Trondhjemites,tonalites and diorites in the South Portuguese Zone and their relations to the vulcanites and mineral deposits of the Iberian Pyrite Belt

In the South Portuguese Zone close associations of diorites, tonalites and trondhjemites occur north of the Pyrite Belt. The period of their emplacement is Pre-Carboniferous and not, as has generally been assumed, Variscanpostorogen. The trondhjemitic intrusive suite and the Lower Carboniferous spilite-keratophyre association are related through their comagmatic derivation. Both series share sodium dominance, low concentrations of large-ion lithophile elements, indicators of a water-rich original magma and a deficient scorification of the element potential, which was acummulated in the sulphide and manganese deposits of the Pyrite Belt through post-volcanic hydrothermal processes. The mineralogically and geochemically primitive composition of both plutomtes and vulcanites, their mode of eruption from acid to basic facies as well as their position in the orogenic process indicate that they represent products of a successively proceeding partial melting of subducting oceanic crust. The trondhjemitic intrusives are the initialites in the magmatic-orogenic development.

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Zusammenfassung In der Südportugiesischen Zone treten nördlich des Pyritgürtels enge Vergesellschaftungen von Dioriten, Tonaliten und Trondhjemiten auf. Der Zeitraum ihrer Platznahme ist päkarbonisch und nicht, wie bisher allgemein angenommen, variszisch-postorogen. Die trondhjemitische Intrusivsequenz und die unterkarbonische Spilit-Keratophyr-Assoziation sind in weiten Bereichen durch komagmatische Herkunft verbunden. Gemeinsamkeiten beider Abfolgen sind ihre Na-Dominanz, die niedrigen Gehalte an gro\ionigen Elementen, die Anzeichen für ein wasserreiches Ausgangsmagma und die mangelhafte Verschlackung des Elementpotentials, das in den Sulfid- und Manganlagerstätten des Pyritgürtels während postvulkanischer, hydrothermaler Proze\e angereichert wurde. Die mineralogisch wie geochemisch primitive Zusammensetzung sowohl der Plutonite wie der Vulkanite, ihr Eruptionsmodus von saurer zu basischer Fazies sowie ihre Position im orogenen Ablauf deuten darauf hin, da\ sie die Produkte einer sukzessiv fortschreitenden Aufschmelzung subduzierender, ozeanischer Kruste darstellen. Die trondhjemitischen Intrusiva sind die Initialite innerhalb der magmatisch-orogenen Entwicklung.

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Résumé Dans la zone sud du Portugal, il existe, au nord de la ceinture pyriteuse, des associations étroites de diorites, de tonalites et de trondhjémites. Leur mise en place date de la période pré-carbonifère et non, comme on l'avait généralement admis jusqu'à présent, de la période varisque post-orogénique. Le faisceau intrusif trondhjémitique et l'association spilite-kératophyre du Carbonifère inférieur sont liés par une origine comagmatique. Les points communs des deux successions sont leur dominante sodique, les faibles teneurs en éléments à gros ions, les indices d'un magma de départ aquifère et la scorification réduite du potentiel d'éléments qui, dans les gisements de sulfure et de manganèse de la ceinture pyriteuse a été enrichi par des processus post-volcaniques et hydrothermaux. La composition primitive, tant minéralogique que géochimique, de la plutonite comme de la vulcanite, leur mode d'éruption qui va du type acide au type basique, ainsi que leur position dans le processus orogénique indiquent qu'il s'agit de produits successifs de la fusion partielle d'une croûte océanique en voie de subduction. Les intrusions trondhjémitiques représentent le stade initial dans l'évolution orogéno-magmatique.

