The quartz-sericite schist of the Saint Barbe Volcanic unit is highly acidic (avg. 70 wt.% SiO2) and compositionally plots in the field of rhyodacite/dacite-rhyolite (Section 3.2.1). Similarly to the rocks of the Akjoujt Metabasalt unit, the composition of the quartz-sericite schist is characterized by the selective HFS and LREE enrichment i.e. Th, Ce and Ta (avg. 16 ppm, 48 ppm, and 1.5 ppm, respectively; Table 12.42). Similar to the metavolcanics of the Akjoujt Metabasalt unit, when plotted in the Th/Yb-Ta/Yb discrimination diagram (after Gorton and Schandl 2000) the dacitic-rhyolitic rocks of the Saint Barbe Volcanic unit show an active continental margin signature (Figure 3.17).
3.7.3 Metacarbonate
The metacarbonate body, which hosts the Fe oxide-Cu-Au-Co mineralization of the Guelb Moghrein deposit, has been recently interpreted to represent marine sediment (Kolb et al. 2006) in accordance with earlier works on Guelb Moghrein (Ba Gatta 1982; Pouclet et al. 1987). It has been linked genetically to the banded iron formations that contain siderite in that area, deposited as ironstone on a continental shelf setting (Martyn and Strickland 2004; Kolb et al. 2006). The postulation of a sedimentary origin of the metacarbonate is based mainly on its stratiform morphology, its stratigraphic framework (close association with pelitic sediments, BIF and marine volcanics), and the absence of any metasomatic textures that could imply an origin from metasomatic replacement of a carbonate precursor (Ba Gatta 1982; Pouclet et al. 1987; Martyn and Strickland 2004; Kolb et al. 2006).
Based solely on the major element composition, Mozley (1989) demonstrated that early diagenetic siderites from marine and nonmarine environments are characterized by distinctive elemental compositions. Nonmarine siderites are commonly relative pure (i.e. greater than 90 mol% FeCO3) whereas siderites from marine environments are always extremely impure, with extensive substitution of Mg for Fe in the siderite lattice (Mozley 1989). Coarse siderite crystals from the least altered metacarbonate of the Guelb Moghrein deposit show a relative homogeneous composition with high Mg-contents (XMg=31.6-38.5) and very low Ca- and Mn-contents. According to the classification of Mozley (1989), the relative Mg-rich composition of the siderite implies that this has been initially deposited in marine environment.
Chondrite (C1) normalized REE distribution patterns of the siderite, display a weak to moderate REE fractionation, distinct negative Ce anomalies (Ce/Ce*=0.7-0.9) and no significant Eu anomalies. Despite the low concentration of REE in siderite, the REE signature is compatible with formation of siderite from marine solutions. In particular the REE patterns of the siderite can be explained by interaction of marine or pore water in coastal sediments. Such processes would produce weakly fractionated REE patterns with no positive Eu anomaly, but with negative Ce anomaly (Hu et al. 1988; Möller 1989; Bau 1991; Bau and Möller 1992). The lack of distinct positive Eu anomalies in the REE signature of the siderite of the Guelb Moghrein deposit, gives evidence that there was no contribution from high-temperature solutions to the REE budget of the water column from which the carbonates precipitated (Möller 1989; Bau 1991). The presence of negative Ce anomaly is characteristic of marine sedimentary carbonates (Hu et al. 1988). According to Möller (1989) and Bau and Möller (1992), two processes of carbonate formation can produce negative Ce anomaly: deposition from seawater and from hydrothermal fluids equilibrated with oxidized sediments. Obviously the second case is rejected because of the required reducing conditions needed for the transport of Fe+2.
Figure 3.17 Th/Yb-Ta/Yb discrimination diagram for the volcanic rocks of the Akjoujt Metabasalt unit and the Saint Barbe Volcanic unit, respectively (after Gorton and Schandl 2000).
Figure 3.18 Hf/3-Nb/16-Th discrimination diagram for the volcanic rocks of the Akjoujt Metabasalt unit (after Wood 1980).
3.8 Metamorphism
Three metamorphic mineral assemblages are distinguished in the metavolcanic rocks of the Akjoujt Metabasalt unit. They are characterized by a peak metamorphic ferro/an pargasite- albite paragenesis (amphibolite), which is replaced by an biotite-actinolite-quartz paragenesis (biotite-actinolite schist) and subsequently by a chlorite-quartz paragenesis (chlorite schist). This suggests a retrograde evolution from peak metamorphic amphibolite facies to lower greenschist facies conditions. The fact that actinolite is aligned parallel to the S2 foliation and chlorite is parallel to the S3 foliation indicates that the D2 reverse shearing was associated with retrogression in the upper greenschist facies and that the D3 reverse shearing occurred during lower greenschist facies conditions. In contrast, the mineral assemblages in the quartz-sericite schist and the biotite-garnet-quartz schist of the Saint Barbe Volcanic unit are typical of the upper greenschist facies. The well-preserved quartz porphyroclasts with vitreous inclusions in the quartz-sericite schist again indicate that peak metamorphism did not exceed upper greenschist facies conditions to any extend. A retrograde chlorite-quartz assemblage is also developed in these lithologies parallel to the S3 foliation, which points to lower greenschist facies conditions during the D3 deformation event.
In order to constrain the metamorphic conditions, two different geothermometers were applied to mineral assemblages of the wall rocks. The hornblende-plagioclase geothermometer of Holland and Blundy (1994) was applied to the peak metamorphic assemblage in the amphibolite. For the temperature calculation of the retrograde upper- greenschist facies metamorphism (D2), was used the garnet-biotite geothermometer for the biotite-garnet-quartz schist of the Saint Barbe Volcanic unit. Four different calibrations were used, which take into account the Fe-Mg-Ca-Mn and Al-Ti solid solutions in garnet (Hodges and Spear 1982; Pertchuk and Lavrenteva 1983; Dasgupta et al. 1991; Bhattacharya et al. 1992).