2009-09-25_PERMIT FILE - M2009076 ‎(31)‎

• Permit No: M2009076
• IBM Index Class Name: PERMIT FILE
• Doc Date: 9/25/2009
• Doc Name: Ex. T- EPP
• From: Venture Resources
• To: DRMS
• Media Type: D
• Archive: No

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. 35° 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.
REGIONAL JOINT SYSTEM
Four sets of joints are almost uniformly present in all rocks except the Tertiary porphyries and are
remarkably consistent in attitude despite differences in Precambrian structure. These sets, termed the "regional-
joint system," are summarized in figure 4, a stereodiagram that was constructed from the regional joint maxima
on Schmidt contour diagrams of joints in the Idaho Springs district. The prevalence of these joint sets
throughout the Idaho Springs and surrounding areas, the angular relations between the sets, and the fact that
they intersect on a nearly common line suggest that they are genetically related to one another. The regional
joint system in turn shows systematic relations to the broad archlike form of the Front Range.
The uplift and arching of the Front Range in Paleozoic and Laramide times (Lovering and Goddard,
1950, p. 57-63) are recognized major disturbances after the emplacement of the biotite-muscovite granite to
which the regional joint system might logically be related. The Laramide arching was the more pronounced of
the two disturbances. This and the fact that the geometric relation of the regional joint sets to the known shape
of the Laramide arch is similar to relations shown between joints and folds elsewhere (Harrison and Moench,
1961, fig. 8) indicate that the arch and the regional joint system are genetically related. The arch trends about N.
15° W. near the Idaho Springs district; accordingly, the joint set that strikes N. 19° W. is called the "regional
longitudinal set" because it is subparallel to the arch. The arch disappears southward near Cripple Creek, Colo.,
which indicates that it plunges gently southward. A set of cross joints related to the arch should dip steeply
north and strike east-northeast; accordingly, the set that strikes N. 70° E. and dips steeply north is termed the
"regional cross joint set." So-called diagonal joints are common in folded areas. Ideally, two sets are present, in
each, the acute angle of intersection is bisected by the cross joints and the obtuse angle of intersection is
bisected by the trend of the folds. The regional cross-joint set shown in figure 4 bisects the acute angle between
two intersecting sets, which are accordingly termed the "regional diagonal joint sets"
The regional joint system probably formed as a result of the Laramide arching of the Front Range
under compressional stresses that were oriented east-northeast. The two diagonal joint sets have the position of
shear fractures for such a stress system and probably originated by shear. The cross joint set has the position of
tension fractures for this stress system and may have originated by elongation or bending of the axis of the arch.
Tension fractures may also form approximately parallel to the axial plane of a fold owing to the stretching of
layers across the top of the fold, and they may form owing to the release of the principal stress. The regional
longitudinal joint set may have originated in both ways. (See Harrison and Moench (1961) for a review of the
joints expected on folds.)
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