 5 ft. above the breccia contact <br />between the Leadville and the Weber [Belden]. <br />Sequence of events. (1) A granite batholith that did <br />not reach the surface was intruded; (2) this intrusion <br />was followed by the intrusion of related porphyries from <br />cupolas at the top of the batholith; (3) folding and <br />faulting related to magmatic'Sforces occurred throughout <br />the intrusion period; (4) the ore deposits were formed; <br />and (5) there was a renewal of faulting.76 <br />Structure. The principal structural feature is the <br />Castle Creek fault which bounds the Aspen district on <br />the west. Related to the fault is a narrow disturbed <br />zone containing folds, overturned strata, and both flat <br />and steep thrusts. The Castle Creek fault trends north- <br />south to immediately west of Aspen, then turns north- <br />westerly. Along the curve the dip flattens to about 50° <br />east. The central Aspen district occupies a syncline on <br />the east side of the fault, starting 3 miles south of <br />town as a tight fold with vertical limbs and plunging <br />under the valley to form a broad asymmetric fold, with <br />the east limb dipping 45°. Paleozoic formations in the <br />hanging wall of the fault diverge from it and the two <br />are about a mile apart in the vicinity of the town. The <br />main ore deposits have been found in the Carboniferous <br />formations along the east limb of the syncline. In <br />Smuggler mountain the formations dip uniformly 45° to <br />60° to the northwest. <br />The fold structures are cut by two sets of faults, one <br />approximately parallel to the axis of the syncline, the <br />other crossing the axis at 45° to 90°. In Smuggler mountain <br />the faults dip 35° to 50° southwest. In general their <br />footwalls moved upward, with displacements ranging from a <br />few feet to over 200 ft. Brecciation or shattering in zones <br />up to 50 ft. wide adjacent to the faults has great bearing <br />on ore deposition; the amount of brecciation is unrelated to <br />amount of displacement. <br />Now considered aplite porphyry, see ANNOTATED GLOSSARY. <br />ss Intrusive. <br />'6 This sequence has been significantly altered, see STEGEN REPORT 294. <br />Bruce A. Collins - 21 - BISLIOGRAPxx <br />