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Chert in the Iberian Pyrite Belt

Since lenses of chert are common within the volcano-sedimentary succession hosting the massive sulphide deposits of the Iberian Pyrite Belt (Spain and Portugal), we examined numerous chert occurrences, both petrographically and geochemically, to test their possible value for massive sulphide exploration. The chert is found at two main lithostratigraphic levels (upper and lower) that are also interpreted as massive-sulphide bearing. In both cases the chert is located at the top of acidic volcanic sequences or in the associated sediments; we have not been able to observe the relationships between massive sulphides and chert, but some of the large orebodies of the Province (Lousal, La Zarza, Tharsis, Planes-San Antonio body of Rio Tinto, Neves) are described as being locally capped by chert facies. Four main types are recognized among the chert and associated facies: (1) red hematitic chert?±?magnetite; (2) radiolarian and/or sedimentary-textured (conglomeratic) chert with hematite and/or Mn oxides; (3) pale sulphidic chert; (4) rhodonite and/or Mn carbonate?±?magnetite facies. In the Spanish part of the Province the radiolarian chert is confined to the upper level; the distribution of the other types appears to be haphazard. The hydrothermal origin of the South Iberian chert is shown by its high Fe-Mn and low Co-Ni-Cu contents. The presence of small positive Ce anomalies indicates a shallow marine environment (shelf or epicontinental sea), which is consistent with the volcanological and sedimentological data. The chert was emplaced below the sea floor through chemical precipitation and/or through alteration and replacement of the country rock, residual traces of which are ghost phenocrysts and high Al, Ti and rare earth contents. Macro- and microscopic relationships indicate that the oxide facies (hematite?±?magnetite) formed first, probably providing a protective insulating cover against the marine environment and enabling an evolution towards sulphide facies; a phase of Mn?carbonate and silicate + quartz?±?chlorite + sulphides appears to be even later. It was not possible, through discrimination, to isolate a chert that could be considered as representing a lateral marker of massive sulphides; moreover, both field observations and geochemical data seem to indicate a relative independence of this siliceous sulphide hydrothermal activity from the hydrothermal activity giving rise to the massive sulphides. Such is also indicated by the lead isotopic signature of the chert, which is appreciably more radiogenic than that of the massive sulphides; the lead enrichment in the sulphidic chert facies indicates the participation of a different source (sediments, sea water) from that of the massive sulphides. The hypothesis of an independent hydrothermal “chert” event can thus be envisaged, wherein the chert reflects submarine low-temperature hydrothermal activity that is most apparent during a “break” within the volcano-sedimentary succession and which may locally have competed with the high-temperature hydrothermal activity giving rise to the massive sulphides. The interest of the chert thus rests in its palaeodynamic significance, as a marker of periods of volcanic quiescence, and in its possible role as a protective insulating cap favourable to the deposition of massive sulphides.     

Gold mineralisations in the SW Iberian Pyrite Belt

The occurrence and distribution of gold in the massive sulfide orebodies of the upper Palaeozoic volcano-sedimentary environment of the SW Iberian Pyrite Belt have long been assumed to be quite uniform. Recent investigations in several working mines indicate that the gold content of the ores varies between ore types, and is controlled by physico-chemical and time factors during deposition. Above-average gold grades occur in the following ore types: lead-zinc-rich ores, situated laterally and on top of the massive sulfide lenses; copper-rich siliceous basal ore facies; footwall stringer sulfides; and gangue-rich massive sulfides (siliceous, carbonate-rich, baryte bearing, or carbonaceous matrix). Exploration possibilities therefore are promising in foot- and hanging-wall environments within an orebody, as well as in disseminated and stringer ores in its footwall.     

Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt,Iberian Massif)

A low-pressure metamorphic zonation ranging from biotite tomigmatite zones occurs in the Sierra Albarrana area (VariscanBelt of southwestern Iberian Peninsula) in uppermost Precambrianto Lower Palaeozoic metasedimentary rocks. The principal deformationin this area is related to a major ductile shear zone whosecentral part is localized immediately to the southwest of theSierra Albarrana Quartzites. The metamorphism is synchronouswith respect to this deformation. The metamorphic zones aresymmetrically distributed with respect to the Sierra AlbarranaQuartzites. Pressure–temperature (P–T) conditionsare 3.5–4 kbar and range from 400°C (biotite zone)to 500°C (staurolite–garnet zone) up to 650–700°C(migmatite zone). We have not detected pressure variations alongthe different metamorphic zones. Relic kyanite is observed inthe form of inclusions in andalusite within veins in the lower-gradepart of the staurolite–andalusite zone. The low-pressuremetamorphism of the Sierra Albarrana area arises from a two-stagehistory including moderate crustal thickening followed by subsequentlocalization of deformation in a transcurrent shear zone duringpeak P–T conditions. Channelized fluid flow within themajor ductile shear zone may have contributed to the heat budgetof the low-pressure metamorphism. KEY WORDS: fluid flow; Iberian Massif; low-pressure metamorphism; shear zone; Sierra Albarrana area     

