Asra Minerals Limited announced further exciting Rare Earth Element (REE) results from 113 drill holes resulting in significant REE 4 occurrences together with critical mineral, Scandium (Sc) at the company's Yttria regolith-hosted discovery at its Mt Stirling Project, located in the Eastern Goldfields of Western Australia. Professor Ken Collerson's Technical Explanations: Discovery of ionic clay hosted REE deposits requires both an understanding of nature of geochemistry of the basement lithologies below the regolith, as well as the behaviour of REEs during oxidation. Cerium (Ce) provides an important clue.

Oxidation at the Earth's surface enables Ce to occur as the mobile tetravalent state Ce4+. As a result, in regolith profiles the uppermost weathering zone develops an excess of Ce (expressed as a positive Ce anomaly; expressed as Ce/Ce>1). However, in deeper zones in the profile generally shows a Ce deficit (i.e. Ce/Ce <1), because of Ce4+ loss.

This process causes the concentration of HREEs to increase in deeper parts of the weathering profile. The removal of tetravalent Ce4+ produces a strong `negative' Ce anomaly relative to the neighbouring LREEs, La and Pr in the chondrite normalised plots. Chondrite normalisation is used to smooth out the variable concentrations in sequential plots caused by the `Oddo- Harkins' effect, i.e., the fact that elements with even atomic numbers >5 are more stable and therefore, are more concentrated than elements with odd atomic numbers.

Most igneous rocks, show smooth Chondrite-normalised REE patterns extending from the LREE between lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd) samarium (Sm), gadolinium (Gd) gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) to lutetium (Lu). Europium (Eu) in many igneous rocks shows a negative or positive spike in the shape of the REE pattern which is commonly interpreted to reflect plagioclase or fluorite removal or gain. Both show smooth LREE depleted and HREE enriched profiles.

However, the sample from the upper part of the regolith (MSC0290-10m) shows a positive spike in Ce that reflects gain of Ce4+. By contrast, the deeper sample (MSC0290-16m) shows a negative spike that reflects loss of Ce4+. As a result, sample MSC0290-10m therefore has a Ce/Ce of 12.5 while MSC0290- 16m has a Ce/Ce of 0.4. Ce/Ce ratios <1 reflect the loss of mobile Ce4+ from deeper parts of weathering profiles while Ce/Ce ratios >1 reflect the gain of Ce4+ at shallower regolith levels.

This understanding has important metallurgical consequences, because REE leachable ionic clays and regolith's where REEs are also associated with hydroxyl complexes, typically have Ce/Ce << 1. TREYO concentrations versus Ce/Ce anomaly, for all assayed samples from Yttria are plotted against comparative data for ionic clay mineralisation at Makuutu5 in Uganda, and for ionic clay REE mineralisation in China, Brazil, Madagascar 6,7 and Thailand8. Assays for the Yttria and Makuutu samples occupy overlapping fields, with the majority of samples plotting with Ce/Ce <1 in the same quadrant as ionic clay REE mineralisation from China, Brazil, Madagascar and Thailand. Note: samples with Ce/Ce >1 with elevated TREYO concentrations, simply reflect the presence of high concentrations of low value Ce.

The majority of Yttria assays have negative Ce/Ce anomalies and are chemically similar to other regolith hosted ionic clay REE systems. However, the Yttria regolith with Ce/Ce <1, differs from many of the other global examples because of its extreme enrichment in heavy REEs, with a mean HREYO/TREYO ratio of 62%, indicating the presence of significant high value Dysprosium and Terbium DyTb. The overlap of a significant number Yttria assays with a Ce/Ce of ~1, a value typical of unoxidised primary igneous rocks, and TREYO concentration of ~ 80 ppm in Figure 5 is nterpreted to reflect the composition of the primary mafic to ultramafic alkaline source ntrusion for the Yttria REE enriched regolith. Thus, the enrichment REYs in the regolith profile at Yttria from close to surface to 20m is entirely consistent with concentration profiles seen in ionic clay REE systems on other continents.

Below ~20m from surface at Yttria, the REYs basically emulate concentrations in the mafic to ultramafic alkaline source. The presence of significantly elevated Sc throughout the entire regolith column averaging 74 ppm Sc2O3, but with values up to 174ppm Sc2O3, shows that Sc2O3 occurs at potentially at economic levels throughout the entire regolith profile at Yttria. Scandium behaviour therefore appears to be decoupled from the other REEs in the weathering profile.

The mean Nb/Ta ratio of 17.9, of more than 2000 Yttria regolith assays with Nb/Ta >13, unequivocally shows that the mafic to ultramafic intrusion below Yttria is a comparatively rare plume generated alkaline intrusion. This alkaline magmatic event was likely associated with an early Proterozoic plume track that has been postulated to exist between Lynas Rare Earths Mt Weld Carbonatite near Laverton and Victory Metals North Stanmore alkaline intrusion near Cue.