COPPER ISOTOPE COMPOSITION OF PARAÍBA TOURMALINE FROM SAO JOSE DA BATALHA

Vivid neon blue Paraíba tourmaline (tur) is one of the most expensive colored stones in the gem trade. Its highly desired color is caused by the presence of the transition element Cu which produces the unique colors found in Paraíba tur, including the coveted electric blues and purples. Cu-bearing tur is rare, and the geologic source of Cu within Paraíba tur remains unknown. Previous studies have been unable to distinguish between the two most likely Cu sources for the tur: (1) Cu enrichment during pegmatite evolution and tur crystallization, or (2) tur crystallization from Cu-bearing hydrothermal fluids due to the dissolution of Cu sulfides at depth. Transition metal stable isotopes, such as Cu, offer a potential avenue to fingerprint the origin of Cu in Paraíba tur. About five carats of cuprian elbaite tur from the Sao Jose da Batalha Mine, Paraíba, Brazil, were purchased from a dealer as a parcel of melee rough. The tur are small fragments ranging in size from 2 to 7 mm. All stones are electric blue to blue-green. Sixty-five Paraíba tur grains were analyzed for major and minor elements by EMPA and trace element concentrations by LA-ICP-MS. Samples have Cu concentrations ranging from 0.27 to 4.62 wt.% CuO (0.21 – 3.69 wt.% Cu). Five of those samples were chemically separated and their Cu isotopic compositions were measured using a MC ICP-MS. The δ⁶⁵Cu values of the Brazilian Paraíba elbaite range from +1.28‰ to +1.82‰, with an average of +1.65 ± 0.25‰ (1σ; n = 5). The Cu isotope composition of these tur is much higher than that of granites (δ⁶⁵Cu = +0.00 ± 0.45‰; Li et al., 2009) suggesting their formation is not associated with magmatic processes. Instead, the higher δ⁶⁵Cu_tur values likely reflect oxidation of Cu sulfides, reactions that release Cu and preferentially incorporate ⁶⁵Cu in the fluid phase in the absence of S. This data suggests the source of Cu in the Paraíba tur is likely related to a hydrothermal fluid that oxidized and dissolved Cu sulfides rather than a primary igneous signature.