Paleogeography and structure of the central Mediterranean: Sicily and its offshore area

The geology of the mainland and offshore of Sicily is illustrated by a few geologic sections and seismic profiles across the late Cenozoic orogenic belt of central and western Sicily and across the Sardinia Channel and Sicily Straits. This belt is the result of several tectonic events. Deformation involved mainly the sedimentary cover of the old African continental margin characterized by a broad basinal domain, flanked along its external (southern) margin by a shallow-water carbonate platform attached to Africa in the Triassic. Compressional deformation started in the more internal basinal rock assemblages overlying a thinned crust. The most important structural characteristic of the early phase of thrusting is the duplex pile forming the bulk of the chain in Central Western Sicily. The structure consists of a basal allochthon, made up of Permian to Middle Triassic layers, an intermediate duplex wedge, composed of competent Mesozoic carbonates, and a roof complex, including Upper Mesozoic-Lower Tertiary less competent rocks. Large-scale clockwise rotation of the thrusts accompanied transpressional movements in the hinterland during the Pliocene. Right oblique reverse faults modified the previous tectonic contacts between the allochthons in the hinterland zones. Contemporaneous south-directed imbrications affected the southern external areas, progressively incorporating foreland and piggyback basins. Development of the Gela Thrust System appears to be linked to the transpressional event; its accretion is also related to contemporaneous underthrusting at deeper levels of Mesozoic carbonate substratum. The older buried thrust sheets were pushed up to the surface breaching the deformed Tertiary cover of the Gela TS. Northwards in the belt post-Messinian normal growth faults opened half graben whose sedimentary fill underwent structural inversion. Alternation of extension and compression tectonics characterizes the Sicilian continental margin in the last million years.     

Overview of in-situ thermomechanical experiments in clay: Concept, results and interpretation

The HADES project (High Activity Disposal Experimental Site) aims at demonstrating the technical feasibility and the long-term safety of geological disposal of reprocessed HLW (High-Level Wastes) radioactive wastes. This disposal could be realised in the Tertiary Boom clay formation below the Mol/Dessel nuclear site. Previous studies in the 80's on the geomechanical behaviour of Boom clay, at host rock temperature (15°C), have demonstrated the mining capabilities of this clay. European partners have collaborated to increase the number of in-situ tests to be developed and operated from the Underground Research Facility (URF). Integrated large-scale experiments have been developed during the last four years, within the framework of the Commission of the European Communities (CEC) research contracts, in order to gain more insight into the thermal influence of heat-emiting wastes on the clay behaviour in the near field.

This paper summarizes the knowledge gained from the in-situ hydro-thermo-mechanical experiments realised from the URF. The objectives and main issues of the tests are described. Information regarding selection of parameters and representativeness of their measurements is discussed. Despite their different aims and designs, three hydro-thermo-mechanical experiments are compared in order to emphasize similarities in the general behaviour of the clay massif submitted to a thermal load. It is observed that its behaviour is qualitatively similar to the reaction upon a constant total stress increase. With the information now available, it is not obvious to draw conclusions with respect to the irreversibility of some phenomena.

Simple in-house analytical codes are able to simulate with a good accuracy the hydro-thermo-mechanical behaviour of clay. Nevertheless, these codes are not sufficient to describe the complexity of the phenomena that are involved in hydro-thermo-mechanical processes as pointed out by field data. In order to validate more complex models, the possibilities to improve the representativeness of the measurements are investigated.     

Thermomechanical properties of deep argillaceous formations

A difficulty in the interpretation of mechanical and thermo-mechanical tests on specimens drawn from large argillaceous formations is the strong inhomogeneity of void ratio, clay minerals and carbonates content.

In this paper a relationship is developed to link strength and the maximum preconsolidation stress to the initial void ratio and carbonate content. Compressibility is also correlated to carbonates.

Thermal strains in drained and undrained conditions for a Spanish, a belgian and an Italian natural clay are compared. In the elastic state strains are comparable, while in the plastic range thermal strains are highest for the Belgian clay, lower in the Spanish cemented clay and lowest in the Italian clay, very stiff and cemented.     

