1 Identification and interpretation of ferruginous rock in tropical environment
1.4 Pertinence of geochemical prospections
a Definition of indexes
Even if general knowledges gathered from petrographic and structural analyses entitled us to define geodynamic environments favorable to the expression of mineralizations, it is nevertheless the case that it is important for mineralizations indexes to be pointed out in order to justify their interest. If geochemical environments are a truth at a given moment of the state of knowledges, the history of geology shows that there is nearly always a choice among the arguments dependant on Scientific societies or even modes. Thus the perception of the geodynamic frame is evolutive, when it must ensure a “legitimate” status to mineralization, should the latter be index or resource.
On the other part, mineralization is an object defined by its composition and constitution giving the latter its interest and whose interpretation will be one of the challenges of the geodynamic truth at the present moment.
The surficial materialization of a mineralization is a fact confirmed by observation or measurement, by mineralogy or geochemistry. Under tropical climate such surficial materialization is frequently a challenge implying setting of different methodologies, which account for surficial climatic specificities or part of the latter. The surficial environment obliterates obviously a great part of the usual diagnostic elements, especially the mineralogical ones, but some memory forms remain during supergene transformations. If the mine prospecting has been oriented toward sightless methods (investigation of indirect or non-outcropping indexes), it is because the investigation results on weathering have been little integrated and their conclusions rapidly neglected without tentative of more appropriate prospecting methods. In these conditions, mineralization reveals by a mono or pluri-elemental geochemical aureole defined by statistical values or contrasts concerning recent sediments or soils unconnected with outcropping rocks.
It seems obviously difficult to create more complete and original petrographic, mineralogical and geochemical indexes, without accounting for tropical supergene specificities.
By keeping the simple conclusions that lead to a lithodependency of weathering and lateritic ironcrusts, which is the more general case, the investigations may be not totally sightless. In this approach, if an ore deposit is considered as a particular rock, its specific weathered products contrasting with the surrounding material are the central point of the prospecting. These weathered products are well known, especially by their ferruginous manifestations likely to be maintained in the considered climatic zone. This could be set at the “order of the day” of observation, provided that prospectors integrate to their competence domain supergene rocks including what is ferruginous.
b Methodologies applied to indexes investigation
The principle of so-called sightless indirect methods is founded of the geochemical signature of clays from soils and fluvial sediments, completed by the concept of regolith including the whole of surficial formations where all of the genetic modes are confounded. This approach seems partly justified in order to characterize alumino-silicic rocks, since it is known that the weathering product of all of the rocks consists of authigenic clay minerals s.l. (hydrated alumino-silicates and metallic oxi-hydroxides). Under similar conditions there is even some specificity of surficial evolutions as related with rocks, with differentiated qualitative and quantitative, mineralogical and geochemical signatures.
A first restriction is directly connected to the very aim of investigation for secondary signatures of mineralizations which are particular rocks with specific weathered products more defined by iron minerals than by clay minerals.
The second restriction concerns the frequent exclusion of iron minerals in the choice and preparation of samples, should they be in the form of ironcrust or hardpan, or constitutive of concretions in soils and sediments. There is no real justification for that, except an ancient idea of iron great mobility whilst all of the landscapes result from the contrary process and that iron oxi-hydroxides are a part of the same neogenetic environment, as clay minerals s.s. in the general weathering context. The balance of the “sightless” methods is contrasted in the whole West Africa and even if the results are known to be successful, other approaches would have brought such assured ones. The choice of the sightless prospection implying clays as indirect targets have often resulted in interpretations rather little useful for the objectives and in deductions non-conformable to the reality on the field.
No logic can be found in clay minerals as mineralization indicators, since they are rarely typical of sulfidic mineralizations. In fact during weathering of sulfidic bodies, the mineral species forming the most rapidly and being preserved for the longest time is iron oxi-hydroxide, which at once is a good trap for numerous kinds of indicatory or typical elements.
Permanence in secondary mineralization is ensured by iron oxi-hydroxides under various forms up to an ultimate term which is gossan. The iron minerals trap in a little reversible way different metallic elements released during hydrolysis of primary and secondary metallic minerals and they represent the most direct memory of the mineralization.
Gossans are the common expression of the weathering of carbonated and sulfidic mineralization, but a confusion with ironcrusts is possible. This ascertainment leads to draw up a succession of criteria allowing a differential characterization of gossans and lateritic ironcrusts at different step of the investigation of indexes (Blot, 2004). A schematic pattern of the defined criteria is proposed, the latter having permitted to discriminate gossans from ironcrusts from the author’s observations in Togo, Burkina and Brazil. These criteria are morphological, faciological, geochemical, mineralogical and crystallo-chemical (Table 16). Among the formations with well-known potential, like the volcano-sedimentary ones, the ferruginous rocks should probably be carefully observed from this point of view.
