Application of structural geology in the exploration for residual gem deposits of Sri Lanka

Geological investigations of some major gem-bearing areas of Sri Lanka have shown that the gem deposits are controlled by the geological structure. Corundum deposits are generally associated with axial plane areas of tight, doubly plunging synclinoria and anticlinoria where occurrences of crystalline limestones and pegmatites are observed. Corundum deposits also occur at sites of heavy structural disturbances such as discontinuities, faults, folds, joints, lensing and necking zones etc. if occurrences of marbles and/or intrusions of granite and pegmatites have taken place. Alluvial gem deposits do not necessarily exist close to the area where they originate, but most concentrations of alluvial gem beds still remain close to the source area. Detailed investigations of the geological structure of a gem-bearing area and the relationship of these structural elements with appropriate source rocks will yield vital clues to the existence of gem deposits