Evolution des hydrocarbures dans des sédiments côtiers méditerranéens prélevés à proximité d'un emissaire d'une raffinerie de pétrole

Coastal Mediterranean sediments highly polluted by refinery effluents have been studied for their hydrocarbon content and bacterial activity. The study has taken into account both the distance from the refinery and depth of sampling. Very high hydrocarbon concentrations have been found (1–250 g/kg dry sediment). Micro-organisms in the sediments have a high bacterial activity and a good correlation exists between hydrocarbon concentrations and bacteria able to use these substrates as a carbon and energy source.     

Gem corundum deposits of Madagascar: A review

Madagascar is one of the most important gem-producing countries in the world, including ruby and sapphires. Gem corundum deposits formed at different stages in the geological evolution of the island and in contrasting environments. Four main settings are identified: (1) Gem corundum formed in the Precambrian basement within the Neoproterozoic terranes of southern Madagascar, and in the volcano-sedimentary series of Beforona, north of Antananarivo. In the south, high-temperature (700 to 800 °C) and low-pressure (4 to 5 kbar) granulites contain deposits formed during the Pan-African orogenesis between 565 and 490 Ma. They accompany mafic and ultramafic complexes (ruby deposits of the Vohibory group), skarns at the contact between Anosyan granites and the Proterozoic Tranomaro group (sapphire deposits of the Tranomaro–Andranondambo district), and shear-zone corridors cross-cutting feldspathic gneisses, cordieritites and clinopyroxenites in the Tranomaro, Vohimena and Androyan metamorphic series (biotite schist deposits of Sahambano and Zazafotsy, cordieritites of Iankaroka and Ambatomena). The circulation of fluids, especially along discontinuities, allowed in-situ alkaline metasomatism, forming corundum host rocks related to desilicified granites, biotitites, “sakenites” and “corundumites”. (2) Gem corundum also occurs in the Triassic detrital formations of the Isalo group, as giant palaeoplacers in the Ilakaka–Sakaraha area. Here, sapphires and rubies may come from the metamorphic granulitic terranes of southern Madagascar. (3) Gem corundum deposits occur within the Neogene-Quaternary alkali basalts from Ankaratra (Antsirabe–Antanifotsy area) and in the Ambohitra Province (Nosy Be, Ambato and Ambondromifehy districts). Primary deposits are rare, except at Soamiakatra where ruby in gabbroic and clinopyroxenite xenoliths within alkali-basalts probably derive from mantle garnet peridotites. The blue-green-yellow sapphires typical of basaltic fields are always recovered in palaeoplacer (in karst formed upon Jurassic limestones from the Montagne d'Ambre, Antsiranana Province) and alluvial and soil placers (Ankaratra volcanic massif). (4) Deposits occur within Quaternary eluvial, colluvial and alluvial concentrations, such as high-quality rubies from the Andilamena and Vatomandry deposits.     

Melt inclusions in olivine from the boninites of New Caledonia: Postentrapment melt modification and estimation of primary magma compositions

Melt inclusions were investigated in olivine phenocrysts from the New Caledonia boninites depleted in CaO and TiO₂ and enriched in SiO₂ and MgO. The rocks are composed of olivine and pyroxene phenocrysts in a glassy groundmass. The olivine phenocrysts contain melt inclusions consisting of glass, a fluid vesicle, and daughter olivine and orthopyroxene crystals. The daughter minerals are completely resorbed in the melt at 1200?C1300°C, whereas the complete dissolution of the fluid phase was not attained in our heating experiments. The compositions of reheated and naturally quenched melt inclusions, as well as groundmass glasses were determined by electron microprobe analysis and secondary ion mass spectrometry. Partly homogenized melts (with gas) contain 12?C16 wt % MgO. The glasses of inclusions and groundmass are significantly different in H₂O content: up to 2 wt % in the glasses of reheated inclusions, up to 4 wt % in naturally quenched inclusions, and 6?C8 wt % in groundmass glasses. A detailed investigation revealed a peculiar zoning in olivine: its Mg/(Mg + Fe) ratio increased in a zone directly adjacent to the glass of inclusions. This effect is probably related to partial water (hydrogen) loss and Fe oxidation after inclusion entrapment. The numerical modeling of such a process showed that the water loss was no higher than a few tenths of percent and could not be responsible for the considerable difference between the compositions of inclusions and groundmass glasses. It is suggested that the latter were enriched in H₂O after the complete solidification of the rock owing to interaction with seawater. Based on the obtained data, the compositions of primary boninite magmas were estimated, and it was supposed that variations in melt composition were related not only to olivine and pyroxene fractionation from a single primary melt but also to different degrees and (or) depths of magma derivation.     

An audiomagnetotelluric study of the Meugueur-Meugueur ring structure, Aïr, Niger: ring dyke or cone sheet?