Criteria for discrimination between ironcrusts and gossans (Blot, 2004)
It is also sure that even if this approach does not permit to settle a geodynamic frame, it makes however possible to define the characteristics of mineralization indicators, such as morphology of the ore deposit’s trace and supergene facies of the mineralization. Following this approach in Burkina should have avoided any preliminar geochemical prospection in order to point out the metallogenic interest of Perkoa, Kwademen and Tiébélé.
Elsewhere and particularly in the case of Upper Proterozoic formations, potentiality perspectives were really considered only very late, whilst evident indicators of sulfidic mineralizations in the form of gossans s.l. have first been defined from ferruginous outcrops.
Consequently, every ferruginous body discriminated from ironcrust should be considered as a likely mineralization indicator and these indicators must be tested in order to define their interest. Since gossans almost at all times are defined a posteriori, likewise better prospection indicatory characteristics will be specified a posteriori. Prospection can reasonably be developed from ferruginous outcrops owing to good analyses, good geophysical fit and wells integrating surficial state and physical, mineralogical and geochemical properties. In other words, it can be said that the typological approach of ferruginous outcrops is necessary. The methods performed are complementary and enter the Australian morphological patterns. In a more restricted manner, the geochemical definition of ferruginous rocks is absolutely essential due to the qualitative and quantitative evolution of standard analyses (Besnus, 1977 ; Pouit, 1987 ; Chaurris and Garreau, 1990 ; Blot, 2004), particularly in the typological approach, but cannot completely take the place of observation
c Surficial formations
This part deals only with rocks’ surficial weathering dynamics, extended to enclaves and ores, i.e. the extrapolation of the perspectives evoked by Zeegers and Leprun (1979). In comparison with the concept of regolith, this may be considered as restrictive, since regoliths involve all of the surficial formations (Butt, 1992 ; Butt and Zeegers, 1992, Butt et al., 2002 ; Teeuw, 2002). But the importance of surficial formations, other than those unquestionably ascribed to weatherings in situ is rarely highlighted. In eastern Senegal, which represents the Proterozoic “boutonnière” of Kéniéba the observations during regional mapping of soils and alluvial formations were collected and show that transported formations represent about 2% of the surface, being characterized by very little thicknesses (Blot, 1980). Well structured weatherings in situ were observed and described in the lower course of rivers, included that of the Falémé (Blot and Leprun, 1973) and Senegal rivers. Beyond this region whose area is more than 20,000 km2, the attested transported formations are little developed as compared with which can be ascribed to weathering cover. It is sure that weathered products are difficult to identify, which gives rise to passionate debates between the supporters of the primauty of autochthonous weathering products and materials, and those who consider that materials as well as elements are predominantly allochthonous. The extension of the regolith concept originates for the most part from these difficulties, adhering to the imprecision of the real knowledge of the surface, in contradiction with the typology proposed by Butt and Smith (figure 7). The materialization of mineralizations’ indexes does exist in the West-African zone, through iron-oxide-bearing minerals and ferruginous rocks. Iron originating from sulfides and carbonates is little migrating in tropical climatic conditions. At the same time, the iron of mafic minerals of the prevailing alumino-silicated rocks generates alumino-ferruginous rocks with the alumina of silicates, which strenghten landscapes by some fossilization of the latter.
d Convergence between prospections and observations
In a certain way, the sightless measuring (geochemical prospections) can be cross-checked by the observation of the outcrop index. In fact, in Burkina, for ores and indexes of Perkoa, Kwademen and Tiébélé, there are at once typical outcrops, and geochemical aureoles in the neighbouring recent weathering. In Togo, the geochemical prospections in stream sediments and soils have validated a posteriori the diagnosis of gossans. However, here once again, surrounding weatherings are recent and have developed after erosion of the ancient lateritic ones. In Burkina as well as in Togo, the surrounding weatherings are little thick and non-weathered sulfidic mineralizations were observed not very high up (15m at Pagala, 45m at Perkoa, 30m at Tiébélé). Surfaces covered by ancient lateritic weatherings are completely lacking in West Africa ; there are no examples, either as for geochemical prospections, or as for the occurrence of mineralized ores.
On the scale of continent as Xie Xuejing et al.(2004) tackle geochemical prospections, the relationships between geochemical anomalies and marks of mineralized ores are common. From 37 millions analyses distributed over 6 millions km2 and from a composite grid pattern under the notion of blocks, all of the surficial states and climates are integrated and prospection confirms observation geographically or genetically speaking in a concentric, eccentric, peripheral or lateral way (figure 11). And so, even at that scale, there is some place for observation, all the more that the relationship between ore and geochemical prospection is highlighted only for a part of mineralizations. This stands also partially for gold, which does not necessarily occur in the form of sulfidic mineralization and for which little more than half of cases show a good concordance between observation and geochemical composition (400 cases among 750 gold ores). The place of transport sites seems not to be determinant and on every scale the specific outcropping, directly or indirectly connected to geochemical anomalies seems to be decisive, as observed at Pagala (Blot and Magat, 1990), Perkoa, Kwademen, Tiébélé.
Spatial relationships between known large metallic deposits and geochemical blocks from Xie Xuejing et al., 2004