The transformation of rocks and minerals is a phenomenon that is widespread in nature. Hypergenetic processes lead to the formation of large accumulations of minerals. Among these phenomena are the formation of alluvial deposits, nickel bearing weathering crusts, oxidation zones of sulfide deposits and, of course, bauxites, from which aluminum—the most important metal of modern industry—is produced. At the same time, it should be noted that minerals associated with weathering crusts occupy a strictly defined place in the weathering profile. Therefore, the study of the zoning of weathering crusts, the causes and mechanisms of the formation of mineralogical and geochemical zones, and their spatial and temporal relationships, as shown by recent studies, are of great importance not only in theoretical but also in applied terms.

Weathering crusts in Fouta Djallon–Mandingo Province (FDM) contain the largest bauxite deposits. All bauxite-bearing lateritic covers of the region, formed on parent rocks of different structure, composition, and occurrence, can be classified as “sublaterite” according to G.I. Bushinsky (1975), “complete” according to A.P. Nikitina (1971), or to the type identified by B.M. Mikhailov (1969) based on the predominant confinement in the relief of the region to the weathering crusts of “high peneplains”. They contain the most complete set of lithological and mineralogical–geochemical zone–horizons (from bottom to top) in accordance with the geological and industrial nomenclature identified at the deposits during prospecting and appraisal and exploration work: saprolite (mainly polymineral clays), lithomarge (kaolinite clays with an admixture of goethite and hydromicas), ferruginous laterites or transition zones (goethite, hematite, gibbsite), bauxites (gibbsite with boehmite, hematite), ferruginous–aluminous laterites, or stony overburden (goethite, gibbsite), land cover, or loose overburden. Unlike other researchers (Bardossy and Aleva, 1990; Boulange, 19894; Braun et al., 2012; Edmond et al., 1995; Eggleton et al., 2008; Fritz, 1988; Giorgis et al., 2019; Gu et al., 2013; Hickman et al., 1992; Peixoto and Horbe, 2008; Schumann, 1993; Sidibe and Yalcin, 2019; Tardy, 1993), we single out a ferruginous horizon as an independent one, which has an important genetic significance.

Vertical zoning is evidence of changes in physicochemical conditions in the profile of lateritic bauxite-bearing weathering crusts (LBWCs). Water that seeps downward (in the case of weathering crusts, this is warm rainwater), due to exchange reactions with rock, organic matter, and its transformation products in the hypergenesis zone, changes its composition (Geological Evolution …, 2007; Shvartsev, 2013). In accordance with this, the mineral and chemical compositions of rocks are transformed. In lateritic bauxite-bearing covers, these are the processes of removal-inflow and redistribution of chemical elements with the formation of a certain zonality.

The term “metasomatosis” was introduced in the middle of the 19th century by Karl Naumann to denote the pseudomorphic replacement of some minerals by others. Metasomatism is the process of interaction of rocks and fluids, leading to a change in the chemical and mineral composition while maintaining the solid state of rocks, i.e., with conservation of volume (Walter Lindgren’s law) (Lindgren, 1912). D.S. Korzhinsky (1955) found that the interaction of fluids with rocks is characterized by differential mobility of components and that the interaction products, metasomatites, are characterized by local chemical equilibrium. As a result, metasomatite bodies often have regular zoning, with alternating zones of different mineral composition. There are two extreme cases of metasomatism: diffusion and infiltration (Korzhinsky, 1955).

Hypergenetic metasomatosis has its own distinctive features. Its essence lies in “the work of the boundary phase, in which there is a transformation of free surface energy into bound energy through the condensation of a new substance and of bound energy into free energy through the elimination of old interfacial surfaces and the formation of new ones”. Hypergenetic metasomatosis proceeds with volume retention during replacement and with metasomatic contraction (Sirotin, 2000). Since substitution reactions conserve the volume of the solid, they must be volume balanced. This is due to the metal ions that are present in the water (Merino and Dewers, 1998).

LBWCs are considered a product of hypergene infiltration metasomatism by B.M. Mikhailov (1976). He noted that there are bauxites that formed after the parent rock without intermediate clay neoformations as a result of alumino-iron supergene metasomatism. He came to this conclusion, among other things, due to the use of an isovolumetric analysis of the balance of matter in the weathering profile, which made it possible to establish, along with the removal of alkaline and alkaline-earth elements and silicon, the absolute accumulation of aluminum and iron. Comparing the LBWC of the Republic of Guinea (West Africa) with those buried under sediments in Russia, he identified one of the most developed impoverishment processes, bauxite resilification (new formation of kaolinite after gibbsite), as silicic metasomatism (Lajoinie and Bonifas, 1961).

This article presents the results of a study of the profile of the lateritic weathering crust of the Fouta Djallon–Mandingo (FDM) province, in which unique bauxite deposits are located. General information about this province, its exploration and resource assessment, as well as the influence of the lithology of the source substrate were considered earlier (Mamedov et al., 2020, 2021). The influence of the geomorphological factor and the age of the relief on the distribution, scale, and quality of bauxite deposits is described by Mamedov et al. (2022). This article substantiates the influence of hydrogeological and gas regimes on the formation of zoning in the lateritic weathering profile due to processes associated with hypergene low-temperature infiltration metasomatism.

Within the FDM province, the source substrate is represented predominantly by essentially argillaceous sedimentary rocks of the silt–argillite type intruded by dolerite sills. The rocks generally lie subhorizontally. However, in connection with the tectonic warping of the platform cover, typical troughlike synclines ranging in size from hundreds of meters to a few kilometers and swell-like anticlines were formed everywhere.

