Regolith geology of the Yilgarn Craton

It is highly appropriate that the first issue of the Australian Journal of Earth Sciences to appear in the 50th anniversary year of the Geological Society of Australia should be devoted to a subject in the field of economic geology. The theme for this anniversary year is ‘celebrating 50 years of service to Australian geoscience’ and what better way to match the theme than to publish in a field that several surveys have revealed to be of priority interest to the majority of the Society’s members. While the Australian Journal of Earth Sciences will have to wait another year to celebrate its 50th volume, the present issue is unique in the 49 years of publishing to date, in that it is the first issue to contain only one paper. However, this is not surprising in view of the breadth of material covered and the amount of detail given for an area—the Yilgarn Craton—of great potential as an exploration target, and a topic—the regolith—of importance in exploration strategy. The Yilgarn Craton is one of the world’s principal mineral provinces, with considerable resources of gold, nickel and bauxite, as well as lesser amounts of a wide range of other commodities. As such, it is a major target for exploration. Despite considerable variation in the total expenditure on exploration for mineral resources (other than coal and petroleum) over the past 5 years, the proportion of Australian exploration expenditure spent in Western Australia has remained constant at approximately 60%, much of which has been for exploration on the Yilgarn Craton, with over two-thirds targeting gold and nearly one-fifth targeting base metals, particularly nickel. Deeply weathered regolith profiles are widely distributed on the Yilgarn Craton. The presence of thick regolith has two main implications for the mineral industry. First, the presence of regolith conceals mineral deposits hosted by the underlying bedrock. Even in the residual regolith, the characteristics of mineralisation are greatly altered and difficult to recognise, although dispersion during regolith formation potentially yields much larger exploration targets than the mineralisation itself. Second, the regolith materials may themselves represent economically significant mineral deposits. The application of many geophysical techniques at prospect scale is hindered by certain properties of regolith. Similarly, the development and evolution of the landscape give rise to several problems in the effective application of geochemical procedures that use soil and other regolith materials as sample media. Effective geochemical exploration can only be achieved by recognition of the problems caused by the evolution of the landscape and regolith, and by the selection of sampling media and data-interpretation techniques appropriate to the target being sought. Thus, it is important to assess the nature and origin of the regolith, geomorphological processes and regolith–landform relationships to devise effective sampling strategies and to interpret the geochemical data sensibly. This issue presents and reviews the processes responsible for the distribution and formation of regolith and associated landscapes on the Yilgarn Craton and highlights their implications to exploration. Many of the studies referred to were originally part of a series of large multiclient research projects conducted by CSIRO and the Cooperative Research Centre for Landscape Evolution and Mineral Exploration, through the Australian Minerals Industry Research Association. Other studies are from the University of Western Australia, the Geological Survey of Western Australia, Curtin University of Technology, mining companies and consultants. Much of this work has been undertaken with the collaboration of numerous mining companies, who are thanked for their past and ongoing support for research. This thematic issue starts with an introduction that surveys the history and background of regolith studies. This is followed by a discussion of weathering and laterite profile terminology. The main section of the issue is devoted to regolith materials, which are dealt with under the headings: lag, soils, sediments, ferruginous materials, calcrete, silcrete and silicification, and multiple duricrusts. The section on regional distribution and patterns of regolith on the Yilgarn Craton looks at mapping of the regolith and discusses regolith–landform mapping units. A discussion on the geomorphic history of the Yilgarn Craton analyses various landscape evolution models and summarises what is known of the regolith–landform history of the craton. Finally, the authors discuss the implications for exploration in terms of the economic significance of the regolith and the choice of sampling media. Although the research presented here is largely aimed at helping explorers understand more about the geochemical and geophysical data, it also has a wider value for environmental research and management. The regolith affects the surface and subsurface patterns of groundwater recharge and discharge, the amount of groundwater and the direction of flow. Different rocks weather to different types of regolith and deep weathering increases the ability of the landscape to store salts. All these are vital factors in understanding and managing the landscape underlain by the Yilgarn Craton.