GEOPHYSICAL IMAGING OF ULTRAMAFIC AND MAFIC ROCK BODIES WITHIN GNEISSIC METASEDIMENTARY COUNTRY ROCK, WESTERN NORTH CAROLINA, USA

To better understand the origin of small (<1.5 km) mafic/ultramafic bodies in the southern Appalachian Blue Ridge province, researchers have employed various geophysical methods to image their extent and contact relationships with surrounding gneissic metasedimentary country rock. A compilation of previous studies shows that both magnetic and gravity methods are quite sensitive to the degree of alteration of the dunite that predominates in the ultramafic zones. As the extent of alteration is variable both within and between individual bodies, it is difficult to predict whether a specific method will prove useful at any given site. Previous surveys generally targeted the overall dimensions of the mafic/ultramafic complexes. With new small-scale surveys in North Carolina’s Webster-Addie, Carroll Knob, and Balsam Gap complexes, we exploit the local magnetic variability within the dunite to resolve the location and attitude of contacts between mafic/ultramafic and host rocks. We interpret 600 nT magnetic highs and lows in the Balsam Gap body as a signal of hydrothermally altered or serpentinized zones a few meters wide. Resistivity profiling, previously undocumented in this setting, proved useful at constraining the location and, in at least one case, attitude of ultramafic contacts. Resistivity contrasts associated with lithology are strongest in the uppermost 15 meters, suggesting that the resistivity lows associated with dunite arise from enhanced weathering within this unit. The utility of resistivity measurements appears to be limited to areas where the ultramafics are found close to the surface and where units have areal extent >~10 m. In the Chestnut Gap Quarry (Webster-Addie Body), for example, a resistivity survey was inconclusive, most likely due to the presence of a several meter thick alluvial fan and small-scale (<~10 m) heterogeneities in lithology and weathering. Resistivity results are independent of survey geometry (Wenner traverse vs. dipole-dipole) and choice of inversion code.