A PETROLOGIC EXAMINAITON OF THE MINES IN THE MINPRO PLUTON OF THE SPRUCE PINE PLUTONIC SUITE, WESTERN NORTH CAROLINA

The value of the granitiods in the Spruce Pine Mining District (SPMD) has been recognized for centuries. Native Americans in western North Carolina mined the pegmatites for the very coarse-grain muscovite prior to European Contact. Today, coarse-grained granodiorite ore is mined from open pit mines for feldspar, muscovite, and ultra high purity quartz. These granitoid rocks are part of the larger Spruce Pine Plutonic Series (SPPS) and consist of hundreds of small dikes and sills enclosed in amphibolite-grade schists and gneisses. Coalescence of smaller intrusions forms several small plutons in the vicinity of the town of Spruce Pine. The Minpro pluton, north of the town Spruce Pine, is the location of recently intensifying mining activity in many mines operated by several different companies. One company, the Feldspar Corporation (FC), began mining here in 2001. A wall in the FC mine was mapped and sampled in the spring of 2001. Exposed in this wall was a typical section of Spruce Pine granitoid rocks including the coarse-grained granodiorite and pegmatite. Samples used for this study come from both the pegmatites and the host granodiorite. The purpose of this study is to compare the composition of minerals in the FC mine to compositions from other mines in the Minpro pluton and to test for regional zoning patterns in the SPPS.

Miocroprobe analyses were done on feldspars, garnet and muscovite. Plagioclase is unzoned oligoclase (Ab 85 to 88). Exsolution lamella of albite (Ab 98) occur in the K-feldspar. The garnet is essentially an almandine-spessertine solid solution (alm+spss = 86%) with a few mole % of other components (grossular = 10%; pyrope= 4%). Some garnet grains with Ca-rich rims were confirmed. Muscovite contains several wt % FeO (4 to 6.17). Mineral compositions in pegmatites are similar to granodiorite at the FC mine.

Mineral compositions from the FC mine are similar to minerals from other mines in the Minpro pluton. A regional pattern of plagioclase compositional variation, recognized almost 100 years ago, fits with the FC mine plagioclase compositions. Garnet and muscovite compositions vary from place to place in the SPMD. Results from this study help to define these variations.