ALBERT

Description: 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 CO 2 -N 2 ±CH 4 (type 1) and H 2 O-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), CO 2 -rich-H 2 O-NaCl-N 2 ± CH 4 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 18 O fluid compositions of 8.8 to 10.7, is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane. Similar fluid compositions were previously reported elsewhere in the Birimian crust of West Africa and in other orogenic gold districts worldwide. The precipitation of gold from the H 2 S-rich metamorphic fluid is primarily linked to phase separation of the ore fluid, which is controlled by P-T fluctuations and/or fluid-rock interaction with carbonaceous host sedimentary rocks (confirmed by low 13 C values of –21.7 to –15.8). Fluid inclusion investigations from Gara and a similar style orebody (Yalea North) indicate the presence of coexisting CO 2 ± N 2 ± CH 4 (type 1) and mixed-salinity (5–21 wt % NaCl equiv) CO 2 -rich-H 2 O-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 H 2 O-rich-CO 2 -NaCl-FeCl 2 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 ( 13 C of –14.4 to –4.5), suggests a magmatic input to the Gara-style hydrothermal system. The coexistence in the fluid inclusion assemblages of the magmatic brines with Yalea-style, CO 2 -rich, metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing. The interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the Gara-style orebodies. This includes the growth of widespread multi-stage Fe 3+ -rich tourmaline (B-rich granite source) and sodic alteration, and ore assemblages consisting of abundant nickeloan pyrite, (REE)-phosphates, Ni ± Co ± Pb ± Zn minor/trace sulfides, and scheelite. Gold deposition in the Gara-style hydrothermal system is related to physical and chemical changes of the two fluids during mixing (e.g., decreases in f O2 and T in the brines and retrograde boiling of the CO 2 component in the metamorphic fluids, a "salting out effect").

Description: Loulo is a world-class orogenic gold mining district in the Birimian terrane of western Mali. Orebodies are located along second or higher order shears associated with the Senegal-Mali shear zone, with gold mineralization largely linked to a transtensional event. The mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics. One group is typified by the Gara deposit, whereas the other by the Yalea deposit. Both deposit styles are hosted by similar rock types (calcareous graywackes and calciticdolomitic marbles). Gara-style orebodies occur as sulfide disseminations or ankerite-rich shear vein stockworks, hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the Senegal-Mali shear zone. These deposits are characterized by intense multi-stage albitization and tourmalinization (pre-, syn- and postmineralization). Gold lodes are Fe-rich (dominated by nickeloan pyrite), contain Cu-Ni ± Co minor and trace sulfides (e.g., chalcopyrite, gersdorffite, pentlandite, cobaltite, millerite), and show consistently high levels of P-REE-W–bearing phases (apatite, monazite, xenotime, and scheelite). Base metal concentrations show a marked increase in marble host rocks, with the formation of nickeloan pyrite-cobaltite-clausthalite ores. In contrast, Yalea-style deposits are associated with quartz ± ankerite vein lodes and disseminated sulfide stringer zones. Mineralization occurs along highly altered (tourmaline-absent), brittle-ductile, shears up to 8 km away from the Senegal-Mali shear zone. Wall-rock alteration is characterized by addition of K 2 O, CaO, CO 2 , H 2 O, and SiO 2 , with mineral assemblages consisting of chlorite-sericite-carbonate-quartz ± albite. Ore paragenesis is enriched in As, mainly as multistage growth of arsenopyrite and arsenian pyrite. Base metal sulfides, scheelite, and (REE)-phosphates are extremely rare. The diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region. The As-rich Yalea-style deposits have characteristics typical of Birimian gold mineralization in Ghana, and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism. On the other hand, the polymetallic, Fe-B–rich, Gara-style orebodies show atypical features for Birimian gold mineralization. Instead, field relations and the mineralogy and geochemistry of the Gara-style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks, with possible links between gold mineralization and iron oxide skarn development in the region. The data collected at Loulo highlight the diverse nature of orogenic gold deposits, especially in West Africa. This style of ore deposit can form from a variety of fluid sources, both metamorphic and magmatic.