PGE distribution in massive sulfide deposits of the Iberian Pyrite Belt

We present the first platinum group elements (PGE) data on seven massive sulfide deposits in the Iberian Pyrite Belt (IPB), one of the world largest massive sulfide provinces. Some of these deposits can contain significant PGE values. The highest PGE values were identified in the Cu-rich stockwork ores of the Aguas Teñidas Este (Σ PGE 350 ppb) and the Neves Corvo (Σ PGE 203 ppb) deposits. Chondrite normalized PGE patterns and Pd/Pt and Pd/Ir ratios in the IPB massive, and stockwork ores are consistent with the leaching of the PGE from the underlying rock sequence.     

The deposition of volcanics and pyritite in the Iberian Pyrite Belt

Stratiform pyritic deposits occur interbedded with sedimentary and volcanic rocks and may be considered to consist of sulphidic rock, called pyritite. In the Iberian Pyrite Belt such deposits are found at different levels and settings in the Volcanic-Siliceous Complex of Lower Carboniferous age, which comprises sediments and felsic to mafic volcanics. The felsic volcanics range from dust tuffs to lapilli tuffs of quartz-keratophyric to rhyolitic composition, and are interpreted as submarine ashflow tuffs laid down by sliding and flowing down volcanoes at the eruptive centres in the Hercynian geosyncline. The pyritite bodies are likewise regarded as redeposited: they originated on the volcano flanks from emanations towards the end of the felsic vulcanicity, to slump and move down into deeper water among tuffs or muds.

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Résumé Les gisements pyriteux stratiformes se trouvent intercalés dans des roches sédimentaires et volcaniques, ce qui mène à les considérer comme constitués d'une roche appelée »pyritite«. Dans la Ceinture Pyriteuse Ibérique, ces dépôts se présentent dans des niveaux et des milieux variés, dans le Complexe Vulcano-Siliceux, d'âge Carbonifère inférieur, qui comporte des sédiments et des roches volcaniques acides et basiques. Les volcanites acides sont des tufs variant de très fins à grossiers, de composition quartz-kératophyrique à rhyolitique. Nous admettons un mode de dépôt par des coulées sous-marines de cendres, à la suite d'éboulements sur les volcans aux centres éruptifs dans le géosynclinal hercynien. Nous estimons également que les masses de pyritite se sont déposées en deux étapes: engendrées par des exhalations sur les flancs des volcans, vers la fin du volcanisme acide, elles glissaient et coulaient dans des eaux plus profondes, parmi des tufs ou des boues.

     

Geological constraints on massive sulphide genesis in the Iberian Pyrite Belt

The Iberian Pyrite Belt (IPB), SW Iberian Peninsula, Spain and Portugal, one of the most famous and oldest mining districts in the world, includes several major concentrations of massive sulphides, unique on Earth (e.g., Riotinto, Neves Corvo), as well as a large number of smaller deposits of this same type. All these deposits, in spite of their general similarities, show significant differences in geological setting, age, relations to country rocks, hydrothermal alteration, mineralogy and geochemistry. As a consequence of a review of the available data in the IPB, together with new findings on volcanism, hydrothermal alteration and ore mineralogy, we propose a modified genetic scenario, that can account particularly for the diversity of the geological situations in which sulphide deposits occur, as well as for their mineralogical and petrological diversity. It is concluded that there is no direct genetic relationship between felsic volcanic activity and massive sulphide deposition in the IPB, and that most of the massive sulphide bodies, including all of the giant ones, are closely related to hydrothermal vents, being therefore proximal. The available isotopic data yield additional genetic information: (a) Homogeneous lead isotope values indicate a single (or homogenized) metal source; (b) sea and connate water are the fluid reservoirs for hydrothermal input, and (c) bacterial reduction of sulphur is the most probable cause of differences in δ³⁴S between stockwork and massive sulphide mineralizations. Finally, current geodynamic models suggested for the IPB are discussed. It is suggested that an intracontinental, ensialic rift or pull-apart environment is the most probable genetic environment for the IPB mineralizations.     

