Stable isotope study of the igneous, metamorphic and mineralized rocks of the Edough complex, Annaba, Northeast Algeria

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Abstract

The petrogenesis of igneous, metamorphic and mineralized rocks in the Edough massif, NE Algeria, indicates an interplay between crustal and magmatic sources, and magmatic and surface fluids, as determined by sulphur and oxygen isotopic analyses. The Tertiary igneous rocks (microgranites and rhyolites) show a tendency towards I-type granitoids with δ34S values of +5.4±2.2 (1σ) and δ18O between +5.6 and +6.9, with the most hydrothermally altered rocks having the lowest δ18O values. This is indicative of seawater being a major component of the hydrothermal fluids. Seawater interaction with the granitoids produced enrichment in 34S of sulphide-sulphur in both the microgranites and associated skarn mineralization. However, in the Beleleita W–Sn–(Au) deposit, magmatic fluids dominated the mineralization, reflected by δ34S values of +1.7 and +1.8. The basement amphibolites of Kef Lakhal indicate derivation from a basaltic magma with δ18O ranging from +4.9 to +8.6. Locally altered amphibolite has the lowest δ18O values suggestive of meteoric water interaction, whereas the highest δ18O indicate the incorporation of crustal material during the genesis of the amphibolites. Crustal contamination is also reflected in their δ34S of −18 to +2.1, with the most contaminated rocks having the lowest δ34S. Host Palaeozoic mica schists with δ18O of +12.7 and δ34S of −13.2 and associated marbles (δ34S −9.9 to −17.4) are thought to be the most likely contaminants. At the Ain Barbar Fe–Cu–Pb–Zn deposit, δ34S values range between −9.6 and −10.8, indicating that the sulphide-sulphur and base-metals were likely leached from the host Cretaceous flysch and/or the underlying mica schists, with local Tertiary magmatic rocks providing the heat for local convection cells. At Boumaiza Fe-deposit, δ34S values of the sulphide-sulphur extend from −1.2 to −8.1 indicative of magmatic sulphur with the incorporation of substantial amounts of sulphur derived from the host mica schists.

Introduction

The Edough Massif is the easternmost metamorphic complex of the Algerian coast, and has been the subject of several geological studies (e.g. Gleizes et al., 1988; Hammor and Lancelot, 1998; Hilly, 1962; Marignac, 1985; Marignac and Zimmermann, 1983; Vila, 1970). The core complex of the Edough is composed of a variety of Neoproterozoic and Palaeozoic metamorphic rocks, distributed in a NE-striking antiform (Fig. 1). The gneisses which form the core are calc-alkaline rocks (Ahmed-Said and Leake, 1993). The metapelites overlying the gneisses consist of two units: garnet- or, more commonly, kyanite-mica schist alternating with metric slabs and layers of marble and skarns; and a Palaeozoic upper unit (Ilavsky and Snopkova, 1987) comprising andalusite-bearing aluminous schists alternating with feldspathic quartzites. These rocks have undergone a polycyclic metamorphism, from HP–HT conditions (cycle 1) to medium P–T conditions (cycle 2) and finally to LP–HT (cycle 3) conditions (Hammor, 1992).

The Edough Massif is part of the internal zone of the North African Alpine belt, whose current position is believed to be a consequence of Oligo-Miocene subduction and collision in the western Mediterranean, between the African and European plates (Auzende et al., 1975; Bouillin, 1979, Bouillin, 1986; Cohen, 1980; Carminati et al., 1998). The core complex is locally overlain by a cover of sedimentary nappes resulting from the Oligo-Miocene subduction and collision. The nappes consist of Cretaceous sandstones, shales and marls, and Oligo-Miocene sandstone flyschs.

The metamorphic complex and the overlying sedimentary rocks were intruded by igneous rocks around 16 Ma (Marignac and Zimmermann, 1983). These igneous rocks are divided into two groups: rhyolitic domes and dykes, and subvolcanic microgranites. The geology, geochemistry and petrogenesis of these igneous rocks have been documented by Ahmed-Said et al. (1993), who showed that they are metaluminous lithologies with high Sri ratios (0.71947–0.72404) indicative of crustal melting. The igneous events and related hydrothermal alteration are thought to have resulted in four types of mineralization in the Edough Massif: (1) vein base-metal mineralization: (e.g., that at the largest mine of the area, Ain Barbar which was recently closed; Fig. 1) and El-Mellaha (Fig. 2, Bolfa, 1948; Marignac, 1976); (2) W–As–Au deposits related to skarns (e.g., Beleleita (Karezas), see Fig. 1 and Aissa et al., 1995); (3) Fe–Pb–Zn–Cu deposits related to the skarns and amphibolites of Boumaiza and Berrahal region (Fig. 1); and (4) Sb–Au veins occurring within the metamorphic complex (e.g., Koudiat El Ahrach, Fig. 2; and Saf-Saf, Ain Barbar, Fig. 1). The various ore deposits of the Edough Massif have been extensively exploited in the past, and veins in the core complex are currently being prospected for Au–W–As.

