Session 3

Beach placer deposits along wave-dominated coastlines are critical sources of economically valuable heavy minerals. However, conventional coastal studies often analyze bulk sediment samples, obscuring the precise hydrodynamic pathways of dense mineral fractions. This study evaluates the seasonal granulometric variations of isolated heavy mineral concentrates along a 40 km stretch of the high-energy southern coast of Sri Lanka, extending from the Walawe Ganga river mouth to Yala National Park. Nine composite samples were collected from the active swash zone during the monsoonal hinge periods of October 2025 and April 2026. Heavy fractions were isolated using an electronic jig separator and analyzed via standard ASTM sieving and grain size statistical analysis using GRADISTAT. Results reveal high spatial-seasonal variability. Following the Southwest monsoon, very coarse heavy mineral fractions (636.4 µm) were observed near the river mouth, consistent with enhanced cross-shore winnowing under high-energy wave conditions. Following the Northeast monsoon, finer heavy mineral fractions occurred near the river mouth, while the coarsest and best-sorted fractions shifted to downstream locations, suggesting localized hydrodynamic controls, possibly influenced by rocky bathymetry, that modify the expected down-drift fining trend. These findings provide valuable baseline information for coastal engineering, sediment management, and targeted heavy mineral exploration.

Ground Electrical Resistivity Surveys (GERS) are widely used in geotechnical and geophysical investigations due to their reliable low-cost spatial quick results. However, the quantitative relationship between soil moisture content and resistivity remains insufficiently characterized for tropical soils, and soil moisture is a critical factor that affects data interpretation. This study investigates the relationship between soil moisture content and electrical resistivity of lateritic soils collected from selected locations in the Kalutara District of Sri Lanka. Laboratory experiments were conducted to determine basic soil parameters and electrical resistivity using both the Wenner probe method and the open container method as per BS 1377 to measure resistivity across controlled moisture variations ranging from 0.5% to 25% for laterite soil. Results indicate that laterite soil exhibits a strong logarithmic relationship between moisture content and resistivity, achieving a high correlation coefficient (R² = 0.9826), expressed as ρ = -41.02 ln (w) + 152.21, which significantly exceeds typical values reported in the literature. Based on the findings, standardized resistivity ranges were developed to serve as practical tools for field and laboratory interpretation. The outcomes provide guidance for Sri Lankan engineers in non-destructive moisture estimation, compaction quality assessment, and improving the reliability of geophysical surveys.

Sri Lanka is well renowned for its vast gemstone treasure, specially in gemstones rich Ratnapura District which is known for producing a diverse range of precious and semi-precious gemstones. The demand for gemstone provenance determination has been increasing in recent years, requiring scientific methods that able to differentiate gemstones by their depositional localities. This study investigates the geochemical properties of selected gemstones and gem bearing ores from various gem mining localities in Ratnapura District. The selected gemstone samples (Green Zircon and Pink Sapphire) were then subjected to geochemical analysis using Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDX) and the trace elements analysis of gem-bearing ore samples were conducted by using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The study compares the concentration of the major and trace elements present in gemstones and gem bearing ores from various depositional localities, and investigates the correlation between the gemstones and the related gem bearing ores. Green zircon exhibited variations in compositions of elements among depositional localities, particularly in impurity content and Zr/Si atomic ratios, whereas pink sapphire exhibits differences in impurity levels and minor elemental composition despite the limited number of specimens. Variations in major and trace element concentrations among the gem-bearing ores were revealed by ICP-OES analysis, reflecting differences in source lithology and depositional environments. The combined interpretation of gemstones and gem bearing ore chemistry demonstrates the applicability of geochemical characterization for determination of preliminary provenance of gem deposits in Rathnapura District. This study provides baseline geochemical data that can be useful for future investigations on characterization of gemstones and source determination using advanced trace-element analytical techniques.

Alluvial sediments can concentrate economically valuable heavy minerals, including gold, Ti-Zr minerals and rare earth element-bearing phases. This study targets the upper Walawe River Basin, with the focus on its tributary rivers and main-channel corridor, to characterize the geochemistry and mineralogy of the sediments and to evaluate their economic mineral potential. Nine sediment samples were collected from tributary-influenced and main-channel settings, panned in the field, and the <63 µm fraction of the panned concentrates was analyzed by inductively coupled plasma mass spectrometry (ICP-MS) and X-ray diffraction (XRD). Gold was detected in all analyzed samples at 0.18-1.22 ppm, exceeding 1 ppm at two sites, where visible gold grains were also recovered by panning. The same sites carried elevated nickel, cobalt, zinc and arsenic, suggesting possible contribution from mafic and/or sulphide-bearing source rocks upstream. One main-river sample displayed a strong rare earth-thorium-uranium anomaly, consistent with monazite and zircon enrichment. Normalized to upper continental crust, gold showed strong enrichment relative to upper continental crust values, indicating effective placer concentration. The most prospective signatures were concentrated along the main-channel corridor, while the sampled tributaries carried comparatively lower values. Therefore, the main-channel sediments hold combined gold and heavy-mineral potential, warranting detailed sampling, total-digestion analysis and grade evaluation.

Pegmatites are important geological formations that host a variety of industrial and strategic minerals. This study investigates the mineralogical zonation, geochemical characteristics, and economic mineralization potential of pegmatite deposits in the Balangoda area of Sri Lanka. Field observations, X-ray diffraction (XRD) analysis, and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) were used to characterize the pegmatite bodies and evaluate their economic significance. Twelve representative samples were collected from different zones of the pegmatite. XRD results revealed the presence of quartz, feldspars, muscovite, biotite, spodumene, lepidolite, beryl, garnet, and accessory minerals. Mineralogical zonation was identified as wall zone, intermediate zone, core zone and border of the pegmatite body. ICP-MS results showed a significant enrichment of trace elements, particularly Rare Earth Elements (REEs) with absolute abundances ranging from 10 to 1,000 times chondrite values, alongside pronounced negative europium anomalies and the enrichment of incompatible elements. The occurrence of spodumene and beryl, together with specific geochemical characteristics such as high REE enrichment, extreme positive Gadolinium (Gd) anomalies, and severe Strontium depletion (0.1 to 0.01 times Upper Continental Crust)suggests that the pegmatites may represent an evolved rare-element-bearing system. The occurrence of spodumene and beryl, together with trace-element geochemical characteristics, suggests that the pegmatites may represent an evolved rare-element-bearing system. However, further mineralogical and geochemical investigations are required to confirm the pegmatite classification. The study indicates that the Balangoda pegmatites have promising potential for industrial minerals and possible rare-element mineralization, although detailed resource evaluation is required.

This final segment invites reflections from presenters and attendees, synthesizing key insights from the session. The session chair will conclude the discussion by summarizing thematic threads, highlighting interdisciplinary contributions, and outlining potential collaborative directions.