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

The Gara, Yalea, and Gounkoto Au deposits of the >17 Moz Loulo mining district, largely hosted by the Kofi series metasediments, are located several kilometers to the east of the 650-Mt Fe skarn deposits in the adjacent Faleme batholith. The Au deposits are interpreted to have formed through phase separation of an aqueous-carbonic fluid, which locally mixed with a hypersaline brine of metaevaporite origin. Recognition of an intrusive relationship between the Faleme batholith and Kofi series opens the possibility that the Fe skarns and Au deposits are part of the same mineral system. In this paper, we combine new delta (super 13) C, delta (super 18) O, and delta (super 34) S 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. We combine these with existing data from the Loulo Au deposits to model the contribution of magmatic volatiles to Au mineralization. C and O isotope compositions of auriferous carbonate-quartz-sulfide veins from the Loulo Au deposits have wide ranges (delta (super 13) C: -21.7 to -4.5 ppm and delta (super 18) O: 11.8 to 23.2 ppm), whereas values from carbonate veins in Kofi series Au prospects close to the Faleme batholith and the Karakaene Ndi Fe skarn deposit have more restricted ranges (delta (super 13) C: -16.8 to -3.7 ppm, delta (super 18) O: 11.4 to 17.2 ppm, and delta (super 13) C: -3.0+ or -1 ppm, delta (super 18) O: 12.6+ or -1 ppm, respectively). Kofi series dolostones have generally higher isotopic values (delta (super 13) C: -3.1 to 1.3 ppm and delta (super 18) O: 19.1 to 23.3 ppm). Pyrite from Kofi series Au prospects adjacent to the Faleme batholith have a wide range of delta (super 34) S values (-4.6 to 14.2 ppm), similar to pyrite from the Karakaene Ndi skarn (2.8 to 11.9 ppm), whereas delta (super 34) S values of pyrite and arsenopyrite from the Loulo deposits are consistently >6 ppm. 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 devolatilization of sedimentary rocks with an organic carbon component. Isotopic data are permissive of the 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.