1 Identification and interpretation of ferruginous rock in tropical environment

1.1 Rock weathering

As the most part of rocks are composed of silicates, the alumino-silicic cycle was the reference for several authors and particularly Pédro (1966, 1968) to define the frame of a climatic zonality of the weathering processes on a global scale.

 

The great domains of the alumino-silicic cycle are: (i)allitisation corresponding to intense genesis of Al-hydroxides (gibbsite and boehmite) ; (ii) monosiallitisation meaning intense genesis of an aluminous silicate (kaolinite) ; (iii) bisiallitisation resulting in illite and smectite genesis ; (iv) and last, podzolisation generating free silica, which is a minor process in West Africa.

 

The ancient weatherings, lasting from the Cretaceous or even the Jurassic times are characterized, as for Si and Al, by predominant allitisation and monosiallisation due to the rate and intensity of hydrolytic processes over long periods (Pédro et al., 1975). This is typical of the tropical zone, where deep and thick transforming weatherings prevail. In that zone, the identification of small-sized formations such as orebodies represents a major problem for prospecting.

 

As for Si and Al too, the recent weatherings are typically concerned by monosiallitisation and bisiallitisation.

 

A weathering mantle or weathering cover was defined in the sixties and described for all of the rock families (figure 1) as follows from top to bottom : ironcrust, hardpan, mottled clays, argillaceous arenas which are smectitic in many cases and often observed in the recent weatherings at the base of ancient profiles. Other characterizations are used, such as saprolites among which coarse saprolites, an equivalent of argillaceous arenas including residual fragments of weathered rocks (Trescases, 1973), and fine saprolites, a frequent equivalent of the mottled clays. By contrast, the concept of regolith (Butt and Zeegers, 1992) which gathers all of the surficial formations, is more imprecise than the others mentioned within the notion of weathering cover.

 

Fig. 1

Compared schematic weathering profiles of alumino-silicated rocks and sulfidic mineralizations under tropical climate. Thickness of profiles is between 10 and 100 meters.

 

The iron cycle (content of around 7% Fe2 O3 in the Earth’s crust) is in keeping with the zonal scheme but with its own behaviours, distinct or not from Si and Al. In the allitisation and monosiallitisation domains, ferritisation prevails, which means that iron is only in an oxidized or hydroxidized form (goethite and hematite). The silicated form of iron, in association with Al is typical of the bisiallisation domain, giving rise to ferriferous smectites, beidellites and nontronites (Paquet, 1969 ; Bocquier, 1976).

 

The three principal elements of the earth’s crust, about 83% of the latter, represent after oxidation and hydroxylation, thus after supergene weathering of all the rocks of the earth’s crust at least 95% of the ancient weathering formations in the intertropical zone. This increased predominance of Si, Al, Fe is expressed by a geochemical uniformisation together with a mineralogical simplification (Blot, 1980). Concentrations of Al and Fe are particularly spectacular in lateritic crusts, typical of intense supergene weatherings. The more ancient ones are aluminous (bauxites) and the more recent are ferruginous (Grandin, 1973 ; Boulangé, 1984), Al being in the silicated form for the major part (kaolinite). Their simplified mineralogical composition consists of Fe, Al, eventually Mn-oxi-hydroxides, alumino-silicates and quartz which is partly neogenetic but above all residual. The lateritic crusts display a specific picture of the underlying rocks, which has been investigated (Blot et al., 1976 ; Leprun, 1979 ; Zeegers and Leprun, 1979; Tardy et al., 1988, Boski and Herbosch, 1990) or is legible through results already obtained (Boulangé, 1984 ; Bamba, 1996) (figure 2). In specific conditions that is in the case of so-called “preconcentrated rocks”, similar Mn-accumulations can be observed in ancient weathering profiles developed from manganesiferous carbonated or silicated protores (Grandin, 1973 ; Weber, 1973, 1997) ; likewise, in nickeliferous ones, the so-called lateritic-nickel formations (Trescases, 1973; Blot et al., 1976 ; Nahon et al., 1982), nickel originates essentially from the high contents usually found in the silicates of dunites, peridotites and pyroxenites.

 

Fig. 2 

Al and Fe-crust - bearing laterite profiles typical of different parent rocks (from the results of Blot 1980, Trescases 1973, Boulangé 1984, Boski and Herbosch 1990, Bamba1996)

 

In non-silicated rocks, as well as beyond the manganesiferous carbonates ore deposits, the alumino-silicic zonal frame does not work so fundamentally any more. In short, ancient weathered formations of the intertropical zone are characterized by simple dissolution of carbonates and subsequent laterizing evolution of dissolution-derived silicated residues, whilst recent weatherings can generate calcareous soils (Leprun et Blot, 1978). In phosphatic rocks, Ca-phosphate transforms into Al or Fe-phosphate, or can lead to formation of “phosphated lateritoids” (Flicoteaux, 1980). The tropical weathering of sulfidic rocks is not well-defined and it will be attempted to specify all its aspects below.

 

Whatever high the weathering rate may be, elements or minerals remain the geochemical and mineralogical memory of parent rocks across ancient or recent weathering covers – the small bodies comprised in a more larger domain included – keeping an individuality during weathering, should the latter be ancient or recent and displaying at least a contrasted surficial picture (Blot et al., 1973 a,b, 1976). In the course of investigations comparing weathering of granitic massifs and their enclaves, I decided to consider orebodies as enclaves, because of their reduced dimensions as compared to those of the other formations. The composition of sulfidic orebodies, bearing minerals with high reactivity to the weathering agents, leads one to consider them as enclaves of a special type, whose supergene evolutions are not those usually known for alumino-silicated rocks. Consequently, each type of rock has its particular weathering, keeping memory of residual and neogenetic minerals at once, with elements removed or maintained in situ in every weathering level.