Kapila Dahanayake

General geology and petrology of some Precambrian crystalline rocks from the Vijayan Complex of Sri Lanka

Abstract Detailed field studies of the Precambrian Vijayan Complex terrain, Sri Lanka, reveal the occurrence of granites, gneisses and migmatites in association with calc-silicate gneisses, quartzites and dolerites. Microcline-rich granites and gneisses show both sharp and gradational contacts with the adjacent migmatites. Petrological observations favor a magmatic origin for the granites and gneisses. These bodies seem to have intruded into pre-existing metasedimentary rocks which were subsequently subjected to retrograde metamorphism under amphibolite facies conditions.

Secondary phosphate mineralization in a karstic environment in Central Sri Lanka

At Eppawala in central Sri Lanka secondary phosphate mineralization is intimately associated with laminated fabrics within depressions (sinkholes and smaller cavities) formed in the thick weathering profiles of a hilly terrain underlain by a Precambrian apatite-bearing formation. The lowermost levels of the profile show extensive zones of leaching where derived apatite crystals occur within fine-grained, laminated stromatolite sequences. The stromatolitic groundmass, which diagenetically formed by percolating oxygenated phosphate and carbonate-rich groundwaters, is impregnated by the phosphate minerals francolite and collophane. Scanning electron microscopy (SEM) reveals that fine filaments, characteristic of microorganisms, are associated with the secondary phosphate mineralization. Continuous degradation and fragmentation of the stromatolitic mat has produced pellets, peloids, and intraclasts all enriched in secondary apatite. Degrading recrystallization around the edges of the primary apatite crystals has developed coated grains. The widespread occurrence of phosphate-enriched allochems in stromatolitic groundmasses is a unique development of a modern phosphorite in a karstic environment.

A modern terrestrial phosphorite-an example from Sri Lanka

Abstract At Eppawala in Central Sri Lanka, a thick weathering profile is mined for use as phosphate fertilizer. Recent tropical subaerial weathering processes have produced a phosphate-enriched horizon in the weathering profile which has developed on a Precambrian apatite marble formation. The enriched horizon with P2O5 compositions ranging from 10 to 40% is formed essentially of laminar phoscretes occurring in a network of sinkholes. Detailed examination of the laminar phoscretes reveals that primary metamorphic apatite crystals occur in association with phosphatic allochems such as coated grains, ooids, pisoids and intraclasts in a finely layered groundmass. It has been observed that the primary apatite grains released from the parent apatite marble due to tropical weathering processes have been deposited in sinkholes where nutrient-rich percolating meteoric waters were precipitating secondary apatite. Subsequent diagenetic processes active in such sinkhole sedimentary environments have produced the phosphatic allochems characteristic of secondary phosphate mineralization. The allochems together with their mineralized groundmass and the associated primary apatite grains have given rise to the Eppawala phosphorite deposit.

A permo-triassic(?) plant microfossil assemblage from Sri Lanka

Abstract Palynological observations on peaty and sandy clay beds of some sedimentary sequences from the Kurunegala district of Sri Lanka reveal the occurrence of plant microfossils of probable Permo-Triassic age. The microfossils are found in sediments considered to be of glacial origin deposited on the Sri Lankan fragment of the Gondwanaland.

Mineralogical, chemical and solubility variations in the Eppawala phosphate deposit of Sri Lanka — a case for selective mining for fertilizers

The Eppawala phosphate deposit of Northcentral Sri Lanka is a large deposit with an estimated reserve of more than 40 000 000 metric tonnes of the ore. Field observations at Eppawala have helped to recognize several types of phosphate material on the basis of colour, texture and structure. Laboratory studies have established variable apatite mineralogies, P2O5 compositions and consequent variations in solubility in different segments of the phosphate body. In some selected phosphate samples from Eppawala the combined aluminium and iron percentages (Al2O3 + Fe2O3 = R2O3) and MnO2 showed remarkably low values in contrast to earlier reported high values. The single apatite crystals which form at least 30% of the ore with P2O5 values of more than 40% and R2O3 contents in the range of 1% suggest their suitability for wet process production of phosphoric acid. On the basis of our findings selective mining is recommended for the Eppawala ore which is currently mined indiscriminately without due consideration being given to the heterogeneity and the anatomy of the deposit.

Glacial sediments from Weuda, Sri Lanka

Abstract Evidence for probable Paleozoic glaciation is reported for the first time from Sri Lanka. Rhythmic sequences containing clay/peat, sand, gravel and pebble layers occur in valleys underlain by Precambrian metamorphic rocks. Varve-like sediments and discshaped striated pentagonal pebbles characterize these sequences. This interpretation is also supported by granulometric data.

Potential use of naturally occurring sulphuric acid to beneficiate poorly soluble phosphate from Eppawala, Sri Lanka

A 29 000 yr old high-sulphur peat deposit occurs in the western district of Colombo, Sri Lanka. This permanently submerged deposit has several pyrite-rich layers in 8 to 10 m sequences and is connected to the open ocean. Sea water intrudes to the peat deposit during tidal movements. Villagers have excavated the peat to form islands which serve as foundations for their dwellings. The areas around these peat-rich man-made islands are inundated daily due to tidal movements. The interstitial waters of these tidal zones indicate low pH (2 to 5) and high Eh and are rich in sulphuric acid.Selected samples from the poorly soluble phosphate deposit at Eppawala were left buried in the sulphate-rich zones for fixed periods of time. Some phosphate samples when retrieved from the peat deposit showed four-fold increases of solubility. Preliminary agronomic experiments to grow paddy in P-deficient soils with the beneficiated phosphate fertilizer have given favourable results.