Geochemistry and mineralogy of Early Archean spherule beds, Barberton Mountain Land, South Africa: evidence for origin by impact doubtful

Abstract

Spherule layers in the ∼ 3.4 Ga Barberton Greenstone Belt, South Africa, have been interpreted as being the result of large asteroid or comet impacts on the early earth. This interpretation was based, among other arguments, on the enrichment of siderophile elements, especially the platinum group elements. We made a detailed mineralogical, petrological and geochemical study of spherule bed samples taken from drill cores and underground exposures at the Princeton, Mt. Morgan and Sheba gold mines, as well as surface localities. The macrostructure of each sample (from within different spherule layer units) shows evidence for multiple (more than five) events over about 30 cm. This would require multiple impacts within a few million years, which is unlikely. The mineral phases are almost exclusively of secondary origin. The mineralogy provides evidence for extensive hydrothermal and metasomatic alterations of the spherule beds. Geochemical analyses of alternating spherule, shale and chert layers show no correlation between the siderophile elements (e.g., Ir, Co, Ni and Au), contrary to that which would be expected if the siderophile elements had an extraterrestrial source. Furthermore, no significant variation in the content of the siderophile elements was detected between spherule layers and shale layers; however, siderophile element contents are high only in layers containing abundant sulphide minerals and having high As, Sb, Se and Cr contents. We suggest that complex mineralizations, similar to those that have formed the Barberton Archean gold deposits or the Bon Accord deposit, were responsible for the siderophile element enrichments in the spherule beds. The mineralogical and geochemical data provide no direct evidence in favour of their initial origin by impact. Nowhere else in the world have such multiple (or even single) spherule beds been observed, and none of the numerous known impact crates (or the Cretaceous-Tertiary boundary) is associated with comparable spherule beds. Known impact debris usually contains < 1% meteoritic component, if any at all, while the Barberton spherules are anomalous in being extremely enriched compared to any known impact deposits.