The volcanic-hosted massive sulphide deposits of the Iberian Pyrite Belt Review and preface to the Thematic Issue

The Iberian Pyrite Belt (IPB) has, over the past decade, been an area of renewed mining activity and scientific research that has resulted in a wealth of new data and new geological and metallogenic concepts that are succinctly presented in this Thematic Issue. The reason for this interest in the IPB, which forms part of the Hercynian orogenic belt, is that its Late Devonian to Middle Carboniferous rocks host a huge quantity of volcanic-hosted massive sulphide (VMS) mineralization (1700 Mt of sulphides, totalling 14.6 Mt Cu, 13.0 Mt Pb, 34.9 Mt Zn, 46100 t Ag and 880 t Au). The mineralization and its environment display a number of typical signatures that can be related to the mineralogy and zoning of the sulphide orebodies, to the lead isotopes of the mineralization, to the geochemical and mineralogical variations in the hydrothermal alteration halos surrounding the orebodies, to the geochemical characteristics of the bimodal volcanics hosting the VMS, to the complex structural evolution during the Hercynian orogeny, to the presence of palaeofaults and synsedimentary structures that acted as channels and discharge traps for the metalliferous fluids, and to the gossans developed over VMS. Discriminant geological criteria have been deduced for each domain which can be helpful in mineral exploration, complementing the more traditional prospecting techniques. Although the question of the IPB's geodynamic setting is still under debate, any interpretation must now take into account some incontrovertible constraints: for example, the geochemical characteristics of a large part of the basic lavas are comparable to those of mantle-derived basalts emplaced in extensional tectonic settings, and the associated acidic rocks were produced by melting of a basic crustal protolith at low- to medium-pressures and a steep geothermal gradient, thus, the sulphide-bearing volcano-sedimentary sequence differs strongly from recent arc-related series. It is considered here that the tectonic setting was extensional and epicontinental and that it developed during the Hercynian plate convergence, that culminated in thin-skinned deformation and accretion of the South Portuguese terrane to the Iberian Paleozoic continental block.

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Resumen (translated by E. Pascual) Durante la década pasada, la Faja Pirítica Ibérica (FPI) ha sido un área de actividad minera e investigación cientifica renovadas, lo que ha conducido a la obtención de nuevos datos y conceptos geológicos y metalogénicos, que se exponen sucintamente en este Número Especial. La razón de este interés en la FPI, que forma parte del cinturón orogénico hercínico, es que sus rocas, cuyas edades abarcan desde el Devónico tardío al Carbonífero Medio, albergan una enorme cantidad de mineralizaciones de sulfuros masivos ligados a vulcanismo (1700 millones de toneladas de sulfuros, que totalizan 14,6 Mt de Cu, 13,0 Mt de Pb, 34,9 Mt de Zn, 46100 toneladas de Ag y 880 toneladas de Au). Las mineralizaciones y su entorno muestran signaturas que se pueden relacionar con la mineralogía y la zonación de las masas de sulfuros, con los isótopos de plomo de la mineralización, con las variaciones en los halos de alteración hidrotermal alrededor de las mineralizaciones, con los caracteres geoquímicos de las rocas volcánicas bimodales que albergan los sulfuros masivos, con la compleja evolución tectónica del conjunto durante la orogenia hercínica, con la existencia de paleofallas y estructuras sinsedimentarias que actuaron como canales y trampas de descarga para los fluidos metalíferos y los gossans que se desarrollaron sobre los sulfuros. Se han deducido criterios geológicos discriminantes para cada área de conocimiento, que pueden ser útiles para la exploración minera, complementando las técnicas más tradicionales de prospección. Aunque la cuestión del entorno geodinámico de la FPI todavía es materia de debate, cualquier interpretación tiene que tener ahora en cuenta algunas restricciones incontrovertibles: por ejemplo, los caracteres geoquímicos de una gran parte de las rocas básicas son comparables a los de basaltos derivados del manto y emplazados en entornos tectónicos extensionales, y las rocas ácidas asociadas se produjeron a partir de un protolito cortical básico, a presiones bajas o intermedias y asociadas a un abrupto gradiente térmico. Por consiguiente, la secuencia vulcanosedimentaria que contiene los sulfuros masivos difiere claramente de las series recientes relacionadas con entornos de arco. Consideramos aquí que el entorno tectónico fue extensional y epicontinental y que tuvo lugar durante la convergencia de placas hercínica, que culminó en deformación “thin-skinned” y acreción del terreno constituído por la Zona Sudportuguesa al bloque continental paleozoico ibérico.