Three radial audiomagnetotelluric (AMT) sounding profiles were carried out across the narrow, 65-km diameter troctolitic Meugueur-Meugueur ring structure, central Aïr, Niger, to study its electrical configuration; one profile extended across the bedrock into the large Ofoud complex situated slightly off geographical centre within the ring. Apparent resistivity data from 27 sites ranged from isotropic to strongly anisotropic. In nearly all soundings, one- and two-dimensional modelling indicated the presence of a major zone of low resistivity (60–600 Ωm), about 200 m thick, dipping steeply inwards at an angle of 65–80° and extending to a depth of at least 2–5 km. This layer, overlain and underlain by rocks of higher resistivities in excess of 5000 Ωm, is taken to be the outer contact. A highly resistive body, about 200 m in width, dipping inwards to a depth of at least 4 km is taken to be the Meugueur-Meugueur intrusion, which is thus interpreted to be a cone sheet.     

The trace element geochemistry of Corsican ophiolites

Ultramafic rocks, gabbros, plagiogranites, dolerites and basaltic dikes and pillows lavas of the ophiolite slices of eastern Corsica have been metamorphosed in both oceanic and orogenic environments. The trace element content of the metabasaltic rocks which exhibit a tholeiitic differentiation trend towards a ferrobasaltic composition, is similar to that of oceanic basalts. The cumulate sequence is interpreted to have been formed by fractional crystallization from an olivine-tholeiite magma. The troctolites representing cumulates derived from melts representing liquid fractions in the range F=1-0.85, gabbros from melts with F=0.85-0.45, and ferrogabbros and plagiogranites from melts with F<0.45. An oceanic environment with moderate spreading rates and magmatic processes similar to present-day normal oceanic ridge segments is considered the more probable original setting of the Corsican ophiolites.Financial support by C.N.R. (Italy) and A.T.P. Géodynamique de la Méditerranée Occidentale C.N.R.S. (France)     

Trace elements in the Merensky Reef and adjacent norites Bushveld Complex South Africa

Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits.     

Platinum mineralization in the Alaskan-type intrusive complexes near Fifield,New South Wales,Australia part 1. platinum-group minerals in clinopyroxenites of the Kelvin Grove prospect,owendale intrusion

Summary Several Alaskan-type complexes intrude metasedimentary rocks of the Cambro-Ordovician Girilambone Group near Fifield, New South Wales, Australia. These intermediate to ultramafic circular intrusives, emplaced during the Devonian, are composed of monzodiorite, gabbro-norite, clinopyroxenite, hornblendite, peridotite and dunite. Several types of PGE mineralization associated with these complexes are known. A new type described here occurs within pegmatoid, biotite- and magnetite-poor clinopyroxenites (P-units) which form irregular lenses and vein-like bodies within biotite- and magnetite-rich, fine grained clinopyroxenites. The mineralized "P-units" are not enriched in base metal sulphides with respect to the surrounding barren clinopyroxenites.The following PGM were identified: erlichmanite, isoferroplatinum, tetraferroplatinum, cooperite, cuprorhodsite, malanite, geversite, sperrylite, stumpflite, and several unidentified palladium antimonides. The early PGM (erlichmanite, (Pt-Fe) alloys, cooperite) were deposited from a fluid-rich system slightly before the end of clinopyroxene crystallization. The early mineralization stages are characterized by equilibrium crystallization whose order corresponds to the stability of PGE-sulphides as a function of temperature and fS₂. Late PGMs are dominated by sperrylite-geversite solid solution resulting from the reaction of early PGM with a fluid phase. These minerals extensively replace cooperite and locally remobilize the PGE stock. Isoferroplatinum appears as a reaction product during this metasomatic replacement indicating a high iron activity in the fluid phase. The deposition of PtSb₂ preceded that of PtAs₂. Sperrylite and geversite are later than exsolution ilmenite lamellae in the interstitial Cr-rich magnetite.Base metal sulphides are very rare, slightly later than PtAS₂-PtSb₂. Cobalt pentlandite and cobaltian pentlandite are the most common species, accompanied by pyrrhotite, pyrite, chalcopyrite, sphalerite and cobaltite. They are extensively replacing cooperite, but are in equilibrium with geversite-sperrylite.The presence of the equilibrium association OsS₂ + PtS implies a maximum temperature of 860°C, while the composition of cobalt-rich pentlandite indicates the highest stability limit of 670–710°C. Thus the temperature range of the formation of the Kelvin Grove mineralization can be estimated at about 850 to 650°C. The mineralization originated in a fluid-rich system which lowered the crystallization temperature of clinopyroxene. The precipitation of PGM is probably related to the appearance of a reducing fluid phase during final evolution stages of P-units, themselves emplaced in the most oxidized pyroxenites of the Owendale Complex.