Under conditions of a favorable climate and maternal substrate, the morphology of the relief and the degree of its dissection are considered significant factors. The processes of geomorphological inversion, in which watershed spaces or individual hills are often formed along synclinal structures and valleys along anticlines, cause a more complex hydrogeological situation in bauxite deposits. In areas of trough-shaped synclines, which are more common on the upper platforms of boval zones, groundwater stagnates. Rainwater is poorly discharged, and the groundwater table (GWT) during the rainy season is very close to the day surface or even comes to the surface, forming temporary small lakes. The aeration zone under such conditions is almost not manifested. The bauxite horizon is practically not formed in such cases. The lower ferruginous horizon merges with the upper (cuirass), and together they form a continuous high-ferruginous lateritic cover. Of course, it contains a significant amount of alumina (from 20–25 to 38–39%), but mostly less than the accepted cutoff content of this component (40%) for calculating the resources and reserves of bauxite in this province.

Along with hydrogeological zonality, the gas regime in the underground atmosphere of the weathering profile plays an important role. Under equal climatic conditions, the role of the geomorphological factor for leveled surfaces acts as an indirect factor that affects the processes of lateritic weathering indirectly through the hydrogeological and hydrodynamic regimes under specific conditions (Anderson, 2007; Brantley, 2009).

REGULARITIES OF CHANGING THE HYDROGEOLOGICAL AND GAS REGIMES IN THE LBWC PROFILE

Detailed exploration work carried out at a number of fields in the province with regime hydrogeological year-round observations made it possible to establish patterns of changes in the hydrogeological situation during the annual cycle.

Based on tens of thousands of interval samples, data were obtained on the average thickness and chemical composition of the lateritic weathering products. Preservation of the textural and structural features of the original parent rock made it possible to study the change in the balance of matter on an isovolumetric basis and calculate concentration coefficients Rc of rock-forming components for each horizon, which is the LBWC zone in relation to the contents in the underlying horizons (Table 1).

Table  1.   Chemical composition, thickness, and volumetric mass of individual zones of the lateritic weathering crust
Full size table

Figure 1 summarizes the data on the typical zonality of the LBWC and the spatial relative position of horizon–zones of vertical hydrogeological zonality and gas-contamination zones. The right side of this scheme shows the change in the balance of matter in the concentration coefficients of rock-forming chemical components from zone to zone from bottom to top along the vertical section. This scheme is a clear illustration of the interaction of the lithosphere with the hydrosphere, atmosphere, and biosphere in the hypergenesis zone.

Fig. 1.
figure 1

Lithological zonation and its comparison with hydrogeological zones and changes in the gas regime in the LBWC profile (Rc is calculated with respect to parent rock).

Full size image

In the world’s largest bauxite-bearing province, the FDM province, bauxite-bearing covers are confined mainly to the gently sloping relief of peaks and upper slopes of local lateritic uplands and dissected watershed massifs—bowals. Their excess above the lower pedestal level of the relief is from tens to hundreds of meters, which provides a hydraulic gradient for an active flushing regime.

In a generalized form, the LBWC can be divided into two significantly different united horizons (from top to bottom):

— the laterite horizon proper, which includes products of intense lateritization, represented mainly by essentially ferruginous, aluminous, or aluminum–ferruginous rocks of stony composition, from which the mobile components of alkaline and alkaline-earth elements and silicon are almost completely removed; and

— the horizon of eluvial clays, from polymineral at the bottom to predominantly kaolinite at the top, which are characterized by the removal of mobile components increasing from bottom to top.

These two parts of the weathering profile, as can be seen in Fig. 1, during the rainy season, when active lateritic weathering occurs, they differ significantly in hydrogeological conditions. The lower part of the profile is a generalized horizon of pseudomorphic clays, which spatially coincides with the hydrogeological zone of constant watering. The upper part, the lateritic part of the LBWC proper, coincides with the hydrodynamic zones of GWT fluctuations (below) and aeration and infiltration in the middle and at the top of this horizon.

The hydrodynamic setting and hydrochemical characteristics of the waters within the LBWC also differ significantly in these two generalized parts of the weathering profile.

Rainwater falling on the surface of the bowls form three hydrodynamically different streams. One, which represents a significant amount of it, is discharged over the surface, directly feeding local streams and rivers. The second part of rainwater, which is also very significant, up to 35% in the landscape-climatic zone of humid savannahs, relatively quickly falls down to the GWT through numerous cracks, canals of the dead root system, and shrew passages (Seliverstov, 1983).

The pseudomorphic clays that are located lower down, and the parent rock that is even lower, due to the relatively low coefficient of filtration, cannot provide a vertical discharge of such a large amount of rainwater. Therefore, an active lateral fault comes into force along the boundary of ferruginous clays to the edges of the bowels. During the rainy season, one can observe how numerous streams flow out from under the stony horizon of the weathering crust, sometimes gushing. On the paths of these underground flows, suffosion-karst phenomena are widely developed with the formation of clastic lateritic breccia-like rocks. This lateral discharge leads to a lowering of the groundwater level, ensuring the functioning of the hydrogeological zone of GWT fluctuations.

Some of the rainwater saturates the pores of the underlying lateritic rocks and their microcracks, forming a hydraulic front, which, under the pressure of each new portion of rain, slowly filters down to the GWT.

The clays of the weathering crust are in a state of constant flooding. However, this flooding flows. Water falls from top to bottom in accordance with a decrease in porosity and an increase in their bulk density at a rate proportional to the filtering ability of these clays (Table 1).

With the onset of the dry season, the water table drops relatively slowly down to the parent rock. The presented data on hydrogeological zonality in comparison with the generalized zonality of the LBWC allow us to conclude that two columns develop in parallel in the weathering profile under the action of the same rainwater: the upper one is lateritic proper, and the lower one is clayey.

Many years of detailed exploration and mapping work has made it possible to obtain unique information on the relationship and mutual influence of hydrogeological and gas regimes in vertical zonality in the lateritic bauxite cover.

To study the composition of waters and solutions, special sampling and hydrochemical studies were carried out (Makarova et al., 2019). In addition, special observations were made at the fields for changes in the gas regime of the LBWC underground atmosphere (Mamedov and Vorobyov, 2011), with periodic formation of gas-contamination zones, representing the accumulation of carbon dioxide and, partially, carbon monoxide.

