Geophysics and Drilling Collaborations Program Greenfields Drilling Round 16, 2023-2024 Final Report - Callista Rare Earth Element Project

Abstract

The important role of critical metals in clean energy technologies has increased interest in exploration for rare earth element (REE) deposits of all kinds and, in particular, regolith-hosted REE deposits of types with potential for economically favourable REE extraction pathways. The best known REE-in-regolith deposits are the so-called ionic-adsorption on clay (IAC) style of REE mineralisation, as exemplified by deposits in southern China, but other REE-in-regolith styles with different mineralogies and extractabilities are recognised. The latter includes the weak acid extractable (WAE) style of REE-in-regolith deposit, which is the most common deposit style known in Australia. IAC and WAE deposits are developed on weathered and altered granitic rocks, typically have grades of 0.03 to 0.5% total rare earth oxides (TREO) and are leachable with mild reagents. GSW Resources (GSWR) identified the incompatible-element-enriched Southwark granite suite of the central Australian Aileron Province, as having potential for hosting REE mineralisation in granitederived regolith materials. Subsequently, GSWR acquired title EL 33195 to explore for REE mineralisation of this type. Following a promising soil geochemical campaign, in which significant REE anomalies in clay-rich surficial materials were identified, a successful application was made under Round 16 of the NT Geophysics and Drilling Collaborative Program to fund an initial aircore drilling program. The program objective was to test the REE potential of three identified prospects: Callista, Callista North and Thorntona, which collectively form the Callista REE Project. No mineral exploration drilling of any kind had previously taken place on the tenement. The aircore program, which utilised a low-impact, mobile rig, comprised 122 holes for a total of 2,902m drilled. Average hole depth was 20m at Callista, 31m at Callista North, and 17m at Thorntona. Drill spoil materials consisting mainly of clay-rich granitic saprolite were logged in the field and single-metre or two-metre composite samples were sieved to -2mm and dispatched to the laboratory for assay by a microwave-assisted acid digest method. The results indicate that significant thicknesses of REE mineralisation are present. At the 900ppm TREO cut-off level, average mineralisation thickness and grade at Callista is 17.5m at 1,851ppm TREO, at Callista North 18.4m at 1,503ppm TREO, and at Thorntona 10.2m at 1,470ppm TREO. For the valuable Magnet REEs: NdPr-oxides are present at average grades of 328ppm, 266ppm and 291ppm, and DyTb-oxides are present at average grades of 47, 43 and 35 ppm for Callista, Callista North and Thorntona, respectively. Hole CA036 has the best intercept at Callista of 18m at 0.45% TREO (747ppm NdProxides, 98ppm DyTb-oxides). Unmineralised cover amounts to only a few metres on average with mineralisation often occurring from surface. The results indicate that the Callista REE project is one of the highest grade REE-in-regolith prospects currently known from a granitic terrain in Australia, and contains superior levels of the valuable DyTb-oxides compared with other deposits. In the regolith column, mineralisation typically straddles the boundary between light-coloured kaolinitic clays of an upper saprolite unit and darker grey to yellow-brown clays of a lower saprolite unit. Chemically, the upper saprolite is characterised by depleted Rb levels (<300-350 ppm) compared to the lower saprolite, which is more potassic and Rb-rich. The lower saprolite transitions into granitic saprock, which may be partly silcretised. Mineralisation is often but not always associated with a positive Ce-anomaly (Ce/Ce* >1, and >2 in some cases) which differs from the negative Ce anomalies seen in the REE-accumulation zones of IAC deposits. In some holes such as CA052 more than one strongly mineralised horizon is present. Other complexities include downhole variations in the ratio of the light REE (La to Sm) to the heavy REE (Gd to Lu) and in the Y/Ho ratio (Y-anomaly). All these features suggest that the REE have been mobile to some degree in the regolith column. Finally, some initial bottle-roll leach extractions (50C, 6hr, 10% pulp density) were undertaken using 10% HCl on 60 samples resulting in an average 70% extraction of the Magnet REEs. This result is comparable to the average 68% extraction of Magnet REEs obtained by OD6 Metals Ltd for 10% HCl leaches on samples from its Splinter Rock project (Esperance, WA), which is a REE deposit of the WAE mineralisation style. The Callista project prospects are also believed to be of this style.