Finding blind orebodies: geochemical exploration for large nickel-copper and PGE sulphides on the Western Gawler Craton.

Abstract

The search for economically viable ore deposits focuses increasingly on deeply buried deposits. This study was designed to highlight specific mafic/ultramafic igneous bodies in the western Gawler Craton, near Streaky Bay, South Australia, through the analysis of the behaviour of pathfinder elements within the regolith above anomalous aeromagnetic targets. In particular, the possibility of developing a rapid and inexpensive means of characterising the intrusions at depth by looking within the calcareous sediments located within the top two to twelve metres of regolith was evaluated. Data from 26 of the 53 holes drilled were analysed, covering an area of approximately 214 km2. This area covers a diverse lithological basement including ultramafics, gabbros, granitoids and felsic gneisses and is located near a strong magnetic anomaly. Given the significant difference in basement lithology of the target bodies (mafic to ultramafic) versus the variably magnetic felsic to intermediate granitoids, pathfinder elements including Ni, Cu, Cr, Mn and V, which are elevated in mafic to ultramafic rocks, were targeted. Depth plots and ratios of the transition elements are shown with simple graphing techniques are used to illustrate the behaviour of geochemical signatures throughout the profile and to display any correlation between basement rocks and the regolith. There was no discernible anomaly in any trace metals throughout the calcrete of the uppermost regolith unit. Calcrete pathfinder element abundances are uniformly low, which is to be expected, as the sediments are up to 75% carbonate, and any basement detrital signature is highly diluted. In the majority of holes, however, an abrupt increase in these element values occurred at the base of the calcrete or a few metres deeper within ferruginous sediments. The increase in values occurred in Ni, Cu and Cr, but was most prominent in V. This pattern is reflected in the plots for the basement saprolitic material. Unfortunately, sampling of the oxidised zones requires expensive and time­consuming air-core drilling through up to twelve metres of calcrete, and in places soft, unconsolidated sands. More detailed geochemical analyses of the calcrete layers in the 26 holes were undertaken to try to establish a method of identifying the basement lithology from the calcrete chemical data. Absolute abundances of pathfinder elements are too low in the calcrete to be useful in distinguishing differences in basement lithology. While calcareous sediments may contain subtle geochemical indicators of the differences in basement lithology, it alone is not adequate to confidently predict the basement lithology for drilling. Below the calcrete, within the oxidised zone, the geochemical anomalies are large enough to confidently conclude whether the basement is mafic or felsic.