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Classification of the gem deposits of Sri Lanka

Figure shows below illustrates the classification scheme for the gem deposits of Sri Lanka proposed by Dissanayake and Rupasinghe (1995). The scheme follows the general classification of the three main rock types and classification is based on deposit genesis. The advantage of the genetic classification of gem deposits lies in its predictive value. For example, contact metamorphic zones associated with calcium rich rocks are likely loci for certain gem deposits in Sri Lanka and identification of such features assists in the location of target areas for detailed exploration.

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Sedimentary gem deposits

Sedimentary gem deposits are by far the most important of all gem deposits in Sri Lanka and were classified by Dahanayake et al. (1980) into residual, eluvial and alluvial types. The sedimentary placer gem deposits occur in thin layers or lenses of gravel and sand, termed locally as Illam, in river beds and alluvial plains and on hillslopes and hillsides. Among the most important factors that govern the depositional nature of these gem deposits are the intensity and distance of transportation from the source and the topographical suitability of the sites for deposition.

The residual gem deposits occur as beds containing gem minerals mostly deposited in-situ and are found at depths ranging from a few centimeters to about 10 meters. These deposits mostly occur on the flood plains of rivers and streams and their sources are assumed to be in the close vicinity. A characteristic feature of the residual gem deposits is the presence of layers of alternating sand, clays and laterites containing angular fragments, as exemplified by the Elahera gem deposit.

The eluvial sedimentary gem deposits are found on hillslopes and flat areas incised by valleys. Often, the eluvial deposits grade into alluvial deposits making identification difficult. The presence of rock fragments and the angular to sub-rounded nature of the gem minerals are characteristic of the eluvial beds.

Alluvial gem deposits are the most widely distributed gem deposit type in Sri Lanka, the Ratnapura gem deposits being a good example. They often reach depths of more than 20 metres and usually contain two or three gem bearing layers. They occur mostly in old stream terraces and flood plains and are characterized by well-rounded grains. The gem-bearing layers in these alluvial deposits are markedly heterogeneous exhibiting a variety of shapes and sizes that indicate frequent changes in stream courses and velocity.

Metamorphic gem deposits

Most of the gem deposits in Sri Lanka are clearly of metamorphic genesis reflecting the fact that approximately 90% of Sri Lanka comprises high grade metamorphic rocks. Intense tropical weathering has decomposed and disintegrated the gem bearing rocks to form sedimentary gem deposits.

Skarn and calcium-rich rock types

Recent research has shown that calcium-rich bed- rock is a source for gem minerals within the metamorphic terrain of Sri Lanka. The mineral and fluid inclusions in gem corundum from Sri Lanka and noted that all the fluid inclusions are pure CO2. Thus CO2 is an important indicator of the genesis of the gem minerals. The microthermometry results for the primary inclusions suggested that these corundum formed under granulite facies metamorphism (> 630 ̊C, 5.5 kbar), while the presence of secondary fluid inclusions indicated retrograde post metamorphic cooling and uplift of the source areas. Further, the high density of the fluid inclusions (average density d = 1.05 g/cm3) was considered as being compatible with the formation of corundum under granulite facies metamorphism. Gem deposits located at Bakamuna near the main Elahera gem field an example of a corundum-bearing skarn deposit, and some other examples of this type, at Elahera and at Ohiya respectively.

Aluminous metasedimentary rock types

One of the characteristic features of the Highland Complex is the abundance of aluminous metasedimentary rocks. These have the chemical composition required for the formation of corundum and other aluminous gem minerals. It is clear that in the Highland Complex a combination of the P-T conditions and a suitable chemical composition has yielded voluminous sources for gems of this type. It is known as these gemstones have an origin related to granulite facies metamorphism involving CO2 flooding, the purging of H2O-rich fluids and partial melting.

Gems of pegmatitic origin

Pegmatites are common in the Highland Complex and  they  are  also  considered  as  important sources of gem minerals. One of the best known pegmatitic gem deposits is the moonstone deposit at  Meetiyagoda,  southern  Sri  Lanka . In addition, moonstones have been located in regions around Balangoda and Kundasale near Kandy. Pegmatites in Sri Lanka also contain gem minerals such as beryl, chrysoberyl, zircon and corundum.