Sea water basalt interaction in spilites from the Iberian Pyrite Belt

Low grade hydrothermally metamorphosed mafic rocks from the Iberian Pyrite Belt are enriched in ¹⁸O relative to the oxygen isotopic ratio of fresh basalt (+6.5±1). The observed ¹⁸O whole rock values range from +0.87 to +15.71 corresponding to positive isotopic shifts of +5 to +10, thus requiring isotopic exchange with fluids under conditions of high water:rock ratios at low temperatures. The lowest ¹⁸O observed corresponds to an albitized dolerite still and is compatible with independent geochemical data suggesting lower water: rock ratios for the alteration of these rocks.The isotope data are consistent with the hypothesis that the spilites from the Pyrite Belt were produced by interaction of basaltic material with sea water.Significant leaching of transition metals from the mafic rocks during alteration coupled with available sulphur isotopic data for the sulphide ores also suggest that sea water may have played an important role in the formation of ore deposits in the Iberian Pyrite Belt.     

Blue amphiboles,metamorphic regime and plate tectonic modelling in the Iberian Pyrite Belt

Riebeckite-arfvedsonite amphiboles occur in very low-grade metamorphosed doleritic sills at various localities within the Iberian Pyrite Belt. The alkali-amphiboles grew during sub-solidus hydrothermal spilitization of basalt associated with submarine massive sulphide ore formation. The riebeckite-arfvedsonite is only very rarely preserved, being converted to albite-chlorite during regional metamorphism. In the South Portuguese zone Hercynian regional metamorphic grade increases in a northward direction from zeolite facies south of the Pyrite Belt through prehnite-pumpellyite facies to the greenschist facies in its northernmost zone. Compositional and mineralogical data indicate a geothermal gradient in the order of 40–50 °C/km.Volcanism in the Pyrite Belt is essentially representative of a bimodal association of twoleiitic to alkalic basalt and dacite/rhyolite. Geochemical data for the Pyrite Belt mafic meta-volcanics contrast with available data for subduction related volcanic suites in orogenic belts but exhibit similarities with the basaltic members of basalt-rhyolite associations found in areas of extensional tectonics. It is proposed that the Iberian Pyrite Belt volcanism represents magmatic activity in an intra-continental basin undergoing rifting during the late Devonian and lower Carboniferous times. On leave from: Mineralogia e Geologia, Faculdade de Ciencias, Lisboa-2, Portugal     

A study of inherited zircons in granitoid rocks from the South Portuguese and Ossa-Morena Zones, Iberian Massif: support for the exotic origin of the South Portuguese Zone

Trace element and U–Pb isotopic analyses of inherited zircon cores from a sample of Gil Márquez granodiorite (South Portuguese Zone, SPZ) and Almonaster nebulite (Ossa-Morena Zone, OMZ, in the Aracena Metamorphic Belt) have been obtained using laser ablation-inductively coupled plasma-mass spectrometry. These data reveal differences in the age of deep continental crust in these two zones. Inherited zircon cores from the Ossa-Morena Zone range at 600±100 Ma, 1.7–2 Ga and 2.65–2.95 Ga, while those from the South Portuguese Zone range at 400–500 and 700–800 Ma. These data support the “exotic” origin of the South Portuguese Zone basement relative to the rest of Iberian Massif. The young ages of inherited zircon cores and Nd model ages of magmatic rocks of the South Portuguese Zone are comparable to results from granulite facies xenoliths and granitic rocks from the Meguma Terrane and Avalonia and support a correlation between the basement of the southernmost part of the Iberian Massif and the northern Appalachians.     

