Evolution, classification and use of ferruginous regolith materials in gold exploration, Yilgarn Craton, Western Australia

Abstract

Ferruginous regolith materials are abundant and widespread in the Yilgarn Craton of Western Australia and have been successfully used as sample media in Au exploration. However, their formation has been complex. They are developed in residual and transported materials of various ages. Four general types are recognized: ferruginous duricrust and gravel, ferruginous mottles, ferruginous saprolite and iron segregations. Ferruginous duricrusts include lateritic residuum and ferricrete. Lateritic residuum has evolved by partial collapse of mottled or ferruginous regolith, involving local vertical and lateral (generally 10–50 m) movements after chemical wasting. Ferricretes are ferruginized sediments. Some are detrital clasts cemented by Fe oxides and others are authigenic pisoliths and nodules in sandy or clayey sediments. Ferruginous mottles are formed by accumulation of hematite and goethite in saprolite, residual clays or sediments. Ferruginous saprolite is formed by the uniform ferruginization of saprolite. Iron segregations form by the replacement and/or modification of sulphide-rich lithologies or as exotic accumulation of Fe oxides along preferred pathways, such as fractures, faults and lithological contacts within saprolite. Gold dispersion in the ferruginous zone of the profile is commonly considered to have a ‘mushroom’ shape formed by downward weathering, where the surficial Au halo is derived from the mineralization by a combination of chemical, residual and mechanical processes. In this study, appreciation of regolith–landform history provides an understanding of the nature of Au dispersion (or lack of it) in ferruginous materials. It has been possible to relate the mechanisms of dispersion of Au to the material being ferruginized, the environment of ferruginization and its position within a weathering profile. The distribution of Au at the micro-morphological scale provides evidence of a mobility related to the formation of particular facies of ferruginous duricrust. Accordingly, it is essential that careful attention is paid to characterization of potential ferruginous sample media and their location in the landscape in order to deduce their relationship to bedrock.