A Sulphur Isotope Study of the Caledonian Granites of Britain and Ireland

The Caledonian fold belt is thought to have formed as a result of the closure of the 'Iapetus' ocean basin during Lower Palaeozoic times. Granitic magmas were intruded into Precambrian and Lower Palaeozoic country rocks between approximately 640 and 390 Ma. Most important, spatially and volumetrically, are the late Caledonian granites, the 'Newer' and 'Last' granites of Read (1961). In general, the late Caledonian granites have the calc-alkaline characteristics typical of plutonic rocks emplaced at destructive plate margins. They range in composition from diorite and tonalite, through granodiorite, to peraluminous granite with chemical variation continuous among the various rock types. Petrographically and chemically, the granitoids span the range from S 'sedimentary' to I 'igneous' types as defined by Chappell and White (1974) with no discernible geographical pattern to their distribution. There have been few sulphur isotope studies on primary sulphides in granites; yet such data have considerable potential in understanding the petrogenesis; that is in terms of granites derived either from igneous or metasedimentary protoliths. From about 50 British and Irish Caledonide granites, including both 'Newer' and 'Older' intrustions, conventional separation techniques have yielded sulphide separates from only 19 granites for delta34S analyses. Mineralogically, the sulphides consist of mainly pyrite and pyrrhotite, although chalco-pyrite is present in a few intrusions. It was noticeable that granites with high delta18O and high initial 87Sr/86Sr yielded very little sulphides. S analyses proved most informative when compared with the following: mol Al2O3/(CaO + Na2O + K2O) , K/Na, Fe2O3/FeO ratios, initial 87Sr/86Sr and delta18O. Granites with high to intermediate delta18O (11 to 10%) and high initial 87Sr/86Sr (> 0.709) are accompanied by g 34S from +5.0 to +16.0%. and are indicative of crustally derived granites (S-types). These are mainly 'Older' Caledonian granites (e.g. the Oughterard granite and the Cashel microgranite sill in Connemara, Ireland; the Aberdeen granite in Scotland). Intermediate to low delta18O (10 to 6%.) and low initial 87Sr/ 86Sr (0.703 to 0.708) correspond to delta34S values between -4.5%. and +4.4%. indicative of granites derived from the mantle or lower crust; these intrusions are mainly 'Newer' Caledonian granites. delta34S analyses also confirm the diverse protolithic derivation of zoned granites; a good example being the Strontian granite in the northwest Highlands of Scotland with a variety delta34S value of -0.1%. and, therefore, considered to be derived from a basic igneous protolith, and a more leucocratic member showing a spread delta34S values between +5.5 and +8.4%. which is considered to be derived from a crustal component. Sulphur isotope analyses of coexisting sulphide minerals are also useful as a geothermometer as long as the sulphide-bearing phases are in equilibrium. The putative temperature of formation of two coexisting mineral pairs from the Oughterard granite has been calculated; 41

Hydrochemistry and stable isotopes (δ 18 O and δ 2 H) tools applied to the study of karst aquifers in Southern Mediterranean basin (Teboursouk area, NW Tunisia)

Karst aquifers receive increasing attention in Mediterranean countries as they provide large supplies water used for drinkable and irrigation purposes as well as for electricity production. In Teboursouk basin, Northwestern Tunisia, characterized by a typical karst landscape, the water hosted in the carbonates aquifers provides large parts of water supply for drinkable water and agriculture purposes. Groundwater circulation in karst aquifers is characterized by short residence time and low water-rock interaction caused by high karstification processes in the study area. Ion exchange process, rock dissolution and rainfall infiltration are the principal factors of water mineralization and spatial distribution of groundwater chemistry. The present work attempted to study karstic groundwater in Teboursouk region using hydrochemistry and stable isotopes (δ18O and δ2H) tools. Karst aquifers have good water quality with low salinity levels expressed by TDS values largely below 1.5 g/l with Ca-SO4-Cl water type prevailing in the study area. The aquifers have been recharged by rainfall originating from a mixture of Atlantic and Mediterranean vapor masses

Hydrogeochemical and stable isotope data of Groundwater of a multi-aquifer system: Northern Gafsa basin - Central Tunisia

