This paper focuses on the geochemical controls and associated patterns of element dispersion in aeolian sand/calcrete regolith overlying the Goldridge Au mineralization in the Kraaipan greenstone belt, South Africa, and discusses possible sampling strategies.
Regolith in the Goldridge area consists of in-situ weathered basement rocks, overlain by a colluvium and valley-filled sedimentary sequence comprising carbonate-impregnated pebbly to coarse/medium-grained sands at the base, followed by an upper reworked friable/laminated calcrete. These units are unconformably overlain by a fine- to medium-grained ferruginized aeolian sand. The thickest and most laterally extensive regolith sequence occurs along major drainages. Broad valleys and deep channels permitted widespread development and preservation of thick colluvial and alluvial sediments, which were subsequently overlain by aeolian sand.
Gold shows variable dispersion patterns in aeolian sand, often with peak Au concentrations occurring in the central portions of the profile, coinciding with the transition zone between deep red and reddish-brown sands, that is geochemically characterized by a c. two-fold increase in the Fe/Mn ratio. The underlying calcite-rich laminated carbonate facies are accompanied by enhanced contents of CaO, MgO, Fe2O3, LOI, H2O− and a decrease in SiO2 contents. The laminated calcretes are associated with the highest accumulation of Au in the entire regolith, whereas the calcrete-impregnated basal pebbly to sandy zone shows generally low but sporadically high Au content.
Results of multivariate analysis of geochemical data suggest an association of Au with Fe–Mn oxides/ oxyhydroxides in the upper zones of aeolian sand. Increased influence of ferricrete nodules and underlying calcrete in the distribution pattern of Au and associated base metals occurs with depth. The use of stacked or additive element plots (e.g Cu+Ni+Co+Mn) show patterns that reflect various rock types or lithologies concealed beneath aeolian sand.
The laminated to hardpan carbonate horizon and the basal zone of the aeolian sand horizon should therefore be sampled in exploration. An understanding of the spatial distribution of these regolith components is essential to maintain a consistent sample.
- © 2005 AAG/The Geological Society of London