Characterizing Structural Features and Hydrothermal Alteration in the Spavinaw Granite of Northeastern Oklahoma

Abstract

Northeastern Oklahoma's basement rocks, particularly the Spavinaw Granite Group, have undergone episodes of hydrothermal alteration and tectonic activation, significantly influenced by regional tectonic events and fault dynamics. This study investigates the hydrothermal and tectonic evolution of the Spavinaw Granite using core samples from the AMAX SP-1 well located in Mayes County, employing petrographic analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), fluid inclusion analyses, and apatite fission-track thermochronology. Different hydrothermal episodes were identified due to the alteration of primary minerals and textures in the Spavinaw, 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. Apatite fission-track dating identified thermal episodes with central ages of 227 ± 26 Ma, 199 ± 23 Ma, and 147 ± 40 Ma, corresponding to significant tectonic events. These include slow exhumation attributed to isostatic rebound or flank uplift resulting from the rifting of the Gulf of Mexico, as well as subsequent compressional regime that caused faulting. Fluid inclusion data indicated migration paths for hydrothermal fluids, likely connected to broader regional systems such as the Tri-State district, resulting in mineral precipitation along fractures. These findings highlight the interplay of thermal, mechanical, and chemical processes that shape fault zone behavior. Hydrothermal fluid-rock interactions, structural reactivation, and mineralization of fractures created 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.