Across the West African craton — from the Birimian greenstone belts of Ghana and Burkina Faso to the deeply weathered terranes of Guinea and Côte d'Ivoire — supergene enrichment has concentrated gold into lateritic profiles that are both an opportunity and a puzzle. The opportunity is real: some of the continent's most significant deposits, including Fekola and Syama, have substantial near-surface laterite resources. The puzzle is that laterite horizons are geochemically complex, spatially variable, and visually monotonous from the ground. A classic soil sampling grid will tell you where gold is, but it rarely tells you why it is there — and without that structural and mineralogical context, you are drilling on faith rather than evidence. Remote sensing, applied correctly, can change that calculus before a single sample bag is collected.
Why Laterite Profiles Respond to Multispectral and Hyperspectral Imaging
A mature laterite profile in the Birimian is not a homogeneous red cap. It is a vertically zoned sequence — from mottled clay at the base through ferruginous pisolithic horizons to a hardened cuirasse at surface — and each zone carries a distinct mineralogical signature. Goethite, haematite, kaolinite, gibbsite, and smectite each absorb solar radiation at characteristic wavelengths between 1.6 and 2.5 microns (SWIR) and in the thermal infrared. ASTER, with its 14 spectral bands covering VNIR through TIR at 15–90 metre resolution, has been the workhorse for mapping these assemblages at regional scale for two decades. Band ratio combinations — particularly ratios isolating the 2.16 and 2.20 micron absorption features — allow discrimination of kaolinite-rich versus gibbsite-dominant zones, which in turn reflects the degree of profile maturation and drainage conditions that favour gold retention.
The practical implication is stratigraphic: where the ferruginous hardcap is intact and the kaolinite zone beneath it is thick and laterally continuous, residual gold from the underlying primary source is most likely to be preserved. Where deep incision or stripping has removed the profile, secondary dispersion halos may mask primary targets. ASTER mapping lets you rank your licence blocks by profile integrity before committing to ground programmes.
What EMIT Adds That ASTER Cannot Deliver
The EMIT (Earth Surface Mineral Dust Source Investigation) instrument, launched to the International Space Station in 2022, was designed to map arid surface mineralogy for climate modelling — but its 285-band hyperspectral coverage from 380 to 2500 nm at approximately 60-metre resolution is directly applicable to mineral exploration. Unlike ASTER's discrete bands, EMIT captures continuous spectra, enabling full spectral unmixing rather than band-ratio approximations. In a laterite context, this means you can distinguish between partially and fully kaolinised saprolite, map smectite alteration halos that indicate proximity to original sulphide lodes, and identify manganese-oxide phases associated with oxidised gold-bearing veins — none of which ASTER resolves cleanly.
Early applications of EMIT across the Sahel and savannah zones of West Africa are demonstrating that smectite-to-kaolinite transition zones — areas where hydrothermal fluid pathways have driven localised clay alteration deeper into the bedrock — correlate spatially with known gold occurrences in the Houndé Belt of Burkina Faso. This is not coincidental: hydrothermal fluids that originally deposited gold also altered wallrock mineralogy, and that alteration signature survives weathering better than the gold itself in many cases. EMIT gives you a proxy for palaeo-fluid pathways that surface geochemistry alone cannot reconstruct.
Integrating Spectral Data with Structural Frameworks
Spectral mineralogy without structural context is incomplete. Gold in Birimian terranes is overwhelmingly structurally controlled — hosted in shear zones, extensional jogs along D2 transpressive faults, and dilational breccia bodies at lithological contacts. Laterite gold targets are no different; the richest pisolithic accumulations overlie structural traps where gold-bearing fluids ponded during primary mineralisation. The most effective workflow combines ASTER and EMIT mineral maps with lineament analysis derived from SRTM or Copernicus DEM data. Fault intersections and restraining bends identified from satellite elevation models define the structural nodes most likely to underlie anomalous laterite geochemistry. Where a kaolinite–smectite alteration halo identified by EMIT coincides with a fault intersection identified from the DEM, you have a convergent target — and convergent targets are where you prioritise your auger or RAB drilling.
The Exploration Value: Smarter Prioritisation, Lower Early-Stage Costs
The West African craton covers millions of square kilometres, and licence areas routinely exceed 200 square kilometres. Ground-truthing every anomaly is not financially viable at early stage. A combined ASTER–EMIT workflow, integrated with structural lineament mapping, can reduce a regional target inventory from dozens of anomalies to four or five structurally and mineralogically coherent priority zones — the ones worth mobilising a field team for. That prioritisation can be completed for a fraction of the cost of a single RAB programme, and it means your drilling budget is deployed where the geology is actually telling you to look.
About Orex: Orex is a mineral exploration intelligence platform headquartered in Tanzania, providing geospatial tools, structural analysis, and remote sensing workflows tailored to African greenstone and cratonic terranes. Our products are built by exploration geologists, for exploration geologists.
Want to see fault structures and intersection targets on your area of interest — for free? Install GoldRadar Faults on your phone or desktop: it maps lineaments and automatically flags fault intersections derived from satellite elevation data, giving you a structural framework for preliminary exploration before you spend a dollar on the ground.