U–Pb Geochronology of VMS mineralization in the Iberian Pyrite Belt

A geochronology study using U-Pb isotope dilution TIMS analyses of zircon has been conducted to determine the ages of volcanic-associated massive sulfide (VMS) deposits in the Iberian Pyrite Belt (IPB), the world's most prolific VMS province. Ages have been determined for host rocks to four VMS systems that span the IPB: the giant Rio Tinto and Aljustrel districts in the central region, Lagoa Salgada to the west, and Las Cruces to the east. A sample of chloritized quartz porphyritic dacite/rhyolite in the footwall of the San Dionisio massive sulfide deposit of the Rio Tinto district is 349.76ǂ.90 Ma. This is taken as the best age estimate of the mineralization in the Rio Tinto district, probably the world's largest volcanogenic massive sulfide system. Two xenocrystic zircons from the same sample yielded ²⁰⁷Pb/²⁰⁶Pb ages of 414 and 416 Ma, which provide a minimum estimate for the age of the inherited component. A biotite tonalite from the Campofrio area, 3.5 km north of the center of the Rio Tinto district, is chemically similar to the felsic host rock protolith at Rio Tinto. The Campofrio sample has an age of 346.26ǂ.81 Ma, slightly younger and outside of the 2C error for the Rio Tinto age; therefore, this phase of this intrusion was not a heat source for the hydrothermal system that formed the deposits of the Rio Tinto district. The Campofrio sample also has three zircon analyses with ²⁰⁷Pb/²⁰⁶Pb minimum ages of 534, 536, and 985 Ma, indicating inheritance from Ordovician and Neoproterozoic sources. In the Aljustrel VMS district, a U-Pb zircon age of 352.9ǃ.9 Ma characterizes the altered Green Tuff host rock of the Algares deposit, which is slightly older than the Rio Tinto age. Two zircons with ²⁰⁷Pb/²⁰⁶Pb ages of 531 and 571 Ma from this sample indicate inheritance from a Cambrian or older source. The age of mineralization at Lagoa Salgada is given by essentially identical ages of 356.21ǂ.73 and 356.4ǂ.8 Ma, for footwall and hanging wall samples, respectively. The hanging wall sample has two zircon analyses with ²⁰⁷Pb/²⁰⁶Pb ages of 464 and 466 Ma, indicating inheritance from an Ordovician or older source. The age for an altered dacite tuff sample from the hanging wall of the Las Cruces deposit is 353.97ǂ.69 Ma. One zircon analysis from the Las Cruces sample has a ²⁰⁷Pb/²⁰⁶Pb age of 1048 Ma, indicating inheritance from a Neoproterozoic source. These U-Pb ages refine the IPB geochronology provided by previous studies, and they suggest that either volcanism progressed toward the center of the IPB, or that volcanism was broadly static and the strata were progressively rifted to the margins during transtensional basin formation. The zircon inheritance provides direct evidence for Proterozoic to Ordovician sources, reflecting either basement rocks beneath the Phyllite-Quartzite Group during VMS formation in late Tournaisian times, or a Proterozoic to Ordovician detrital component in Phyllite-Quartzite Group source rocks. The presence of an older crustal component is consistent with VMS formation during rift development at a continental margin.     

Convergence in a thermally softened thick crust: Variscan intracontinental tectonics in Iberian plate rocks

The subduction phase in the development of the Variscan Orogen in SW Europe was followed by an extended period of ‘intracontinental’ tectonics. The progressive temperature rise in the hinterland during plate convergence was accompanied by widespread partial melting in the lower crust and the nucleation of kilometric buckle folds and crustal‐scale shear zones in the stronger upper crust. Thermal mechanical weakening in the core of the orogen was contemporaneous with shortening and thickening in the foreland fold‐and‐thrust belt. We evaluate lithospheric strength profiles in the hinterland and foreland based on the metamorphic and structural record for three tectonic stages. We find that lower crustal strength varied in space as well as in time during orogenesis. Strength contrasts between the foreland and the hot hinterland during convergence may have led to the additional indentation of the foreland into the hinterland of the Ibero‐Armorican Arc.     

