Preliminary investigations on some bottom sediments from the continental shelf, south of Sri Lanka
In the study sediment samples collected from an area of 120 km2in the southern coastal belt of Sri Lanka between 5° 31' 35" N - 60 11' 16" N and 81° 07' 34" E - 81° 31' 52" E, were analyzed. Initially, organic and carbonate fractions were digested and density variability of the placer minerals was used to differentiate the heavy minerals from lighter minerals. Later, magnetic separation was carried out and identification" of placer minerals was done by microscopic analysis.
The sediment distributions and its considerable variations appeared to be due to bottom morphology and carbonate content. Visual observations revealed that the carbonate fraction consists of gray whitish and reddish coloured coral and shell fragments. The paleonthological study reveals that they are mainly gastropod and pelicypod shells. Grain sizes of sediments varied from —0.13Ø, to 2.22Ø) with a mean value of 1.04Ø. The study area mainly consisted of coarse grains (~40 %) and medium grains (~30 %). Sorting map indicate the poor sorting conditions. The results indicate placer minerals that are not economically much valuable such as garnet, sillimanite and scheelite are abundant in the study area. Of these, only garnets could be used as abrasives. Most of the mineral grains have angular morphology while garnet grains are of both angular and rounded nature, indicating the differences in transportation lengths and sources. Iron oxide inclusions were present in garnet grains. The concentration of placer minerals varied from 0.5 % to 30 %. However, in most of the areas, the concentrations were less than 5 %. The levels of economically viable placer minerals such as ilmanite, rutile and monozite were found to be low in the study area.