PETROGRAPHIC AND SCANNING ELECTRON MICROSCOPY INVESTIGATION OF PEGMATITE MINERALS FROM SENTINEL ROCK, PIKES PEAK BATHOLITH

The Pikes Peak batholith west of Colorado Springs is 1 billion year old granite that contains complex pegmatites, with microcline, amazonite, biotite, and smoky quartz predominantly. The purpose of this research is to understand the mineralogy, composition, and zoning in order to better understand the evolution of the pegmatites. Field work was completed over a course of two days at two different locations in October, 2010. At locality one, Specimen Rock, quartz, microcline and fluorite crystals were collected. The second day, collecting was done near Sentinel Rock, where amazonite crystals were collected in large quantities, in association with biotite and quartz. In the field we observed the gradation from graphic granite to biotite rich layers, to quartz with white to tan microcline that changes into rich green euhedral amazonite in the open pockets. The biotite in the biotite layers are up to 3.5 cm in diameter, and also included quartz and feldspar. The biotite layer is missing in some samples. Other crystals in the pocket are clear to smoky quartz, zoned biotite, and goethite pseudomorphs after siderite. The pegmatite was also highly fractured in the direction of crystal growth.

Using the Sentinel Rock samples, data was collected by making a continuous series of polished thin sections through the massive pegmatite towards the open pockets. A crystallization history was determined by study of the numerous fractures and mineralogical relationships. The observed sequence is graphic granite with a coarsening upward trend, growth of the biotite layer if present, microcline/albite containing perthitic textures intergrown through the quartz, then microcline that zones into amazonite and smoky quartz along with amazonite crystals in the open pocket. After the initial formation of the amazonite, severe fracturing occurred and a possible second crystallization of quartz and amazonite occurred. Future data collection will be done using a Scanning Electron Microscope, and will focus on the compositional zoning within each mineral, as well as the pegmatite itself, and host rock/pegmatite boundary composition.