The geology, structure and metallogenesis of the world class Loulo-Bambadji Au district in Mali and Senegal, West Africa

The 2.1 Ga Kedougou-Kenieba Inlier in West Africa hosts outstanding mineral wealth, with ~45 Moz of gold and 630 Mt of iron ore hosted along the Senegal-Mali Shear Zone (SMSZ). To the west of the SMSZ the Faléme' Volcanic Belt (FVB). Detailed petrography and analysis of igneous rocks by solution lCP-MS and AES show that the FVB is comprised of calc-alkaline volcaniclastic sediments, lavas and plutonic rocks, hosts iron ore in a series of magnetite skam deposits. To the east of the SMSZ, the Kofi series is comprised of clastic sedimentary rocks and peraluminous granite plutons. Orogenic Au hosted in the Loulo-Bambadj i district of the Kofi Series (including the Gara, Yalea and Gounkoto mines) is spatially associated with epigenetic tourmaline alteration, while widespread albite alteration is associated with early stages of mineralisation in both the FVB and the Kofi Series. A >400 ppm boron soil anomaly along >100 km strike length of the SMSZ is related to widespread tourrnalinisation of sedimentary rocks in the Kofi Series. The Au deposits of the Kofi Series are characterised by a Fe-As-Cu-Au-Ag i REES-W-Ni-Te metal association. Ore assemblages are pyrite and arsenian pyrite dominated with accessory chalcopyrite, Ni-sulphides, scheelite and REE phosphates. Two distinct hydrothermal fluids were involved in Au mineralisation in the Kofi Series: 1) a moderate temperature (315-340 °C), low salinity (<10 wt. % NaCl equiv.), low density (31 gcm -3), HzO-COZ-NaCl-HZSiNz-CH4 fluid; and 2) a high temperature (up to 445 oC), hypersaline (~40 wt. % NaCl equiv.), high density (~1.3 gcm -3), H2O-CO2-NaCl+-FeCl2-B fluid. This hypersaline fluid has been interpreted as being sourced from crystallising magmatic bodies, new data presented here suggests otherwise. Stable isotope data (0, C and S from silicate, carbonate and sulphide minerals) indicate that volatiles involved in ore formation were generated through metamorphic devolatilisation reactions within the Kofi Series; this is interpreted to be the source of the low salinity fluid. Isotopically heavy delta 34 values (+25 %o) from diagenetic pyrite together with 11 B-enriched isotopic ratios in hydrothermal tourmaline (-4.6 to +19.8 %), measured by SIMS, suggest that the hypersaline fluid formed through dissolution of evaporite units formerly present in the Kofi Series. Furtheirnore, dating of magmatic zircons from nearby plutons by LA-ICP-MS shows a considerable time gap between magmatism at ~2100 Ma and mineralisation at ~2030 Ma. Fluid inclusion data from the Gounkoto deposit indicates that phase separation in the metamorphic sourced fluid was an important mechanism for ore deposition. In addition, discrete sections of the deposit exhibit evidence that partial mixing of the high salinity and low salinity end member fluids enhanced phase separation through retrograde boiling. The hypersaline fluid is widely distributed along the SMSZ, with increasing abundance toward the FVB in the west. Hypersaline inclusions at the Karakaene Ndi magnetite skam deposit in the FVB imply that the hypersaline fluid played a significant role in the development of the magnetite skarn deposit. The NaCl content allowed efficient transport of Fe to sites of ore formation in F6C120 complexes.