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Received: 4 April 1996 / Accepted: 10 April 1997     

The western part of the Gulf of Cadiz: contour currents and turbidity currents interactions

Recent multibeam bathymetry and acoustic imagery data provide a new understanding of the morphology of the western part of the Gulf of Cadiz. The gulf is under the influence of a strong current, the Mediterranean Outflow Water (MOW). This current is at the origin of the construction of the giant Contourite Depositional System. Canyons and valleys with erosive flanks are observed. Only the Portimao Canyon is presently connected to the continental shelf. Channels occur on the continental shelf but are presently disconnected from the deeper network of channels and valleys. Slumps are localized in steep slope areas. They are caused by oversteepening and overloading, sometimes probably associated with earthquake activity. Slumps transform sharply into turbidity currents, depositing turbidites on the floor of deep valleys. Interaction of the MOW and gravity currents is suggested by the filling of the incisions located on the drifts below the present seafloor, the shifting of valleys and canyons in the direction of the MOW flow inducing an unusual phenomenon of capture of submarine valleys.     

Geologic and metamorphic evolution of the basement complexes in the Kontum Massif, central Vietnam

This paper presents a regional scale observation of metamorphic geology and mineral assemblage variations of Kontum Massif, central Vietnam, supplemented by pressure–temperature estimates and reconnaissance geochronological results. The mineral assemblage variations and thermobarometric results classify the massif into a low- to medium-temperature and relatively high-pressure northern part characterised by kyanite-bearing rocks (570–700 °C at 0.79–0.86 GPa) and a more complex southern part. The southern part can be subdivided into western and eastern regions. The western region shows very high-temperature (> 900 °C) and -pressure conditions characterised by the presence of garnet and orthopyroxene in both mafic and pelitic granulites (900–980 °C at 1.0–1.5 GPa). The eastern region contains widespread medium- to high-temperature and low-pressure rocks, with metamorphic grade increasing from north to south; epidote- or muscovite-bearing gneisses in the north (< 700–740 °C at < 0.50 GPa) to garnet-free mafic and orthopyroxene-free pelitic granulites in the south (790–920 °C at 0.63–0.84 GPa). The Permo-Triassic Sm–Nd ages (247–240 Ma) from high-temperature and -pressure granulites and recent geochronological studies suggest that the south-eastern part of Kontum Massif is composed of a Siluro-Ordovician continental fragment probably showing a low-pressure/temperature continental geothermal gradient derived from the Gondwana era with subsequent Permo-Triassic collision-related high-pressure reactivation zones.     

Phytoremediation of Sb,As, Cu,and Zn from Contaminated Water by the Aquatic Macrophyte Eleocharis acicularis

Sb, As, Cu, and Zn toxicity and contamination have become a growing concern in recent years. Phytoremediation, a plant based and cost effective technology, may be an effective approach in the cleanup of water contaminated by these metals. In this study, the aquatic macrophyte Eleocharis acicularis was used in laboratory and field experiments to assess its capability to accumulate Sb, As, Cu, and Zn, and thereby investigate its potential application in phytoremediation. The results showed that E. acicularis adapted well to water contaminated by these metals. The removal rates of Sb, As, Cu, and Zn in the laboratory experiment were 3.04, 2.75, 0.417, and 1.49 μg/L/day, respectively. The highest concentrations of these metals accumulated in E. acicularis after 10 days of the laboratory experiment were 6.29, 6.44, 20.5, and 73.5 mg/kg dry weight, respectively. Only 8% of As, 12% of Sb, 87% of Cu and 93% of Zn removed from the water were used by E. acicularis. The highest concentrations of Sb, As, Cu, and Zn accumulated in E. acicularis after 10 wk of the field experiment were 76.0, 22.4, 33.9, and 266 mg/kg dry weight, respectively. The results indicate that E. acicularis has the ability to accumulate Sb, As, Cu, and Zn from contaminated water.     