All these studies, combined with a detailed mineralogical, petrographic, and chemical study of weathering products, made it possible for the first time to reliably establish the spatial correspondence and genetic relationship of the LBO zoning with changes in the hydrogeological and gas regimes, as well as the metasomatic essence of weathering processes.

WEATHERING CONDITIONS AND COMPOSITION OF ROCKS IN LATERITE BAUXITE-BEARING COVER

Figure 1 shows the lithological zoning and the quantitative assessment of the input–removal of matter in the lateritic cover itself, as well as the presence of gas content inside the section of the weathering crust. Analysis of the redistribution of matter (the main rock-forming components) was carried out on an isovolumetric basis (in kg/m3) in absolute quantities. This made it possible to express the input and output of matter in terms of concentration coefficients Rc in each horizon of the weathering crust relative to the underlying one. With absolute enrichment, with introduction of one or another component, Rc >1, and, with removal, Rc < 1.

Ferruginous laterites, the lower horizon of lateritic bauxite-bearing covers, are represented by ferruginous laterites, which are formed in the hydrogeological zone of the GWT fluctuation. The main results of weathering processes are the active hydrolytic decomposition of kaolinite with the new formation of gibbsite, on the one hand,

$$\mathop {{\text{A}}{{{\text{l}}}_{{\text{2}}}}{\kern 1pt} {{{\left( {{\text{OH}}} \right)}}_{{\text{4}}}}{\kern 1pt} {\text{S}}{{{\text{i}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{5}}}} + {\text{5}}{{{\text{H}}}_{{\text{2}}}}{\text{O}}}\limits_{{\text{kaolinite}}} = \mathop {{\text{2Al(OH}}{{{\text{)}}}_{{\text{3}}}} + {\text{2}}{{{\text{H}}}_{{\text{4}}}}{\text{Si}}{{{\text{O}}}_{{{\text{4(aq)}}}}}}\limits_{{\text{gibbsite}}} ,$$

and, on the other hand, a very significant input of iron (742–198 = 544 kg/m3) and, to a lesser extent, aluminum (644–396 = 248 kg/m3) (Table 1). The ferruginous laterites of this horizon, in contrast to clays, become stony and lithified due to ferruginous cement and, correspondingly, the volumetric mass of these rocks increases to an average of 1900 kg/m3.

In this horizon of ferruginous laterites, highly ferruginous dense and strong slab and lenticular bodies 0.2–50 cm thick are observed almost everywhere in the FDM province. In accordance with their predominantly goethite or hematite composition and morphology, they are called “ferriplantites” (Shipilova et al., 2022). Their bulk density reaches 2650 kg/m3, and their porosity is reduced to 8%. Most often, they are developed in the middle part of the zone of fluctuations in the groundwater level, where the GWT is statistically the longest.

Three types of LBWC groundwater occur in the GWT fluctuation zone. These are ground waters, into which fissure waters and pore solutions rush through the overlying hydrogeological zone of aeration and infiltration during the rainy season.

Fissure waters quickly fall to the GWT. The closeness of hydrochemistry to rainwater indicates that fissure waters do not have time to lose oxygen, react with overlying rocks of the weathering crust, and significantly change their composition (Table 2). The content of iron and aluminum in fissure waters averages 35 and 0.023 mg/L, respectively. They also show a low content of silica, which is not in equilibrium with the kaolinite of the underlying horizon of the weathering crust, along which the lower ferruginous horizon is formed (Shvartsev, 2013; Helgeson et al., 1968). Therefore, from bottom to top, the amount of kaolinite in the lower ferruginous laterites actively decreases. The amount of silica also decreases (Table 1, Fig. 1).

Table 2.   Chemical composition of different types of waters in the FDM province
Full size table

The strong absolute accumulation of oxide and hydroxide forms of iron indicates that an intense oxidative geochemical barrier operates in the zone of fluctuations in the GWT. The meeting of fissure waters with ground waters replenishes their upper part with oxygen and leads to the rise of the GWT.

The pore solutions saturate the pores and microcracks in the upper part of the weathering profile, including the soil, and slowly descend to the GWT. Despite the fact that it was difficult to obtain pore solutions undiluted with fissure waters from the stony rocks of the aeration and infiltration zone - bauxites and laterites, nevertheless, four samples were taken. Their composition fundamentally differs from all other types of LBWC groundwater (Table 2). The content of iron and aluminum, as well as a large group of trace elements, is an order of magnitude higher than in other types of groundwater (Makarova et al., 2019). This is explained by the fact that prolonged contact of pore solutions with the surrounding medium, with dead vegetation and the root system, at high temperature (24–28°С) and with the active participation of biota leads to the formation of organic acids, leading with the complete transformation of organic matter to the formation of carbon dioxide and carbon monoxide. As a result, organomineral complexes are formed, primarily compounds with iron and aluminum (Hao et al., 2010). With the seepage (filtration) front, these elements move along the weathering profile.

The accumulation of CO and CO2, which is especially active in the daytime after the end of the rain and warming up of the air and surface, leads to the appearance of a gas-contamination zone above the GWT. The consumption of oxygen for the oxidation of organics leads to a decrease in its content to 4–5%, while the content of CO2 increases to 12–14%. Closer to the surface, the intensity of gas contamination decreases, and, near the surface, the composition of the atmosphere becomes close to a normal atmospheric. Pore solutions, after passing through the zone of gas content, in which due to the lack of oxygen a gley geochemical environment should be created, additionally mobilize iron.

When gley solutions encounter the GWT enriched with oxygen in fissure waters, an oxidative geochemical barrier is formed. Iron precipitates as hematite and goethite. The amount of introduced iron at an average thickness of the lower ferruginous horizon of 3.6 m is 2099 kg (581 kg/m3 × 3.6 m3). Such an amount of iron could not have accumulated only due to capillary uplift in the dry season from the clay horizon, including the upper ferruginous red kaolinite clays (McFarlane, 1976). At the same time, no significant removal of iron is recorded (Table 1). Even if almost all iron was removed from light kaolin and polymineral clays, then in this case its amount (175 kg/m3 × 10.3 m3 = 1802 kg) would not be enough to form the lower LBWC ferruginous horizon. Therefore, it can be argued that iron and some free alumina were introduced from above by pore solutions. Another amount is formed due to the hydrolytic decomposition of kaolinite (Tardy and Nahon, 1985).

