C.B. Dissanayake

Importance of Grain Size Factor in Distribution of Trace Elements in Stream Sediments of Tropical High Grade Terrains - A Case Study from Sri Lanka

Abstract Determination of optimum grain size fraction is critical in geochemical exploration of stream sediment. In terrains with various geological, climatological, and geomorphological conditions different size fractions are customarily used. For geochemical exploration in tropical high-grade metamorphic terrains of Sri Lanka stream sediment samples collected from 6 river basins representing different geological, geomorphological, hydrological, and climatic conditions were studied. Four selected grain size fractions,

Weathering of apatite under extreme conditions of leaching

Abstract Apatites occurring in lateritic residues derived from high-grade metamorphic rocks from Sri Lanka are hydroxy-apatite as indicated by EPMA and XRD. Weathered phosphates and weathered crystalline products are composed of carbonate fluorapatite, crandallite, wardite, fluellite, kaolinite, goethite and gibbsite. EDAX, DTA and SIMS show the presence of amorphous materials, such as oxides of Fe, Al, Si, Al+Si, and S, on the surface of weathered phosphate. Using SEM, TEM, EDAX and electron diffraction techniques, the presence of these amorphous and crystalline materials with several transitions has been clearly identified in weathered phosphate. Recrystallization of phosphate occurred during weathering by dissolution, nucleation and precipitation. The lattice images of hydroxyapatite are gradually increased from 8.3-A basal spacings by weathering, and form mosaic bound fragments. Chondrite-, shale- and phosphorite concretion-normalized REE patterns of these apatites, known to have sedimentary origins, suggest enrichment and remobilisation during post-depositional reworking.

Geochemistry of endemic goitre, Sri Lanka

Abstract A large section of the population living in the wet climatic zone of Sri Lanka is prone to endemic goitre, which is mainly caused by the lack of I. In Sri Lanka, an area in which 40% of the inhabitants are afflicted by goitre, the I content in soil and water was still observed to be high relative to other goitrous regions of the world, indicating the possible importance of other factors such as goitrogens in the aetiology of endemic goitre. Cluster analysis of the geochemical data showed that the endemic goitre region lies in the group with lowest I, alkali earths, Cl, NO3-, Fe and Mn.

Gold-graphite association in granulite terrains―Implications for ore genesis

Abstract The gold occurrences of Sri Lanka are closely associated with the graphite deposits and are genetically related. The quartz-graphite margins of the vein graphite are shown to be metal accumulating. The movement of CO 2 -rich fluids, formation of graphite and the concentration of Au are closely related. Whereas the CO 2 -rich fluids may well be the “carbon-carriers” in the formation of graphite, the original source of CO 2 is still debated. Hydrothermal solutions, when in contact with the graphite tend to activate the carbon, thereby producing sites for the enrichment of gold. The genetic link between the graphite veins and gold deposition has useful implications in the investigation of ore genesis in high-grade metamorphic terrains and the exploration for gold.

Rocks as fertilizers: preliminary studies on potassium availability of some common rocks in Sri Lanka

Abstract Preliminary investigations of the K availability of some commonly occurring rocks of Sri Lanka showed that granulitic gneiss, pink granite, migmatitic gneiss and microcline granite release relatively high amounts of K indicating the capability of using these to provide K in plant nutrition. The finer crushed sizes are more effective in releasing K, and the availability of the K-reserves under appropriate soil conditions makes them possible substitutes for synthetic K fertilizers.

Application of Rb-Sr ratios to gem exploration in the granulite belt of Sri Lanka

Abstract The geochemical distribution of rubidium and strontium in the central granulite belt of Sri Lanka, where many of the gem deposits are found, was studied. The Rb-Sr ratios, particularly in the stream sediments, were found to be useful in delineating gem-bearing areas from the non gem-bearing or low potential areas. Among the main gem minerals that are mined at present are corundum, spinel, zircon and tourmaline. It was observed that higher Rb-Sr ratios correspond to high gem potential and, even within areas of good potential, barren areas could be delineated using these ratios. During pegmatite formation, Rb is enriched, and there is a marked depletion of Sr yielding a high Rb-Sr ratio. Pegmatites, granites and other magmatic bodies are associated with gem formation under granulite facies conditions and, when used in conjunction with geology, structure, and mineralogy, the Rb-Sr ratio could be used effectively to delineate target areas for further exploration.

The geochemistry of mercury in some coastal sediments from Sri Lanka

Abstract The first data on the abundance of mercury in some coastal sediments from Sri Lanka are presented. Sediments from the tidal flats of Mannar, situated in the northwest of Sri Lanka, showed a maximum Hg content of 8 ppm while the marshland peaty sediments contained a maximum of 95 ppm Hg. In the intertidal sediments, Hg appears to be bound to grain coatings mainly of Fe-Mn-oxides and -hydrates and to undecomposed organic matter. In the marshland sediments, the organic matter is completely decomposed and the Hg originally found in the undecomposed organic matter has been released and accumulated on detrital clay minerals.

Trace elements in vein graphite of Sri Lanka

Abstract The abundances of 23 elements in vein graphite of two producing mines from Sri Lanka have been studied. The vein-type graphite deposits are located in a Precambrian granulite-grade metamorphic setting. Even though the origin of the graphite is still being debated, field observations suggest a fluid phase playing a major role in the transport of the carbonaceous matter. The abundance of elements in the graphite indicates the extent to which metals and other elements are transported in a fluid phase involving a carbonaceous matrix under granulite-grade metamorphic conditions. Fe, Ca and Mg are particularly abundant in the vein graphite. Transition elements such as Co, Ni, Cu and Zn though present in lesser concentrations behave in a similar manner geochemically as observed from their correlation patterns. The rare-earth elements - La and Y - are closely associated with phosphorus, indicating a possible genetic association.

Weathering of phosphatic marble to exploitable apatite deposit, Sri Lanka

Abstract Weathering of a granulite-amphibolite facies phosphatic marble from Eppwala, Sri Lanka is described. During the weathering process, apatite is concentrated to an exploitable grade. Primary metamorphic phosphates in the marble from Sri Lanka are hydroxyapatite, as indicated by EPMA and XRD. Weathered phosphates and weathered crystalline products are composd of carbonate fluorapatite, crandellite, wardite, fluellite, kaolinite, goethite and gibbsite. DTA, SIM, SIMS and SAM show the presence of amorphous materials, such as oxides of Fe, Al, Si, Al + Si, and S, on the surface of weathered phosphate. Using SEM, TEM, EDX and electron diffraction techniques, the presence of these amorphous and crystalline materials with several transitions have been clearly identified in weathered phosphate. Secondary crystalline phosphate, consisting of P, Ca and Fe, has partially developed on the surface of the primary phosphate. Recrystallization of phosphate occurred during weathering and calcite dissolution, with nucleation and precipitation of secondary phosphates. The lattice images of hydroxyapatite are gradually increased from 8.3 to 9.1Abasal spacings during weathering.