The hydrodynamic of the multi-aquifer system (the Continental Intercalaire “ C.I ” and the Complex Terminal “ C.T ”) of the North Gafsa basin is largely determined by tectonics (Tebessa - Gafsa fault). The composition of groundwater is controlled by complex reactions at gas-liquid-solid “mineralogical composition of associated rocks” interfaces, which depend on the natural surrounding and potential anthropogenic impact. The hydrochemical data (major ion geochemistry) indicate that these groundwaters are characterized by the dominance a Ca-Mg-HCO3/SO4 and Na-Cl-NO3 water types. Geochemical pattern is mainly controlled by the dissolution of halite, gypsum and/or anhydrite as well as by the incongruent dissolution of carbonate minerals. The pH of these samples range from 6.54 to 8.89, supporting the conclusion that the H2CO3/HCO3 couple control pH buffering. Oxygen-18 (δ18O‰SMOW) and deuterium (dD‰SMOW) isotopic data show the exchange between the groundwater and the rock (water-rock interaction) and the evaporation effect. The isotopic content of the boreholes waters is of mixed Mediterranean - Atlantic origin and is opposite to the quantity of rainwater distribution, both in space and time in the study area. This is due to its geographical situation in the southern and south-western of the Mediterranean Sea and between the Atlas area and the Sahara Platform. The concentrations of the isotopic composition of the groundwater are significantly higher than the rainwater. This is indicative of the dissolution of salts and other processes modifying the rainwater geochemical composition during infiltration into the vadose zone. The hydraulic interconnection of these components of the system has led to the evolution of these interesting groundwater types

Preliminary data of REE in Algerian phosphorites: a comparative study and paleo-redox insights

This study deals with the preliminary data of rare earth elements (REE) obtained on northeastern Algerian phosphorites from the Tébessa region. These phosphorites are located in two different basins: the northern basin represented by Dj. El Kouif, Dj. Dyr and Tazbant showings and the southern basin with the giant Dj. Onk phosphate deposit. The host sedimentary formation is Late Paleocene to Early Eocene. Twenty-six (26) samples from the four (4) localities were collected and analyzed for their REE contents using the ICP-MS technique. Phosphates from the southern basin (Dj. Onk deposit) show ∑REE contents ranging from 174.41 and 906.39 ppm (average ∑REE = 623.01 ppm), while the northern phosphorites have lower ∑REE contents (from 125.45 to 472.44 ppm; average = 265.57 ppm). PAAS-normalized REE patterns and binary Box plot of (Sm/Pr)N vs (Sm/Yb)N show HREE enrichments for samples from the northern localities while most samples from the southern basin are HREE depleted. Normalized (La/Yb)N vs (La/Sm)N plot shows that (La/Sm)N ratios of all samples are similar to those of modern seawater (from 0.83 to 1.55). However, the (La/Yb)N ratios of the two basins are significantly higher (from 0.67 to 1.18), which indicates an early diagenesis. The Ce/Ce* vs Pr/Pr* diagram shows that the observed Ce anomaly was not affected by diagenesis and more likely represents a proxy for redox conditions. The obtained results substantiate that the northern phosphorites were formed in more oxic environment with more pronounced negative Ce anomalies, whereas the southern phosphorites have lower Ce anomalies. Northern phosphorites are different from those from the south probably because the northern basin was more connected to an open sea as did the Sra Ouartan basin in northern Tunisia. These results have also been confirmed by statistical method studies, such as factorial discriminate analysis

