Abstract Paleoproterozoic (Rhyacian) gold deposits of the Loulo district in western Mali contain >17 million ounces (Moz) Au and form part of the second most highly endowed region within West Africa. The deposits are located within siliciclastic, marble, and evaporitic rocks of the ca. 2110 Ma greenschist facies Kofi series, which were folded and inverted between ca. 2100 and 2070 Ma, prior to gold mineralization. Deposits at Yalea and Gounkoto are located along discontinuous, low-displacement, albite- and carbonate-altered shear zones, whereas Gara is confined to a tourmaline-altered quartz sandstone unit. Lodes typically plunge gently to moderately, reflecting the attitude of folds in the adjacent rocks and bends in the host shear zones, both of which influenced their location. Gold mineralization in the Loulo district was broadly synchronous with emplacement of the Falémé batholith and associated Fe skarn mineralization, which intrude and overprint the western margin of the Kofi series, respectively. However, hydrothermal fluids generated during metamorphic devolatilization of the Kofi series rocks appear responsible for gold mineralization, albeit within a district-wide thermal gradient associated with emplacement of the Falémé batholith. The regional-scale Senegal-Mali shear zone, commonly cited as an important control on the location of gold deposits in western Mali, is absent in the Loulo district.

Abstract As the functionality and speed of 3-D geologic modeling software have improved over the last 30 years, it has become a core tool for identifying, understanding, and modeling the structural controls on ore deposits. This chapter attempts to summarize some of the key considerations involved in the 3-D modeling of structurally controlled ore deposits and establishes a basic three-step workflow that can be applied to almost any deposit style: establish a geologic framework through field work and 3-D visualization, model the project-scale geology, and finally identify, model, and understand the controls on ore shoots. Importantly, the geologic understanding of a project is not a static concept. Each step in the modeling process should add to it, highlighting which aspects of the model fit the current geologic understanding, and thus increase confidence, and which require further review and possible modification. This chapter also provides guidance on preparing data for 3-D modeling, basic 3-D visualization techniques, selecting a modeling approach, and model validation, as well as commentary on some of the more common pitfalls encountered in 3-D modeling. Finally, case studies of the Tuzon gold deposit in Liberia and the Yalea gold deposit in Mali are provided as examples of the process involved in building a 3-D geologic model, from field work to final model.