Primary gold deposition linked toaqueous fluids in the Inata gold deposit(Burkina Faso) : Evidence from sulphidesand fluid inclusions geochemistry

Abstract

International audienceThe Inata gold deposit is hosted by metavolcano-sedimentary units thatwere affected by several pre-, syn- and post-mineralization deformationstages and greenschist-facies metamorphism during the Eburnean orogeny.Early-D1 E-W oriented structures host anhedral pyrite of metamorphicorigin, rich in Ni, Co, As and Cu (concentrations measuredby in-situ LA-ICPMS). The Au/Ag ratio of this early pyrite generationis very low (1). Concentrations of arsenopyrite are highestclose to quartz-ankerite-albite veins and decrease gradually withdistance, whereby pyrite becomes the dominant sulphide furthest fromthe veins. Visible gold is essentially found within fractures crosscuttingsulphides associated to late stage transpressive D3 structures. This goldis interpreted to be the result of remobilization of early, disseminatedgold, related to the D2 ore stage.Analyses of fluid inclusions (FI) revealed different H2O/CO2 ratios aswell as D values of H2O for each deformation stage. Fluid geochemistry,obtained by in-situ LA-ICPMS analysis of FI, allowed distinguishingdifferent fluid compositions. Early D2 ore-stage fluids are characterizedby low salinities, low CO2, and low K and Ca concentrations,and are rich in B, Ag, Pb, As and REE. Zones in quartz veins containingthese aqueous fluids are also marked by higher 18O values (>15‰)than D1, late D2 and D3 related quartz veins. In contrast, late D3 fluidsrelated to remobilization are rich in CO2 and K and do not contain mostof the latter trace elements. Trapping conditions of FI related to earlyD2 stage are of 410 (30) °C and 1.75 (0.25) kbar, whereas those relatedto the D3 event where trapped from 320 to 360°C and between 1.1to 1.5 kbar. The Inata gold deposit thus represents a very rare examplewhere nearly pure aqueous mineralizing fluids are documented