Application of geochemical ratios for delineating gem-bearing areas in high grade metamorphic terrains

Trace element data for stream sediment samples collected from 6 river basins situated in geomorphologically and climatologically different terrains of Sri Lanka were analyzed to identify a suitable geochemical ratio to delineate potential areas for gems. All the areas are located in high grade metamorphic terrains with different lithologies. Correlations between different elements were studied using Spearman rank correlation and elements showing consistent high correlations were selected to calculate element ratios that can be used to discriminate areas with different potentials for gems. It was noted that the Rb, Ba, Sr, and the Ti, Nb, Zr element groups have high correlations between them. From the calculated ratios Ba/Sr, Rb/Sr and RbxBa/Sr2 were identified as being useful. The best ratio was determined considering the discrimination %, and representation of within category gem potential variation. Results show that RbxBa/Sr2 is the best ratio and a value greater than 4 represents gem potential areas while a ratio less than 1 represents low/no potential areas. Application of these results to the locations where test gem pits were validated this empirical rule.