Regolith geology of the Yilgarn Craton, Western Australia: Implications for exploration

Abstract

This paper presents and reviews the processes responsible for the distribution and formation of regolith and associated landscapes of the Yilgarn Craton and highlights their implications for mineral exploration. It provides an analysis of regolith geology investigations that were conducted in many districts of contrasting contemporary geomorphic and climatic conditions.The Yilgarn Craton is composed of Archaean rocks, predominantly granitoids, that are crossed by north‐northwest‐trending belts of greenstones. It has an arid to humid climate at present. The gently undulating landsurface forms a partial etchplain and the topography is largely related to bedrock lithologies and a complex history of valley development and aggradation. Deep weathering has affected most lithologies and geological provinces across the craton. The depth of the weathered mantle may be as much as 150 m, but it varies considerably and rock outcrop may occur in any part of the landscape. The main factors influencing extent of weathering are rock type, mineralisation and deformation. Palaeomagnetic dating of deeply weathered regolith profiles suggest that they formed throughout the Phanerozoic.An idealised profile commonly comprises fresh bedrock, grading upwards into saprock and saprolite, commonly bleached towards the top, especially on felsic or sheared mafic rocks. This is overlain by a clay‐rich and/or quartz‐rich zone, a mottled zone and a ferruginous, bauxitic or siliceous upper zone. These horizons are formed by a combination of weathering and landscape processes. Landscape processes would have been continuous throughout the weathering period, with major environmental changes triggering particular erosional and depositional events. Thus, upper horizons, mottled zone, ferruginous duricrust and silcrete have developed in residuum, colluvium and alluvium of various ages.Weathering is the result of interaction between the hydrosphere, biosphere and lithosphere. During weathering some of the components of primary minerals are leached and secondary minerals are formed as residua. The pathways by which these minerals form are varied and complex. Biota were present in the regolith and it is likely that they and their associated chemical reactions played a significant part in the weathering process, as well as inorganic chemical processes. The final product of weathering of all rocks is a mineral assemblage of least soluble minerals (kaolinite, hematite, goethite, maghemite, gibbsite, anatase and boehmite) and the most resistant primary minerals (quartz, zircon, chromite, muscovite and talc), although neoformation of several generations of hematite, goethite, kaolinite and gibbsite may occur. Poorly crystalline minerals are an important constituent in surface or near‐surface materials. In addition, the more soluble minerals, including carbonates, sulfates and halides, occur in arid environments.The principal effects of weathering on element distributions relates leaching and retention of a range of elements to mineral transformations in the principal regolith profile horizons. However, the chemical patterns may not be consistent from profile to profile even for similar lithologies due to differences in groundwater regimes and topography. Minor and trace elements may be displaced from their primary host mineral. Thereafter, they may occur camouflaged in the newly formed secondary minerals, as major components of accessory new minerals, or in resistant minerals.In the present landscape, deeply weathered profiles may be preserved or partly eroded and buried beneath a variety of sediments. Three major regolith‐landform terrains were identified and the extents of these regolith‐landform terrains vary across the Yilgarn Craton. The first is dominated, in its upper part, by sands, ferruginous gravels and duricrust that commonly overlie mottled zones and saprolites. Ferruginous duricrust is developed in weathered Archaean bedrock and younger sediments that have been extremely weathered and/or indurated by Fe oxides. A second regolith‐landform terrain comprises saprolite with fresh rock in places. These may have been exposed by erosion of a pre‐existing weathered material, or may represent the most weathered form of the parent rock, which had never been capped with a ferruginous duricrust. A third terrain is of sediment‐dominated areas with fluvial, aeolian and/or lacustrine deposits, commonly several metres thick, that may be underlain by ferruginous duricrust, saprolite or bedrock. Sediments are highly variable in genesis, provenance, composition and thickness and were either derived from erosion of fresh and weathered Archaean bedrocks or the reworking of older pre‐existing sediments. They vary according to region, topography and age.Numerous buried palaeochannels occupy the lower parts of the landscape and are up to several hundreds of metres wide and many kilometres long. Drainage incision along palaeovalleys on the weathered landsurface res