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Platin-Vererzung in intrusiven Komplexen des Alaska-Types bei Fif eld, New South. Wales, Australien Ted 1. Platingruppen-Minerale in Klinopyroxeniten des Kelvin Grove Prospektes, Owendale Intrusion

Zusammenfassung Intermediäre his ultrarnafische, konzentrisch gebaute Intrusionen des Alaska-Types intrudierten im Devon metasedimentare Gesteine der Karnbro-Ordovizischen Girilambone Gruppe in der Ndhe von Fifield, New South Wales, Australien. Sie bestehen aus Monzodioriten, Gabbro-Noriten, Klinopyroxeniten, Hornblenditen, Peridotiten und Duniten. Mit diesen sind verschiedene Typen von Platin-Vererzung assoziiert. Ein neuer Typ, der hier beschrieben wird, tritt in Pegmatoiden, Biotit und Magnetit-armen Klinopyroxeniten (P-Einheiten) auf, die unregelmäßige Linsen und gangförmige Körper innerhalb Biotit- und Magnetit-reicher, feinkörniger Klinopyroxenite bilden. Die mineralisierten "P-Einheiten" sind im Vergleich zu den sic umgebenden erzfreien Klinopyroxeniten nicht mit Buntmetall-Sulfiden angereichert.Folgende PGM wurden bestimmt: Erlichmanit, Isoferroplatinum, Tetraferroplatinum, Cooperit, Cuprorhodsit, Malanit, Geversit, Sperrylit, Stumpflit, und einige Palladium-Antimonide bisher nicht bekannter Zusammensetzung. Die frühen PGM (Erlichmanit, (Pt-Fe) Legierung, Cooperit) wurden von einem fluid-reichen System kurz vor dem Ende der Klinopyroxen-Kristallisation abgesetzt. Die fruhen Vererzungsstadien sind durch Gleichgewichts-Kristallisation charakterisiert, deren Abfolge der Stabilität der PGE-Sulfide als einer Funktion von Temperatur und fS₂ entspricht. Spdtere PGM werden von Sperrylit-Geversit Mischreihen dominiert, die aus der Reaktion früh gebildeter PGM mit einer fluiden Phase entstanden sind. Diese Minerale verdrangen Cooperit extensiv und rernobilisieren lokal das Reservoir an PGE. Isoferroplatin erscheint als ein Reaktionsprodukt während dieser Phase metasomatischer Verdrdngung; es weist auf hohe Eisenaktivität in der fluiden Phase hin. Der Absatz von PtSb₂ erfolgte vor dem von PtAs₂, Sperrylit und Geversit sind junger als Entmischungslamellen von Ilmenit im chromreichen Magnetit der Matrix.Buntmetallsulfide sind außerordentlich selten, und etwas junger als PtAs₂-PtSb₂ gebildet. Kobalt-Pentlandit und Kobalt-führender Pentlandit sind die verbreitetsten Typen und werden von Pyrrhotit, Pyrit, Kupferkies, Zinkblende und Kobaltit begleitet. Sie verdrangen extensiv den Cooperit, sind aber im Gleichgewicht mit Geversit-Sperrylit.

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With 4 Figures and 2 Plates     

Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field,California

The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (⁸⁷Sr/⁸⁶Sr=0.7036,²⁰⁶Pb/²⁰⁴Pb=19.05,²⁰⁷Pb/²⁰⁴Pb=15.62,²⁰⁸Pb/²⁰⁴Pb= 38.63). The (initial) isotopic composition of typical rhyolite (⁸⁷Sr/⁸⁶Sr=0.7053,²⁰⁶Pb/²⁰⁴Pb=19.29,²⁰⁷Pb/²⁰⁴Pb= 15.68,²⁰⁸Pb/²⁰⁴Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.     

Spatial variation of metal bioaccumulation in the hydrothermal vent mussel Bathymodiolus azoricus

The variability of the bioaccumulation of metals (Ag, Cd, Cu, Fe, Mn and Zn) was extensively studied in the mussel Bathymodiolus azoricus from five hydrothermal vent sites inside three main vent fields of increasing depth along the Mid-Atlantic Ridge: Menez Gwen, Lucky Strike and Rainbow. Metal bioaccumulation varied greatly between vent fields and even between sites inside a vent field with B. azoricus showing a great capacity to accumulate metals. The bioaccumulation of these metals also varied significantly among tissues. The main target was the gills where metals were mainly associated with soluble compounds whereas in the digestive gland they were mainly associated with insoluble compounds. Storage of metals under insoluble forms in B. azoricus seems to be a major pathway for the detoxification of both essential and non-essential metals. Mussels from the studied fields can be discriminated following their metallic load but the segregation relies partially on the composition of the metal-enriched fluids.     

Lithostratigraphic and structural controls of ‘tsavorite’ deposits at Lemshuku,Merelani area,Tanzania

The first study of the Lemshuku ‘tsavorite’ mining district is presented. From bottom to top, the lithostratigraphic column corresponds to a metasedimentary sequence composed of quartzite, fine-grained graphitic gneiss, kyanite-graphite gneiss, biotite-almandine gneiss, metasomatized graphitic gneiss and dolomitic marble. ‘Tsavorite’ occurs in quartz veins and rarely as nodular concretions. Two factors control mineralization: (1) lithostratigraphy, with ‘tsavorite’ in association with pyrite and graphite confined to quartz veins within the metasomatized graphitic gneiss; and (2) structure, with the mineralized veins characteristically controlled by tight isoclinal folds associated with shearing.