Chemical and boron isotope composition of tourmaline from the Kiaka orogenic gold deposit (Burkina Faso, West African Craton) as a proxy for ore-forming processes

Abstract

The Kiaka orogenic gold deposit (Burkina Faso), located in the Paleoproterozoic domain of the West African Craton, is characterized by a two-stage gold mineralization hosted in volcano-sedimentary metamorphic rocks that was formed during the Eoeburnean (2.20–2.13 Ga) and Eburnean (2.13–2.05 Ga) orogenic cycles. These two stages include an early disseminated low-grade gold mineralization and a late vein-hosted high-grade gold mineralization. Paragenetic studies indicate that the first gold stage was coeval with the deposition of hydrothermal tourmaline. The aim of this paper is twofold: (i) to determine the processes responsible for deposition of the economic disseminated gold mineralization and (ii) to identify the source of the mineralizing fluids. For this purpose, we performed an in situ study on tourmaline by combining electron probe microanalysis and secondary ion mass spectrometry measurements of boron isotopes. Hydrothermal tourmaline hosted in metamafic volcanic rocks and metagreywackes has a dravite composition but shows different δ11B values falling within the two intervals of − 25.1 to − 22.0‰ and − 19.6 to − 15.1‰, respectively. Our results suggest that tourmaline formed from a distal, high-temperature (ca. 400 °C), reduced, and low-salinity hydrothermal fluid that interacted with the local host rocks. Based on the modeling of tourmaline–fluid boron isotope fractionation, we propose a metamorphic fluid origin derived from devolatilization of deeply buried muscovite schists during the regional prograde to peak metamorphism prior ca. 2.13 Ga. This metamorphic fluid–rock interaction model may possibly extend to other orogenic gold deposits in the West African Craton.