A traverse through the western Kunlun (Xinjiang,China): tentative geodynamic implications for the Paleozoic and Mesozoic

The northern part of the western Kunlun (southern margin of the Tarim basin) represents a Sinian rifted margin. To the south of this margin, the Sinian to Paleozoic Proto-Tethys Ocean formed. South-directed subduction of this ocean, beneath the continental southern Kunlun block during the Paleozoic, resulted in the collision between the northern and southern Kunlun blocks during the Devonian. The northern part of the Paleo-Tethys Ocean, located to the south of the southern Kunlun, was subducted to the north beneath the southern Kunlun during the Late Paleozoic to Early Mesozoic. This caused the formation of a subduction-accretion complex, including a sizeable accretionary wedge to the south of the southern Kunlun. A microcontinent (or oceanic plateau?), which we refer to as “Uygur terrane,” collided with the subduction complex during the Late Triassic. Both elements together represent the Kara-Kunlun. Final closure of the Paleo-Tethys Ocean took place during the Early Jurassic when the next southerly located continental block collided with the Kara-Kunlun area. From at least the Late Paleozoic to the Early Jurassic, the Tarim basin must be considered a back-arc region. The Kengxiwar lineament, which “connects” the Karakorum fault in the west and the Ruogiang-Xingxingxia/Altyn-Tagh fault zone in the east, shows signs of a polyphase strike-slip fault along which dextral and sinistral shearing occurred.     

Quaternary minettes and associated volcanic rocks of Mascota,western Mexico: a consequence of plate extension above a subduction modified mantle wedge

 Pleistocene-Holocene volcanism in the Jalisco block of western Mexico is confined to two conspicuous grabens, where potassic eruptives range from absarokites (48–52% SiO₂) and minettes (49–54% SiO₂) through basaltic andesites (53–57% SiO₂), the most voluminous type, to andesites and their lamprophyric equivalent spessartite (58–62% SiO₂); there are no contemporary rhyolitic rocks. This suite has high concentrations of Mg, Cr (<550 ppm) and Ni (<450 ppm) accompanied by large concentrations of K, P, Ba (<4000 ppm) and Sr (<5000 ppm) and elements such as LREE and Zr (<600 ppm). No combination of crystal fractionation and/or crustal contamination can reproduce the compositional range of these magmas, which nevertheless are believed to be genetically related because of their proximity in time and space. Hydrous minerals in the lamprophyres and the typical absence of plagioclase phenocrysts in both basaltic andesites and andesites reflect the relatively high concentrations of water in the magmas, which suppressed the crystallisation of feldspar. Experimental verification of the minimal amounts of water required to reproduce the phenocryst assemblages in selected rocks range from 3.5 to 6%. During ascent in a volcanic conduit, andesitic magma may lose water and consequently precipitate plagioclase, or it may ascend more rapidly, retaining more of its initial water, which stabilises phenocrysts of hornblende at the expense of plagioclase. Our estimates of water concentrations, which are consistent with the various low pressure phenocryst assemblages, will be minimal for the magmas in their source regions, and the process of magmatic dewatering on ascent may be typical in well established volcanic conduits. In accord with the compositions of phenocrystic olivine in the basaltic andesites and the minettes, the values of FeO and Fe₂O₃ of the bulk lavas and scoriae are demonstrably pristine. As a consequence, there are two characteristic features of the Mascota suite: the high range of relative oxygen fugacities (ΔNNO=1–5) and the high Mg# (MgO/MgO+FeO) that ranges from 0.70 to 0.91 (with only one andesite as low as 0.66). From the evidence of phlogopite phenocrysts, a partial melt involving phlogopite would have a higher Mg# than one from olivine (Fo₉₀) and pyroxene alone. As the Mascota series shows a correlation between K₂O and Mg#, we conclude that it was generated by partial fusion of the mantle wedge, with a variable contribution of phlogopite and apatite from veins throughout the lherzolitic assemblage. In conformity with an origin by varying increments of partial fusion of a phlogopite-bearing mantle, all incompatible elements vary linearly with Ti (or K) as if phlogopite (+apatite) in the source dominated their contribution to the partial melts. Fluids from dehydration of the subducting slab presumably deposit hydrous and other minerals in veins in the mantle wedge and also increase its redox state. As the Mascota volcanism occurs in grabens closer to the trench than the main andesite arc, it is concluded that the eruption of these small volumes of hydrous magmas require the tectonically favored ascent paths offered by the extensional grabens to reach the surface from their mantle sources. Received: 24 January 1995 / Accepted: 21 February 1996     

