Primary gold deposition linked to aqueous fluids in the Inata gold deposit (Burkina Faso) : Evidence from sulphides and fluid inclusions geochemistry

Luc Siebenaller (1) , Stefano Salvi (1) , Philippe Boulvais (2) , Sékou Sangaré , Et Al.

1 GET - Géosciences Environnement Toulouse
2 Terre, Temps, Traçage

The Inata gold deposit is hosted by metavolcano-sedimentary units that were affected by several pre-, syn- and post-mineralization deformation stages and greenschist-facies metamorphism during the Eburnean orogeny. Early-D1 E-W oriented structures host anhedral pyrite of metamorphic origin, rich in Ni, Co, As and Cu (concentrations measured by in-situ LA-ICPMS). The Au/Ag ratio of this early pyrite generation is very low (<0.05). During the ore stage (D2, N-S oriented shearzones), sulphides occur as euhedral pyrite and arsenopyrite, characterised by high Au/Ag ratios (>1). Concentrations of arsenopyrite are highest close to quartz-ankerite-albite veins and decrease gradually with distance, whereby pyrite becomes the dominant sulphide furthest from the veins. Visible gold is essentially found within fractures crosscutting sulphides associated to late stage transpressive D3 structures. This gold is interpreted to be the result of remobilization of early, disseminated gold, related to the D2 ore stage. Analyses of fluid inclusions (FI) revealed different H2O/CO2 ratios as well as D values of H2O for each deformation stage. Fluid geochemistry, obtained by in-situ LA-ICPMS analysis of FI, allowed distinguishing different fluid compositions. Early D2 ore-stage fluids are characterized by low salinities, low CO2, and low K and Ca concentrations, and are rich in B, Ag, Pb, As and REE. Zones in quartz veins containing these aqueous fluids are also marked by higher 18O values (>15‰) than D1, late D2 and D3 related quartz veins. In contrast, late D3 fluids related to remobilization are rich in CO2 and K and do not contain most of the latter trace elements. Trapping conditions of FI related to early D2 stage are of 410 (30) °C and 1.75 (0.25) kbar, whereas those related to the D3 event where trapped from 320 to 360°C and between 1.1 to 1.5 kbar. The Inata gold deposit thus represents a very rare example where nearly pure aqueous mineralizing fluids are documented.

Domaines: Sciences de la Terre

Luc Siebenaller, Stefano Salvi, Philippe Boulvais, Sékou Sangaré, Et Al.. Primary gold deposition linked to aqueous fluids in the Inata gold deposit (Burkina Faso) : Evidence from sulphides and fluid inclusions geochemistry. 24 ème Réunion des sciences de la Terre 2014, Oct 2014, Pau, France. pp.354. ⟨insu-01080853⟩