Chemical, Mineralogical and Textural Characterization of Red Earth Formation in the Northwest Coast of Sri Lanka

The Red earth (RE) deposit that exists along the northwest coastal belt of Sri Lanka is a significant geological formation. Fine-grained sand coated with iron-aluminum oxides brings the characteristic reddish color and the presence of important heavy minerals such as magnetite, ilmenite, and rutile enhance its economic importance. This work reports the detailed characteristic features of Sri Lankan RE exposed in two different regions; Puttalam (southern part of the deposit) and Mannar (Middle-top part of the deposit). The collected samples were analyzed for particle size distribution, mineralogy, heavy mineral content, geochemistry, and morphology by performing sieve analysis (wet and dry), powder X-ray diffraction, isodynamic magnetic separation, ICP-MS analysis followed by acid digestion and stereo microscopic analysis, respectively. The results indicate two types of clustering of RE in the two regions; moderately sorted (σ = 0.8) RE in Puttalam region and poorly sorted (σ = 1.3) RE in Mannar region. Rounded quartz grains with bulbous edges and disc-shaped concavities resemble the features of dune sand in Puttalam RE. Sub-angular quartz grains with abundant conchoidal fractures and arcuate steps on grains reflect the influence of high-energy subaqueous depositional environment of Mannar RE. Magnetite and ilmenite are found as the most abundant heavy minerals in both regions. However, significantly higher amounts of heavy minerals, clay minerals (kaolinite) and lighter rare earth elements (La, Ce, Pr, Nd, Sm) are found in Puttalam RE than in Mannar RE. Therefore, though the Sri Lankan RE is considered as a single unit, it has spatial variation in geochemistry, mineralogy and texture.