The age and thermal history of Cerro Rico de Potosi, Bolivia

 Cerro Rico de Potosi, Bolivia, is the world’s largest silver deposit and has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite, and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extruded at 13.8±0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and alteration occurred within about 0.3 my of dome emplacement, as indicated by a ⁴⁰Ar/³⁹Ar date of 13.76±0.10 Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5±1.1 Ma (1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and widespread volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing events; the exceptionally flat thermal release pattern of ³⁹Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough, and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal system. Received: 14 October 1995/Accepted: 29 January 1996     

Field assessment of acid mine drainage contamination in surface and ground water

Both sulfate and conductivity are useful indicators of acid mine drainage (AMD) contamination. Unlike pH, they are both extremely sensitive to AMD even where large dilutions have occurred. The advantage of using sulfate to trace AMD is that unlike other ions it is not removed to any great extent by sorption or precipitation processes, being unaffected by fluctuations in pH. These two parameters are also closely associated as would be expected, as conductivity is especially sensitive to sulfate ions. Therefore, as sulfate analysis is difficult in the field, conductivity can be used to predict sulfate concentration in both AMD and contaminated surface waters using regression analysis. Most accurate predictions are achieved by using equations given for specific conductivity ranges or AMD sources. There is also potential to use conductivity to predict approximate concentrations of key metals when the pH of the water is within their respective solubility ranges.     

Pore-water response on seasonal environmental changes in intertidal sediments of the Weser Estuary,Germany

In order to determine time-dependent changes in estuarine pore-water chemistry and flux variations across the sediment-water interface, sediment cores of an intertidal mud flat in the Weser Estuary were taken monthly over a one-year period. Sediment temperature, pH, Eh, Cl^–, O₂, NO ₃ ^– , and SO ₄ ^2– pore-water concentrations were measured and showed variations that relate to the changes of surface temperature and estuarine water composition. Fick's first law was applied to quantify diffusive fluxes from concentration gradients in the diffusive boundary layer and in the pore water. Total nitrate fluxes were calculated from flux chamber experiments. Diffusive oxygen fluxes increased from 5 mmol m^–2 d^–1 in winter to 18 mmol m^–2 d^–1 in early summer, while nitrate fluxes into the sediment increased from 3 mmol m^–2 d^–1 in winter to 60 mmol m^–2 d^–1 in early summer. Oxygen and nitrate fluxes into the sediment correlated linearly to sediment temperature. Sulfate fluxes increased from 0.5 mmol m^–2 d^–1 in winter to 10 mmol m^–2 d^–1 in August and September. Converted into carbon fluxes, the sum of these oxidants ranged from 10 mmol m^–2 d^–1 in winter to 80 mmol m^–2 d^–1 in summer. An estimation of the upper limit of the annual nitrate flux into the sediment showed that about 10% of the 250,000 t of nitrate discharged annually by the river may be decomposed within the inner Weser Estuary.     

Platinum-group minerals in chromitites from the ojen lherzolite massif (Serrania de Ronda,Betic Cordillera,Southern Spain)

