OBSERVATIONS OF COMPOSITIONAL AND TEXTURAL VARIATION IN PEGMATITIC MICA FROM COLORADO SPRINGS, COLORADO USING LA-ICP-MS, SEM-EDS, AND PETROGRAPHY TECHNIQUES

The Pike’s Peak Batholith is a billion year old granite body that contains numerous pegmatites. The pegmatite at the center of this research is a miarolitic NYF pegmatite found at an elevation of 8200 to 8000 feet, below Sentinel Rock, just west of Colorado Springs, Colorado. This pegmatite is amazonite-bearing and also contains quartz, fluorite, zircon, thorite, cassiterite, and an unusual variety of mica that has been the focus of recent study.

There are distinct relationships between the textural occurrence of the mica and their morphology and growth habit. Where the mica is in contact with albite, a mica-quartz symplectite is observed. There are four types of mica, each classified by location within the pegmatite and zoning pattern. The degree of compositional overlap and similarity of textures, such as the formation of symplectite, suggest that these mica varieties are chemically and perhaps temporally related.

The mica is compositionally zoned and not altered by weathering processes. Using the Monier and Robert (1986) approach to estimating Li content, the data suggests that the Li content in the mica ranges as high as 3.89 formula units, trending from near lepidolite to the siderophyllite join; this is comparable with the Li-mica studied by Brigatti et al. (2000) from the same locality. Direct measurement of Li, to be completed using LA-ICP-MS analyses, is needed to better constrain Li content in these micas.