The effect of sample medium on regolith chemistry over greenstone belts in the northern Eastern Goldfields of Western Australia

Abstract

Gold and Ni concentrations in regolith samples collected from a 16 km 2 grid over Archaean granite–greenstones of the Yilgarn Craton show a strong correlation with lithology and areas of known mineralization, even though most samples are from transported regolith. The influence of sample medium on regolith composition has been assessed by statistical comparison of stream sediment, soil and sheetwash data from four of the largest greenstone belts. Changes in composition according to sample medium can be explained by periodic recharge of stream sediments by fresh or ferruginized bedrock, and different degrees of weathering and sorting during transport, with enrichment or depletion in groups of analytes showing the progressive breakdown of rock-forming minerals. The behaviour of Ti, La, Ce, Cr, Nb and Zr is consistent with the concentration of resistate minerals such as allanite, monazite, chromite, rutile and zircon, due to the combined effects of weathering of less stable phases and mechanical sorting. A statistical assessment shows that comparison of regolith composition between greenstone belts must be made using the same sample medium type. Furthermore, this approach can be used to assess the appropriateness of different sample media in regional geochemical surveys, and to refine analytical programs.