The source of placer gold in the Walawe ganga basin, Sri Lanka

Abstract

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.