Ann: Mineral Resource Estimate Increased Threefold at Makuutu, page-43

This may help your understanding Scarpa. The process is entirely different than Hard Rock deposits.

Clay minerals are part of the phyllosilicate class, containing layered structures of shared octahedral aluminum and tetrahedral silicon sheets; water molecules and hydrated cations can move in and out of the interlayer spaces. Very often, isomorphous substitution of one cation with another (of similar size but with lesser charge, e.g. Al 3+ for Si 4+ or Mg 2+ for Al 3+ ) within crystal structures leads to a charge imbalance in silicate clays, which accounts for the permanent negative charge on clay particles, thus the ability of clays to attract cations to the surface. Amphoteric ‐OH groups at the surface/edge of clays (i.e., silanol and aluminol) may also contribute to surface charge (pH‐dependent reversible charge). 16

According to Bradbury and Baeyens 17 as well as Piasecki and Sverjensky, 18 for acidic and near‐neutral conditions (pH < 6.5‐6.8), most of the surface‐adsorbed lanthanides occur as simple or hydrated cations such as “clay‐REE” or “clay‐REE(H2 O)n ” species derived from straightforward cation‐exchange reversible reactions at the permanent negative charge sites on the clays (physisorption); for pH > 7 the prevalent form is the hydrolyzed “clay‐O‐REE 2+ ” species derived from permanent complexation reactions at the amphoteric surface hydroxyl groups (chemisorption) or soluble carbonate/bicarbonate complexes. 8

Due to various weathering conditions (i.e. nature of host rocks, water and soil pH, temperature, pressure, redox conditions) there are three main categories of REE present in the ion‐adsorption clays, as described by Chi and Tian :

1. Colloid phase: REE deposited as insoluble oxides or hydroxides or as part of colloidal polymeric organometallic compounds. These species have low occurrence in ores at the slightly acidic natural conditions and can be recovered only by acid leach.

2. Exchangeable phase: REE occur as soluble free cations/hydrated cations or part of positivelycharged complexes in solution adsorbed species on clays. These species account for 60‐90% of total content of rare earths in ores and can be recovered by ion‐exchange leaching with monovalent salts.

3. Mineral phase: REE part of solid fine particles with same mineral matrix as the host rocks (REE part of the crystal lattice). This phase usually accounts for the balance from the ion‐exchangeable phase towards the TREE content and can be recovered only by aggressive conditions (alkaline bake and acid leach).
The vast majority of the ion‐adsorption ores present the “negative cerium anomaly”, meaning that, contrary to the majority of lanthanide elements which are usually physically adsorbed as trivalent ions, Ce 3+ can be easily oxidized by atmospheric oxygen (O2 ) to Ce 4+ , 19 and precipitates as cerianite, CeO2 . Consequently, the formation of the mineral cerianite facilitates a natural separation of Ce from the other adsorbed trivalent lanthanides, as described by Bao and Zhao, 5 and makes it impossible to be recovered by ion‐exchange leaching.