Importance of 3-D regolith-landform control in areas of transported cover: implications for geochemical exploration

Abstract

Two-dimensional (2-D) regolith-landform mapping has gained greater importance and wider acceptance and usage in the minerals industry over the last 20 years. This 2-D method is generally adequate in residual areas but is a poor indicator of the complexity of the deep regolith and thickness of transported cover in depositional environments. This paper highlights the importance of three-dimensional (3-D) regolith-landform control as an aid to: (a) identify major controls on metals dispersion; (b) assist anomaly interpretation; and (c) selection of suitable sample media in areas of transported cover from the Yandal greenstone belt (YGB; 260 × 40 km) of the Yilgarn Craton. It represents the first fully integrated study of the regolith on a single greenstone belt. This greenstone belt has been intensively drilled and its regolith units systematically logged. The YGB has less than 10% fresh and weathered rock outcrop and c . 90% of the area is covered by deep regolith consisting of weathered bedrock (locally up to 160 m) under sediments (up to 100 m thick). The application of 3-D mapping in the YGB is restricted to areas of precise and accurate subsurface regolith data-sets comprising c . 50,000 drill-holes covering c . 70% of the belt. The data-sets for the 3-D mapping comprise transported cover, oxidized saprolite, lower saprolite and bedrock and gold geochemistry. The regolith surfaces show that the variability of transported cover, the depth of the oxidized saprolite and the lower saprolite are controlled by the palaeorelief, lithology, mineralization and deformation. The nature and thickness of transported cover determines the suitability of a particular sampling medium. Thus, prototype derivative maps showing residual areas and areas with varying thickness of transported cover (e.g. <5 m, 5–20 m, >20 m) are required for devising a suitable sampling strategy program in depositional environments. Three-dimensional relationships between regolith and gold distribution both at regional and deposit/prospect scales have provided information that may indicate the size of geochemical dispersion and suggest sampling strategies in different transported cover domains. It also allows testing of possible vectors to mineralized sources for anomalies in transported materials. An important feature of the palaeosurface reconstructions below deep transported cover is its considerable relief which shows striking anomalies at a number of deposits and prospects in the YGB. This reinforces the importance of precisely defining this palaeosurface through 3-D mapping as it may host the best direct geochemical indicators of buried mineral systems in areas of deep cover.