Rutile in Precambrian sillimanite-quartz gneiss and related rocks, east-central Front Range, Colorado

Abstract

Unusual rutile-bearing gneisses are thinly interlayered with metasedimentary and metavolcanic gneisses of Precambrian age in the east-central part of Colorado's Front Range. Layers and lenses of rutile-bearing gneiss range in thickness from 15 em to 30 m and in rutile content from trace amounts to about 5 percent. Rutile is most abundant in a sillimanitic topaz-quartz gneiss but is also found in sillimanite-quartz gneiss and biotite-quartz-plagioclase gneiss . The rutile-bearing sillimanitic gneisses have a simple but unusual mineral and chemical composition because they contain little else but quartz, sillimanite, topaz, and accessory rutile. Spectrographic analyses of rutile from the east-central Front Range and from eleven other localities in the United States indicate that the Front Range rutile is relatively pure, that is, it contains fewer metallic cations.