GEOCHEMICAL CHARACTERIZATION OF IMPACT MELT GLASS FROM THE RECENTLY DISCOVERED ORA BANDA IMPACT STRUCTURE, WESTERN AUSTRALIA

The Ora Banda structure is a partially buried impact structure located in an Archean greenstone terrane of the Eastern Goldfields of Western Australia, 50 km NW of the Kalgoorlie gold mine. The structure is mostly covered by regolith and lacks topographic expression. It is defined by concentric gravity and magnetic anomalies reaching 4–5 km in diameter. Geophysical surveys and drilling data reveal a central uplift and annular trough. Target rocks consist of ca. 2.7 Ga Archean mafic-ultramafic greenstones. The presence of shatter cones, planar deformation features in quartz, and an extraterrestrial geochemical component in impact melt glass provide diagnostic evidence of hypervelocity impact.

In-situ geochemical analysis of impact glass in two thin sections of lithic impact breccia (suevite) from a drill core located within the annular trough was conducted using laser ablation inductively coupled plasma mass spectrometry at Curtin University, and electron probe microanalysis at the University of Western Australia.

The glass composition is basaltic andesite (avg = 54 wt% SiO₂), and major oxide compositions reflect mixing of target rocks. Average abundances of Co (205 ppm), Ni (2640 ppm), Ir (290 ppb), and other PGEs (Rh, Pd, Pt) in glass are significantly higher than in target rocks and are interpreted as meteoritic in origin. Cr and Ir abundances indicate a likely iron meteorite impactor. Large variations in Ni, Co, and Cr, and minor variations in Fe, suggest fractionation of Fe from other siderophile elements may have occurred during glass formation. Widespread carbonate globules in silicate glass provide evidence carbonate and silicate melt mixtures, similar to those in other impact sites (e.g., Graup 1999; Jones et al. 2000; Osinski and Spray 2001; Pratesi et al. 2005; Osinski et al. 2015; Anders et al. 2013; Walton et al. 2019). Textural observations of carbonate globules may suggest liquid immiscibility, which can explain low CaO in the silicate glass (<1 wt%), when target rocks average ~10 wt% CaO.

Several features of the Ora Banda structure stand out. The structure preserves the first occurrence of ‘Ries-type’ suevite described from Australia, is one of the oldest known sites with convincing geochemical evidence for an iron meteorite impactor, and is one of the only known impact structures in Archean greenstones.