GEOCHEMISTRY AND MINERALOGY OF EARLY ARCHEAN SPHERULE BEDS, BARBERTONMOUNTAIN LAND, SOUTH-AFRICA - EVIDENCE FOR ORIGIN BY IMPACT DOUBTFUL

Spherule layers in the approximately 3.4 Ga Barberton Greenstone Belt, South Africa, have been interpreted as being the result of large aste roid or comet impacts on the early earth. This interpretation was base d. among other arguments. on the enrichment of siderophile elements, e specially the platinum group elements. We made a detailed mineralogica l, petrological and geochemical study of spherule bed samples taken fr om drill cores and underground exposures at the Princeton, Mt. Morgan and Sheba gold mines, as well as surface localities. The macrostructur e of each sample (from within different spherule layer units) shows ev idence for multiple (more than five) events over about 30 cm. This wou ld require multiple impacts within a few million years, which is unlik ely. The mineral phases are almost exclusively of secondary origin. Th e mineralogy provides evidence for extensive hydrothermal and metasoma tic alterations of the spherule beds. Geochemical analyses of alternat ing spherule, shale and chert layers show no correlation between the s iderophile 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 si derophile elements was detected between spherule layers and shale laye rs; however, siderophile element contents are high only in layers cont aining abundant sulphide minerals and having high As, Sb, Se and Cr co ntents. We suggest that complex mineralizations, similar to those that have formed the Barberton Archean gold deposits or the Bon Accord dep osit, were responsible for the siderophile element enrichments in the spherule beds. The mineralogical and geochemical data provide no direc t evidence in favour of their initial origin by impact. Nowhere else i n the world have such multiple (or even single) spherule beds been obs erved, and none of the numerous known impact craters (or the Cretaceou s-Tertiary boundary) is associated with comparable spherule beds. Know n impact debris usually contains <1% meteoritic component, if any at a ll, while the Barberton spherules are anomalous in being extremely enr iched compared to any known impact deposits.