Regolith-landform processes and geochemical exploration for base metal deposits in regolith-dominated terrains of the Mt Isa region, northwest Queensland, Australia

Abstract

The regolith in the Mt Isa region of Queensland consists of a variety of saprolites and duricrusts developed on Proterozoic basement rocks and fresh to weathered Mesozoic, Tertiary and Quaternary cover, all of which has impeded base metals exploration. This paper presents an overview of some of the regolith-geochemical work conducted in the Mt Isa region as part of an industry-supported three year CRC LEME/AMIRA Project. A complex weathering and landscape history has produced a landscape of (a) continuously exposed and exhumed basement rocks that have undergone varying intensities of weathering and partial stripping; (b) weathered and locally eroded Mesozoic cover sequences and (c) areas with younger transported cover concealing basement and Mesozoic cover. Various regolith sample media have been evaluated at a number of prospects and deposits which represent different regolith-landform terrains and landscape history. Geochemical dispersion processes and models are presented and false anomalies explained.Where ferruginous duricrust or ferruginous nodular gravel are preserved on weathered bedrock on an eroded plateau, they exhibit large (>500m) multi-element (As, Pb, Sb) dispersion haloes and are useful sampling media. Dispersion haloes in truncated profiles on weathered bedrock covered with colluvium are restricted, are limited to tens of metres from subcrop of the source, and contrast to the extensive anomalies in ferruginous duricrust and nodules. Geochemical exploration in covered areas depends on the possible presence of dispersion through the sediments or leakage along faults or fractures, but may be complicated by high metal backgrounds in the sediments themselves. Some of the most prominent anomalies occur in ferruginous materials and soils representing emergent residual terrain developed on Mesozoic sediments. These are largely due to weathering of sulfide mineralization that continued during submergence in a marine environment, with hydromorphic dispersion into the sediments as they accumulated. Multi-element (Cu, As, Zn, Sb, Au) anomalies occur in basal sediments and at the unconformity, due to a combination of clastic and hydromorphic dispersion and represent a useful sample target. Metal-rich horizons in weathered sediments, higher in the sequence, can also be targeted, particularly by specifically sampling ferruginous units and fragments. However, these are less certainly related to mineralization. Zinc and Cu, concentrated in Fe (and Mn) oxides at redox fronts, may be derived by leaching from the sediments with concentration in the sesquioxides, and be unrelated to any proximal basement mineralization. In all these regolith-dominated terrains, a clear understanding of local geomorphology, regolith framework, topography of unconformities and the origins of ferruginous materials is essential to sample medium selection and data interpretation.