Laterite geochemistry applied to diamond exploration in the Yilgarn Craton, Western Australia
This paper describes an exploration programme based on regional laterite sampling (1 to 9 km spacing) aimed at locating geochemical anomalies of kimberlite pipes. Orientation studies at the Aries pipe, Western Australia, and several pipes in the Coromandel area in the state of Minas Gerais, Brazil, demonstrated that lateritic residuum on kimberlite reflects the characteristic geochemical signature of the bedrock. Data interpretation by identifying and ranking geochemical anomalies of alkaline ultramafic rocks in regional multi-element datasets was done using univariate and multivariate statistics. These techniques can also test the datasets for other mineral occurrences, such as rare metals. Although this study did not locate a kimberlite pipe, regional geochemical sampling identified an area (46 Gate Road anomaly near Lake Brown) in the northern Merredin region where lateritic ferruginous materials appear to have formed on mafic–ultramafic alkaline bedrock, possibly an ultramafic lamprophyre that had not previously been recognized. Other regional geochemical features identified by this study, and yet to be followed up, include a Ta–Sn–Nb anomaly SE of Meekatharra, a Ni–Cu–Zn–Co anomaly in an area NE of Lake Barlee, an As–Cr–Sb–(Ni–Co–Mn–Mg) anomaly east of the Youanmi greenstone belt, and a Mg–Cr–Co anomaly in the very southern part of the survey area. The deeply weathered terrains of the Yilgarn Craton and adjacent Proterozoic belts in Western Australia present difficult environments for diamond exploration and to date no economic diamond pipes have been found in the Yilgarn Craton. With the exception of diamond and chromite, indicator minerals are destroyed through intense weathering, with very short dispersion trails of any remaining minerals in a dominantly low-relief landscape. The use of aeromagnetic data is limited by deep weathering causing noisy magnetic backgrounds in areas with preserved lateritic residuum due to contained maghemite. Additional problems arise from variably magnetic granites and gneisses, and greenstone remnants within them. Electromagnetic techniques are often adversely affected by highly conductive overburden. Laterite geochemistry applied within a regolith–landform framework has the potential to be a complementary technique for diamond exploration in deeply weathered terrain. It can be used to test geophysical targets in areas with a preserved lateritic mantle and to explore for pipes that may have obscure geophysical signatures. Orientation studies, using ferruginous gravels and duricrust formed on alkaline ultramafic and ultramafic rocks, show characteristic geochemical signatures with a halo about three times larger than the pipe itself due to mechanical and hydromorphic dispersion within the regolith.