JultikaUniversity of Oulu repository

Abstract

Sadiola Hill is an ~8 Moz gold deposit located in western Mali within a ca. 2200–2050 Ma tectonic window known as Kédougou-Kénieba inlier (KKI), exposing the Western African craton. The deposit is hosted in a metasedimentary package made up of impure carbonate rocks, wackes, and arenites intruded by three distinct igneous phases. A N-S-trending Sadiola shear zone, related to the regional Senegalo-Malian shear zone, and NNE-trending third order fault splays acted as conduits for auriferous hydrothermal fluid flow (Hein and Tshibubudze 2007; Cameron 2010; Masurel et al. 2017). This study determined that gold is associated with ferroan dolomite/dolomite+/-ankerite- muscovite to Al-poor muscovite- phlogopite, cogenetic with sulphide precipitation. The intensity of white mica replacement of biotite, and abundance of Fe-bearing carbonates in association with phlogopite, are good predictors of gold mineralization. Within well-mineralized samples phlogopite occurs as gangue silicate that recrystallized after sulphidation reactions took place. Geochemical data agree with the hyperspectral interpretation of ore-stage alteration assemblages, as K/Al molar, Mg/Al molar, and total C enrichment positively correlated with increasing gold grade.

Differences in spectral expression of carbonate alteration in the greywacke, diorite, and impure marble were well-constrained during this study. Syn-mineralization carbonate is dolomite in the impure marble unit and attains ferroan dolomite-ankerite composition in the greywacke and diorite. The differences in the Fe content between the ore-stage carbonates expressed in different rock types can be explained by progressive rock-buffering with the surrounding Fe-rich minerals within the diorite and greywacke. As the impure carbonate unit is Mg-rich, the ore-stage dolomite consistently shows low Fe-content. Furthermore, Fe in biotite was preferentially consumed by sulphidation reactions, leaving no residual iron for the dolomite.

Muscovite and Al-poor muscovite occur in all host rocks and are associated with gold grade in the impure carbonate and the greywacke. Investigation of protolith control revealed that white mica is consistently more aluminous and attains higher crystallinity values in tonalite, while cogenetic white mica within the greywacke is Al-poor illite. These variational differences are attributed to availability of Al³⁺ and lower Mg²⁺ content in the tonalite to produce muscovite. Phengite (Al-OH at ~2225nm) occurs in the system as a minor post-mineral alteration phase. Finally, Illite crystallinity has a common trend of increasing veinward, which was attributed to lower degree of fluid-rock interaction in the vein centre and higher fluid/rock ratio.