FINGERPRINTING CU-BEARING TOURMALINE USING TRACE ELEMENTS AND LI AND B STABLE ISOTOPES

Cu-bearing (“Paraíba”) tourmalines are by far the most valuable and popular of the gem tourmalines, commanding up to $20,000/carat. Traditionally, the name “Paraíba” is reserved for Cu- and Mn-bearing tourmalines that are bright blue to green from the da Batalha Mine in Paraíba state, north-eastern Brazil. Saturated and less saturated blue and green tourmalines have been mined from other deposits near the da Batalha mine as well as in Morocco, Mozambique and Nigeria. The saturated variety of these gem-quality tourmalines cannot be distinguished from Paraíba tourmalines using standard gemological testing or semi-quantitative EDXRF analysis. Trace-element analysis by LA- ICP-MS has achieved some success at distinguishing tourmalines from Brazil, Mozambique and Nigeria. The ICP-MS study by Abduriyim et al. (2006) used Be, Pb, Mn and Cu to discriminate between Cu-bearing tourmalines; there is slight overlap in trace-element composition between tourmalines from Brazil and Nigeria and significant overlap between tourmalines from Nigeria and Mozambique. Here we use secondary ion mass spectrometry (SIMS) to measure the stable isotopes of Li and B and trace-element compositions of Cu-bearing tourmaline from Brazil, Morocco and Nigeria in order to differentiate between sources. Secondary ion mass spectrometry is an in situ micro-analytical technique that is relatively non-destructive and uses a primary focused ion beam to analyze elements and isotopes of interest. δ¹¹B values of tourmalines from Brazil range from -1.1‰ to 1.7‰, whereas δ¹¹B values of tourmaline from Nigeria and Morocco range from 0.1‰ to 3.8‰ and 10.4‰ to 14.6‰, respectively. Tourmalines from each country have relatively homogenous δ¹¹B values. Although the δ¹¹B values of tourmaline from Brazil and Nigeria overlap, tourmalines from Brazil, Morocco and Nigeria can be easily fingerprinted by coupling Pb and Be contents with δ¹¹B values. δ⁷Li values of tourmaline have greater overlap between mines from the various countries. However, Li isotopes can be used to distinguish between tourmalines within a single country. Ref: Abduriyim, et al. (2006) Gems & Gemology, 42:4-21.