Formation of tourmaline in the Bakoshi-Gadanya Goldfield, Nigeria: Insights from elemental compositions and boron isotopes

Abstract

Tourmaline and gold mineralization form an association in the Bakoshi-Gadanya (BAG) Goldfield on the northern West Nigerian Subshield, located in the southern Trans-Saharan Orogenic Belt of West Africa, although the cogenicity of this association remains untested. In this study, we report the results of an integrated study of the boron isotopic and major- and trace-elemental compositions of BAG tourmalines, from which we infer the nature and origin of the hydrothermal parent fluids and their role in the associated gold mineralization. Tourmalines in the BAG Goldfield are of four types: altered granite-related (Tur I), wallrock-hosted (Tur II), Gadanya Tourmalinite (Tur III) and Shanono Tourmalinite (Tur IV). The tourmalines mainly belong to the alkali group with dravite (i.e., Mg-rich) compositions. The MgFe₋₁ and □Al(NaR)₋₁ exchange vectors are the dominant substitution mechanisms for all BAG tourmalines with contributions from deprotonation AlO[R(OH)]₋₁ substitutions. Except for Tur I, which is enriched in Rb (0.02–21.9 ppm) and Cs (0.01–0.63 ppm), due to high fluid-rock reaction, the BAG tourmalines are enriched in Cr (2.0–3908 ppm), Ni (5.0–222 ppm), Co (0.05–27.4 ppm), Sr (41.8–3031 ppm), Sc (1.6–281 ppm), V (32.0–701 ppm), Al (0.01–0.35 apfu), Fe (0.59–1.47 apfu), and Mg (0.67–2.43 apfu), suggesting metasedimentary-derived components. Boron isotopes display bimodal populations from −16.8 to −12.0 ‰ (Tur II to IV) and from −23.0 to −19.0 ‰ (Tur I). We propose that greenschist to amphibolite facies metamorphism during the Pan-African Orogeny devolatilized country-rock metapelites and produced a metamorphic-hydrothermal fluid responsible for BAG tourmalinization. These metamorphic-derived Tourmalines show no relationship to the magmatic-hydrothermal fluid derived BAG gold ore. Our results show that the BAG tourmalines may not serve as indicator minerals in exploration for gold mineralization in the BAG Goldfield in northern West Nigerian Subshield.