Abstract:
The most well-known Eheliyagoda alluvial gem deposits in Rathnapura District lies on
the Highland Complex (HC) rocks of Sri Lanka. It has been discovered that Sri Lanka's
gem-bearing gravels are incredibly rich in valuable gem quality minerals as well as
minerals that are rich in Rare Earth Elements (REE). Though Eheliyagoda is famous for
its high quality gems, the origin and the base mineralogy of Eheliyagoda gem fields
have not yet been reported comprehensively. Hence, present study investigated the
possible mineralogy of 20 different gem mining sites (paddy lands, forests, cultivated
lands and river banks) located in Eheliyagoda, with a specific focus on the REE
concentrations present in the gem-bearing gravel. Inductive coupled plasma mass
spectroscopic (ICP-MS) analysis was carried out on the samples with the purpose of
identifying the quality and quantity of REE while fourier transform infrared
spectroscopic (FTIR) analysis and X-ray diffraction (XRD) analysis were performed to
determine the auxiliary minerals. The gravels and the sandy sediments of the gem
bearing layers were angular to sub angular in shape indicating its less reworked nature
contrast to the well-known alluvial characteristics in most of the other gem fields in Sri
Lanka. The average Total REE concentrations for the studied samples is 462 (Min:55
Max: 1442) ppm with about 75 % of Light REE (LREE) content. Results indicate that
the study area is rich in LREE concentrations, which is comparable to the published
data on other gem mining regions of Sri Lanka. The Pearson correlation between Heavy
REE (HREE) shows strong positive correlations between each other and further they
show a strong positive correlation with arsenic (average 10 ppm) indicating its regional
metamorphic origin where the arsenic concentrations are comparable. High to moderate
correlation is observed between LREE and HREE of the samples which might be
controlled by the minerals such as garnets. Mineralogical analyses confirm the presence
of gem or gem associated targeted minerals such as ilmenite, kyanite, zircon,
sillimanite, corundum, xenotime and monazite which might have given the origin for
the REE contents. Hence, the study concludes that the origin for the Eheliyagoda gem
fields are the provincial rocks, specifically garnet-sillimanite-biotite gneisses and
charrnokitic gneisses which directs the in-situ weathering products.
