Geology of the Empire quadrangle, Grand, Gilpin, and Clear Creek Counties, Colorado

The Empire quadrangle, in the central part of the Front Range, is about 35 miles west of Denver, Colo. The bedrock of the quadrangle is made up predominantly of Precambrian met-. asedimentary, igneous, and metaigneous rocks. The metasedimentary rocks are composed of calc-silicate gneiss and biotite gneiss. A comparison of the mineralogy of the biotite gneiss with mesonorms of common sedimentary rocks indicates that the original sediments were probably interbedded shale, graywacke, and subgraywacke. The gneisses have been metamorphosed to mineral assemblages typical of the upper part of the amphibolite facies. Microcline gneiss ( microcline-quartz-plagioclase-biotite gneiss) may be metamorphosed sodic arkose or metamorphosed lava flows or intrusions. Hornblende gneiss and amphibolite may be metasedimentary or metaigneous rocks.