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

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The north-trending faults consistently show small left-lateral offset that took place at a late stage of • mineralization. Interestingly, the northwest-trending faults in or near the Idaho Springs fault and the J. L. Emerson-Gem fault similarly show late, but right-lateral, offset. This offset is opposite to the dominant movements on the northwest-trending faults. These relations indicate that the late movements on the two fault sets were about contemporaneous. The north-trending and northwest-trending sets have the expected position and movement patterns caused by compression that was oriented north-northwest. These patterns, however, are also attributable to tensional stress that was oriented east-northeast. The tensional stress may have formed subsequent to the Laramide orogeny and seems more probable than a dominant north-northwest oriented compression. JOINTS All rocks of the district contain two or more sets of joints. A study of joints in the rocks of the Idaho Springs and adjacent areas by Harrison and Moench (1961) revealed that many are related to the emplacement and cooling history of the Precambrian and Tertiary intrusive rocks; other joints may be related to the two Precambrian deformations, and still others may be related to the Laramide deformation. The joints believed to be of Laramide age provide much insight to the Laramide deformation. Detailed descriptions of the joints and the methods used to smoke them were presented by Harrison and Moench (1961) and are only summarized here. Harrison and Moench (1961) measured the attitudes of joints in outcrops throughout the area and plotted the data on Schmidt equal-area projections. The projections were then contoured-according to the method described by Billings (1954, p. 111-114) except that Harrison and Moench used the upper hemisphere - to reveal the dominant attitudes of the joint sets. The diagrams and the data on the geologic occurrence of joints were then examined to determine, as far as possible, those joint sets which may be related to the intrusive rocks, those which may be related to the two Precambrian deformation systems, and those, if any, which may be independent of these features. Some of the problems that arise in the interpretation of contour diagrams were . discussed by Harrison and Moench (1961). Harrison and Moench (1961) found that certain joints are largely confined to the intrusive rocks, others vary in attitude and abundance according to variations of the Precambrian structure, and still others, termed the "regional joint system," are nearly ubiquitous and consistent in attitude in areas of contrasting lithology and structure. PRIMARY IGNEOUS JOINTS All the Precambrian and Tertiary intrusive rocks contain primary joints that probably formed during the emplacement and cooling of these rocks. Such joints may extend slightly into the wallrocks, but these joints are largely confined to the intrusive bodies. Tertiary dikes contain longitudinal joints that parallel the dike walls and "ladder" joints that are normal to the walls (Balk, 1937, p. 34-36, 97). The larger plutons of Tertiary porphyry undoubtedly have their own joint patterns, but they were not studied. The Precambrian granodiorite and biotite-muscovite granite have distinctive joints that commonly contain pegmatite or granitic dikes. (See Harrison and Wells (1959, p. 36) for a complete description of the primary igneous joints in Chicago Creek area.) JOINTS RELATED TO PRECAMBRIAN FOLDS Some joint sets can be related to the Precambrian folds. A few joints are normal to the axes of folds of the older deformation - flattening in dip where the fold axes steepen in plunge - and apparently are cross joints that are related to the older fold system. Longitudinal joints, which parallel the fold axes, and diagonal joints, which form a small acute angle to the cross joints, apparently are related to older Precambrian folds. Cross joints that are possibly related to the younger Precambrian deformation strike about N. 351 W., about normal to the trend of the younger Precambrian fold axes; such joints are most abundant in areas that were strongly affected by the younger Precambrian deformation. • 16