Ceylon Journal of Science (Physical Sciences)

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End date: 1999Subject: search.filters.subject.Mineralogy

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    The source of placer gold in the Walawe ganga basin, Sri Lanka
    (University of Peradeniya. Peradeniya, 1995) Nawaratne, S.W.; Wijeratne, G.N.
    Significant quantities of alluvial gold occur in the zone lying between the gem-bearing gravel layer and the decomposed bed rock of Walawe Ganga, a river draining a Precambrian Highland Complex rock terrain of Sri Lanka. In such zones, the gold content varies from place to place and concentrations up to 9g per tonne of sediment were observed. The gold grains show different morphological features and occur as dust, flakes and nuggets. Some grains contain 100%. Au whereas others have lesser amounts with the balance being constituted of Ag, Cu and Mn. The highest alluvial placer gold concentrations are found in zones of intense shearing and fracturing in the country rock which is commonly intruded by quartz veins and pegmatites. Both shears and fractures show sulphide mineralization. Hydrothermal solutions with high CO2 and S components had been active during or after the deformational phase that formed the shears and the fractures. Such solutions appear to have been responsible for the gold mineralization in the Walawe Ganga basin.
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    Chemical and structural studies on natural spinels, MgAl2O4, of Sri Lanka
    (University of Peradeniya. Peradeniya, 1995) Udawatte, C.P.; Gunawardena, R.P.; Annersten, H.
    Natural Spinels from Sri Lanka (Ratnapura and Bibile) have been found to be mainly in the form, MgAl204 , with minor substitutions of Fe (0.7-3.5 wt% FeO) and Zn (0.07-4.4 wt% ZnO). Concentrations of other elements such as Cr, Co, Ni, V, and Mn were very low and, less than 0.1 wt% oxide. The natural spinels were found to have a complete normal cation distribution, A II[ ,(B III2)O4, suggesting an origin from a slow cooling rock forming process of the igneous and metamorphic rocks. Available experimental data of the Sri Lankan natural spinels indicate that complete ordering occurs at 280 ±175oC. The lattice constants were in the range of 8.087-8.091 A. Heating of the natural spinel results in disordering which may lead to a decreasing cell edge. Disordering of Al at 1000oC, as determined from AI-NMR spectroscopy, shows 13% of the Al occupying the tetrahedral sites. Optical spectra in the visible region are dominated %y spin forbidden transitions in Fe2+ and in some Cr-rich samples by spin allowed transitions in chromium. These are the main features that determine the colour of natural spinels investigated in the present study. The study indicates that the natural spinels have ordered cation distribution and the cation distribution in tetrahedral and octahedral sites depends on the thermal history of the crystal. The presence of transition elements, mainly Fe and Cr upto about 0.01 wt% oxide is sufficient to produce intense colour in spinels.
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    Some geological and geochemical attributes of the phosphate deposit at Ridigama, N.W. Sri Lanka
    (University of Peradeniya. Peradeniya, 1995) Hewawasam, A.L.T.; Dahanayake, K.
    Ridigama phosphate deposit occurs in the form of apatite rich veins that have intruded into the Precambrian granitic gneisses. Weathering processes active in the tropical Sri Lankan environment have given rise to a residual deposit. This deposit which is a weathering profile contains large primary apatite crystals in a fine matrix composed of ferruginous, siliceous, aluminous secondary phosphate minerals. Chemical studies have revealed that the primary apatite crystals of Ridigama have total P2O5 contents varying from 39% to 41% whereas it is less than 35% in the matrix. The 2% citric acid and water solubilities of primary apatite cystals are around 7% P2O5, and 0.05% P2O5 respectively; for the matrix the values were approximately 4% P2O5 0.04% P2O5 . Neutral ammonium citrate (NAC) solubility exceeds 2.5% P2O5, for crystals and less than 2% P2O5 for the matrix. The primary apatite crystals of this deposit showed higher solubility than those of Eppawala. However, the matrix at Eppawala was more soluble than that of Ridigama. The major components in the primary crystals are hydroxyl chlorapatite and carbonate fluor-apatite. Both these apatite types are associated with crandallite, quartz, feldspar, hematite and magnetite in the fine matrix.