Salama, Walid1; Le Vaillant, Margaux1; Schoneveld, Louise1; Schlegel, Tobias1; Anand, Ravi1
1CSIRO Mineral Resources, Kensington, Perth, Western Australia
Mineral exploration in weathered and covered terrains has given preference to geophysical and geochemical methods over mineralogical analyses. Airborne and ground geophysical surveys identify magnetic, electromagnetic and gravitational anomalies and allow the rapid delineation of exploration targets. However, the methods don’t indicate whether a target is mineralised or not. Geochemical surveying, based on the analysis of soils, vegetation, termites or calcrete are well-established techniques for locating and identifying variable types of mineralisation. However, their application in areas of deep cover is limited. Indicator minerals, useful for exploration in the weathered and covered terranes, are those that 1) resist chemical weathering in weathered profiles; 2) undergo chemical changes to form secondary minerals in the weathering profile; 3) precipitate within organic-rich sediments during diagenesis; and 4) resist physical weathering during erosion, transportation and deposition.
Mineral explorers are interested in knowing the fertility of a mineral system, in minerals indicating the presence or absence of mineralisation and in vectors towards mineralisation. Within the CSIRO Discovery Program, research projects continue to focus on the identification and characterisation of indicator minerals. Examples of such studies are the potential use of trace element zoning patterns in pyroxenes as fertility indicators for magmatic Ni-Cu-Co-(PGE) deposits, white mica composition in hydrothermal alteration halo as a vector toward Au mineralisation, trace element composition of chromite and arsenide as exploration tools for Ni and Au deposits, and the REE composition of fluorite which reflect the fluid type involved in IOCG mineralisation.
In weathered terrains, rutile, gahnite and cassiterite with base metal sulfide inclusions are residually enriched in the leached and lateritic zone over the Scuddles massive sulfides forming Bi, Sb and Pb anomalies. The use of heavy indicator minerals is also a practical exploration tool in areas of Quaternary glacial till in Canada and Fennoscandia and in the Permian glacial diamictites in Australia. In Australia, the diamictites deposited immediately above the Permian unconformity are largely unweathered and contain detrital sulfides. The distance that indicator minerals disperse from the source depends mainly on the topography of the unconformity between the cover and the bedrock. In northeast Yilgarn Craton, the Permian diamictites were deposited on a rough topography and indicator minerals are expected to be derived from proximal source rocks. Iron, Cu, Zn, As, Ni and Co sulfides were identified at the base of the Permian cover above mafic-ultramafic rocks and used as vectors toward Au mineralisation at Agnew and Lancefield in Western Australia.
In summary, trace element contents of indicator minerals are used to vector toward mineralisation and the source fluids involved in its formation. Residual and supergene indicator minerals in weathered profiles can indicate mineralised bedrock underneath the cover. Detrital and diagenetic indicator minerals in transported cover are a potential vectoring tool for various types of mineral deposits.
Walid Salama is a senior research scientist working for CSIRO Mineral Resources. He joined the regolith geoscience group as a postdoc fellow in 2012. His research focused on geochemical exploration of Au, base metals, Ni and Fe in the weathered and covered terrains in Western Australia, Queensland and Botswana.