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 th...

Mineralogia, geoquímica e evolução da lateritização em Apuí, sudeste do Amazonas

“…Em outros locais da Amazônia, Nordeste do Brasil, Guianas, Austrália e África, o emprego de paleomagnetismo, Ar-Ar e U-Th-He (Théveniaunt & Freyssinet 2002, Colin et al 2005, Shuster et al 2005, Lima 2008, Horbe 2014) ratificam vários pulsos da lateritização ao longo dos trópicos e a idade cenozoica para as crostas lateríticas. Essas características das crostas, por serem dependentes das condições ambientais, são utilizadas no reconhecimento e na interpretação de paleosuperfícies e na evolução da paisagem (Thomas 1994, Twidale & Bourne 1998, Anand & Paine 2002.…”

“…Tais fatores acarretam agradação e degradação das crostas. Formação de hematita e caulinita implicam agradação, e clima mais seco e geração de goethita, Al-goethita e gibbsita sob a forma de córtex ou plasma, aliadas à pedoturbação, apontam para degradação, pois modificam o arcabouço original em condições mais úmidas (Beauvais 1999, Anand & Paine 2002, Bitom et al 2003. Anand e Paine (2002) advogam que a crosta vermiforme é proveniente do endurecimento do horizonte mosqueado em clima mais seco, enquanto crostas pisolíticas (B1A, B1B e B1C) e brechóides (C1, C3, C5 e C6) resultam da degradação dessas crostas em clima úmido.…”

“…Em períodos úmidos, correlatos à bauxitização, há remobilização do ferro, a hematita é hidratada, há precipitação de goethita, que promove a individualização de nódulos, pisólitos e oólitos de hematita e/ou goethita com córtex goethítico (Clarcke & Chenoweth 1996, Anand & Paine 2002. O acúmulo de hematita, goethita e/ou gibbsita entre os nódulos, pisólitos e oólitos origina crostas nodulares, pisolíticas e oolíticas, respectivamente, como observado nas bauxitas pisolíticas (B1A, B1B e B1C) e na matriz oolítica das crostas brechóides (C1, C5 e C6).…”

“…2F, 3G, H e I). Nas bauxitas sobre as vulcânicas, há resquícios de feldspatos, sugerindo a formação de gibbsita pela hidrólise direta de feldspatos, o que, segundo Anand & Paine (2002), significa intemperismo químico agressivo. A crosta ferro-alumino-silicosa representa a fase intermediária entre a formação da bauxita e a crosta ferruginosa.…”

“…As paisagens lateríticas desenvolvidas ao longo do cinturão intertropical entre as latitudes 35°N e 35°S, onde a temperatura, em geral, é superior a 20°C e a umidade supera 1.200 mm/ano, são interpretadas como produto final de um longo tempo de exposição subaérea das rochas (Anand & Paine 2002). Nessas condições desenvolvem-se crostas lateríticas ferruginosas, bauxíticas, fosfáticas, niquelíferas e manganesíferas que constituem importantes jazidas minerais (Bardossy & Aleva 1990, Costa 1990, Costa 1997, Lucas 1997, Boulangé & Carvalho 1997, Horbe & Costa 1999, Kostchoubey et al 2005, Costa et al 2005, Silva et al 2012.…”