Thus, in the weathering profile in the zone of GWT fluctuations in the ferruginous horizon, from its lower part to its upper part, there is an almost complete removal of mobile elements and a relatively absolute accumulation of iron in the trivalent form and, to a lesser extent, aluminum. The lower part of the lateritic bauxite-bearing cover proper is formed.

Bauxites. The lower ferruginous horizon is overlain by a bauxite horizon (Fig. 1) confined to the hydrogeological zone of aeration and infiltration of the weathering profile. In the lower part of the section, the rocks have a more high-alumina and low-iron composition. Higher in the section, starting from 3–3.5 m from the daylight surface, bauxites become red-colored, more ferruginous, and, starting from 2–1.5 m, pass into high-ferruginous laterites: the cuirass.

Bauxites are formed by replacing polymineral clays with gibbsite:

$$\begin{gathered} \mathop {{\text{2KA}}{{{\text{l}}}_{{\text{3}}}}{\text{S}}{{{\text{i}}}_{{\text{3}}}}{{{\text{O}}}_{{{\text{10}}}}}{{{{\text{(OH)}}}}_{{\text{2}}}}}\limits_{{\text{muscovite}}} \,\, + {\text{2H}}_{{\left( {{\text{aq}}} \right)}}^{ + } + {\text{8}}{{{\text{H}}}_{{\text{2}}}}{\text{O }} = \mathop {{\text{2Al(OH}}{{{\text{)}}}_{{\text{3}}}}}\limits_{{\text{gibbsite}}} \,\, + {\text{4}}{{{\text{H}}}_{{\text{4}}}}{\text{Si}}{{{\text{O}}}_{{{\text{4}}\left( {{\text{aq}}} \right)}}} + {\text{2K}}_{{\left( {{\text{aq}}} \right)}}^{ + }, \\ \mathop {{\text{A}}{{{\text{l}}}_{{\text{2}}}}{{{{\text{(OH)}}}}_{{\text{4}}}}{\text{S}}{{{\text{i}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{5}}}}}\limits_{{\text{kaolinite}}} \,\, + {\text{5}}{{{\text{H}}}_{{\text{2}}}}{\text{O }} = \mathop {{\text{2Al(OH}}{{{\text{)}}}_{{\text{3}}}}}\limits_{{\text{gibbsite}}} \,\, + {\text{2}}{{{\text{H}}}_{{\text{4}}}}{\text{Si}}{{{\text{O}}}_{{\text{4}}}}{{_{{\text{(}}}}_{{{\text{aq}}}}}_{{\text{)}}}. \\ \end{gathered} $$

In the case in which relics of source rocks are preserved in clays, gibbsite immediately develops over them. An example is the reaction of replacing anorthite with gibbsite. Since the volume of the solid is conserved during substitution, the reaction must be balanced by volume, where three molecules of gibbsite equal one of anorthite. This reaction also requires the addition of one aluminum ion present in water (Merino and Dewers, 1998):

$$\mathop {{\text{CaA}}{{{\text{l}}}_{{\text{2}}}}{\text{S}}{{{\text{i}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{8}}}}}\limits_{{\text{anorthite}}} \,\, + {\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}} + {\text{5}}{{{\text{H}}}_{{\text{2}}}}{\text{O}} = \mathop {{\text{3Al}}{{{\left( {{\text{OH}}} \right)}}_{{\text{3}}}}}\limits_{{\text{gibbsite}}} \,\, + {\text{C}}{{{\text{a}}}^{{{\text{2 + }}}}} + {\text{2Si}}{{{\text{O}}}_{{{\text{2(aq)}}}}} + {{{\text{H}}}^{{\text{ + }}}}.$$

For the laterite cover itself, an important regularity should be emphasized: the greater the thickness of the hydrogeological zone of aeration and infiltration, the greater the thickness of the bauxite horizon.

For example, on the Zagota Ridge in southwestern Guinea, the weathering crust is developed along subvertically occurring sheared phyllites. A 20 m-thick bauxite horizon was formed along the phyllite layer lying between the ferruginous quartzites, which is on average 3 times greater than the bauxite thickness in the lateritic bauxite-bearing cover on the platform cover within the FDM province. Such an increase in the thickness of bauxites is explained by the fact that with high water permeability and shearing of rocks, all hydrogeological zones have higher thicknesses. At the same time, due to the increase in the thickness of the GWT oscillation zone, the lower ferruginous horizon is less pronounced. The amount of iron introduced from above is distributed in the GWT oscillation zone, which is 15–25 m thick.

The influence of the occurrence of source rocks on the thickness of the zones is manifested even more clearly in, for example, the Pachpatmali field in the Eastern Ghats of India. The occurrence of folded source rocks, condalites, varies from subvertical to slightly inclined. Correspondingly, the thickness of bauxites varies in individual crossings over a wide range from 6–8 to 40 m or more. The GWT fluctuation zone is also extended in steep rocks for a few tens of meters, and the lower ferruginous horizon is less clearly expressed. However, the upper ferruginous horizon, the cuirass, is extremely consistent and averages 3 m.

In addition to the above examples, there are many intermediate variants in nature, which are reflected in the features of the structure and composition of lateritic weathering products.

Changes in the mineral and chemical composition of rocks also occur depending on changes in the gas regime. During the formation of the lower horizon of light high-alumina bauxites due to ferruginous laterites, a significant accumulation of aluminum and an equally significant removal of iron occur (Table 1). The content of carbon dioxide in this zone is maximum. Higher up, when red bauxites are formed, the reverse process begins. The amount of aluminum decreases and the amount of iron increases, reaching a maximum at the very top—at the cuirass—under conditions of a gas regime close to atmospheric, that is, with a high oxygen content.

