2 Ferruginous rocks of Pagala (Togo) and Tin Akoff (Burkina Faso)
2.3 Outcrops of ferruginous rocks
Pagala (Blot and Magat, 1989, 1990 ; Togbé, 1991 ; Togbé et al., 1993 ; Togbé and Besnus, 1994)
- Outcrops draw out in schists with a thickness of metric order, and pluri-decametric to pluri-hectometric extension. They occur in more or less chaotic beds, composed of discontinuous decimetric to pluri-metric blocks.
- At Pagala new terms of faciologic descriptions as compared to those used in the observations of previous prospections were introduced: onion-like, massive, scoriaceous, geodic, schistose, sandstone-like, conglomeratic and brecciated facies.
- On fresh sections, colors are often bright, based on brown, ochre red, purple, orange, gray, black, white.
- The spectacular goethite geodic crystallizations may be macroscopic, pluri-centimetric and often euhedral.
- All the gossans are dislocated or continuous beds, 50 to 500 m long, 2 to 30 m hick. More than 300 gossans were identified in a sector of 100 km2.
From a textural point of view, Togbé defined four principal facies (structureless massive ferruginous facies, scoriaceous ferruginous facies, ash-tray-like ferruginous facies, ferruginous breccias) distinct from those of ferruginized rocks (eventually partitioned ferrruginized schists, ferruginous quartzites and phosphatic alterations). However, by the microfacies study, he exclusively privileged the filiations originating from zinciferous carbonated rocks.
Picot et al. (1988) increased the inventory started by ORSTOM and BRNM. They defined four types of gossans after the context, that is types Pagala, Tchoun-Tchoun, Mazala Mount, Kpawa. The Pagala type corresponds to a geological environment of graphitic schists and micaschists; mineralization has a sulfidic origin (poly-metallic, with high Zn contents). In the case of the Tchoun-Tchoun type, there is an environment of trachytic metalavas (high P and RE contents). The Mazala Mount type is associated to jaspoids (As, Ba, B, Co and Cu high contents). The Kpawa type is different, because situated on the basement (low Ag and Cu anomalies).
For Theron (2002), there is some distinction between the interstratified gossans, the facies of brecciated schists and the other forms with facies of collomorphic iron oxi-hydroxides, thus exotic.
Tin Akoff (Blot, 2002 a and b)
- The outcrops, which are chaotic are elongated and small-sized (decametre to hectometre). They seem to be organized in chapelets.
- The ferruginized facies are various even for a same ferruginous body, without reaching such a complexity as at Pagala ; that is massive, sandstone-like, brecciated, micro-scoriaceous, but rarely onion-like.
- Boxworks are observed but are not frequent.
- Colours are the most frequently lustreless, tints being more or less brown, black, red, orange, variegated, greenish grey, mustard, white, violaceous. The goethitic recrystallizations are millimetric to centimetric.
- More than one hundred ferruginous bodies were observed and a part of them were collected in an area extending over 150 km east-west, at the Extreme North Burkina.