You hold a licence over a Birimian greenstone belt (the Palaeoproterozoic volcanic and sedimentary belts of the West African craton) in Burkina Faso or Mali. Under it lie hundreds of square kilometres of weathered regolith, with few outcrops and a lot of laterite. Which of the many structures on the map deserves the first soil lines and drill holes? This article shows how to describe a belt's structural architecture, rank shear zones by how well they could localise gold, and test that ranking against independent data.

The underlying principle

In plain terms, gold in these belts was carried by hot, watery fluids that moved upwards through the crust. They flowed along large faults and shear zones because those are the easiest paths. The gold did not drop out everywhere along the way. It precipitated where the fluid lost pressure, changed chemistry or met a contrasting rock. Those places are usually small and predictable once you know the geometry.

The technical detail runs as follows. Orogenic gold is gold deposited from metamorphic fluids during mountain-building. Most gold on the West African craton sits in Birimian rocks and is linked in time and space to structures formed during the Eburnean Orogeny, between 2200 Ma and 2088 Ma. The Birimian began as an immature volcanic arc setting, later metamorphosed during the Eburnean orogeny. The belts therefore contain basalts, felsic volcanics and sediments that have been folded and sheared.

Three ideas help you read the architecture:

• Hierarchy. Crustal-scale shear zones act as fluid highways. Gold usually sits in smaller, second- and third-order structures branching off them. In southern Mali, for example, the Birimian is described as three N–S trending volcano-sedimentary belts and two regional shear zones. The regional zones are the plumbing, and the deposits are in the splays.
• Dilation. Where a shear zone bends or steps in the direction that opens space during movement, a dilational jog forms. Fluid pressure drops there and quartz veins fill the void. Studies in Burkina Faso describe gold-pyrite in quartz veins in dilational jogs along narrow discontinuous shear zones.
• Rheological contrast. Rheology is how a rock deforms. Strong rocks such as basalt and granite fracture, while weak graphitic shales flow. Strain and fluid focus at their contacts.

Timing also matters. Detailed work on single Burkina districts shows more than one gold event, with mineralisation in pulses over roughly 150 million years. A structure can therefore be a good host and still carry gold from only one of several stages.

The workflow

1. Build the regional frame. Download a free DEM (SRTM or Copernicus) and national geological maps. Trace the belt margins and the largest shear zones. Record their trends, because later steps depend on knowing which directions are regional.
2. Interpret magnetics. Airborne magnetic data is the best tool under cover where it has been released. Reduce to the pole, then apply a tilt derivative or first vertical derivative to sharpen edges. Mark lithological contacts, offsets in magnetic units and linear breaks. Free QGIS plugins or Python libraries are enough for this.
3. Map surface clues with Landsat or Sentinel-2. On Landsat 8/9, band 6 covers 1.57–1.65 µm and band 7 covers 2.11–2.29 µm, both at 30 m. Clay and hydroxyl minerals absorb in band 7, so a 6/7 ratio highlights them. A 4/2 ratio flags iron oxides and clays respectively. In lateritic Sahel terrain, treat these ratios as weathering maps first and alteration maps second.
4. Classify structures by geometry. Decide the sense of shear from offsets and fold asymmetry. Then locate bends, stepovers, splays and fold-hinge intersections that would open under that movement.
5. Rank, then test. Score each candidate for the hierarchy, dilation and contrast criteria above. Check the ranking against soil geochemistry, artisanal workings and any drilling, and keep those datasets out of the ranking itself so the check stays independent.

Worked example (illustrative only)

This is a hypothetical case and is not a real project. Suppose a belt trends roughly north–south, bounded by granitoids. Magnetic data shows a 40 km regional shear zone along the eastern margin. Outcrop and magnetic offsets suggest dextral (right-lateral) movement.

Working through the steps:

• Along the zone, one 10 km segment swings from 000° to about 025°. For dextral movement, a clockwise swing like this is dilational: the east block moves south and away from the bend.
• The tilt derivative shows a magnetic basalt unit pinched against a non-magnetic, probably graphitic, sedimentary unit exactly at this bend. That gives a rheological contrast at the dilational site.
• A 6/7 ratio image shows a weak clay signature over the bend. Field checks show it is weathering of the sediments, so it earns no extra score.
• Two other bends on the same shear zone are restraining, meaning they close under dextral movement. They score low.

Suppose the soil data later shows gold values above 50 ppb on three of the five top-ranked sites and on none of the five lowest-ranked ones. Those numbers are invented for the exercise. The logic is what matters: a ranking made from structure alone is judged against data it did not use.

Common mistakes and limitations

• Treating every lineament as a shear zone. Dolerite dykes, joints and drainage patterns all show up as lines. Confirm shear zones with foliation, offsets or alteration.
• Mixing deformation generations. Structures from different events can look alike on a map but may not carry the same fluid history. Some post-Eburnean thrusts, for example, postdate the Eburnean Orogeny.
• Getting kinematics wrong. A bend that is dilational for dextral movement is restraining for sinistral movement. Check the sense of shear before ranking any geometry.
• Over-trusting spectral ratios. Landsat sees only the surface. Laterite and transported cover can mimic or hide alteration, and 30 m pixels cannot resolve narrow zones.
• Ignoring data quality. Airborne survey line spacing sets the smallest feature you can honestly interpret. Features smaller than the line spacing are interpolation.

To check your work, compare against independent data, such as soil or termite-mound sampling and mapped artisanal sites. Have a colleague interpret the same data blind and see where you disagree. Revise the ranking whenever new structural measurements arrive.

Key points to remember

• Gold is localised where fluids from regional shear zones meet dilational sites and rheological contrasts.
• Rank structures by hierarchy, geometry and rock contrast, using the sense of shear to decide which bends open.
• Magnetics define the architecture under cover, and satellite ratios give only surface clues.
• Gold may have arrived in several pulses, so one good structure does not guarantee gold.
• Test the ranking against data you did not use to build it.

Sources

• Episodes Vol. 39, no. 2 155 West African Goldfields (pdfs.semanticscholar.org)
• West African Craton (en.wikipedia.org)
• Mineralogical, fluid inclusion and stable isotope characteristics of Birimian orogenic gold mineralization at the Morila Mine, Mali, West Africa - ScienceDirect (sciencedirect.com)
• Birimian lithological succession and structural evolution in the Goren Segment of the Boromo-Goren Greenstone Belt, Burkina Faso (researchgate.net)
• Structural and gold mineralizing evolution of the world-class orogenic Mana district, Burkina Faso: Multiple mineralizing events over 150 million years - ScienceDirect (sciencedirect.com)
• Landsat 8 (usgs.gov)

About Orex — Orex is a mineral exploration intelligence platform based in Mwanza, Tanzania. We combine satellite remote sensing, elevation-derived structural analysis and open geoscience data to help explorers, licence holders and investors focus their fieldwork on the ground that matters.

Want to see fault structures and intersection targets on your area of interest — for free? Install GoldRadar 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.