Cuirass. The mobilization and redistribution of aluminum, especially where ferruginous bauxites are replaced by ferruginous laterites in the cuirass, is evidenced by brushes of crystalline gibbsite in some caverns (Figs. 2a, 2b).

Fig. 2.
figure 2

(a) Crystalline newly formed gibbsite-3 on the walls of cavities in the cuirass and (b) alternation of goethite, gibbsite and hydrohematite layers inlaid with tabular gibbsite crystals in the cuirass. Gib is gibbsite, Hgem is hydrohematite, Het is goethite.

Full size image

Above the cuirass lies the modern soil–vegetative horizon. When the movement of pore solutions is directed from top to bottom, only soil with fragments and blocks of collapsing lateritic stony rocks is the site of mobilization of both iron and aluminum, which are redistributed along the section. The fact that processes of periodic dissolution and precipitation of iron are taking place in the soil is evidenced by the formation of so-called soil “pisoliths”. These are rounded formations from 0.5 to 2–2.5 cm in diameter. Collomorphic cryptocrystalline material of predominantly goethite and alumogethite composition forms thin concentric microzones around laterite fragments.

THE MECHANISM FOR THE FORMATION OF THE ZONING OF THE LATERITE BAUXITE-BEARING COVER

The lateritic bauxite-bearing cover itself is characterized by a zonal structure, which is due to changes in the physicochemical conditions in the weathering profile, due to which the composition of pore solutions changes, which leads to the formation of either significantly ferruginous laterites or bauxites. From the above material, we can conclude that the formation of bauxite occurs due to the processes of spatial separation of aluminum and iron. This occurs only in the hydrogeological zone of aeration and infiltration, in the middle and lower parts of which the gas regime creates a gley geochemical environment. Starting from the lower laterites of the GWT oscillation zone and upwards toward the cuirass inclusive, the system of weathering products, in terms of chemical composition, can be considered as a binary system with two main rock-forming elements: iron and aluminum.

For this upper LBWC column (Fig. 1), the pore solutions are the main reagent and transport of the inflow–outflow and redistribution of matter. The underlying kaolinite clays, rather than parent rock, should be considered as the source substrate, since pore solutions, after meeting and interacting with groundwater, are completely transformed and cease to exist in the section as an independent type. Due to their small volume, they have practically no effect on the composition of groundwater, which is filled mainly with fissure waters, which are analogous to rainwater.

The lower horizon of ferruginous laterites is just formed by the hydrolytic decomposition of kaolinite and quartz inclusions, which are often preserved in large segregations together with the remains of hydromicas, and the formation of oxides and hydroxides of iron and aluminum (Fig. 3a).

Fig. 3.
figure 3

(a) Traces of ophitic texture in the lower horizon of ferruginous laterites, flow of ferrialum gel and mixing of material, formation of gibbsite in place of quartz grain, ophitic structure, kaolinite develops along plagioclase laths. (b) Inset shows flakes of preserved kaolinite. Gib is gibbsite, Kaol is kaolinite, Hem is hematite, Het is goethite.

Full size image

The replacement of some minerals by others occurs with the preservation of textural and sometimes structural features (Fig. 3b).

A distinctive feature of metasomatism is a tendency toward a decrease in the amount of minerals, which occurs towards the upper part of the lower ferruginous horizon, where iron and alumina minerals become rock-forming minerals. Based on changes in the balance of matter, this part of the lateritic bauxite-bearing cover can be considered a zone of aluminum–iron metasomatism occurring in an oxidizing environment.

Higher in the section, due to the significant removal of iron and the equally significant introduction of aluminum with the replacement of iron minerals with gibbsite, light high-alumina bauxites are formed. This zone should be considered a zone of aluminum metasomatism occurring in a gley setting with the participation of pore solutions of predominantly aluminum composition.

Closer to the day surface, due to a decrease in the intensity of gas pollution, the situation in the profile changes from gley to oxidizing with the active influence of organic acids, which dissolve alumina minerals, in place of which iron oxides and hydroxides are deposited. This process reaches its maximum at the surface, and the cuirass zone can be considered a zone of ferruginous metasomatism.

The model of infiltration metasomatism is based on the principles of local equilibrium (Korzhinsky, 1955). However, especially among low-temperature metasomatites, due to uneven permeability or low reaction rates, relict minerals of previous zones are preserved among them. In the LBO, this phenomenon is developed almost everywhere. Kaolinite is preserved up to the upper boundary of the zone of aluminum–iron metasomatism, goethite with hematite in the zone of aluminum metasomatism, and, in bauxites, gibbsite in the cuirass.

The most convincing example is the preservation of ferriplantite bodies, which have the highest density and low water permeability, in the zone of aluminum metasomatism. The very fact of the preservation of ferriplantite bodies up to the cuirass testifies to the continued development of the lateritic bauxite-bearing cover and to the fact that, at one time, today’s cuirass was at the bottom of the section in the hydrological zone of the GWT fluctuation, while the upper parts of the profile have not been preserved, since they were cut off by the processes of chemical and physical weathering.

The soil horizon with ferruginous pisoliths and ferruginous crusts on blocks and fragments indicates that iron is the least mobile element in the uppermost part of the supergene infiltration metasomatic column.

In the Kogon–Tomine FDM area most favorable for bauxite formation, the following data were obtained. Concentration coefficient Rc of Fe2O3 was about 4 with respect to the average source substrate, while Rc of Al2O3 was only about 2. This means that, on the surface of the autonomous landscape of wet savannahs, only two parts of Al2O3 remained, which had been, mobilized from four parts of source rocks actually processed by hypergenetic processes. Consequently, about 50% of the total original alumina was removed from the lateritic cover, while iron remained almost completely. From this, we can draw conclusions. First, in the soil horizon of the LBWC sections, the destruction of the cover occurs not only by physical processes, but also by chemical ones. Second, for landscapes of wet savannahs, it is Fe3+ that is the predominant element.

