Hyperspectral Mineral Mapping of Drill-Core Samples at Sadiola Hill Gold Deposit, Mali, West Africa: The Paragenesis of the Polyphase Alteration Mineralogy

The Sadiola Hill gold deposit is located in Mali in the Kédougou–Kénieba Inlier (KKI) of the West African craton. The gold deposit is hosted in metamorphosed carbonaceous and clastic sedimentary rocks that are intruded by diorite dykes and granitoids. The deposit records a multistage history of alteration, including complex compositional variation and speciation of carbonate group minerals and white and dark micas. We present the results of spectral and petrographic studies of 36 drill core samples from six diamond boreholes from the Sadiola Hill gold mine to (1) refine paragenetic studies and (2) define alteration assemblages accompanying the main gold event. The methodologies used included high-resolution short-wave infrared (SWIR) hyperspectral imaging, electron microprobe analysis, and conventional petrography. The alteration assemblages that correspond with the gold mineralizing event include ferroan dolomite, dolomite, ankerite, Al-poor muscovite/illite to muscovite/muscovitic illite, and phlogopite. Pervasive and vein-type carbonate alteration includes ferroan dolomite–ankerite in greywacke and dolomite in the marble (with minor Fe2+ substitutions). The texture and spectral character of pervasive hydrothermal dolomite differ from metamorphic dolomite. In the greywacke, it is dominantly expressed as Au-stage Al-poor illite and Al-poor muscovite. In marble, it is expressed as moderately crystalline muscovitic illite and muscovite. In well-mineralized marble samples, white mica is notably more crystalline and aluminous. Pre-gold Mg-rich illite is fully consumed. Phengite, calcite, and chlorite occur late in the paragenetic sequence irrespective of lithology and post-date the main gold event. Results demonstrate that the integrated study of hyperspectral imaging with petrography and microprobe analysis characterizes the alteration mineralogy of the gold-mineralizing event and refines the paragenetic sequence of the gold deposit.