Stratification Pattern and Geochemical Distribution of Gem Mine Sediments in Rathnapura Gem Fields, Sri Lanka

The geomorphic history of Sri Lanka originates in the Precambrian, when the basement rocks were formed, while the much more recent events of subsequent weathering, erosion and sedimentation processes that led to the development of gem-bearing strata. In the hill country, mineral-rich sediments were carried by rivers and streams and deposited in depressions and floodplains, creating favorable conditions for gem concentration. This study investigates the stratification of gem deposits in the Ratnapura region, focusing on how sediment depth, particle size distribution and geochemical signatures reflect long-term geomorphic and climatic dynamics. Ten gem mines were examined to assess vertical and lateral variations in sediment characteristics. Results show that stratigraphic layering corresponds to Quaternary climatic oscillations, with alternating wet and dry phases producing two to three distinct gem-bearing horizons at varying depths. Geochemical analysis indicated relatively stable concentrations of P₂O₅ and CaO, while K₂O, MgO and Fe₂ O₃ showed considerable variability. Grain size decreased with depth, with higher clay and silt proportions suggesting deposition in low-energy basins, whereas sandier deposits reflected higher-energy fluvial transport. Variations across sites point to the influence of paleo-river channel shifts and localized geomorphic processes.