In the lower part of the lateritic cover itself, it builds up in a downward direction due, in this case, to the transformation of kaolinite clays against the background of a downward trend in the groundwater level and the involvement of clays in the GWT fluctuation zone. Specifically, this is expressed in the formation of red kaolinite clays at the boundary of laterites and clays due to their impregnation with hydroxides and, to a lesser extent, oxides of iron brought here.

Two opposite processes—degradation at the top and creation at the bottom—dialectically ensure the long-term preservation on the day surface of this bauxite-bearing laterite cover that is fractured, of relatively small thickness, and not very resistant to the processes of physical and chemical destruction of the geological body.

WEATHERING CONDITIONS AND COMPOSITION OF ROCKS IN THE CLAY HORIZON OF THE WEATHERING PROFILE

The lowest part of the clay horizon is composed of products of initial decomposition and hydration in the form of newly formed polymineral clays with inclusions of remains of slightly altered source rocks (Fig. 3). Kaolinite and muscovite develop after orthoclase; after muscovite, kaolinite does:

$$\begin{gathered} \mathop {{\text{2KAlS}}{{{\text{i}}}_{{\text{3}}}}{{{\text{O}}}_{{\text{8}}}}}\limits_{{\text{orthoclase}}} \,\, + {\text{2H}}_{{{\text{(aq)}}}}^{{\text{ + }}} + {\text{9}}{{{\text{H}}}_{{\text{2}}}}{\text{O}} = \mathop {{\text{A}}{{{\text{l}}}_{{\text{2}}}}{{{{\text{(OH)}}}}_{{\text{4}}}}{\text{S}}{{{\text{i}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{5}}}}}\limits_{{\text{kaolinite}}} \,\, + {\text{4}}{{{\text{H}}}_{{\text{4}}}}{\text{Si}}{{{\text{O}}}_{{{\text{4(aq)}}}}} + {\text{2K}}_{{{\text{(aq)}}}}^{{\text{ + }}}{\text{,}} \\ \mathop {{\text{3KAlS}}{{{\text{i}}}_{{\text{3}}}}{{{\text{O}}}_{{\text{8}}}}}\limits_{{\text{orthoclase}}} \,\, + {\text{2H}}_{{{\text{(aq)}}}}^{{\text{ + }}} + {\text{12}}{{{\text{H}}}_{{\text{2}}}}{\text{O}} = \mathop {{\text{KA}}{{{\text{l}}}_{{\text{3}}}}{\text{S}}{{{\text{i}}}_{{\text{3}}}}{{{\text{O}}}_{{{\text{10}}}}}{{{{\text{(OH)}}}}_{{\text{2}}}}}\limits_{{\text{muscovite}}} \,\, + {\text{6}}{{{\text{H}}}_{{\text{4}}}}{\text{Si}}{{{\text{O}}}_{{{\text{4(aq)}}}}} + {\text{2K}}_{{{\text{(aq)}}}}^{{\text{ + }}}, \\ \mathop {{\text{2KA}}{{{\text{l}}}_{{\text{3}}}}{\text{S}}{{{\text{i}}}_{{\text{3}}}}{{{\text{O}}}_{{{\text{10}}}}}{{{{\text{(OH)}}}}_{{\text{2}}}}}\limits_{{\text{muscovite}}} \,\, + {\text{2H}}_{{{\text{(aq)}}}}^{{\text{ + }}} + {\text{3}}{{{\text{H}}}_{{\text{2}}}}{\text{O}} = \mathop {{\text{3A}}{{{\text{l}}}_{{\text{2}}}}{{{{\text{(OH)}}}}_{{\text{4}}}}{\text{S}}{{{\text{i}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{5}}}}}\limits_{{\text{kaolinite}}} \,\, + {\text{2K}}_{{{\text{(aq)}}}}^{{\text{ + }}}. \\ \end{gathered} $$

The balance of matter shows the predominant removal of alkaline-earth elements and silicon, as well as an absolute increase in the amount of water of crystallization (Fig. 1, Table 1).

Eluvial clays retain textural and, to a lesser extent, structural features of source rocks. Along with changes in the chemical and mineral composition of clays, there is a significant change in their physical and mechanical properties. The rocks become loose and easily abraded with fingers. The volumetric mass of rocks also decreases from bottom to top, from 1.8 g/cm3 of polymineral clays to 1.45 g/cm3 of light kaolinite clays. Due to the removal of a significant part of the substance, an increase in porosity occurs.

The clay part of the section spatially coincides with the hydrogeological zone of flooding during the rainy season. However, the filtration rate changes in accordance with the changing porosity in kaolin clays of the upper and in illite–hydromicaceous clays after silty–argillites and in montmorillonite clays after dolerites of the lower part of the horizon. Accordingly, the rate of water renewal also changes, and due to this, the composition of the waters.

Table 2 shows data on the hydrochemistry of various waters in the LBWC, including rainwater. Fissure waters, which replenish and renew the upper part of the groundwater, are very similar in composition to rainwater. Accordingly, in its upper part, in the GWT fluctuation zone, groundwater is nonequilibrium in terms of the content of silicon and kaolinite in it. Kaolinite clays undergo hydrolytic decomposition to form gibbsite. Below, inside kaolinite, kaolinite–hydromica or kaolinite–montmorillonite clays, the silicon content in water increases to 5.5 mg/L (Table 2). These contents are in equilibrium relative to kaolinite (Bronevoi et al., 1975; Kashik, 1981; Zhukov and Bogatyrev, 2012; Shvartsev, 2013; Tardy and Nahon, 1985), but hydromicas and montmorillonite continue to decompose down the profile. Pore waters approach the surface of unaltered source rocks and with an even higher concentration of silicon and alkaline and alkaline-earth elements, but not in equilibrium with aluminosilics of the source substrate. Their decomposition begins, as does the removal of mobile elements with the formation of hydrated forms—mixed-layer clay minerals. At the same time, the contents of aluminum and iron change in very small absolute amounts (Table 1).

