MICA COMPARISON OF THREE AMAZONITE BEARING PEGMATITES: EVIDENCE FOR ZONING VERSUS ALTERATION IN MICA

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

The mica is compositionally zoned and possibly altered due to weathering. Calculated Li content (Monier and Robert, 1986) based on microprobe spot analyses suggests that the mica is Li-bearing, comparable with the Li-mica studied by Brigatti et al. (2000). Where the mica is in contact with albite, a mica-quartz symplectite is developed. There are four types of mica that occur in different portions of the pegmatite: open pocket mica, close to the pocket mica, graphic granite mica, and aplite mica. All four types appear to be interrelated due to compositional overlap and similarity of textures such as symplectite.

To test the possible role of late-stage alteration in this mica, micas from two additional pegmatites from Lake George, Colorado will be used for comparison. These samples have similar mineralogy (amazonite, albite, fluorite, quartz, and mica), textures (graphic granite, open pocket, and aplite) and exposures to climate. The textures, compositional variations, and weathering characteristics will be compared to the previously-studied pegmatite using light microscopy, SEM, electron microprobe, and ICP-MS analytical techniques.