PETROLOGY AND WEATHERING OF THE IRON DIKE, BOULDER AND LARIMER COUNTIES, COLORADO

The Iron Dike in Boulder and Larimer counties, Colorado, is magnetite-rich diabase and can be traced diagonally across the central part of the Front Range of Colorado for nearly 38 miles. The fabric and mineralogy indicate emplacement by a single, rapid eruption originating in a deep, laterally wide-spread source. The magma was partly crystallized by the time it reached the level of the present surface exposures. Intrusive pressure continued during consolidation and produced fracturing and dislocation of previously formed crystals and promoted deuteric alteration by providing channels for concentration and movement of late residual solutions. The dike is chemically unlike and predates probably all the intrusive and extrusive rocks in the normal Cenozoic igneous sequence of the central Front Range.

Weathering of the dike near Sugarloaf, Colorado, caused extensive oxidation and hydration of iron-bearing minerals, especially pyroxene and chlorite, and downward transportation and deposition of colloidal limonitic material. Leaching has removed considerable calcium and magnesium from the upper part of the weathered zone. Crystallized clay minerals are notably absent.

Comparison of the weathering of the diabase and Precambrian granodiorite into which the dike was intruded indicates approximately the same amount of decomposition as measured by changes in the ferromagnesian minerals in both rocks. Deeper penetration of weathering effects in the granodiorite as compared to the diabase is attributed to a higher permeability in the granodiorite.