Sedimentological Characteristics, Paleoenvironmental Reconstruction, and Metallurgical Characterisation of Beach Sand Deposits in the Ankilimitraha–Analapetsa–Belay Area, Anosy Region, Southeastern Madagascar

Abstract

Heavy-mineral sand (HMS) deposits along the southeastern coast of Madagascar, particularly within the Anosy Region, represent globally significant titanium, zirconium, and rare-earth-element (REE) placers. This study integrates multi-parameter sedimentology, Quaternary stratigraphy, detrital heavy-mineral provenance indices, and metallurgical characterisation of bulk littoral sands from the Ankilimitraha–Analapetsa–Belay coastal prism. Bulk densities of the beach deposits span 1,789 to 2,106 kg m⁻³. Grain-size distributions are characteristically bimodal to polymodal, comprising a tractional coarse mode/lag (+1 mm fraction of 9.5–28.2 wt.%), a dominant moderately well-sorted medium-to-fine sand population (D₅₀ ≈ 192–256 μm), and a minor ferruginous slime fraction (−45 μm, 0.3–6.6 wt.%). Passega C–M projections and Visher log-probability profiles demonstrate a high-energy, wave-dominated foreshore to upper-shoreface environment dominated by swash–backwash saltation overprinted by episodic storm traction. Run-of-mine (ROM) process testwork indicates an 87.9 wt.% treatable sand fraction containing 52.9 wt.% total heavy minerals (HM), yielding a whole-ore HM grade of 46.5 wt.%. Densimetric fractionation confirms that 93.0 wt.% of the total HM suite comprises very heavy minerals (VHM, >4.05 sg), hosting >96% of total TiO₂ and >99% of ZrO₂. The suite is dominated by ilmenite (71.0 wt.% altered, 7.4 wt.% primary), zircon (4.8–5.1 wt.%), monazite (2.9–3.0 wt.%), magnetic Hi-Ti (1.7–4.0 wt.%), and garnet (6.0–7.8 wt.%). A zircon–tourmaline–rutile (ZTR) index of 28.65%, coupled with elevated garnet–zircon (GZi ≈ 61.9%) and monazite–zircon (MZi ≈ 37.04–37.66%) ratios, indicates moderate mineralogical maturity and derivation from the ultra-high-temperature metamorphic and felsic crystalline basement of the Anosyen Domain. The deposits represent Karimbolian residual strandlines reworked into Flandrian barriers. Dry physical upgrading indicates amenability to conventional gravity, magnetic, and electrostatic circuits, producing sulphate-grade ilmenite (~51% TiO₂) alongside commercial zircon and monazite concentrates.