The change in the chemical and mineralogical–petrographic composition of weathering products from the source substrate up the profile to essentially kaolinite clays can also be considered as a metasomatic process. The predominant is the removal of mobile elements, the intensity of which increases from bottom to top in proportion to the hydrodynamic factor, which is an increase in the rate of renewal of pore water. Depending on the different filtration rates of solutions, the concentration of components in the solution changes. Changes in clays of the weathering crust occur in accordance with the classical patterns of formation of infiltration metasomatic zoning (Metasomatism …, 1998).

The most characteristic feature of metasomatic processes is the decrease in minerals in metasomatic zones as the intensity of the process increases. The same is observed in the LBWC section from parent rock to polymineral clays.

In the clay horizon of lateritic weathering, as well as in low-temperature metasomatites, due to uneven permeability and low reaction rates, relics of previous zones are preserved among the overlying horizons—for example, fragments of weathered source rocks in the zone of polymineral clays (Figs. 4a, 4b), partial preservation of hydromicas in the zone of kaolinite clays (Fig. 5), etc.

Fig. 4.
figure 4

Relics of source rocks in polymineral clays: (a) kaolinite blades developed after feldspars; (b) uralite and hydromica developed after clinopyroxene. Mont is montmorillonite, Kaol is kaolinite, Cpx is clinopyroxene, Uralit is uralite, Hdmicas is hydromica.

Full size image
Fig. 5.
figure 5

Replacement of polymineral clay with kaolinite. Montmor is montmorillonite, Kaol is kaolinite.

Full size image

The horizon of pseudomorphic clays of the zone of constant watering is a classic residual product of weathering—an eluvial weathering crust with the preservation of textural and partly structural features of the source substrate.

It should be emphasized that the clayey part of weathering crusts in the region is a huge reservoir of rainwater for the dry season in the annual cycle. Due to the low filtration capacity of pseudomorphic clays, rainwater from the weathering crust continues to feed surface runoff with a significantly decreasing debit almost until the next rainy season. Therefore, the FDM plateau is called the “water tower” of this part of West Africa, from under which the sources of the largest rivers in the region begin: Niger, Senegal, Gambia, Falem, Tomine–Kobul, Konkouré, and many others.

If there had been no such rainwater storages in the weathering crust, then, in 4–6 months of the dry season, all vegetation would have most likely be destroyed and the landscapes would have been desert and semidesert.

APPLICATION OF THE THEORY OF ZONING TO SOLVING THE QUESTIONS OF FORMATION OF BAUXITE DEPOSITS AND BAUXITE POTENTIAL OF THE TERRITORY

The theory and patterns of zoning formation are of great importance in the issue of predicting bauxite content. In the LBWC, the thickness of the potentially bauxite-bearing part of the laterite cover is strictly controlled by the depth of the groundwater level. This is confirmed by the example of relatively large deposits. In the Debele deposit, a relationship was traced between the presence and thickness of bauxite-ore bodies and the depth of occurrence from the day surface of the groundwater table (Fig. 6). It is characteristic that, in areas where the groundwater level is close (2–4 m) to the day surface or comes out onto it, with the formation of marshy swampy places, there are no bauxite ore bodies in the lateritic cover. Mapping made it possible to identify the most extensive barren area in the region of the western summit of the Debele Boval with a gentle troughlike fold, which determined the high position of the groundwater level. On the contrary, in the core of high-order anticlines and in tectonically disturbed zones, where the groundwater level is deep, the thickness of bauxites noticeably increases.

Fig. 6.
figure 6

Depth maps of (a) the position of the groundwater level in the rainy season and (b) the thickness of bauxites at the Debele deposit.

Full size image

The revealed hydrogeological control of the presence and thickness of bauxite-ore deposits in the lateritic covers also explains the absence of commercial bauxite in vast areas of lower and less dissected surfaces. For example, for geomorphological levels II and III in the area of the Boe deposit (Guinea-Bissau), the groundwater level lies close to the day surface.

Thus, at geomorphological levels of relief at different ages—the Eocene in the FDM Debele province and Pliocene–Lower Quaternary in Boe—a similar hydrogeological situation causes a similar chemical composition of weathering products, but the reasons that determined the high position of the groundwater level are different (Mamedov et al., 2020). In Debele, this is a local structural control, a synclinal fold, while in Boe it is predominantly geomorphological; the degree of dissection of the relief, i.e., the ratio of the perimeter to the area, was higher, and there was a relationship with the inflow of water from the associated higher relief levels (Mamedov et al., 2022). The faster the discharge of water over the water-resistant clay horizon, the lower the static level of groundwater and more favorable conditions that are created for the formation of thicker and better bauxites. For example, at the N’Dangara deposit, highly ferruginous bauxites and laterites are found as patches in troughlike synclines, where the GWT almost comes to the surface (Fig. 7).

Fig. 7.
figure 7

Geological map with (a) isolines of the position of the groundwater table during the rainy season and (b) longitudinal and (c) transverse sections of the N’Dangara deposit. (1, 2) sedimentary lateritic bauxites after deposits of the Sangaredi series: (1) bauxite conglomerate; (2) sandstone and gravel–bauxites; (3) lateritic in situ and gelified bauxites after parent rock; (4) substandard ferruginous bauxites and ferruginous laterites; (5) clastic ferruginous bauxites (after young deluvial and deluvial–proluvial formations); (6) lateritic pseudomorphic kaolinite and polymineral clays after parent rock; (7) Devonian silty mudstone, Faro formation; (8) diolite of the Mesozoic trap formation; (9) isolines of the position of the groundwater table in the rainy season: (a) isolines of the depth of occurrence from the day surface (on the map), (b) position on the sections; and (10) lines of geological profiles.

Full size image

Consequently, from the position of the groundwater table and by the composition of the rocks of the lateritic cover, one can indirectly assess the nature of the plicative structures and their sizes.

