The Geology and Mineralogy of the Loulo Mining District, Mali, West Africa: Evidence for Two Distinct Styles of Orogenic Gold Mineralization

Abstract

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.