PEGMATITE, GRANODIORITE, GRANITE, WHITE ROCK; AT SPRUCE PINE ITS ALL GOOD!

Initially, mining of muscovite (and later feldspar) focused on the coarse-grained parts of pegmatites in the Spruce Pine District. Typical mining operations were small and confined to individual dikes or sills of pegmatite.

Decreased demand for muscovite and introduction of flotation at the end of WWII ended mining operations in many of the small pegmatites. Today, mining at Spruce Pine is concentrated in the larger masses (stocks) of granitoid rocks around the town of Spruce Pine. Published maps showing the internal structure of these stocks are not available, but cursory examination reveals a number of individual intrusions within the stocks resulting in a mixture of textures ranging from aplitic to coarse-grained granitic to pegmatitic, often exposed on the same quarry wall. Subsolidus deformation obscures internal contacts and results in a mortar texture with larger feldspar, quartz and muscovite crystals enclosed in a fine-grained fabric of these same minerals. The complexity of these textures accounts for the confusion over the naming of the rocks as pegmatites or granites or granodiorites. Area of pegmatite texture do exist within the stocks, but much of the rock currently mined at Spruce Pine is coarse-grained granodiorite.

Plagioclase + K-feldspar + quartz + muscovite + garnet is the dominant assemblage in the Spruce Pine granodiorites. Biotite occurs in some rocks. Major element chemistry of feldspars, mica and garnet is remarkably uniform. There is no systematic variation in mineral chemistry between pegmatites and host granitoids, nor is there evidence of compositional variation of individual minerals within a pegmatite. Quartz from the Spruce Pine District shows little variation in stable isotope composition.

Despite the similarity in major mineralogy among the mines in the Spruce Pine District, there are differences in the product produced. Quartz from some mines produces a clean product, while quartz from other mines does not. Differences are probably related to something other than the original mineralogy and may reflect some post-magmatic deformation and/or recrystallization event.