Latent and Sensible Energy Flux Over Deforested Land Surfaces in the Eastern Amazon and Northern Thailand

Land cover change may impact watershed hydrology and regional climate by altering land-atmosphere exchanges of energy and water. Conversion of forest to pasture has previously been shown to decrease the rate of evaporation (including transpiration) because of reduced aerodynamic roughness, less canopy rainfall interception and greater canopy resistance during dry periods. However, less is known about the effects of forest replacement land covers other than pasture. In this study, field measurements of meteorological processes, stomatal resistance, and Leaf Area Index (LAI) were taken over various deforested land surfaces in the eastern Amazon Basin and in northern Thailand. Based on these measurements, evaporation and sensible heat flux were estimated over each site. Results show the degree to which different land covers replacing tropical forest affect energy partitioning. In comparison with forest, non-irrigated, actively and recently cultivated sites had sharply lower rates of evaporation. Secondary vegetation generally had higher rates of evaporation than actively or recently used sites. The proportion of energy used for evaporation increased rapidly with age in secondary vegetation. Evaporation rates at sites eight and 25 years after abandonment in the Thailand study area were enhanced by positive sensible energy advection from nearby cultivated areas, and were similar to those of primary forest. Differences in dry season evaporation rates of 3-, 8- and 25-year secondary vegetation are explained, in part, by the differences in canopy resistance. Minimum canopy resistance was 223, 61 and 58 s m ⁻¹ , respectively at the 3-, 8- and 25-year sites.     

A cross-system analysis of sedimentary organic carbon in the mangrove ecosystems of Xuan Thuy National Park, Vietnam

A cross-system analysis of bulk sediment composition, total organic carbon (TOC), atomic C/N ratio, and carbon isotope composition (δ¹³C) in 82 surface sediment samples from natural and planted mangrove forests, bank and bottom of tidal creeks, tidal flat, and the subtidal habitat was conducted to examine the roles of mangroves in sedimentation and organic carbon (OC) accumulation processes, and to characterize sources of sedimentary OC of the mangrove ecosystem of Xuan Thuy National Park, Vietnam. Sediment grain sizes varied widely from 5.4 to 170.2 μm (mean 71.5 μm), with the fine sediment grain size fraction (< 63 μm) ranging from 11 to 99.3% (mean 72.5%). Bulk sediment composition suggested that mangroves play an important role in trapping fine sediments from river outflows and tidal water by the mechanisms of tidal current attenuation by vegetation and the ability of fine roots to bind sediments. The TOC content ranged from 0.08 to 2.18% (mean 0.78%), and was higher within mangrove forests compared to those of banks and bottoms of tidal creeks, tidal flat, and subtidal sediments. The sedimentary δ¹³C ranged from − 27.7 to − 20.4‰ (mean − 24.1‰), and mirrored the trend observed in TOC variation. The TOC and δ¹³C relationship showed that the factors of microbial remineralization and OC sources controlled the TOC pool of mangrove sediments. The comparison of δ¹³C and C/N ratio of sedimentary OC with those of mangrove and marine phytoplankton sources indicated that the sedimentary OC within mangrove forests and the subtidal habitat was mainly composed of mangrove and marine phytoplankton sources, respectively. The application of a simple mixing model showed that the mangrove contribution to sedimentary OC decreased as follows: natural mangrove forest > planted mangrove forest > tidal flat > creek bank > creek bottom > subtidal habitat.     