The purpose of this study was to use O- and S-isotope analyses to assess the relative contribution of (1) magmatic and country-rock crustal components, and (2) magmatic and surface fluids to the Tertiary granites and the associated mineralisation, and the basement amphibolites. Our results show that there is a contrast between the S- and O-isotope systematics of the core complex, the Mesozoic sediments, and the granites. Despite geochemical evidence which indicates that crustal melting was important in the petrogenesis of the granites (e.g. high Sri ratios; Ahmed-Said et al., 1993), there appears to have been little contribution of local crustal sulphur to the granite system. Instead, the S- and O-isotope systematics are interpreted as indicative of hydrothermal alteration of the granites by seawater. In contrast, the S-isotope data from ore deposits indicate that remobilisation of crustal sulphur occurred extensively in the hydrothermal systems set up by the emplacement of the granites. Core complex amphibolites also show evidence of incorporation of low δ34S crustal sulphur, and hydrothermal alteration.

Section snippets

Field relations and petrography

The polycyclic basement of the Edough Massif consists of two major units which are tectonically superposed in the eastern part of the massif (Caby and Hammor, 1992; Gleizes et al., 1988). The lower unit is composed of Neoproterozoic (Pan-African) gneisses (Hammor and Lancelot, 1998) and the upper unit consists of Palaeozoic mica schists (Ilavsky and Snopkova, 1987). Layers of marble, amphibolite, and ultramafic rocks are also associated with these units.

The gneisses, which occupy the central

Ore deposits

The occurrence of ore deposits in the Edough metamorphic complex is related mainly to the magmatic activity and fault systems which were controlled by the Langhian extensional tectonics (Monié et al., 1992).

Four types of mineralization are known: (1) Fe–(Pb–Zn–Cu) deposits hosted by marbles and skarns (e.g., Berrahal, Boumaiza); (2) base-metal veins (Cu–Pb–Zn) hosted by the Maestrichtian flysch (e.g., Ain Barbar, El-Mellaha); (3) W–As–Au skarns hosted by the gneisses (e.g., Beleleita, Bouzizi);

Sampling and analytical procedures

S- and O-isotope analyses were carried out at the Isotope Geosciences Unit, Scottish Universities Environmental Research Centre (formerly SURRC), East Kilbride, UK.

Results

All δ34S and δ18O data of the subvolcanic rocks, the metamorphosed mafic/ultramafic and metasedimentary country rocks, and the ore deposits are given in Table 1, Table 2 respectively. Four O-isotope analyses from Ouabadi (1994) are also listed. Initial strontium isotope values (87Sr/86Sri) given in Table 2 are taken from Ahmed-Said et al. (1993).

Granites

In their study of the petrogenesis of the Edough granites, Ahmed-Said et al. (1993) concluded that these intrusives were generated through the partial melting of a mantle-derived basaltic magma that subsequently interacted with crustal material, probably the gneisses, on the basis of similarities of 87Sr/86Sri. In addition, Marignac and Zimmermann (1983) provided evidence that the granites may have been altered by processes related to younger tectonic events. Notably, they dated hydrothermal

Conclusions

Oxygen and sulphur isotope analyses of the Edough Tertiary granitoids, local metasedimentary rocks, and associated mineralization lead to the following conclusions:

  • 1.

    The Miocene microgranites and rhyolites were probably of I-type affinity as suggested by previous geochemical studies, and exhibit S- and O-isotope evidence for crustal contamination. Instead, there is a significant evidence for the possibility of seawater hydrothermal alteration which generated low δ18O values down to +5.9 and

Acknowledgements

R.L. wishes to thank the Isotope Geoscience Unit, SUERC, for facilitating stable isotope analyses, and the University of Annaba for short period grants to Scotland. We also thank A. Hamzaoui for supplying some sulphide samples from the Edough and Boumaiza skarns, and C. Taylor and P. Gorman for technical support with O- and S-isotope analyses. SUERC is supported by NERC and the Scottish Universities consortium. AJB is funded by NERC support of the Isotope Community Support Facility at SUERC.

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