Polyphase tectonics and late Variscan extension in Austria (Moldanubian Zone,Strudengau area)

New data suggest syn-convergent extrusion and polyphase tectonics followed by late Variscan extension in the Strudengau area of the southern Moldanubian zone in Austria. The tectonic history can be summarized as follows: (1) The oldest ductile event is observed in HT/LP metamorphic pelitic gneisses, which preserve E-dipping foliation planes (D₁-fabric) with NW–SE-trending lineations. (2) The overlying gneisses record HT/HP conditions with decompression-induced anatexis in the central part of the domain. These gneisses exhibit N–S trending, horizontal lineations along steep-dipping foliation planes (D₂-fabric) crosscutting the D₁-fabric of the pelitic gneisses. Along the margin, these rocks have been strongly mylonitized under amphibolite facies conditions (D₂). D₂ is interpreted as a significant vertical shear zone, which juxtaposes the HT/LP rocks against the orogenic lower crust. (3) Lastly, the whole area is overprinted by localized shear zones (D₃-fabric) with top-to-the-NW kinematics. This newly discovered Strudengau shearing event is associated with isoclinal folding that possesses axial planes parallel to the mylonitic foliation and fold axes parallel to the stretching lineations. Initial mylonitization occurred under greenschist facies, representing the latest ductile event of the Strudengau area. The new geochronological data presented here indicate a narrow time frame (c. 323–318 Ma) for the D₃ deformation. Therefore, this event is contemporaneous with the intrusion of the granites of the South Bohemian Batholith (330–310 Ma). The nearby South Bohemian Batholith and generally steep dyke swarms in the Strudengau area and to the north trend in a NE–SW preferred orientation, interpreted to be D₃-synkinematic magmatism. In a regional context, the NW–SE stretching during D₃ together with the synkinematic intrusion of dykes is associated with late orogenic extension in the Austrian Moldanubian Zone. Kinematic data of brittle normal faults and tension gashes are consistent with NW–SE-oriented extension under cooler conditions.     

Ore deposit types and tectonic evolution of the Iberian Pyrite Belt: From transtensional basins and magmatism to transpression and inversion tectonics

A new interpretation of the structural evolution of the Iberian Pyrite Belt (IPB) and volcanogenic massive sulfide mineralization (VMS) is presented in this work, based on a review of the ore deposit types, the analysis of the hosting volcanic sequences and the tectonic evolution. The VMS deposits of the IPB are hosted by volcanic and siliciclastic rocks. Four main volcano-sedimentary sequences (VSC), from VSC₀ to VSC_3, have been assumed, the main deposits being located in the VSC₀ and at the top of the VSC₂.We have defined three main sectors oriented approximately E-W and hosting the VMS deposits. In the Northern sector, which is mostly located in Spain, graben basins and local pull-aparts are the main structures. In this sector, two belts can be distinguished, the deposits being located at the top of the VSC₂ felsic volcanism (Rio Tinto-type IPB deposits). In the Central sector, both in Spain and Portugal, half-graben basins are the most common structures, and the deposits are mostly located in the VSC₀ andesitic volcanic-sedimentary sequence (Tharsis-type IPB deposits). In the Southern sector, which is only located in Portugal, a graben basin with a pull-apart is again the main structure, and the deposits are located in black slates and at the top of a felsic volcanism, Strunian in age (VSC₀). The deposits located in graben basin with a pull-apart are essentially felsic volcanic-hosted with some siliciclastic material, mostly black shales. By contrast, those located in half-graben basins are mainly hosted by black-shales with minor amounts of andesitic rocks.The tectonic evolution shows that as a result of a counterclockwise rotation of the stress axes, the formation of the IPB and the associated ore deposits took place during several episodes, from transtension (with the development of both graben with pull-aparts and half-graben basins), through left lateral E-W shearing, to transpression. At the beginning of the transtensional process, several extensional, roughly E-W trending faults that developed graben and half-graben basins were generated and the first volcanic andesite-rhyolite rocks (VSC₀) formed. The Tharsis-type deposits, mainly hosted by black slates with some volcanic rocks, were formed in the Central sector while the Neves Corvo-type deposit, hosted by black slates and felsic volcanism formed in the Southern one. After a period characterized by barren mafic volcanism (VSC₁), a sinistral shear affected the previous fractures due to the stress axis rotation and felsic crustal volcanism started (VSC₂). Rhyolites and dacites were particularly abundant in two graben basins, which developed rollovers in pull-apart zones, forming the Rio Tinto-type deposits in the Northern sector. The thermal increase associated with VSC₀ and VSC₂ gave rise to the development of crustal-scale hydrothermal convective cells, which generated both types of deposits.After a barren VSC₃ felsic volcanism, subsequently, during the Variscan transpressional phase, the E-W extensional faults were reactivated as reverse faults, affecting the volcanic sequence (VSC₀ to VSC₃) as well as the interbedded sedimentary rocks (mostly black shales). As has been recognized at the Rio Tinto deposit, buttressing must have played a significant role in the geometry of inverted structures, and the VMS ores were intensely recrystallized.It should be emphasized that this new regional geological model for the IPB is an approach to provide a better insight into VMS deposits and could be a key-point for further studies, providing a new tool to improve knowledge of the VMS mineralizations and exploration guidelines elsewhere in the IPB.     

