The stream sediment geochemistry of the Walawe Ganga basin of Sri Lanka - Implications for Gondwana mineralization

A regional geochemical and mineralogical study aimed at investigating the m ineralization in the western-part of the Walawe Ganga (river) Basin in Sri Lanka is represented in this paper. The river basin is the 3(rd) largest in the country and has within it a boundary zone between two geologically dif ferent crustal blocks, which are marked by granulitic grade rocks and amphi bolite grade rocks. Size fractions of stream sediments (< 63 mu m; 63-125 m u m; 125-177 mu m and 177-250 mu m) developed on the granulite-grade metamo rphic terrain have been analysed at their source for their mineralogical an d selected element compositions. Thirty-eight (38) sediment samples and 15 representative probable parent rock samples were chemically analysed giving special emphasis to the High Field Strength trace Elements (HFSE) includin g the Rare Earth Elements (REE). The granulite grade rocks in the study are a is geochemically similar to that of post Archean upper crust. However the stream sediments developed from the high-grade rocks during the intense we athering, are markedly enriched with HFSE and REE. The enrichment of HFSE a nd LREE is accounted for by the presence of HFSE- and REE- rich accessory m ineral phases such as zircon, monazite, apatite, garnet and rutile in the s ediments. in some samples, the content of heavy minerals contributes as muc h as 50 wt. %. These minerals act as a source of elements in the sediments. However, extreme hydraulic sorting of HFSE- and REE-bearing minerals durin g the sediment deposition cannot be expected within a short distance from n ear the sources except From a mineralized occurrence. Therefore, the higher enrichment of these elements presumably indicates occurrences of scattered mineral sources such as highly differentiated granites and associated pegm atites within the Walawe Ganga drainage basin. These granitic pegmatites ar e probably intruded during or soon after the main granulite-facies metamorp hic event and similar events are seen in other terrains of East-Gondwana.