Final Report - West Arunta Carbonatite Search: Innovative Targeting Geochemistry

Abstract

GSW Resources' (GSWR) West Arunta Carbonatite Search Project is located on granted titles EL 33042 and EL 33043, which form part of GSWR's larger critical-metals-prospective West Arunta project area under Group Report GR597. The underexplored West Arunta region is part of the Proterozoic Aileron Province of central Australia and comprises a distinctive magnetic-gravity belt of about 350km east-west extent that straddles the NT-WA border (the 'West Arunta Magnetic Gravity Belt' or WAMGB). High-grade niobium resources, including the globally significant Luni deposit, have been discovered in carbonatite complexes of the WAMGB in Western Australia with high potential for future discoveries in the Northern Territory. GSWR's focus is on exploration for mineralised carbonatites as well as other intrusive bodies with critical metals potential. In the 2024 season GSWR completed a high-resolution (100m-spaced) aerial magnetic and radiometric survey over its West Arunta project area. The magnetic imagery shows strong similarities with the carbonatite-prospective basement terrain across the WA border including the presence of crustal-scale fault zones within variably magnetic basement dominated by the ca 1.85Ga Lander Rock Formation and the ca 1.77Ga Carrington Suite. A first-pass analysis has identified numerous potential intrusion-related magnetic features. Following the successful magnetic-gravity targeting methodology employed in the search for buried carbonatite bodies by explorers in the western WAMGB, GSWR undertook a gravity survey program at 400m station-spacing over an area of 840km2 under Round 18 of the Geophysics and Drilling Collaborations Program (Taylor, 2026). The survey area, deemed the Carbonatite Search project area, represents the assessed most prospective part of GSWR's tenement package based on magneticstructural interpretations, the likelihood of relatively shallow (<50m) regolith cover and the proximity of lamprophyre dykes as an indicator of potential carbonatitic magmatism. A total of 5,305 ground gravity stations were acquired during the survey. Of these stations, a subset of 3,060 sites, located away from major drainages, were selected for soil geochemical sample collection. Whole soils were collected from 10-20cm depth by the gravity field crews. A successful application under Round 18 of the Collaborations Program enabled GSWR to undertake an Innovative Targeting soil geochemistry trial using the Ultrafine (UF) assay method. The UF method provides highly sensitive analysis of multi-elements in the ultrafine (<2micrometre) clay-rich fraction of soil samples. A subset of 768 soil samples from 11 prospect areas covering numerous magneticgravity targets and different surficial geological environments were selected for assay. Other sample subsets were selected for comparison with conventional mixed acid digest (MA) and borate fusion (AF) results. In the search for undercover carbonatites and related rocks, the objectives of the program are to both (a) explore the suitability of these methods and (b) develop new techniques to interrogate the geochemical datasets and hence target anomalous soils. The soil geochemical datasets have been interpreted using the ratios of a number of key pathfinder elements that are characteristic of carbonatites and alkaline rocks. These include: La/Yb, Ce/Yb, Sr/Yb, Nb/Yb, Cr/Yb, Ni/Yb, Cu/Yb, Ti/Al%, Ti/Fe%, Ti/Yb, Mg/Fe%, Ca/U. The combination of ultrafine-fraction targeting and the use of element ratios is designed to minimise the diluting effects of transported regolith materials while enhancing the geochemical signature of buried intrusions. An alkaline index (ALK-Index) and an ultramafic plus copper index (UM-Index) have been developed to incorporate these ratios. High ALK-Index values are associated with a number of coincident magnetic-gravity anomalies including priority target RY15. Other targets show strong UM-Index signatures. The UF method provides useful anomaly definition; this is not always the case for MA assays, and the AF method appears unsuitable. The datasets and interpretations discussed here are expected to provide a useful geochemical reference guide in the evaluation and future drill-testing of magnetic-gravity targets in the Aileron Province.