Sedimentary geochemistry of alluvial overburden in the primary gem deposit of Pelmadulla, Sri Lanka.

Abstract

This study examined the sedimentology and geochemistry of the alluvial gem-bearing succession in Pelmadulla, Sri Lanka. The major aims were to investigate weathering and climatic influences on the provenance and to study the origin of the alluvial formation. A sedimentological study indicated different layers of sand, clay, and organic matter-rich clay (ORC) in the gem deposit overburden, which indicated hydraulic sedimentary facies by saltation, suspension, and storm-type origin, respectively. Notably, the storm-type deposit was dominant with ORC, which concluded the formation under extreme climatic events. Clay layers of the deposit showed bimodal characteristics while sands showed unimodal nature, reflecting the impacts of chemical weathering on different sources. Clay layers evidenced a higher level of inorganic carbon and a strong sedimentation rate. The major element relationships of alluvial succession reflected the presence of aluminous clay, predominantly formed by the weathering of alkali feldspar. The enrichment of ferromagnesian elements and large cations with rare-earth elements in the sediment layers concluded the presence of heavy minerals such as zircon and garnet, while the Chemical Index Alteration (CIA) indicated uniformly higher values (Avg. CIA: 89). Consequently, a steady-state chemical weathering was dominant in the layers with marked depletion of the labile fraction. The negative correlation between CIA and SiO2/Al2O3 of the layers denotes the impact of grain size and sorting, whereas the relationship between Ga/Rb and K2O/Al2O3 reflects the weathering trend from a dry and cool climatic condition to a warm and humid condition. In contrast, the provenance of sediments concluded quartzose to mafic nature formed under a passive margin condition.