Origin of the Tala Hamza Igneous Rocks-Hosted Zinc-Lead Deposit, NE Algeria

The Miocene igneous rocks that host the Tala Hamza Zn–Pb deposit are investigated in order to shed light on their petrogenesis and to provide a brief metallogenic description of associated sulphides mineralization. 800 m depth drill hole reveals from top to bottom the following succession: pyroclasitic rocks, andesite, volcanic tuff, microgranite, silicified tuff, kaolinized andesite cross-cut by aplite, metasomatized granodiorite and granodiorite. These rocks are composed of plagioclase, hornblende, biotite, alkali feldspar, sphene, apatite and zircon. Major elements geochemistry shows that these igneous rocks exhibit calc-alkaline, high K to shoshonitic, metaluminous to slightly peraluminous, I-type characters. Multi-elemental spectra, as well as chondrite-normalized rare earth spectra, show LILE and LREE enrichment relative to HFSE and HREE, respectively. The REE shows a slight negative anomaly in Eu. All these features indicate that the rocks originate from an igneous protolith, probably from a metasomatized mantle material that was contaminated by crustal material and emplaced in a post-collision context during the Miocene. Metallogenic study for both massive sulphides and stockworks reveals the presence of replacement textures, open-space-filling texture and pseudomorphism. These textural patterns, in addition to the presence of the anhydrite layer, are common in deposits of VHMS (volcanic-hosted massive sulphides) group formed in the submarine environment. They also indicate hydrothermal processes throughout two main stages: (1) an early stage in which disseminated pyrite and chalcopyrite hosted by metasomatized granodiorite were formed, and (2) a late-stage or mineralizing stage with the deposition of economic ore (mainly sphalerite and rare galena) along faults and fractures

The Algerian Glauconite-bearing Phosphorites: REE Variation and Insights on the Depositional Environment

International audienceNew data on rare earth element (REE) analyses are reported here on northeastern Algerian glauconite-bearing phosphorites. These rocks of Paleocene-Eocene age are located in the Eastern part of the Saharan Atlas, where two localities were investigated: the Djebel El Kouif in the north and the Kef Essenoun in the south. The latter belongs to the world-class Djebel Onk mining deposit. Hand-picked glauconite grains and other phosphate particles (pellets, coprolites, enamel and dentine of marine fish teeth) were embedded in epoxy resin and polished. They were imaged by SEM and major, trace and REE element contents were analyzed “in-situ” by Electron Microprobe and LA-ICP-MS techniques respectively. For comparison, bulk phosphorite samples were also analyzed by solution ICP-MS. The results show that phosphorite from the main layer of the Kef Essenoun deposit has a significant abundance in glauconite grains, while their occurrence in the Djebel El Kouif is restricted to the basal levels. These glauconite grains exhibit higher ∑REE contents than the co-existing particles (Kef Essenoun: min = 654 ppm, max = 1760, average = 1146 ppm; El Kouif: min = 543 ppm, max = 623 ppm, average = 584 ppm). The overall whole rock REE content also shows substantial differences between the two deposits. Therefore, whole-rock REE enrichment in the Kef Essenoun main sub-layer can be linked to the abundance of glauconite grains. PAAS normalized-REE patterns of the glauconite grains display signatures similar to those of co-existing particles and whole-rocks in each locality indicating a similar origin for the REE. It is noteworthy that phosphorites grains from the northern deposit show a REE uptake source from oxic-modern seawater, while those from Kef Essenoun deposit exhibit bell-shaped patterns with weak Ce anomaly suggesting a different environment of deposition (i.e. more reduced conditions). These geochemical results along with previous petrographic studies, also confirm the allochthonous character of the main phosphorite sub-layer in Kef Essenoun, i.e., winnowed, moved and re-deposited phosphorites as proposed by some authors

Glauconite-bearing sedimentary phosphorites from the Tebessa region (eastern Algeria): Evidence of REE enrichment and geochemical constraints on their origin

