PHYSICAL DEFORMATION AND CHEMICAL ALTERATION IN PRECAMBRIAN BASEMENT GRANITE, SOUTHWESTERN MISSOURI

An exploratory well drilled in southwest Missouri offers an interesting look at both physical deformation and chemical alteration of the uppermost ten feet of the granitic basement of the Mesoproterozoic Southern Granite-Rhyolite Province. Our petrographic analysis is based on four thin sections at approximately two foot intervals, with the first sample taken from 2147.2 feet below the surface.

Structural deformation is common in each sample. Microfaults are visible within some K-feldspar crystals. Folding is evident in magmatic muscovite grains, with some of these grains folded in excess of 100 degrees. Other muscovite grains show evidence of shearing, as cleavage planes become discontinuous and break off at angles from the original crystal. Shearing is also evident elsewhere, as the recrystallization of original quartz grains to polygonal quartz is common for each of the four samples.

Each of the four samples displays chemical alteration from fluid interaction. The replacement of plagioclase crystals with epidote is evident in each sample; in some cases epidotization is extreme, leaving only quartz and alteration minerals including epidote, hematite, and fine-grained phyllosilicates visible in thin section. In this case, it is unclear whether both K-feldspar and plagioclase crystals existed prior to epidotization. These epidote veins also align preferentially with albite stringers in perthitic K-feldspar crystals. Epidote and hematite alteration preserve original zoning within some former plagioclase crystals. There is also evidence of secondary calcite alteration cross-cutting the epidote alteration.

The samples for this study came from an exploratory well drilled with the financial support of the Department of Energy National Energy Technology Laboratory under Award Number DE-NT0006642 to City Utilities of Springfield, MO.