Preliminary X-ray Diffraction Evidence of Hydrothermal Alteration in the Bosumtwi Imapct Crater

Introduction: The Bosumtwi Impact Crater in Ghana, Africa (6°32’N, 1°25’W) is a large complex crater. The crater is 10.5 km in diameter, with a central uplift 2 km in diameter. It is the source of the Ivory Coast Tektite field, found to the southwest of the crater over 200 km away. This study investigates the mineralogy, the petrology, and the geochemistry of drill core samples taken near the center of the crater. In preliminary studies, X-Ray Diffraction (XRD) and petrographic analyses proved to be most conclusive and interesting. Crater Structure & Core Sites: This study focus on two drill cores taken in 2004. Drill core 7 is located on the floor of the crater at the base of the central uplift. Basement rock has been covered by lake sediments and crater collapse. Drill core 8 is located well onto the central uplift. Artemieva [1] has modeled the formation of the crater and has determined that the meteor that made this impact weighed 400-1100 kg and impacted the ground at a high velocity (~20 km/sec) at an angle of 30-50°, creating pressures of 30-50 Pa, thus presumably high temperatures. The 1 Ma crater was emplaced in the 2 Ga Birmian Super group, a sedimentary gray-wacke [2]. Major geologic processes have not affected the crater or the surrounding area after the impact occurred. Mineralogy: Preliminary XRD analysis found chlorite, quartz, and feldspars as the major mineralogical components of the samples. The chlorite peaks are asymmetrical meaning the heavy ions are unevenly distributed throughout the chlorite structure. Major peaks are quartz, feldspar, and chlorite (Fig 3, 4). In most samples analyzed, Illite was discovered in a 2M (Fig 1, 2) stacking pattern. The 2M structure forms only at temperatures greater than 200° C [3]. Illite has also been found at the Manson crater in Iowa [4] which has other mineralogical evidence of hydrothermal alteration. Illite and other clay minerals were discovered in impactites in the 1973 drill core near the center of the Ries Crater [5]. In contrast, only Montmorillionite was found in ejecta outside of the Ries crater [6].