Summary Chromitites (Cr ores) of the Ojen lherzolite massif (Serranía de Ronda, Betic Cordillera, Southern Spain) were found to contain platinum-group minerals (PGM) as discrete inclusions in the chromite and in the associated silicates. The PGM mineralogy consists of sulfides [laurite, erlichmanite, malanite, unnamed (Ni-Fe-Cu)₂ (Ir, Rh) S₃, unidentified Pd-S], sulfarsenides (irarsite, hollingworthite, ruarsite, and osarsite), arsenides [sperrylite, unidentified (Pd, Ni)-As], one unidentified Pd-Bi compound, and native platinum group elements (PGE) consisting of Ru and Pt-Fe alloys. Textural considerations suggest that the PGE chalcogenides with S and As were formed in the high-temperature magmatic stages, as part of the chromite precipitation event (primary PGM), in contrast with the native PGE, which originated during the low-temperature serpentinization of the ultramafic host of the chromitites (secondary PGM).The primary PGM inclusions in the Ojen chromite are unusual compared with PGM inclusions in chromitites from tectonitic upper-mantle of ophiolites and other alpine-type complexes in that i) they display a great variety of mineral species sulfides, sulfarsenides and arsenides, and ii) comprise specific phases of all six PGE. The singularity of the primary PGM mineralization probably reflects high activities of both S and As during chromite precipitation at Serrania de Ronda to be related with particular physico-chemical conditions during uplifting of sub-continental, astenospheric mantle.The nature, composition, and paragenetic association of secondary PGM at Ojen confirm the relatively-high mobility of the PGE at low temperature, and indicate that remobilization can be selective under appropriate redox conditions causing separation and redistribution of the PGE in the rocks as a result of the alteration process.

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Platingruppen-Minerale in chromititen aus dem ojen-lherzolithmassiv (Serranía de Ronda, Betische Kordillere, Süd-Spanien)

Zusammenfassung Platingruppen-Minerale in Chromititen aus dem Ojen-Lherzolithmassiv (Serranía de Ronda, Betische Kordillere, Süd-Spanien) In den Chromititen (Cr-Erzen) aus dem Ojen-Lherzolithmassiv (Serranía de Ronda, Betische Kordillere, Süd-Spanien) warden Platingruppen-Minerale (PGM) als einzelne Einschlüsse im Chromit and in den begleitenden Silikaten gefunden. Die Mineralogie der PGM setzt sich aus Sulfiden [Laurit, Erlichmanit, Malanit, einem unbenannten (Ni-Fe-Cu)₂ (Ir, Rh)S₃ und einem nicht identifizierten Pd-S], Sulfarseniden (Irarsit, Hollingworthit, Ruarsit und Osarsit), Arseniden [Sperrylit, einem nicht identifizierten (Pd, Ni)-As], einer nicht identifizierten Pd-Bi-Verbindung sowie gediegenen Platingruppen-Elementen (PGE) bestchend aus Ru and Pt-Fe-Legierungen, zusammen. Texturelle Untersuchungen haben ergeben, daß die PGE-Chalkogenide mit S und As im Zuge der Chromitfällung (primäre PGM) in den hochtemperierten, magmatischen Stadien gebildet warden, während die gediegenen PGE während der niedriggradigen Serpentini sierung des ultramafischen Nebengesteins der Chromitite (sekundäre PGM) gebildet warden.Die primären PGM-Einschlüsse in den Ojen-Chromiten sind im Vergleich zu PGM-Einschlüssen in Chromititen aus dem tektonisierten oberen Mantel in Ophiolithen und anderen alpinotypen Komplexen ungewöhnlich: i) Einerseits zeigen sie eine große Vielfalt an Mineralarten aus der Gruppe der Sulfide, Sulfarsenide und Arsenide. ii) Andererseits enthalten sie spezifische Phasen aller sechs PGE. Die Einzigartigkeit der primären PGM-Mineralisation könnte hohe Aktivitäten von S and As während der Chromit-Fällung in Serranía de Ronda widerspiegeln, die mit besonderen physiko-chemischen Bedingungen während der Hebung des subkontinentalen, asthenosphärischen Mantels zusammenhängen.Die Art, die Zusammensetzung and die paragenetische Vergesellschaftung von sekundären PGM in Ojen bestätigen die relativ hohe Mobilität der PGE bei niedriger Temperatur und zeigen, daß die Remobilisierung unter geeigneten Redox-Bedingungen selektiv wirken kann, wodurch eine Trennung und Neuverteilung der PGE in den Gesteinen als Ergebnis des Alterationsprozesses bewirkt wird.

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With 7 Figures