Color Zonation Associated with Fractures in a Felsic Gneiss Saprolite

Abstract

AbstractDarkly coated fractures are identified as a major type of macropore in most saprolites found in the Piedmont and Mountain regions of North Carolina. Morphological and mineralogical characteristics of various color zones associated with these Mn‐coated fractures were evaluated to determine their nature and assess their role in water flow through saprolite. Four distinct color zones (black, white, yellow, and red) were identified surrounding the Mn‐coated fractures in a typical saprolite derived from gneiss and schist parent rock in the Piedmont region. Two large, vertically oriented intact blocks (30 by 30 by 80 cm) containing fractures and associated color zones were removed from the 1.9‐ to 2.7‐m depth. Small undisturbed samples containing all color zones, and disturbed bulk samples from each color zone were analyzed for mineralogical and chemical composition using x‐ray diffraction, scanning and transmission electron microscopy, Mössbauer spectroscopy, electron microprobe scans, and HF digestion for total elemental analysis. Iron content was highest in the interior saprolite matrix (182 mg/kg of clay‐size fraction in the red zone) and diminished toward the fracture (black zone). The red zone was dominated by hematite (54% of Fe oxide present), the yellow zone was dominated by goethite (92% of Fe oxide present), and the white zone was almost free of Fe. In the black zone, Mn oxides occurred in substantial amounts and Fe was mostly in the form of goethite. Manganese was not detected in the other three color zones. Color patterns and relative amounts of Fe and Mn indicate historical preferential water movement along fractures. Measurement of saturated hydraulic conductivity, however, indicates that these fractures are no longer active macropores allowing rapid water flow through saprolite.