Fathom Geophysics Newsletter 24

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Regolith news: West African exploration gets clearer

STRATEGIES to help avoid mineral exploration pitfalls common in the lateritic pediment-dominated areas of sub-Saharan West Africa, an important metallogenic province, have been put forward by researchers based in France and Burkina Faso.

When exploring for bedrock-hosted mineral resources in this region, lateritic pediments presented significant obstacles for explorers uninitiated in the surface dynamics and sediment-routing systems of such tropical cratonic shields, according to the researchers. [1] Similar tropical shields existed in South America, India, and Australia as well.

They also said that the region's mineral resources may be underestimated, given the prevailing lack of awareness of how to correctly undertake sampling programs in this sort of geomorphological environment.

Pediments: A sampling headache

Pediments are major obstacles to mineral exploration because they mask bedrock and complicate the interpretation of any geochemical anomaly — which can lead to frustration for explorers hoping for either direct access to bedrock or straightforward geochemical dispersion halos in regolith.

A pediment, also known as a glacis, is a gently-sloping ramp-like landform that transports sediment eroding from a massif to areas of sediment deposition or alluvial transportation. Pediment systems, made up of successive generations of pediments, form in a given area by stepwise dissection of the landscape. Extended periods of pediment development create pediplains, which are regions dominated by residual hills formed by coalescing pediments.

Rolling pediplains were the most common regional landform found in today's West Africa and were locally strewn with relict slivers of pediment ramps that evaded being reduced to residual pediplain hills, the researchers said in their paper.

Deciphering the process of regolith re-shaping and understanding geochemical dispersion scenarios in pediment-dominated landscapes was crucial for meaningful mineral exploration, which mostly relied on soil geochemical surveys, they said.

However, no general consensus existed on the recognition criteria, regional correlation, and geomorphological meaning of paleo-pediments, they said.

What has been standing in the way of scientists' deeper understanding of pediments was their very slow rates of development — so that direct observation of their formation wasn't possible — and their often complex, polygenic nature, which hampered the unequivocal identification of paleo-pediments. [2]

Sampling where it makes the most sense

The wide extent and spatial variability of the West African pediment-dominant landscape should prompt healthy caution in explorers wanting to select surface sampling sites that reflect the bedrock geochemistry as accurately as possible, the researchers said.

"Only sampling of the bedrock or the in-situ preserved saprolite is reliable for characterizing the geological substrate's geochemistry," they said.

The upper edges of pediment ramps that expose a massif's weathering horizons under ferricrete were the best places to look.

In all other situations, a surface geochemical anomaly on a pediment can have several meanings, they said. An anomaly found in material that was not clearly bedrock or in-situ saprolite would have an ambiguous meaning. An anomaly of this type wouldn't necessarily imply that a bedrock anomaly was anywhere nearby. Conversely, the absence of a surface anomaly in clastic materials of unconstrained origins wouldn't rule out the occurrence of an anomaly in the underlying bedrock.

Site mapping crucial to understanding sampling results

Accurate mapping by people knowledgeable in pedimentation-related landforms was needed to help pinpoint the original source of any pediment-hosted geochemical anomaly, the researchers said.

They said that when working in the West African pediment-dominated landscape, exploration-related mappers needed to remember important points that included:

  • The relative elevation of a given pediment's ramp surface wasn't a reliable way of determining which pediment generation it belonged to.
  • While a portion of a given pediment's ramp surface may have been preserved, the massif it originally linked up with may have been completely eroded away.
  • At a given locale, the pediment slope direction not only can vary within the same pediment generation (think of a circular apron of pediment ramps surrounding a peak, for instance), but also can vary from one pediment generation to the next if substantial shifts in the watershed network and catchment shapes and sizes occur over time.
  • Duricrusted surfaces and horizons weren't necessarily formed from in-situ material, and could in fact host allochthonous sedimentary clasts that had been transported to the site prior to weathering and duricrusting.
  • In sub-Saharan West Africa, pediments pull sediments away from massifs to expose pre-existing regolith formed from prior deep, extensive weathering (not fresh bedrock).

In cases where the upper portions of glacis weren't preserved, such as incised rolling pediplains, paleo-landscape reconstruction mapping that encompasses the time the sampled pediment was active was also needed to understand the likely transit path taken by sediments. A single pediplain hill produced by pedimentation processes acting on a large pre-existing pediment ramp may contain inherited material originating from several kilometers away, a scale much larger than the current size of the residual hill.

In fact, sediment transportation distances — the line between a given sampling site on a pediment up to the drainage watershed for that pediment, back when it was active — can be substantial in sub-Saharan West Africa. Reconstruction of the region's first generation of pedimentation (known as the High Glacis) suggested that pediment slope runs sometimes exceeded 20 kilometers, the researchers said.

It meant that any geochemical anomaly found on a residual hill in a rolling pediplain may similarly have a quite farflung source. So paleo-landscape reconstruction mapping needed to involve an area large enough to encapsulate sampling sites' likely complex sediment-source history.

"Instead of a cross-sectional approach, a three-dimensional landscape reconstruction is suitable to take into account the entire upslope drainage area that could have supplied mineralized debris to the anomaly," the researchers said. They supplied an example of such mapping in their write-up.

Stream-sediment sampling

Stream-sediment geochemical surveys were still valid in pediment-dominated landscapes provided that upslope source-tracing mapping was done at an appropriate scale once sample sites were selected, the researchers said.

This guideline applied to the full range of exploration targets, such as gold, platinum, diamond, manganese, and copper deposits, irrespective of whether downslope transport of the commodity of interest was particle- or solution-driven, they said.

Careful landscape reconstruction mapping of landforms would permit explorers to establish a chronology of pedimentation, and to constrain the geomorphological context of the anomaly.

They said a further benefit of such mapping was its usefulness in identifying pediment ramps that may be masking potential bedrock-hosted mineral resources.

References

[1] D. Chardon, J.-L. Grimaud, A. Beauvais and O. Bamba (2018) "West African lateritic pediments: Landform-regolith evolution processes and mineral exploration pitfalls", Earth-Science Reviews, 179, 124-146.

[2] C.R. Whitaker (1976) "Pediment form and evolution in the East Kimberleys: Granite, basalt, and sandstone case studies", PhD Thesis, Australian National University.

About Fathom Geophysics

In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.

Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.

We offer automated data processing, automated exploration targeting, and the ability to tailor-make data processing applications. Our automated processing is augmented by expert geoscience knowledge drawn from in-house staff and from details relayed to us by the project client. We also offer standard geophysical data filtering, manual geological interpretations, and a range of other exploration campaign-related services, such as arranging surveys and looking after survey-data quality control.