Magmatism in the Iberian Pyrite Belt: petrological constraints on a metallogenic model

Igneous formations associated with massive sulphide deposits in the Iberian Pyrite Belt (IPB) are essentially composed of basic lavas and dolerites, and dacitic to rhyolitic volcanites; intermediate lavas are subordinate. The basic rocks show variable geochemical characteristics: lavas and dolerites comparable to recent within-plate alkaline basalts seem restricted to the western and southern parts of the IPB, whereas basic rocks comparable to continental tholeiites or arc-related basalts occur across the whole belt. The felsic rocks are classified as calc-alkaline and belong to the “low-Al₂O₃ and high-Yb type”. At given SiO₂, Al₂O₃ and TiO₂ contents, they show variable Zr, Nb, and HREE contents. Heavy-rare-earth element fractionation decreases from the dacites to the rhyolites ([Gd/Yb]_N ∼ 1), whereas the negative Eu-anomaly becomes more pronounced. The characteristics of the rhyolites are typical of sulphide-fertile volcanic packages. Trace-element modelling suggests that the felsic rocks evolved from a dacitic parent magma through fractional crystallization of hornblende and plagioclase. Partial melting of an amphibolite protolith, which appears as the most probable model for the origin of this dacitic magma, requires a high T/P gradient in the crust. The occurrence of alkaline basalts and continental tholeiites is consistent with formation of the IPB in a tensional tectonic setting. However, the associated island-arc tholeiites suggest a location in a domain of plate convergence. Emplacement in a fore-arc basin over a recently accreted crustal segment is envisaged as a possible hypothesis to account for the geological and petrological constraints. A high geothermal gradient and eruption in a submarine tensional basin could have been two key ingredients for the development of massive sulphide deposits within the IPB.