Evaluation of Two Low‐Cost–High‐Performance Adsorbent Materials in the Waste‐to‐Product Approach for the Removal of Pesticides from Drinking Water

This study evaluates the performance of two low cost and high performance adsorption materials, i.e., activated carbon produced from two natural waste products: Bamboo and coconut shell, in the removal of three pesticides from drinking water sources. Due to the fact that bamboo and coconut shell are abundant and inexpensive materials in many parts of the world, they respond to the “low‐cost” aspect. The adsorption capacities of two local adsorbents have been compared with commercial activated carbon to explore their potential to respond to the “high quality” aspect. Two pesticides were selected, namely dieldrin and chlorpyrifos, because they are commonly used in agriculture activities, and may remain in high concentrations in surface water used as drinking water sources. The results indicate that the adsorption of pesticides on activated carbons is influenced by physico‐chemical properties of the activated carbon and the pesticides such as the presence of an aromatic ring, and their molar mass. The activated carbon produced from bamboo can be employed as low‐cost and high performance adsorbent, alternative to commercial activated carbon for the removal of pesticides during drinking water production. The performance of activated carbon from bamboo was better due to its relatively large macroporosity and planar surface. The effect of adsorbent and pesticide characteristics on the performance was derived from batch experiments in which the adsorption behavior was studied on the basis of Freundlich isotherms.     

Mean and turbulent flow fields in a simulated ice‐covered channel with a gravel bed: some laboratory observations

Northern rivers experience freeze‐up over the winter, creating asymmetric under‐ice flows. Field and laboratory measurements of under‐ice flows typically exhibit flow asymmetry and its characteristics depend on the presence of roughness elements on the ice cover underside. In this study, flume experiments of flows under a simulated ice cover are presented. Open water conditions and simulated rough ice‐covered flows are discussed. Mean flow and turbulent flow statistics were obtained from an Acoustic Doppler Velocimeter (ADV) above a gravel‐bed surface. A central region of faster flow develops in the middle portion of the flow with the addition of a rough cover. The turbulent flow characteristics are unambiguously different when simulated ice covered conditions are used. Two distinct boundary layers (near the bed and in the vicinity of the ice cover, near the water surface) are clearly identified, each being characterized by high turbulent intensity levels. Detailed profile measurements of Reynolds stresses and turbulent kinetic energy indicate that the turbulence structure is strongly influenced by the presence of an ice cover and its roughness characteristics. In general, for y/d > 0·4 (where y is height above bed and d is local flow depth), the addition of cover and its roughening tends to generate higher turbulent kinetic energy values in comparison to open water flows and Reynolds stresses become increasingly negative due to increased turbulence levels in the vicinity of the rough ice cover. The high negative Reynolds stresses not only indicate high turbulence levels created by the rough ice cover but also coherent flow structures where quadrants one and three dominate. Copyright © 2012 John Wiley & Sons, Ltd.     

Recent climatic trends and linkages to river discharge in Central Vietnam

In this study, change in rainfall, temperature and river discharge are analysed over the last three decades in Central Vietnam. Trends and rainfall indices are evaluated using non‐parametric tests at different temporal levels. To overcome the sparse locally available network, the high resolution APHRODITE gridded dataset is used in addition to the existing rain gauges. Finally, existing linkages between discharge changes and trends in rainfall and temperature are explored. Results are indicative of an intensification of rainfall (+15%/decade), with more extreme and longer events. A significant increase in winter rainfall and a decrease in consecutive dry days provides strong evidence for a lengthening wet season in Central Vietnam. In addition, trends based on APHRODITE suggest a strong orographic signal in winter and annual trends. These results underline the local variability in the impacts of climatic change at the global scale. Consequently, it is important that change detection investigations are conducted at the local scale. A very weak signal is detected in the trend of minimum temperature (+0.2°C/decade). River discharge trends show an increase in mean discharge (31 to 35%/decade) over the last decades. Between 54 and 74% of this increase is explained by the increase in precipitation. The maximum discharge also responds significantly to precipitation changes leading to a lengthened wet season and an increase in extreme rainfall events. Such trends can be linked with a likely increase in floods in Central Vietnam, which is important for future adaptation planning and management and flood preparedness in the region. Copyright © 2012 John Wiley & Sons, Ltd.