ALTERATION OF BASEMENT ROCKS IN OKLAHOMA BY FLUID INTERACTIONS

Precambrian igneous basement rocks in Oklahoma contain evidence for fluid migration and alteration by fluids along fractures. This study aims to further characterize the nature and timing of the alteration associated with fractures in the basement rock, and to better understand fluid pathways. Previous studies identified alteration and remagnetization of basement rocks due to migration of hydrothermal fluids (Carlton Rhyolite; Elmore et al., 1998) and weathering fluids (Long Mountain Granite; Hamilton et al., 2015) in the late Paleozoic. New petrographic and scanning electron microscope analyses on cores from the top of basement (rhyolite and microgranite; Denison, 1981) in northeast Oklahoma (Osage and Pawnee Counties) have identified extensive evidence of alteration by fluids, including a large vertical fracture that contains calcite microspar with a rim of dolomite. Zoned dolomite, pyrite, and clasts of the host rock are present in the microspar. Feldspars proximal to the fracture are partially dissolved or replaced by calcite and contain abundant secondary porosity. Preliminary d¹³C (~ -4‰) and d¹⁸O (~ -9‰) values from the calcite microspar are distinct from the overlying Arbuckle dolomite but similar to calcite veins in the dolomite. There is evidence for both shear and brecciation along the fracture margin. Several crystals in the host rock show small scale offset. Smaller fractures are filled with quartz, carbonate, or chlorite and contain porosity. Some fractures also contain clay alteration halos. Fine disseminated hematite in feldspars is common and is similar to that in the Long Mountain Granite, suggesting alteration by weathering fluids, perhaps during exposure in the late Precambrian. Other evidence for fluid alteration includes the presence of epidote, apatite, and chlorite replacement of biotite. Some of the alteration in the NE Oklahoma rocks is similar to that observed in other basement rocks of Oklahoma and the timing and origin of the alteration are under investigation. The fracturing in the NE Oklahoma basement rocks, as well as the alteration in and around the fractures, created porosity that could facilitate the movement of additional fluids into the basement.