EVIDENCE FOR DEEP FLUID PENETRATION INTO THE PRECAMBRIAN BASEMENT, OKLAHOMA

Previous studies of cores in the top of the northern Oklahoma basement have shown evidence of fracturing and associated mineralization due to fluid activity. The nature and origins of this alteration have been the subject of ongoing research in order to better understand the increase of earthquakes in Oklahoma. This study, on the Amoco SHADS No. 4 drill core from Rogers Co., OK, employs optical petrography, scanning electron microscopy, stable isotope geochemistry, paleomagnetism/rock magnetism, and x-ray computed tomography. The SHADS recovered approximately 140 meters of igneous rock, initially described as an andesite porphyry. Petrographically, the rock is an andesitic to trachytic ignimbrite with extensive alteration and groundmass recrystallization. Most feldspar phenocrysts are at least partly replaced by sericite and/or clays, with some instances of dissolution and infill with epidote and calcite. Some also show evidence of feldspar overgrowths. Chlorite is present in all samples, likely sourced from alteration of biotite and augite. Numerous fractures visible in thin section are variably mineralized with chlorite, titanite, Ti-oxides, pyrite, albite, quartz, epidote and calcite. Fractures appear to be the only significant source of extant porosity. Anisotropy of magnetic susceptibility measurements show a highly variable fabric. Some variations appear to be associated with alteration, while some may represent a primary fabric. The uppermost igneous section shows a notable decrease in magnetic susceptibility which may be due to weathering at the unconformity. The core exhibits extensive fracturing and mineralization with a large number of near-vertical fractures most commonly mineralized with epidote. Additionally, there are other sets of inclined to horizontal epidote-filled fractures with some intervals of the core only represented by epidote-coated rubble. Some depth intervals contain calcite-filled fractures, which appear to cross-cut the epidotized fractures and have a different orientation. Oxygen and carbon isotope values from the calcite are consistent with a hydrothermal origin. This core documents evidence of multiple episodes of fluid activity penetrating deep into the basement and raises the issue of whether similar fracture networks may be open in nearby areas.