Micro-PIXE analysis of varied sulfide populations from the Ventersdorp Contact Reef, Witwatersrand basin, South Africa

Abstract

Micro-PIXE spot analysis and true elemental imaging of pyrrhotite and pyrite crystals from the Ventersdorp Contact Reef (VCR) of the Witwatersrand basin, the world's most important gold resource, are reported. The objective was to evaluate the use of this technique with regard to its analytical resolution concerning the detection limits for the elements encountered in these samples and their spatial distribution. Furthermore, this project was designed to identify chemical signatures characteristic for particular textural types of VCR sul®des.Micro-PIXE is indeed a highly useful technique for the analysis of such materials and provided results that could not be obtained by optical and SEM microscopy and electron probe microanalysis (EPMA). Micro-inclusions could be made visible through elemental area scanning. Chemical zonation in the interior of apparently homogeneous grains was detected, which has obvious implications for the interpretation of the multi-stage evolution of Witwatersrand mineralization. The available data do not yet allow unambiguous chemical classi®cation of di erent sul®de types (generations), but a distinct late growth of As-enriched sul®de was detected. These late overgrowths may be enriched in Ni or not. No unambiguous relation between As and Au could be observed in these samples. Ó

Quantitative major and trace elemental mapping by PIXE of concretionary pyrite from the Witwatersrand Basin, South Africa

“…Zonation patterns are individually unique. However, several grains have As and Ni enrichments in outer zones or along margins, which have also been detected in other recent PIXE-analytical studies of various types of Witwatersrand pyrites 20,23 and appear to be a significant feature of relatively late precipitated pyrite.…”

“…Similar compositions have also been observed along margins of grains belonging to other pyrite types (such as various types of authigenic pyrites; note that some of the authigenic pyrite crystals analysed here are also enriched in As!). Earlier studies by Foya et al 23 and Reimold et al 20 have related such signatures on Witwatersrand sulphides to late, Bushveld/Vredefort to post-Vredefort, hydrothermal activity. These authors have also argued that gold growth could be paragenetically related to this late sulphidization event.…”

“…Finally, Reimold et al 20 reported first results of a comprehensive microanalytical study of different textural types of Witwatersrand pyrite and of pyrrhotite. They discussed the usefulness of PIXE analysis for the detection of optically invisible chemical zoning and observed that their initial results did not permit unambiguous chemical classification of different Fe-sulphide types or generations.…”

Formation process of pyrite polyframboid based on the heavy-metal analysis by micro-PIXE

“…Although there have been a number of reports, very few quantitative analyses of framboidal pyrite have been done in previous studies (Przybylowicz et al 1995;Reimold et al 1999 and; hence, the formation process of pyrite framboid and polyframboid has been rarely discussed with heavy metal distributions and concentrations. In the present study, quantitative analyses are performed to elucidate the elemental concentrations and distributions of framboidal pyrites and polyframboids within mudstone, and then formation process of polyframboid is revealed by petrography and the results of quantitative analyses.…”

Quantitative micro-PIXE analysis of heavy-metal-rich framboidal pyrite

“…Heavy metals in euhedral pyrite grains have been analyzed previously using micro -PIXE (particle -induced X -ray emission) (e.g., Meyer et al, 1994;Przybylowicz et al, 1995;Reimold et al, 1999Reimold et al, , 2004Belcher et al, 2004), revealing zoning and unique concentration patterns; however, few studies have used micro -PIXE to measure the concentrations of heavy metals in the context of the formation process of pyrite framboid.…”