INTERPRETATION OF F-BASED ALTERATION IN MICAS FROM COLORADO SPRINGS, COLORADO

The Pike’s Peak Batholith is a 1 billion year old granite that contains numerous pegmatites. One of these is a miaroliticNYF pegmatite found at an elevation of 8200-8000 feet, below Sentinel Rock in Colorado Springs, Colorado. The pegmatite is amazonite-bearing and also contains quartz, albite, fluorite, zircon, thorite, cassiterite, a REE mineral, and unusual varieties of compositionally and texturally zoned Li-mica that are the focus of this study.

Petrographic, SEM-EDS, and EPMA techniques were used to correlate the growth of the micas to the crystallization history of the pegmatite and subsequent hydrothermal alteration. Micas occur throughout the pegmatite; some are unzonedand are interpreted to be unaltered. Others are distinctly compositionally-zoned, trending from the siderophylite-polythionite series to an annite composition. Where mica crystals are in contact with albite, a mica-quartz symplectite is developed along the interface. The interior zones of these crystals commonly include fluorite and Fe-oxide; the symplectite zones contain a metamict REE-phase comprised of Si, Al, Ca, Na, Fe, Ce, Nd, La, Sm, Gd, Y, and Th. The albite nearest the pocket is almost completely replaced with a fine grained white mica sericite that is intergrown with fluorite crystals. Li content was calculated using the method described by Monier and Robert (1986). LA-ICP-MS analyses on four select micas were used to gather in-situ trace element and Li data. Calculated and measured Li correspond well with each other, averaging 1.9105 formula units for calculated Li and 1.9766 formula units for measured Li. The symplectite-bearing zones within each mica are much more enriched in Zn, Ti, Sc, Mn, Cs, and Nb than the other compositional zones.

Previous work on a similar variety of pegmatite (Pezzotta et al., 2005) described two possible hydrothermal event models for the growth of fluorite, trace-element enrichment, and sericite replacement. The models require F-saturated fluids to alter the mica and concentrate the trace-elements within REE-bearing phases and within one particular zone of the mica. The formation of mica-quartz symplectite observed along albite-mica boundaries is also consistent with these models.