International audienceRare earth element (REE) analyses are reported on glauconite-bearing phosphorites from northeastern Algeria. These rocks of Paleocene-Eocene age are located in the Eastern part of the Saharan Atlas, where two localities were investigated: the Djebel El Kouif in the north and the Kef Essenoun in the south. The latter belongs to the world-class Djebel Onk mining deposit. Petrographic examination indicates that phosphorite from the main layer of the Kef Essenoun deposit has a significant abundance in glauconite grains, while their occurrence in the Djebel El Kouif is restricted to the basal levels. In both deposits, glauconite grains are richer in REEs than other co-existing particles (pellets, coprolites, enameloid and dentine of marine fish teeth), but the glauconites of the Kef Essenoun deposit exhibit significantly higher REE concentration (min = 654 ppm, max = 1760 ppm, average = 1146 ppm) than those of Djebel El Kouif deposit (min = 543 ppm, max = 623 ppm, average = 584 ppm). The whole-rock REE concentrations also show substantial differences between the two deposits and the REE enrichment in the Kef Essenoun main sub-layer is more likely the result of the high glauconite content. PAAS normalized-REE patterns of the glauconite grains display similar patterns between the northern and southern localities with weak negative Ce anomalies and slight middle REE enrichments. On the other hand, the pellets and whole phosphorite grains from the northern deposit indicate a REE source from oxic-suboxic seawater, whereas those from the Kef Essenoun deposit exhibit slight middle REE enriched patterns with weak negative Ce anomaly suggesting a different environment of deposition (i.e. a tendency to reduced conditions). These geochemical results along with previous petrographic studies confirm the allochthonous character of the main phosphorite sub-layer in Kef Essenoun, where winnowing, transport and re-deposition of previously deposited phosphorites resulted in high accumulation and formation of glauconite grains. The glauconitization process of fecal pellets happened in two stages and was controlled by semi-confined micro-environments. This can explain the abundance of glauconite grains in the phosphorites from Kef Essenoun deposit and their restricted occurrence in the basal levels of the Djebel El Kouif outcrop

The oceanic anoxic event 2 at Es Souabaa (Tebessa, NE Algeria): bio-events and stable isotope study

At the southern margin of the Tethys, the Es Souabaa area recorded traces of Oceanic Anoxic Event 2 (OAE2) around the Cenomanian-Turonian boundary (C/Tb). The dark, laminated, filament- and pyrite-bearing limestones represent the typical facies of this event. In terms of sedimentary environment, these features reflect a transgressive drowning that had induced hypoxia in these sedimentary environments. Such conditions favored the deposition and preservation of organic matter of marine origin, the distribution of which was controlled by paleogeography and halokinetic tectonics at that period. The OAE2 reached a climax between the last upper Cenomanian occurrence of Rotalipora cushmani and the lower Turonian occurrence of Whiteinella praehelvetica. Positive shift of the δ13C excursion along with relatively high total organic carbon (TOC) contents during OAE2 both indicate palaeo-environmental modifications enhanced by a significant change in primary marine productivity. Meanwhile, negative δ18O peaks in carbonates reflect increasing temperatures. Comparison of the data from this study with those from the neighboring Kalaat Senan section (Tunisia) suggests close similarities of events, although OAE2 is much more enhanced in Algeria

The Sidi El Hemissi Triassic “ophites” (Souk Ahras, NE Algeria): petrology, geochemistry, and petrogenesis

International audienceThe Sidi El Hemissi region, Souk-Ahras, is part of the Tellian Atlas where the Triassic sediments tectonically outcrop under the Tellian nappes of the Maghrebide chain. Mafic rocks, mainly gabbros and dolerites, called “ophites,” are interbedded as a ~ 200 × 30-m lenticular body within the Triassic gypsum-rich formation. These rocks show granular, microgranular, and microlithic textures and are composed of plagioclase, amphibole, pyroxene, and scarce olivine crystals. Albitization is the main alteration process, though chloritization, calcitization, and epidotization of ferromagnesian minerals can also be occasionally observed. The major, trace, and rare earth element studies show that these mafic rocks display relatively low P2O5 (less than 0.2 wt%) and moderate to low TiO2 contents (less than 2 wt%) and exhibit low-Ti continental tholeiitic basalt affinity. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) compared with high field strength elements (HFSE) and heavy rare earth elements (HREE). This, along with the observed weak Nb and Ce anomalies and the low-Ti contents, suggests an enriched mantle source for the generation of the magma, which likely underwent crustal contamination before emplacement within the Triassic sediments.The petrological and geochemical features of the Sidi El Hemissi ophites show many similarities with the basaltic rocks emplaced during the Late Triassic–Early Jurassic times, now cropping out in north-western Africa, south-western Europe, north-eastern, and south-eastern America. This large magmatic activity is believed to be related to the continental rifting associated with the early stages of the Pangea breakup