Lithological and mineralogical–geochemical zoning is the most reliable evidence of the lateritic development of clastic formations. The application of this criterion made it possible to substantiate the lateritic sedimentary origin of bauxites from deposits of the Meengi–Fosseka, Balandugu, and other groups, which occur on quartz sandstones of Ordovician age.

Determination of the genesis of bauxite deposits composed of clastic rocks is usually a difficult task; often, the very fact of the presence of clastic structures with traces of redeposition and differentiation of clastic material leads to the assignment of such structures to the sedimentary type (Mikhailov, 1969; Seliverstov, 1983; etc.). However, the change in the quality of bauxites in the sections of the Meengi, Fosseka, South Balandugu, East Debele, and other deposits was not reflected in this theory. They have fundamentally the same structure, which is similar to the weathering crusts formed on parent rocks—shales or dolerites—but with a high content of silica in the form of quartz and clastic structures of bauxites. This is clearly seen from a comparison of the sections with the involvement of data on hydrogeological zonality and changes in the composition of the underground atmosphere. The bauxite-bearing lateritic cover could have been formed only due to the presence of a loose redeposited substrate worked out by lateritic weathering together with the underlying quartz sandstones. This testifies to the laterite–sedimentary, rather than sedimentary, genesis of these deposits.

CONCLUSIONS

Data obtained as a result of detailed exploration work and year-round observations of the hydrogeological regime at large deposits, as well as special hydrochemical studies of various types of water and changes in the gas composition of the underground atmosphere in the LBWC of the largest bauxite-bearing province FDM, allowed a number of important conclusions to be drawn about the conditions and features of the formation of zoning in the structure of the weathering profile for the first time on a reliable factual basis.

(1) On the surface of positive landforms along the aluminosilicate substrate of the lithosphere, as a result of the action of agents of the hydrosphere, atmosphere and biosphere, a single-type vertical hypergenetic metasomatic zoning is formed, including bauxite bodies. These rocks, formed in a certain vertical sequence, belong to the complete or sublaterite type of bauxite-bearing weathering crusts, strictly due to vertical hydrogeological zonality in the rainy season, and periodic production of active reagents of organic acids, as well as carbon dioxide and carbon monoxide due to decomposition and oxidation of plant residues in biologically active environment. The relationship between the zoning of weathering products and changes in the hydrogeological and gas regimes in the weathering profile has been determined.

(2) Changes in the weathering profile occur in accordance with the main patterns of low-temperature infiltration metasomatism, which makes it possible to attribute the LBWC to the products of infiltration hypergenetic metasomatism.

(3) The weathering profile, due to the peculiarities of the hydrodynamic flows of rainwater passing through the LBWC, consists of two metasomatic columns that are spatially combined in plan, but vertically separated:

— the upper, lateritic cover itself, composed mainly of hydrolysis products of the final lateritic weathering, i.e., hydroxides and oxides of iron and aluminum; and

— the lower one, composed of clayey eluvium, products of hydration and decomposition of aluminosilicates—polymineral clays and kaolinite.

In both columns, the processes of change begin with the same rainwater, but differ in hydrodynamic parameters in each of the columns. Reagent water passing through the upper column (pore solutions), actively reacting with organic matter and biota, leading to a powerful redistribution of the substance, after meeting with the groundwater table, due to its small amount, is diluted so much that it has no practical effect on the composition of groundwater, which is filtered through the bottom column.

(4) In the upper and lower hypergenetic infiltration columns, the processes of interaction between solutions and rocks generally correspond to the main patterns of low-temperature infiltration metasomatism:

— there is a dissolution of some minerals, and in their place new minerals are formed while maintaining the solid state of the rocks;

— due to the low reaction rate and different permeability of the medium, local equilibrium is not maintained and relics of previous zones are preserved in each zone;

— as the process intensifies, the amount of minerals decreases; and

— in the upper column in the soil horizon in the rear zone, most of the components become mobile, with chemical degradation of the weathering profile occurring.

At the same time, the conditions, mechanisms, and main processes that occur in the upper and lower columns differ significantly.

In the lower column, the main processes are the removal of alkaline and alkaline-earth elements, as well as silicon, the intensity of which increases from bottom to top as the flow rate increases and water changes and the solubility of mobile elements. The role of organic matter and the products of its transformation is not decisive in this case. It is the clayey horizon that can be considered as a residual eluvial weathering product, without taking into account the absolute accumulation of crystallization water.

In the upper column, the processes of removal–inflow and redistribution of matter occur against the background of a change in the geochemical situation with the decisive role of organic matter and its transformation products—organic acids, carbon monoxide, and carbon dioxide. The environment changes from an oxidizing one, a zone of aluminum–iron metasomatism at the bottom; to a gley one, a zone of aluminum metasomatism in the middle part; and, again, to an oxidizing one, a zone of ferruginous metasomatism closer to the day surface.

Thus, the modern hydrogeological and gas regimes in the bauxite-bearing lateritic covers control the mineralogical and geochemical zoning. Features of biological and soil processes and hydrodynamics and hydrochemistry in the weathering profile lead to a threshold nature of the change in physicochemical and biochemical conditions, providing the action of geochemical oxidative, reductive, gley, and adsorption barriers, this being the main mechanism of zonal distribution of matter in lateritic weathering crusts. Lithological and mineralogical–geochemical zoning is the most reliable evidence of the lateritic development of clastic formations. The application of this criterion makes it possible to substantiate the sedimentary–laterite genesis of many deposits. The application of the theory and patterns of zoning formation is important in predicting bauxite content: in the LBWC, the thickness of the potentially bauxite-bearing part of the laterite cover is strictly controlled by the depth of the groundwater level.

The presence in the weathering profile of a hydrogeological zone of infiltration and aeration with a thickness of at least 4–6 and, preferably, 8–12 m on subhorizontally occurring source aluminosilicate rocks in favorable geomorphological conditions, together with the zoning of the gas regime in the profile, in a variable humid climate, is of course the main factors in the formation of zoning in the LBWC. All other factors act indirectly, through the creation of favorable hydrogeological and gas conditions.