Stable C, O, and S isotope record of magmatic-hydrothermal interactions between the Faleme Fe skarn and the Loulo Au systems in western Mali

New delta13C, delta18O, and delta34S data from the Karakaene Ndi skarn, Au occurrences along the western margin of the Kofi series, and zircons within plutonic rocks of the Faleme batholith are combined with existing data from the Loulo Au deposits to model the contribution of magmatic volatiles to Au mineralisation. Comparison of the C and O isotope data with water-rock reaction models indicates the Loulo Au deposits formed primarily through unmixing of an aqueous carbonic fluid derived from the devolatilisation of sedimentary rocks with an organic carbon component. Isotopic data suggest the presence of a hypersaline brine that enhanced this phase separation including components derived from both Kofi series evaporite horizons interlayered with the dolostones and a magmatic-hydrothermal brine. This magmatic-hydrothermal component is particularly apparent in O, C, and S isotope data from the Gara deposit and Au prospects immediately adjacent to the Faleme batholith. It is concluded that there was a critical role for magmatism in the development of early alteration assemblages in the Loulo district, in the genesis of the Faleme iron skarns, and in those Au deposits that formed in response to fluid mixing.
New delta13C, delta18O, and delta34S data from the Karakaene Ndi skarn, Au occurrences along the western margin of the Kofi series, and zircons within plutonic rocks of the Faleme batholith are combined with existing data from the Loulo Au deposits to model the contribution of magmatic volatiles to Au mineralisation. Comparison of the C and O isotope data with water-rock reaction models indicates the Loulo Au deposits formed primarily through unmixing of an aqueous carbonic fluid derived from the devolatilisation of sedimentary rocks with an organic carbon component. Isotopic data suggest the presence of a hypersaline brine that enhanced this phase separation including components derived from both Kofi series evaporite horizons interlayered with the dolostones and a magmatic-hydrothermal brine. This magmatic-hydrothermal component is particularly apparent in O, C, and S isotope data from the Gara deposit and Au prospects immediately adjacent to the Faleme batholith. It is concluded that there was a critical role for magmatism in the development of early alteration assemblages in the Loulo district, in the genesis of the Faleme iron skarns, and in those Au deposits that formed in response to fluid mixing.