Abstract

The narrow northeast-trending Colorado mineral belt, the site of most of the major mining districts of Colorado, is characterized by intrusive porphyries and associated ore deposits of Laramide age and, in some places, by fissures and faults of northeasterly trend. The belt, about 250 miles long, extends diagonally across the generally north-trending mountain ranges of Colorado, occupies several different geologic environments, and seems to be independent of the present mountain structure.

The mineral belt follows an ancient zone of weakness defined by northeast-trending shear zones of Precambrian age in a belt 10–35 miles wide. Individual shear zones or clusters of zones are spaced from a mile or less to many miles apart. In most parts of the belt, a major northeast-trending shear zone is flanked by lesser shear zones in an echelon arrangement.

Movement occurred along the belt of shear zones through most of the Precambrian time recorded in the region. During this time, deformation progressed from a deep-seated environment characterized by folding and plastic flow to a more shallow environment characterized by fracture and retrograde metamorphism. The earliest movement occurred during regional folding, when folds were oriented parallel to the shear zones or were bent where they impinged upon the shear zones. Cataclastic deformation began after the folds had formed, but the earliest products of cataclasis re-crystallized as new gneisses, indicating that relatively intense metamorphic conditions either still prevailed or recurred. Later, when pressure and temperature were lower, cataclastic gneisses, pseudotachylyte, mylonite, and broad granulated zones formed, and in some places small crossfolds formed in layers of incompetent gneiss between cataclastically deformed competent layers. Still later in the Precambrian, gouge and breccia formed, partly by the degradation of earlier shear products of higher rank.

During Paleozoic and Mesozoic time, minor differential movements occurred repeatedly in the regional zone of shearing, as recorded by thinning, wedgeouts, and changes in facies of several sedimentary formations along the zone.

With the onset of the Laramide orogeny, magma invaded the regional zone of shearing and imparted to it the conspicuous features that characterize the mineral belt—intrusive igneous bodies and ore deposits. Fault movement occurred along the zone at this stage also but was on a smaller scale than it had been previously.

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