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Resumen (translated by E. Pascual) Las formaciones ígneas asociadas con los depósitos de sulfuros masivos de la Faja Pirítica Ibérica (IPB) se componen esencialmente de doleritas y lavas básicas y de rocas volcánicas dacíticas a riolíticas; las lavas intermedias son poco abundantes. Las rocas básicas muestran caracteres geoquímicos variables: lavas y doleritas comparables a basaltos alcalinos intraplaca recientes parecen hasta ahora restringidos a las partes W y S de la IPB, mientras que rocas básicas de caracteres comparables a las de toleítas continentales o basaltos relacionados con arcos aparecen en toda la zona. Las rocas ácidas se clasifican como calcoalcalinas del tipo “low-Al₂O₃, high-Yb”. Para un contenido dado en SiO₂, Al₂O₃ y TiO₂, muestran contenidos variables en Zr, Nb y REE. El contenido en tierras raras pesadas decrece de dacitas a riolitas ([Gd/Yb]_N ∼ 1), al tiempo que la anomalía de Eu se hace más pronunciada. La modelización de elementos trazas sugiere que las rocas evolucionaron a partir de un magma parental dacítico mediante cristalización fraccionada de hornblenda y plagioclasa. La fusión parcial de un protolito anfibolítico, que parece el modelo más plausible para el origen del magma dacítico, requiere un elevado gradiente T/P en la corteza. La existencia de basaltos alcalinos y de toleítas continentales es congruente con la formación de la IPB en un entorno tectónico distensivo. El emplazamiento en una cuenca “fore-arc”, en un segmento cortical de reciente acreción, se contempla como una hipótesis posible para explicar los caracteres geológicos y petrológicos. Un alto gradiente térmico, junto con la erupción en una cuenca submarina extensional, pueden haber sido los dos ingredientes clave en el desarrollo de los depósitos de sulfuros masivos en la IPB.

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Received: 3 March 1996 / Accepted: 7 April 1997     

Structural study of a semiductile strike-slip system in the Central Iberian Zone (Variscan Fold Belt,Spain): structural controls on gold deposits

The Villalcampo shear system is a regional dextral strike-slip fault zone that affects Late Variscan granites and their metamorphic country rocks over an area of about 150 km². The detailed geometry of this subvertical north-west—south-east shear zone is outlined. The system forms an extensional fan to the northwest and extends to the south-east as a broad extensional duplex. Particular attention is focused on the distribution of fault rocks and associated veins in its north-west splay. A structural study of the shear bands (encompassing both geometric and kinematic criteria) and a microscopic study of the fault rocks has led to the interpretation of the system as a brittle—ductile shear zone. Calculations give a shear strain value of = 1.5 and a minimum displacement of s = 3700 m. The localization of gold mineralization in mylonite-filled subvertical extensional veins is a product of the formation of the Villalcampo shear system. The subvertical faults and veins underwent a process of cyclical sealing and reopening. As such they acted as valves controlled by fluid pressure regulating fluid—rock interactions and gold deposition. Conditions favouring these processes occur near the base of the seismogenic zone in the vicinity of the frictional—quasi-plastic transition at mid-greenschist metamorphic conditions (T = 350°C and 10–15 km depth).     

Lead isotope systematics of the giant massive sulphide deposits in the Iberian Pyrite Belt

Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (²⁰⁶Pb/²⁰⁴Pb), 15.622 (²⁰⁷Pb/²⁰⁴Pb) and 38.191 (²⁰⁸Pb/²⁰⁴Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (²⁰⁶Pb/²⁰⁴Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.     

Geochemistry and geothermometry of volcanic rocks from Serra Branca,Iberian Pyrite Belt,Portugal

Volcanic rocks from Serra Branca, Iberian Pyrite Belt, Portugal, consist of calc-alkaline felsic and intermediate rocks. The latter are massive andesites, whereas the former include four dacitic to rhyolitic lithologies, distinguishable on spiderdiagrams and binary plots of immobile elements. Zircon thermometry indicates that two felsic suites may have formed from different magmas produced at distinct temperatures, with only limited fractionation within each suite. Alternatively, all the felsic rocks can be related through fractionation of a single magma if the lower zircon saturation temperature obtained for one suite merely results from Zr dilution, mostly reflecting silicification.The relatively high magma temperatures at Serra Branca ease the classification of felsic rocks based on their HFSE contents and also indicate volcanogenic massive sulfide deposit favorability. This contrasts with other areas of the Belt that register lower magma temperatures and are subsequently barren. However, magma temperatures may have not been high enough to cause complete melting of refractory phases in which HFSE reside during crustal fusion of an amphibolite protolith, implying difficult discrimination of tectonic environments for the felsic rocks. The intermediate rocks were possibly formed by mixing between basaltic magmas and crustal material, compatible with volcanism in an attenuated continental lithosphere setting.