ZINC-ENRICHED FRACTURE-FILL AND FIBROUS OVERGROWTHS IN PARAÍBA TOURMALINE FROM THE SÃO JOSÉ DA BATALHA, BRAZIL

Copper-bearing elbaitic tourmaline is widely prized as a gemstone due to its vivid, saturated, ‘neon’ blue hues. Originally discovered in Brazil near the São José da Batalha Mine (1980s) in the state of Paraíba and later in the nearby state of Rio Grande do Norte (1990s), these Cu-bearing specimens became known as Paraíba tourmalines. Subsequently, other localities hosting similarly colored Cu-bearing tourmalines were found as elbaites in Nigeria (2001) and Mozambique (2004) and as liddicoatites in Mozambique (2017). In the market, the gemstone’s value is related to their source; Brazilian materials are 5-10 times that of African stones. Significant progress for determining their country of origin has been made using trace elements but additional criteria for sourcing is desirable.

Detailed chemical characterization and imaging of a suite of Cu-bearing tourmaline from the São José da Batalha locality, unheated and heated, with the electron microprobe (EMP) revealed two distinctive features: they contained a late generation of fracture filling tourmaline and fibrous outgrowths at the +c-pole of the host tourmaline, both compositionally enriched in Zn. The overgrowths contained higher Zn than the fracture fill. ZnO is ~ 2.2 wt % in the fracture fill and up to 2.5 wt % in the fibers. A representative structural formula of the host Paraíba tourmaline is (Na_0.58 ☐_0.38 Ca_0.04) (Al_1.63 Li_1.17 Cu_0.16 Zn_0.01 Mn_0.01) (Al_6.00) (Si_5.87 Al_0.13) (BO₃)₃ (OH)₃ (F_0.46 OH_0.54) whereas the late-generation tourmaline is relatively enriched in Na, Zn, Mn and depleted in ☐, Al, Cu with a formula of (Na_0.79 ☐_0.15 Ca_0.04) (Al_1.23 Li_1.13 Zn_0.29 Mn_0.28 Cu_0.04 Fe_0.02 Mg_0.01) (Al_6.00) (Si_5.98 Al_0.02) (BO₃)₃ (OH)₃ (F_0.68 OH_0.32). Even with heat treatment and removal of the color zoning, the vein fill and overgrowths remain observable in backscattered electron imaging and in chemical EMP traverses. The fibrous nature and composition of the late generation tourmaline implies introduction of an aqueous fluid with a more saline composition (~0.66 mol/l) that likely enhanced Zn mobility and tourmaline precipitation. Within our small sample suite, these features were lacking in the African Cu-bearing tourmalines. Thus, Zn-enriched fractures and overgrowths may be diagnostic of the original Paraíba materials.