Mineral exploration and basement mapping in areas of deep transported cover using indicator heavy minerals and paleoredox fronts, Yilgarn Craton, Western Australia

Abstract

Glacial sediments have been thoroughly integrated into mineral exploration protocols in the Northern hemisphere (e.g., Canada and Fennoscandia), but have received less attention in Australia. In Western Australia (WA), Permo-Carboniferous diamictites, buried by younger Cenozoic sediments, cover scattered areas that have potential to host gold and nickel mineralization, in the NE of the Yilgarn Craton. A systematic stratigraphic, mineralogical and geochemical study was undertaken to determine whether diamictites in the Agnew–Lawlers gold province have properties that may help target concealed mineralization. At the Agnew–Lawlers District, rocks intersected in 31 drill holes NE of the Waroonga gold mine, were interpreted in terms of lithological, textural and paleolandscape, mineralogical and geochemical variations to select the best sampling media for mineral exploration and provenance assessment. The paleotopographic variation, asymmetry of the depositional basin, polymictic composition, matrix-supported, mixed angular and rounded and poorly sorted texture of the diamictite clasts indicate mechanical dispersal from proximal and distal, heterogeneous source rocks. Ferromagnesian minerals, chromite, Cr-magnetite, magnetite and Ni–Cu–Fe sulfides are abundant in the diamictite close to mafic–ultramafic source rocks. Monazite, apatite, zircon and clasts derived from pyrite-bearing quartz veins increase in abundance in the diamictite close to the Waroonga shear zone that separates granitoid/gneiss terrain from the Scotty Creek metasediments. Ilmenite is extensively distributed in diamictites throughout the study area and its Mn content can be used to differentiate between felsic and mafic source rocks. Alteration of ilmenite to titanite, monazite to apatite and thorite, and replacement and fracture-filling of pentlandite to monazite suggest felsic, intrusive-related hydrothermal alteration of the source rocks prior to mechanical weathering. Late Paleozoic tropical weathering associated with oscillation of water table and icehouse to greenhouse climatic shift has created paleoredox fronts in the Permo-Carboniferous sequence. Below the redox fronts, diamictites are unweathered, rich in detrital sulfide and opaque oxide minerals and can be used in provenance studies and tracing mineralization. These characteristics of unweathered diamictite are the optimal sampling medium target for provenance studies and mineral exploration. These are cemented mainly by calcareous cement where they overlie mafic–ultramafic rocks and by pyrite cement where they overlie the Scotty Creek metasediments. Above the redox fronts, diamictites are variably weathered to ferruginous and bleached kaolinitic saprolites which are stable under oxidizing, circum-neutral conditions. Ferruginous diamictites are rich in recycled redox-sensitive elements derived from oxidation of ferromagnesian, sulfide and opaque oxide heavy minerals. These weathering features can be used to identify weathered diamictite for sampling for hydromorphic metal dispersion studies.