Characterizing regolith materials using geochemistry Emmanuel Arhin and Musah Saeed Zango University for Development Studies, Faculty of Applied Sciences, Department of Earth and Environmental Sciences, P. O. Box 24, Navrongo, Ghana Corresponding Author:

Some Basic Questions that need Answers  Regolith what is it? The materials overlying the coherent bedrock Contain unconsolidated and secondarily cemented units Forms the oxidising environment of the earth May be characterised by simple or complex weathering histories Superficially the surface materials look alike Host the surface geochemical expressions that controls exploration surveys  To continue or  Discontinue

Regolith Type- Simple weathering History- Residual soils

Transported Regolith

More on Transported Regolith

Transported Regolith  Transported soils DO NOT have horizons, they are found in layers or unsorted depending on how they were transported.

Erosional Regolith

Ferruginous Regolith

Ferruginous Regolith Con’t. Ferruginous Duricrust- Ferricrete Lateritic Duricrust- Characterised by Equigranular groundmass

Where do these FOUR regolith landform units reside in the LANDSCAPE and HOW?  Over the coherent BEDROCK and form MOSAIC of different regolith units; changing rapidly from one landscape position to another.  Generally PATTERNLESS  Depend on Climate conditions  and WEATHERING HISTORIES THE Study Area

Fig. 3 Regolith map and sample locations (Arhin et al.2015) Assortment of different regolith classes; nonconforming to any pattern but changes rapidly.

Regolith field identification and characterisation- Not Easy Sometimes  The reason is: Check from Butt and Zeegers (1992) pg. 531:  There is a wide range of materials within the regolith, with great variations in mineralogical and chemical composition, fabric and origin, even within a single profile or toposequence.  From Arhin et al (2015):  Ferruginous (F) regime pertains to regolith having obvious Fe-oxides. This includes lateritic duricrust, lateritic gravel, ferruginous duricrust or ferricrete and all ferruginous materials whose origins cannot be identified easily in the field. The French have cuirasse to represent undifferentiated ferruginous unit.

Our Questions for the Day!!!  How do we identify properly the different regolith units as posed by Arhin et al.(2015) and many other researchers (e.g. Anand and others) in Mineral Exploration?  Why is this important in our search for new mineral deposits? From Butt & Zeegers (1992) and Anand (2001); the declining rates of world-class mineral deposit discoveries in less complex regolith terrains has resulted in broader attention of devising geochemical exploration methods suitable for regolith-dominated terrains.  Again many authors had observed misleading classification of some duricrust (cuirasses) in Africa, whose origin is now disputed (Bolster, 1999; Butt and Bristow, 2013).

What should we do then? If it is not easy to identify the regolith units in the field then the identification using regolith geochemistry as a surrogate for regolith mineralogy can guide the field geologists from misclassifying some regolith units during regolith mapping.

My Motivations are….  We can use regolith geochemistry data to characterise and identify different regolith materials  THE BENEFITS: It helps in devising appropriate geochemical exploration techniques, which place geochemistry in regolith context It reduces surface geochemical data interpretation challenges in complex regolith terrains

The Theoretical Foundation is Based on the Premise: 1.That the geochemical/mineralogical signatures in the regolith profile varies on their path to end-stage regolith formation. 2.That the regolith materials at any point depict mineralogical and chemical evolutions of the regolith on their path to end-stage regolith formation at different stages.

What was Done:  K/Al and Mg/Al ratios measured by XRF analytical method were plotted to test the practicality of McQueen’s simple index to characterise and identify different regolith materials in an area  This was carried out in a known mineralised environment of the Lawra Birimian Gold Belt in the savannah region of NW Ghana (Fig. 1).

The study area is underlain generally by volcanic and metasedimentary units intruded by places Gold mineralisation in the area known since 1945 but no active operational mines except scattered artisan mining activities

The Science of the method used is  In consideration that rock is an aggregate of minerals and  That the geochemistry of the available primary minerals released at the weathering front to the end-regolith-stage during weathering can be used as a broad surrogate for the regolith mineralogy The Geochemistry of the Regolith can also be used as surrogate to Regolith Mineralogy Companies have HUGE Multi element DATA We can Use these DATA to identify the Regolith Types

What we NEED to Understand before making Progress…… 1.What is the degree of preservation of elements, such as Al, Fe, Si, Na, K, Ca, and Mg inherited from previous weathering episodes. 2.What is the geochemical and mineralogical changes of these elements in the uppermost residual horizon,- Check whether modified as soil or buried beneath transported overburden. In tropical soils there are retention of Si 4+, Al 3+, and Fe 3+ in the overlying regolith whilst some base cations such as Na +, K + Ca + and Mg 2+, (some Si 4+ ) are lost

The Results

Standard & Selected Sample Results

Results Con’t

Analysis of Mg/Al and K/Al ratios

Analysis & Interpretation Juxtaposing the regolith sample points with regard to regolith types showed: o 50 samples were collected from ferruginous, o 32 samples in relict, o 8 samples from erosional and o 64 samples from depositional regimes (Slides 11 & 24).

The Assumption that:  Surface regolith is homogeneous every where resulting in wholesale transfer of successful exploration techniques has been identified as incorrect because in Fig. 3 a mosaic of different regolith regimes forming cumulatively and expressed on the landscape from past and present regolith landscape processes characterises the surficial regoliths In Simple and Complex Weathering History Terrains. Fig. 3 (Arhin et al. 2015)

Non Incorporation of Regolith Environments in Surface Geochemical Interpretation in this Case:  Will assume the broad regolith classes (E.g. Fig. 3) to have developed from a simple weathering history whereas  They may be an expression of cumulative effects of long complex weathering histories.  The consequence is a mix surface geochemical expressions from assorted regolith classes as some will represent Real anomalies and others representing ‘false’ anomalies.

If that is the case, then  Reducing geochemical data challenges where ‘False’ and ‘Real’ anomalies could be taken care of require the incorporation of regolith units identifications and  The 1 st Step by novice is characterising and indexing regolith from Mg/Al and K/Al plot A homogeneous regolith characterise by simple regolith will be in conflict with the mix geochemical signatures. That will be devoid with ‘FALSE’ anomaliesA homogeneous regolith characterise by simple regolith will be in conflict with the mix geochemical signatures. That will be devoid with ‘FALSE’ anomalies

Conclusions  The study identified the surface regolith to have undergone heterogeneous mixing as a result of the past and present regolith landform evolutional events.  The ratio plot recognised most of the samples collected for the study were made up of transported clays/sediments

Out of the 154 Samples Collected: Ferruginous Regolith (F) = 6 Ferruginous sediments/Fe rricrete = 6 2 Cuirasse = 2 2 Lateritic Duricrust= 2 Relict Regolith (R ): 8 Lateritic residuum at or close to surface including in situ soils and soils with insignificant lateral movements = 8 Erosional Regolith (E) 6 Saprolite on felsic/mafic rocks = 6 5 Ferruginous Saprolite= 5 Depositional Regolith (D) ) = 123 Transported clays/Sediments = 123

The Disparities or Misclassifications F R E D

Conclusion 1.The variable regolith material features identified from the 154 samples enabled the characterisation and identification of the different sample materials because an overprint of bedrock geochemistry is reflected in the regolith. 2.Plot of Mg/Al and K/Al highlighted the compositional variability of the regolith samples and contested the notion of the homogeneity of all the sampled materials to be the same in the area. 3.The study recognized the ratio plots Mg/Al versus K/Al as useful in supporting field identification of regolith-landform units (RLU). 4.And suggest the use of regolith geochemistry to unravel regolith material types in complex regolith terrains.

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