The world-class Loulo mining district (15.5 Moz resource), in the Birimian terrane of western Mali, contains a range of mineralogically diverse styles of orogenic gold mineralization. The district is distinguished by As-rich orebodies as at Yalea, as well as tourmaline-bearing, Fe-rich, orebodies as at Gara. New fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization, and point towards the role of multifluid sources (metamorphic and magmatic) in the formation of orogenic gold deposits.
Microthermometric and laser Raman studies from Yalea Main and two other similar orebodies (Loulo-3 and Baboto) reveal the dominance of coexisting CO2-N2±CH4 (type 1) and H2O-NaCl (type 2) fluid inclusions. These inclusions show evidence of fluid unmixing from reduced (quartz-faylite-magnetite, QFM, buffered), low-salinity (≤10 wt % NaCl equiv), CO2-rich-H2O-NaCl-N2 ± CH4 primary ore fluids. The combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270° to 340°C at 1.4 to 1.8 kbar. The P-T-X state of the Yalea-style mineralizing fluids, along with the
Fluid inclusion investigations from Gara and a similar style orebody (Yalea North) indicate the presence of coexisting CO2 ± N2 ± CH4 (type 1) and mixed-salinity (5–21 wt % NaCl equiv) CO2-rich-H2O-NaCl inclusions (type 3). Inclusion assemblages also contain common oxidized (hematite-magnetite, HM, buffered), high temperature (>400°C), hypersaline (∼35–50 wt % total dissolved solids), metalliferous (Na+Fe+Ca+Cu+Ni+W+ Pb+Zn), multiphase H2O-rich-CO2-NaCl-FeCl2 inclusions (type 4). This inclusion type has not been previously reported in other Birimian terranes. The composition of the brines, along with carbon isotope data (