The depth at which weathering starts has been studied by others in felsic and mafic protoliths. However, weathering initiation in foliated parent material, which is inherently more permeable, has not, and there is little understanding of what mineral is first to weather at depth in this setting. Nor has the difference in weathering profiles along a hillslope transect been studied to better understand the geochemical evolution of groundwater as it moves down gradient due to deep mineral weathering. Using pXRF and binocular microscopy, elemental and mineral abundances were analyzed in 2 drill cores collected from low and high topographic positions located in a gneiss lithology. Comparative mineral ratios and mass-transport values (τ) were used to identify weathering front depths of biotite, hornblende, and plagioclase. Pearson correlation coefficients for biotite (r = -0.32), hornblende (r = -0.19), and biotite + hornblende (r = -0.4) with respect to iron-oxides show a combination of biotite and hornblende weathering is likely the initiation of weathering at depth rather than weathering of either specific mineral. Greater depth to the water table coincided with formation of thicker saprolite, and the boundary between hard-weathered rock and unweathered rock coincided with oxygen depletion. This study used non-destructive testing to identify the depths over which profile weathering occurs emphasizing ferrous mineral oxidation. The weathering of Fe-bearing minerals at depth releases solutes that directly impact dissolved oxygen, and trace metal solubility, while also enhancing permeability because of mineral grain swelling, both of which impact water quality with corresponding implications for human and environmental health.