Mineral Exploration of the Senegalese Grande Côte heavy minerals placer: QEMSCAN® characterisation of Fe-Ti oxides for ore modelling

Titanium-containing minerals serve a variety of industrial applications. Iron and titanium oxides, ilmenite (Fe²⁺TiO₃), pseudorutile (Fe₂³⁺Ti₃O₉), and rutile/anatase (TiO₂) are notably used in the production of paint, plastic and paper pigments; moreover, titanium metal is considered as a Critical Raw Material (CRM). Grande Côte Operation (GCO), a subsidiary of Eramet, has been operating the Senegalese Grande Côte heavy minerals (HM) placers for zircon and Fe-Ti oxides since 2014. Senegal's placer deposits extend over 100 km in length and 5 km in width and lie alongside the country's north coast. These Quaternary ore-bodies resulted from the erosion of the Mauritanian belt and repetitive episodes of marine transgression and regression, as well as from aeolian dune formations, leading to significant heterogeneity. Related distribution trends in impurities and heavy minerals are yet not anticipated or understood.

This study explores the mineralogical heterogeneities to investigate variations in terms of the distribution and alteration of the titanium-bearing phases. Ten drill cores were selected to investigate three synthetic profiles based on high-resolution sampling. Heavy minerals from composite samples were recovered using dense liquid. The obtained concentrates were prepared as representative thick sections for textural analysis. Semi-quantification investigations were conducted by means of QEMSCAN^® analyses.

The heavy minerals content was not related to sand facies or depth, and the average concentration ranged from 0.1% to 4.2%, with an average of 0.9. From the concentrate, it could be inferred that Fe-Ti phases represented 14.4% for ilmenite, 57.1% for pseudorutile, 1.8% for anatase and 3.7% for rutile. Pseudorutile was the predominant phase, indicating an advanced alteration. A decrease in ilmenite/pseudorutile ratio was observed with increasing depth in all profiles.

Based on these findings, the alteration rate in the ilmenite series was investigated by adding a finely spaced range of Fe/Ti ratios and impurities content (mainly Al) to the QEMSCAN^® database. The weathering process is initiated by the oxidation of Fe²⁺ into Fe³⁺, progressively leading to the formation of pseudorutile, marked by grains with cracking patterns due to topotaxial reactions. The following stage is driven by iron-lixiviation and implies hydroxylian pseudorutile apparition due to intense hydration and hydroxylation processes. Dissolution and reprecipitation reactions led to a final alteration, creating highly Ti-enriched, impurities-rich and porous grains. The evolution with depth of the coefficient of variation between the content of Fe-Ti phases illustrated an authigenic Ti-enrichment. A substantial drop (-40%) in unaltered ilmenites was observed at surface levels. A downward enrichment of pseudorutile proportion (5 to 10%) was observed up to 13m, where the sharp increase (up to 40%) in Ti-rich phases correlates to the water-table depth above 18m, advanced alteration led to the transformation of almost all ilmenite phases into pseudorutile.

QEMSCAN^® analyses contributed to a better understanding of the Grande Côte placer deposits, highlighting the significance of spatial variability and local water table settings for Fe-Ti oxide distribution and alteration processes, allowing a first ore body modelling and a global assessment of HM content.