2 Ferruginous rocks of Pagala (Togo) and Tin Akoff (Burkina Faso)
2.4 Mineralogical composition
The level of minerals identification and quantification is not the same for the ferruginous rocks of Pagala and Tin Akoff. In fact, very numerous and various studies were performed in Togo (Togbé, 1991), whilst in Burkina, studies kept little developed.
Numerous techniques were used on bulk rocks in frequent association to chemical analysis: DRX, DTA-PTA, IRS; SEM, TEM.
The typical facies are usually above all composed of goethite and hematite, which forms a secondary ferruginous phase, embedding a residual silicated phase composed of frequently very corroded quartz and micas scattered or gathered in rock fragments. There are also automorphous secondary quartz in the form of small transparent crystals developed on goethite. Goethite is the most abundant and frequent mineral ; its content is higher than the hematite’s one in 82% cases and its distribution is bimodal (M1=52,5%, M2=77,5%), whilst hematite has an unimodal one (M<10%).
Micas are almost always present ; the most frequent one is barytic pink muscovite. It must be noticed that the accumulations of massive white clays, locally observed at the contact of the gossan CF3 are dissociated from the ferruginous rocks and seem to have been generated by a previous non-meteoric alteration (kaolinite, metahalloysite, gibbsite and gorceixite). It must be noticed that no mineral of the alunite-jarosite family has been identified in ferruginous rocks, which could characterize a tropical general alteration superimposed to the classical sulfo-phosphatic supergene alteration of sulfidic ores. The alunite detected by Anglo American in drilling has not been checked in the weathering profile.
According to the different mineralizations, specific features are observed. In the case of zinciferous sideritites, the agressive conditions are less strong and quartz and micas are rather well preserved. In the case of the phosphorites of Pagala, ferruginizations are original, since besides goethite the known secondary phosphates are above all aluminous and little ferruginous, i.e. typical of alteration of apatite generating wavellite, crandallite, turquoise and gorceixite but weatherings cannot be discriminated from previous alteration. On the hematitite of Tinchro-Lalamila, of probable magmatic origin, alteration is quasi non-existent ; however plumbogummite is observed which can be surely related to a supergene alteration, as a Zn spinel (gahnite) was not affected by any transformation.
The only diffractometric analyses were systematically performed in eventual association with chemical ones.
Similarly at Pagala as at Tin Akoff, iron oxi-hydroxides levels include siliceous elements and clayey pockets likely originating from transformation of preexisting fine-grained rocks. There are diverse siliceous forms, monocrystalline or not, such as quartz, silexite, chert … The clay pockets, composed of kaolinite and quartz are completely dissociated from iron minerals. Except in two samples of ferruginized schists, phyllites (micas) are rarely observed. Goethite is largely predominant as compared to hematite and samples composed of prevailing hematite are primary outcrops scarcely transformed from hematite-bearing quartzite to hematic jaspe or hematitite. The simple mineralogical composition permits to quantity directly the present minerals by chemical analysis. A petrographic study of the principal ferruginizations (Miningou, 2006) highlights the variety of microstructures inherited from primary minerals s.l. : replica figures of numerous sulfids and less frequently of carbonates (pyrite, chalco-pyrite, galena, blende, ankerite, siderite…)
First, it is clear that these ferruginizations from Pagala, of sulfidic or carbonated origin, whose diagnosis is more indefinite than at Tin Akoff are clearly distinguable from lateritic ironcrusts. It is noticeable that the presence of micas in the ferruginous rocks of Pagala corresponds to metamorphic formation, which does not exist at Tin Akoff.
Mineralogical composition in percent of the gossans of Pagala and Tin Akoff - m=average, m*=average, according to the chemical composition s = one sigma.
If considering the only ferruginous rocks gossans, are mineralogically poor, when mature, composed above all of iron oxides and hydroxides, quartz, eventually manganese oxides and hydroxides and little or no clay minerals. This is the principal characteristic of the ferruginizations of central Togo and North Burkina, which is likely generalizable to the climatic regional context.