Exploring the Gebel Kamil impactoclastic fallback deposits in Egypt: Their origin and post-impact alteration using SEM/AM and Cathodoluminescence techniques

A Scanning Electron Microscope (SEM)-based Automated Mineralogy (SEM-AM) analysis and Cathodoluminescence (CL) were used to study the mineralogy and texture of a vertical 60 cm profile within the Gebel Kamil impactoclastic fallback deposits. The SEM/AM technique was employed to quantify mineralogical and textural criteria such as distribution, grain size, sorting, mineral association, and shape of clastic deposits in two units of the profile. Microscopically, these deposits are composed mainly of unshocked and shocked quartz, along with impactogenic particles such as lithic ejecta, glassy and impact melt fragments, and projectile shrapnel. SEM/AM effectively identified taenite as the main projectile debris, distinguished various primary mineral phases and related alterations, and microscopically unresolvable inclusions such as tantalite. The impact melt composition was characterized as a Si-rich lechatelierite white melt surrounded by a dark melt. The SEM/AM analysis delineated the composition variance between (Fe-rich) hematite and (Fe-Al-Si-rich) almandine melts. Quantitative textural aspects of the SEM/AM indicate a higher concentration of shocked and other impactogenic particles in the lower unit of the deposits created by the impact plume, whereas the more sorted, finer, more altered upper unit inferred a higher degree of mixing with non-impact related Holocene sediments. The CL results identified shock-induced features, alteration products, and zoning in specific minerals. They also provided insight into the origin of the target rocks, indicating their derivation from a Precambrian complex. The vertical variation of authigenic minerals suggests the presence of a phyllic, post-impact hydrothermal alteration affecting these porous deposits.