Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

C. B. Dissanayake ¹, P. W. Vitanage ¹

Affiliations
1 Department of Geology, University of Sri Lanka, Peradeniya, Sri Lanka, IN

Abstract

Preliminary geochemical investigations for bauxite in Sri Lanka show the existence of laterite deposits containing bauxitic material approaching the lower limit of commercial grade. The geochemical processes responsible have followed two paths resulting in the formation of lateritic bauxite and lateritic iron ore. While the few samples analysed do not give any information regarding the possible reserves of commercial grade bauxite, the existence of economically important bauxitic material, particularly in the Highlands of Sri Lanka has been confirmed.

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Tissa Munasinghe ¹, C. B. Dissanayake ¹

Affiliations
1 Department of Geology, University of Sri Lanka, Peradeniya, LK

Abstract

Pink granites in the Digana antiform area of the Precambrian of Sri Lanka occupy the marginal zone of migmatites and metasediments of granulite facies, and occur as isolated bodies within the migmatite terrains. Field evidence and petrography of these rocks are suggestive of an anatectic origin. The period of emplacement is interpreted as being correlative with the basement mobilization and migmatisation presumed to have occurred at 1250 Ma which affected the Highland Series metasediments of Sri Lanka.

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Tissa Munasinghe ¹, C. B. Dissanayake ¹

Affiliations
1 Department of Geology, University of Peradeniya, Peradeniya, LK

Abstract

A new model is put forward to explain the geologic evolution of Sri Lanka. The Highland Group is a highly metamorphosed assemblage of a marine deposited volcanic, volcano-clastic and sedimentary rocks. The origin of the Highland Group dates back to 3000 Ma during a permobile phase in the evolution of the earths crust. Archaean tectonics suggest a downfolding of a thin crust forming the Highland Basin and subsequent filling with marine sediments and volcanics. The tectonic characteristics of the Archean suggest that its evolution possibly passed through a transitional phase (during which the crust thickened) to a plate tectonic phase. Plate tectonic processes analogous to those of the present were active from 2000 Ma. The material in the Highland Basin was metamorphosed and deformed repeatedly, in a collisional tectonic environment, to form the present configuration in the Precambrian of Sri Lanka.

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A. Senaratne ¹, C. B. Dissanayake ¹

Affiliations
1 Department of Geology, University of Peradeniya, Peradeniya, LK

Abstract

The study of the tonal variations of aerial photographs showing the vegetational cover in the Jaffna Peninsula of the north of Sri Lanka clearly points to the progressive formation of spits and lagoons. The contrasting tonal variations of the vegetation found in the emerging lands of different ages can be used to some extent effectively in the delineation of lands in a chronological sequence as exemplified by studies in the Karativu island.

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C. B. Dissanayake ¹, B. J. Van Riel ²

Affiliations
1 Department of Geology, University of Sri Lanka, Peradeniya, LK
2 Unesco House, 40.B, Lodi Estate, New Delhi, IN

Abstract

Geological and geochemical investigations of a serpentinized ultramafic body in the south of Sri Lanka have revealed the presence of nickel in the range 0.8-1.9%. Nickel is present in silicate form in the serpentine minerals. The typical tropical climate prevailing has been responsible for the formation of a deep weathering profile conducive to the residual enrichment of nickel. Cobalt, chromium and zinc are also present, though in lesser quantities when compared to nickel. The opaque minerals consisted of magnetite, chromite/picotite and other spinels. The leaching of silica and magnesium had resulted in the formation of chert, agate, chalcedony and carbonate veins respectively.

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C. B. Dissanayake ¹

Affiliations
1 Department of Geology, University of Sri Lanka, Peradeniya, LK

Abstract

Cd has been determined in a suite of rocks and minerals from the Skaergaard intrusion, east Greenland by f1ameless atomic absorption spectrophotometry with the heated graphite atomizer. Cd is progressively enriched in the ultimate residual magmatic fractions. Among the minerals studied, Cd shows a distinct preference for ironrich pyroxene, olivine and titaniferous magnetite.

A study of the mode of entry of Cd into the minerals as discussed in terms of the number of Cd²⁺ ions per appropriate structural site reveals that despite the similarity of the ionic radius of Cd²⁺ and Ca²⁺ in octahedral co-ordination, Cd²⁺ ions appear to substitute more for Fe²⁺ in octahedral positions. It has been found that Cd enters octahedral sites in olivine, pyroxene and magnetite in approximately equal amounts. The poor entry of Cd²⁺ ions into the Fe²⁺ site of ilmenite is attributed to its highly distorted octahedral nature in addition to the presence of Fe²⁺ ions in the lattice.

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C. B. Dissanayake ¹

Affiliations
1 Department of Geology, University of Sri Lanka, Peradeniya, LK

Abstract

Being group lIb elements, both Zn and Cd show similar chemical properties. The chemical analysis of a suite of rocks and separated minerals from the Skaergaard intrusion, East Greenland has enabled a detailed comparison to be made of the geochemistry of Zn with that of Cd.

Both Zn and Cd show an affinity for ferromagnesian minerals, A study of their mode of entry into structural sites however, points to the fact that while Zn shows a definite preference for Fe²⁺ sites in all the minerals studied, Cd shows a lesser tendency to substitute for the Fe²⁺ sites, except in the case of pyroxene. While Zn shows a very weak tendency to substitute for Ca²⁺ sites, Cd displays a much stronger affinity for the Ca²⁺ sites.

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