Tourmaline as a textural, geochemical and isotopic marker of fault valve processes recorded at the Paleoproterozoic Lafigué orogenic gold deposit, Ivory Coast

Abstract

The Lafigué gold deposit (Western African Craton, Ivory Coast) is located in the northern part of the Toumodi-Fétékro greenstone belt, and its formation is related to the development of a NNE-SSW-striking sinistral shear zone during the regional D₂ deformation phase. Transpression is evidenced by a contractional jog expressed by E-W-trending, S-dipping thrusts. Boudinaged fault veins and horizontal extension veins infilled by a quartz-calcite-tourmaline-sulfide-gold assemblage have been developed along these thrusts. Two generations of hydrothermal tourmaline have been identified: (1) Tur_2a relates to a barren event, when (2) Tur_2b is associated with gold endowment. In situ analyses of major elements and boron isotopic ratios in tourmaline reveal that the precipitation of tourmaline and gold might result from multiple fluid discharges in damaged zones of shear zones as a consequence of fault-valve behaviour. It is expressed by a slight oscillatory zoning within the Tur_2b crystals, combined with fluctuations in the #Mg within the growth bands (#Mg from 0.56 to 0.63). The overall homogeneous crystal chemistry of Tur_2a and Tur_2b (up to 2000 µm), combined with a quite homogenous δ¹¹B between the core and the different growth bands (from -20.06 to -18.1 ‰), suggest a crystallisation from geochemically and isotopically relatively homogenous hydrothermal fluids. It suggests that no specific hydrothermal processes (such as fluid mixing, Rayleigh fractionation, changes in temperature or water/rock ratio) were sufficient enough to change the isotopic composition of tourmaline during its growth. We propose that fluid discharge and flash vaporization following the fault failure along the thrusts planes is the main mechanism allowing the crystallization of oscillatory zoned tourmaline and gold at the Lafigué orogenic gold deposit.