NEAR-SURFACE ALTERATION OF THE MOUNT SCOTT GRANITE, WICHITA MOUNTAINS, OKLAHOMA

The Mount Scott Granite is an extensively exposed alkali-feldspar granite in the Wichita Mountains of southwestern Oklahoma, and hosts km-scale and smaller fractures that may have permitted long-lived fluid transport. Surface exposures are appreciably weathered relative to quarries and other deep cuts. Prior work showed decreasing fracture intensity, lighter and pinker color, and increasing magnetic susceptibility with depth, particularly within the first 30 m of the surface. These color and susceptibility changes were linked to martite formation and attributed to increasing fracture-assisted weathering (Price et al., 1998, Basement Tectonics 12). To further document alteration, we examined fine powders from 12 depths using the same shallow borehole (123 m total depth) of the prior study. Each was prepared from ~10 cm long, half-cylinder core segments. These were characterized for mineral content and mode through X-Ray Powder Diffraction (XRPD) using the ICDD PDF-2 database with Rietveld refinement and for MnO mobility by X-ray Fluorescence (XRF).

XRPD analysis found that all samples are dominated by exsolved feldspar and quartz, with lesser biotite and hornblende, as noted in petrographic observations. Results revealed that the overall percentage of hornblende increases with depth. Only deep samples contained measurable magnetite (> 1 vol.%). Conversely, the near-surface samples (<28 m) contain clinochlore and hematite, the likely alteration products of biotite, hornblende, and magnetite. XRF analyses produced identical values for four immobile elements at all depths, demonstrating that the granite igneous composition is unchanged over the drilled interval. It also recorded MnO of 0.04 wt.% at and above 17 m, decreasing from a fairly consistent 0.07 wt.% at and below 28 m. We attribute this to Mn leaching by meteoric waters in the near surface. The presence of alteration minerals and the decrease in hornblende and MnO within the upper 28 m further supports the conclusion that fracture-assisted weathering is the principal alteration process.