STRUCTURAL FEATURES AND ALTERATION IN THE SPAVINAW GRANITE

The Spavinaw Granite Group in northeastern Oklahoma, part of a Precambrian basement complex, offers insights into the interplay of tectonic and hydrothermal processes shaping the Midcontinent's geological evolution. Episodes of tectonic reactivation and hydrothermal alteration can dramatically modify the mechanical and hydrological properties of the rock with significant implications for anthropogenically driven fault slip. This study investigates the structural evolution and mineralogical alteration of the Spavinaw Granite using ~50 ft of core, from 1750 ft measured depth, recovered from the AMAX SP-1 well located in Mayes County. This research focuses on structural characterization of the core while employing petrographic analyses, X-ray diffraction (XRD), and scanning electron microscopy to identify changes in deformation styles and mineralogy. Structural and petrographic analyses revealed the presence of mineralized fractures, slickensides, mineral microfolds, and intergranular microfractures in the granite, which emphasizes its structural complexity and history of repeated fault related processes. Geochemical analyses identified different hydrothermal episodes due to the alteration of primary minerals and textures in the Spavinaw Granite, resulting in the formation of clay-rich phases such as chlorite, kaolinite, illite, and mixed-layer of illite/smectite (I/S), along with mineral veins of quartz, chlorite, and carbonates. Hydrothermal fluid-rock interactions, structural reactivation, and mineralization of fractures would create zones of mechanical weakness, contributing to fault instability and potentially increasing seismic susceptibility. This is particularly relevant for regions like Oklahoma, where fluid injection activities interact with pre-existing fault systems hydraulically connected to altered basement rocks, a dynamic closely linked to the state's recent history of induced seismicity driven by anthropogenic fluid circulation. This work highlights the importance of integrating thermal, mechanical, and chemical data, collected from relevant material from the subsurface, to assess the stability and behavior of basement faults in regions with active fluid injection practices.