Surface and Near-Surface Investigation of the Alteration of the Mount Scott Granite and Geometry of the Sandy Creek Gabbro Pluton, Hale Spring Area, Wichita Mountains, Oklahoma

Abstract

Surface and subsurface investigation of the spatial relationships between the Cambrian-age Glen Mountains Layered Complex, Mount Scott Granite, and Sandy Creek Gabbro in the Hale Spring Area of the Wichita Mountains, shows that the upper portion of the Sandy Creek gabbro pluton is largely steep-walled, with a blunt irregular subhorizontal roof capped by Mount Scott Granite. In the subsurface, the near vertical sidewalls of the gabbro are presumed to truncate an older subhorizontal contact of regional extent, between the Glen Mountains Layered Complex substrate and the Mount Scott Granite cap. This subhorizontal contact is interpreted as an angular unconformity that developed on the layered complex and was subsequently buried by volcanic deposits of the Carlton Rhyolite Formation prior to intrusion of the Mount Scott Granite sheet. The Sandy Creek Gabbro does contain xenoliths of Glen Mountains Layered Complex and Meers Quartzite, a metasedimentary unit associated with this unconformity. Locally, a thin ledge of gabbro, with an irregular floor, protrudes more than 0.5 km to the south from the main body of the intrusion presumably exploiting this subhorizontal contact. Thus, the Sandy Creek Gabbro is a stock, capped by the floor of the Mount Scott Granite sheet, and only locally spreads laterally along the older unconformity, the contact between the Mount Scott Granite sheet and the underlying Glen Mountains Layered Complex.

The Mount Scott Granite was cored to a depth of 87.5 m adjacent to the contact at the Ira Smith Quany (site SQ-1). The core is homogeneous with respect to primary igneous mineralogy and texture. The only exceptions are the presence of a thin aplitic zone at a depth of 47.1 m and sodic amphibole at a depth of 87 m, both of which can be seen in surface exposures in the adjacent quarry. However, subsequent events have altered the primary igneous mineralogy and texture of Mount Scott Granite, and important variations in fracture density, color, oxide mineralogy, and magnetic susceptibility (ϰ) occur within the upper 30 m of the core. Macro-fracture densities are 0.6 cm^-1 at the surface to 0.0 cm^-1 at 30 m depth. Fracture density, although variable throughout this upper 30 m, generally decreases with depth. Oxide mineralogy varies from hematite + iron hydroxide near the surface, to magnetite + hematite (adjacent to microfractures and grain boundaries) + ilmenite, at depths between 23 and 30 m, to magnetite + ilmenite below 30 m. Color changes from grayish reddish orange (2YR 5/6) near the surface to moderate grayish red (8R 4/6) below 30 m due to alteration of sub-microscopic hematite flakes stmcturally trapped within the abundant alkali feldspar grains, to iron hydroxide. Magnetic susceptibility varies from 2 × 10^-4 at the top, to ∼9 × 10^-3 (SI mass) below 30 m. The order of magnitude change in susceptibility values reflects oxidation of the primary igneous mineral assemblage (principally titanomagnetite) to hematite and hemoilmentite, and hydration of secondary hematite to iron-hydroxide, and is likely to be a result of interaction with meteoric fluids. This significant change in magnetic susceptibility in the upper 30 m of Mount Scott Granite was incorporated in geomagnetic modeling of the subsurface. Changes in color, magnetic susceptibility, and oxide mineralogy are directly correlated to changes in fracture density. Thus, brittle fractures provided conduits for fluid-fiow and subsequent alteration of the crystalline basement. The extent of alteration reflects the fluidlrock ratio which, in part, is a function of fracture density and connectivity. The timing of the fracturing and the composition of fluid(s) is largely unconstrained, but is likely to be groundwater in equilibrium with atmospheric conditions. As evidenced by the alteration within the core, circulation of these fluids has been restricted to depths less than 30 m below the present erosional surface. Thus, opening of these fractures is likely to be directly related to unroofing of this basement. The timing of alteration may be a relatively recent phenomenon. However, because the Wichita Mountains were previously exposed to atmospheric conditions in the Permian, the possibility for multiple alteration events cannot be excluded.