Hydrothermal alteration in the Bosumtwi impact structure: Evidence from 2M(1)-muscovite, alteration veins, and fracture fillings

摘要

Drill-core samples from the Bosumtwi impact structure (1.07 Myr old and 10.5 km in diameter) in Ghana exhibit mineralogical evidence for post-impact hydrothermal alteration. Nine samples of drill core obtained through the 2004 International Continental Scientific Drilling Project (ICDP) were studied, including an uppermost fallback layer overlying impactite breccias, and partly deformed massive meta-graywacke bedrock. The petrographic study revealed alteration veins containing secondary sericitic muscovite (comparable to 2M(1)-muscovite) crosscutting original bedding in meta-graywacke and forming a matrix between clasts in impactite breecias. X-ray diffraction (XRD) shows that these impactite samples are rich in 2M(1)-muscovite, consistent with post-impact fluid deposition and alteration. Optical analysis indicates the presence of a pre-impact stratiform chlorite in meta-graywacke samples and a secondary alteration chlorite occurring in all samples. Secondary illite was detected in upper impactites of drill core LB-08A and samples containing accretionary lapilli. The lower temperature constraint for the hydrothermal event is given by 2M(1)-muscovite, secondary chlorite, and illite, all of which form at temperatures greater than 280 degrees C. An absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 degrees C. The presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores LB-07A and LB-08A indicates that a post-impact hydrothermal system was present in and adjacent to the central uplift portion of the Bosumtwi impact structure. A sample containing accretionary lapilli obtained from drill core LB-05A exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor.