Proton microprobe characterization of a unique siderophile element mineral assemblage in the melt sheet of the giant 145 Ma Morokweng impact structure, Southern Kalahari, South Africa

Abstract

The present study deals with mapping of major and trace elements in the Morokweng impact melt sheet which leads to preliminary understanding of its mineralogy. The elements of interest are the siderophile metals, namely, Ni, Co and platinum group metals (PGMs) which are thought to be of meteoritic origin and are highly enriched in the quartz norite and largely concentrated in sulphide–metal oxide intergrowths, which has never been observed before in impact melt sheets. The striking feature of this study is the identification of a new platinum-bearing mineral at the intergrowth of the two major phases, namely nickel sulphide and the iron oxide. The highest concentration of platinum is found to be about 14.3% by weight and is localized in a smaller domain of 20 μm×20 μm . This study not only has metallogenic applications, but also addresses fundamental questions on the behavior or the PGMs at high temperature and very reducing conditions.