MINERALOGY AND GEOCHEMISTRY OF THE MAGETITE ORE AND GRANITOID HOST ROCK IN THE CRANBERRY IRON MINE, WESTERN NORTH CAROLINA

The Cranberry Iron Mine was, at one time, the biggest iron mine east of the Mississippi and extended from Roan Mountain, TN to Cranberry, NC. The magnetite ore was mined from 1826 to 1930 and was vital to the economy of western North Carolina. The ore occurs in lens/boudin-shaped pockets within primarily mafic intrusives. The mafic units are bordered by felsic-rich mylonites which are commonly cut by pyroxenite dikes. These units are all hosted in the pegmatitic Cranberry Granite/Granitoid Gneiss. The complicated textural and mineralogical character of this rock association suggests a complex igneous and/or metamorphic origin. Fourteen samples of the magnetite ore and surrounding host rocks were collected from the Cranberry Mine for analysis by transmitted light microscopy and ICP-MS methods. The magnetite ore bodies and surrounding mafic rocks were found to contain magnetite, hornblende, clinopyroxene, quartz, calcium plagioclase, apatite, epidote, sulphides, some calcite, and small amounts of hematite, garnet, amphibole, and sphalerite. The surrounding felsic mylonites, granites and granitoid gneiss contained plagioclase, quartz, microcline, biotite, and muscovite. ICP-MS analysis of the magnetite ore reveals that it has detectible amounts of Pb, Co, As, Zn, and Ni, while the host granitoid contains Mg, K, Ca, Th, and REE. These results indicate that a fluid immiscibility may have developed between the felsic host magma and an iron-rich magma component. Alternatively, the magnetite ore may have formed in the host granite as a result of hydrothermal remobilization/recrystallization during a later metamorphic event.