Origin of Pan-African mesothermal gold mineralisation at Bin Yauri, Nigeria

Abstract In the Bin Yauri area, mesothermal Au-sulphide-quartz-carbonate veins and associated hydrothermal alteration appear to have accompanied late tectonic brittle faulting which cut across the contact zone of a Pan-African granodiorite batholith intruding phyllites. Fluids of magmatic origin were initially suspected to be responsible for the Au mineralisation, due to proximity to the intrusion. However, studies of fluid inclusions, rare earth elements and tourmaline chemistry tend to suggest mineralising fluids of metamorphic origin. The present study involves the use of large ion lithophile elements (K, Rb, Ba, Sr) and stable isotopes (S, C, O) in further constraining the origin of the Au mineralisation. It is found that the hydrothermal alteration (and Au mineralisation) is characterised by a systematic coenrichment of K, Rb and Ba and inter-element ratios (K/Rb, K/Ba, Ba/Rb, Rb/Sr) characteristic of average (metasedimentary) crustal rocks. These are distinctly unlike the K-Rb-Ba-Sr fractionation patterns associated with magmatic-related mineralisation. Similarly, stable isotope evidence, where δ 34 S = 1.5–9.4‰ and δ13C = −7±0.7‰, are more consistent with crustal sources of S and C, respectively, while δ 18 0 = 13±1.4‰ is within the isotopic range for metamorphic water. The study, therefore, has shown that the direct involvement of magmatic fluids can be ruled out as a source of the Au mineralisation at Bin Yauri, despite the close association with a granodiorite batholith. The fluids involved were probably generated along a regional (transcurrent) fault system and subsequently focused into the metasedimentary country rocks via the late tectonic fault zone.