Didier Béziat

A Paleoproterozoic ultramafic-mafic assemblage and associated volcanic rocks of the Boromo greenstone belt: fractionates originating from island-arc volcanic activity in the West African craton

Abstract The Loraboue Birimian ultramafic-mafic assemblage, located in the Boromo greenstone belt (Burkina Faso), is interpreted as the remains of a magma chamber that crystallized at the base of an island arc. The ultramafic rocks exhibit an heteradcumulate texture, being generally made up of wehrlites and more rarely dunites. The crystallization sequence inferred from the cumulates is olivine+chromite followed by clinopyroxene+amphibole±orthopyroxene±biotite. The gabbroic rocks are mainly layered and grade into more differentiated facies with sub-pegmatitic texture, containing up to 70% modal plagioclase including zircon and, more commonly, apatite crystals. Textural relationships and mineral phase chemistry are indicative of crystallization at elevated pressures (>8 kbar), the parental magma being generated by a moderate to high degree of partial melting of a mantle source affected by previous metasomatic events above a subducted oceanic slab. The Loraboue volcanic formations exhibit a range of geochemical features. They consist dominantly of calc-alkaline basalts, pyroclastites and rhyolite and, more rarely, of basalt, dolerite and gabbro of tholeiitic affinity. These different types of basalt, as well as the dolerite and the isolated massive gabbro, show the classic features of arc magmatic suites, namely LILE and Pb enrichment, depleted HFSE patterns and high Ce/Nb and Th/Nb ratios. Thus, the calc-alkaline plutonic and volcanic assemblages of Loraboue could represent the roots of an island arc and the associated coeval volcanic rocks. The Paleoproterozoic crust of the West African craton was heterogeneous and was not the consequence of a single process of genesis. As some modern igneous province, the Birimian crust was generated by both volcanic arc accretion and oceanic plateau accretion.

Geodynamic implications of geochemical data for the Pyrenean ophites (Spain-France)

Abstract Field observations and radiometric data (K/Ar) of Pyrenean tholeiites issued from numerous large sills in Mesozoic basins indicate a Liassic intrusion. Variations in the chemical composition of these tholeiitic rocks show that the magmatic liquids are slightly differentiated via fractional crystallization and/or in situ density fractionation. Data on the highly hygromagmaphile (HYG) elements exhibit a noteworthy homogeneity of compositions and display, relative to chondritenormalized abundances, a distinctly negative Ta anomaly and a slight enrichment of highly HYG elements. These geochemical features are significantly different from ocean-type tholeiites due to the geochemical and mineralogical properties of their mantle source. The peculiar composition of the Pyrenean mantle source can be interpreted as the result of the Hercynian convergence process in which the dehydration of the subducted slab induced hydrothermal transformation and high-P-T hydrous pyroxenite segregation. The opening of the North Atlantic Ocean could have produced the ophites by inducing melting in this subcontinental mantle; with the enlargement of the new ocean basin, such a magma source would become progressively less available.

Cation site distribution in clinochlores: a NIR approach

Abstract A near infrared (NIR) spectroscopy approach was undertaken to collect information on the Al cationic distribution in tetrahedral and octahedral sites in natural chlorite (clinochlore) samples. Structural formulae were established using electron microprobe and Mossbauer spectroscopy. A band located near 7115 cm-1 was attributed to the overtone of OH fundamental stretching mode of Mg2AlOH and increases with the total Al amount. Good correlation was obtained between the (SiAl)O-OH vibration band area and tetrahedral Al content, making it easy to partition Al (and thus Fe3+) between octahedral and tetrahedral sites.

The Paleoproterozoic Copper‐Gold Deposit of Gaoua, Burkina Faso: Evidence for A Polyphased Mineralization

Due to increasing gold prices, gold exploration has intensified in the last 10 years, especially in the West African craton where many propects exist. Among these, the three copper-gold occurrences Dienemera, Gongondy and Mont Biri, of the Gaoua district (B2Gold), southern Burkina Faso, are thought to be part of a porphyric system (Sillitoe, 2007) but have not been so far subjected to detailed studies to confirm this hypothesis. This study presents mineralogical and fluid inclusion data for the mineralization and associated gangue phases that allow us to propose a metallogenic model for the formation of the Gaoua copper-gold system. In addition, the age of the mineralizing event was determined by Re-Os dating of sulfides.

Origin of rare earth element variations in clinopyroxene from plutonic and associated volcanic rocks from the Fouldé basin, Northern Kédougou inlier, Sénégal, West Africa

Abstract: The Paleoproterozoic basin of Foulde´, in the northern part of the Ke´dougou inlier (eastern Se´ne´gal), comprises bothvolcanic and plutonic suites belonging to the same K-rich calc-alkaline series. Plutonic and volcanic rocks are coeval and they showstrong compositional similarities in major and trace elements including rare earths (REE). Clinopyroxene is ubiquitous in the maficrocks and has the same major-element (except CaO) and trace-element (except LREE, Th and U) contents in both suites. Theclinopyroxenefoundinbasaltsandbasalticandesiteshasconvexupwardchondrite-normalizedREEpatterns,withaLREEdepletion[(Ce/Yb) N ¼0.64–0.67)].Clinopyroxenefromgabbroshaschondrite-normalizedREEpatternscharacterizedbyaLREEenrichmentwith(Ce/Yb) N ¼1.86–1.66.Thesedifferencesarerelatedtotheemplacementmodeofthetwomagmaticsuites,i.e.tothecoolingrateof the magma. In volcanic rocks, very rapid cooling allows clinopyroxene to retain its primary magmatic composition. In plutonicrocks the lowercooling rate allows late-crystallization andsubsolidus processes, which modify the early trace-element composition.Themagmaticprocessesaffectingthemonzogabbrosareoftwotypes:(i)reactionbetweenthemainigneousphases(clinopyroxeneand plagioclase) and an interstitial melt crystallizing quartz and K-feldspar, and (ii) exsolutions of amphibole, ilmenite andorthopyroxene in the clinopyroxene. The core of monzogabbros clinopyroxene that shows no exsolution has a higher CaO contentthanthatofclinopyroxeneinthevolcanicrocks.WeproposethatexsolutionofCa-and-LREEdepletedorthopyroxeneintheinnerrimofthe clinopyroxeneinmonzogabbrosinduces anincrease oftheCa content ofitscore andmay explain itscompositionaldifference(Ca and LREE) with the clinopyroxene in volcanic rocks.Key-words: clinopyroxene, in situ trace elements, monzogabbros, basalts, exsolution texture, Paleoproterozoic, Se´ne´gal.

Iron production in Northern Benin: excavations at Kompa Moussékoubou

In the context of the “Crossroads of Empires” project led by Anne Haour, one strand of enquiry aims to understand the history of blacksmith groups and the development of iron production in Dendi country, in the northern Republic of Benin. Numerous remains of iron production have been discovered, showing a great variability in furnace design and waste assemblages. At least three smelting traditions can be distinguished. In this paper, we present the smelting site of Kompa Moussekoubou (10th/11th c. AD) which has been investigated by archaeological and archaeometric methods. Beyond the archaeometallurgical results, the excavation of a 1 x 2 m trench on a settlement mound nearby and survey work, which place the site within its wider context, are also discussed. In particular, we offer a detailed analysis of the ceramics recovered during test pitting and within one of the furnaces itself. This paper thus offers a rare opportunity to combine archaeometallurgical and ceramics data.