Characterisation and evolution of Au mineralisation in the Loulo mining district, Western Mali
Loulo is a world-class orogenic gold district in SW Mali. The mininig district is comprised of two major operational mines at Yalea (6.5 Moz at 4.7 g/t) and Gara (3.7 Moz at 4.1 g/t), plus numerous satellite deposits and other prospective targets, which takes Loulo's overall resource to 14.4 Moz. Two distinct types of ore deposit are recognised. Gara-style deposits are Fe-rich and characterised by a Fe-Cu-REE-Au-Ni-W-As ± Co-Ag-Pd metal association. Yalea-style, As-rich, deposits exhibit more typical orogenic gold features with a metal signature consisting of Fe-As-Cu-W-Au-Pb-Sb-Ag. Investigations into the source and composition of the auriferous fluids indicate two styles of ore fluid were responsible for mineralisation: (1) a 'Gara-style' oxidised (FM-buffered), high temperature (>400 °C), hypersaline (35-55 wt.% NaCl equiv.), Na-Fe-Cl-B-bearing magmatic fluid; and (2) a 'Yalea-style' reduced (QFM-buffered), lower temperature (300-350 °C), low-salinity (<10 wt.% NaCl equiv.) C0[sub]2-N[sub]2-H[sub]2S-rich metamorphic fluid. The identification of hypersaline Na-Fe chloride-rich fluid inclusions questions the metamorphic fluid paradigm for orogenic gold deposits. Field, petrographic and isotopic studies also provide convincing evidence that magmatic fluids played an integral role in gold mineralisation. These include: (1) the presence of a significant boron anomaly along the length of the Senegal-Mali Shear Zone (which hosts Loulo and other large gold deposits such as Sadiola), typified in Loulo by the presence of large volumes of replacive epigenetic tourmaline; (2) widespread pink sodic alteration; (3) the spatial association with iron oxide-copper-gold (IOCG) deposits in the adjacent Falémé district in Senegal; (4) the presence of Ni-Co sulphides and abundant REE-W minerals in the ore paragenetic assemblage; and (5) carbon isotopes overlapping the magmatic field (e.g. Gara vein ankerites show [delta][sup]13C compositions from -14.4 to -4.5%). Both metamorphic and magmatic fluids were capable of carrying gold as either bi-sulphide complexes or oxidised chloride complexes, respectively. In the Gara-style orebodies, mineralisation was controlled by the mixing of these two fluids, which resulted in changes in temperature, pH and f0[sub]2, and caused retrograde boiling ofthe CO[sub]2 component. This style of Loulo orebody contains distinct mineralogical and fluid inclusion characteristics, and it is therefore proposed that Gara-style orebodies represent a new sub-class of orogenic gold deposit, with evidence suggesting possible links with IOCG/skarn mineralisation. In the Yalea-style orebodies, mineralisation is confined to metamorphic fluids with the principal mineralisation trap linked to fluid immiscibility during fluctuations in pressure and/or temperature, and fluid-rock interactions with reduced host sediments. Chemisorption processes may also have played a role in gold deposition to account for the high levels of refractory gold in arsenopyrite.