Bismuth and cobalt minerals as indicators of stringer zones to massive sulphide deposits, Iberian Pyrite Belt

The stringer zones and commonly the interaction zone at the base of the massive sulphide mounds in the Iberian Pyrite Belt contain bismuth and cobalt minerals that are not found in the overlying massive sulphides. These are fairly rare cobalt sulphoarsenides (cobaltite, alloclasite, galucodot) that were formed at the beginning of the massive sulphide genesis, and fairly common bismuth sulphides (bismuthinite, hammarite, wittichenite, cosalite, kobellite, joseite, etc.), including species rare at world scale (nuffieldite, giessenite, jaskolskiite) that were deposited from last stage high-temperature (> 300 °C) copper-bearing fluids containing Bi (Te, Se). The last stage fluids precipitated chalcopyrite containing Cu, Bi, Te, (Se) sulphosalts at the base of the sulphide mound to form a high cupriferous zone. Their interaction with the massive sulphides is reflected by the formation of an exchange zone, a few metres thick, showing chalcopyrite disease textures, at the base of the mound; this zone forms the upper limit of potentially economic copper enrichment and of bismuth minerals. Gold is undoubtedly in part, if not totally, related to this last phase. The bismuth concentrations being equivalent in the massive sulphides and the stringers, the presence of bismuth minerals in the stringer zones results from high-temperature conditions combined with a rarity of galena, which impedes absorption of available Bi. The distribution of these bismuth minerals provides a basic mineralogical zoning in the stringer zone, with a deep, low-aS₂ zone containing native bismuth and tellurides and a shallow, higher-aS₂ zone in contact with the massive ore sensu stricto and containing complex bismuth sulphides. These results make it possible to distinguish between sulphide veinlets belonging to stockwork zones of massive orebodies and veinlets of an ambiguous nature, and provide mineralogical criteria for the proximity of copper-rich zones. They enrich the very complex mineralogy of the Iberian Pyrite Belt.     

Héritage et sources des métaux d'après la géochimie isotopique du plomb

Résumé Les galènes et sulfoantimoniures de plomb des filons quartzeux, barytiques et fluorés du Haut-Allier (district de Brioude-Massiac) ont fait l'objet d'analyses isotopiques du plomb. Les résultats montrent l'existence de deux populations de plomb appartenant à chacune des deux phases minéralisatrices reconnues dans la région.La première population (18,108<²⁰⁶Pb/²⁰⁴Pb<18,181) est rapportée à la phase mésothermale permienne. Elle comprend les sulfosels précoces des filons antimonifères ainsi que la galène des filons plombo-stannifères. L'homogénéité isotopique confirme la liaison génétique existant entre ces différents types filoniens et leur appartenance à un même processus métallogénique. Le socle métamorphique régional apparaît comme la source possible du plomb.La relative dispersion des valeurs isotopiques de la seconde population (18,215<²⁰⁶Pb/²⁰⁴Pb<18,472), d'âge liasique, est corrélable avec la situation géographique et géologique des filons. Elle est due à deux phénomènes complémentaires: le lessivage d'un segment crustal comprenant le socle métamorphique avec une participation graduée du granite uranifère de la Margeride et la réassimilation partielle du plomb des filons permiens. Ce dernier phénomène d'héritage métallique se traduit au niveau des paragenèses par la formation de filons à sulfosels dominants.

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Lead isotope analyses are reported for galena and lead sulfosalts from quartz, barite and fluorite vein deposits from the Haut-Allier (Brioude-Massiac district). The data define two lead populations reflecting the two mineralization episodes in this region.The first population (18,108<²⁰⁶Pb/²⁰⁴Pb<18,181) is related to the Permian mesothermal episode. It includes early sulfosalts from antimony veins and galena from lead-stanniferous veins. The Pb isotopic homogeneity confirms the relationship between the different vein types and their formation by a single metallogenic process. The metamorphic basement is the potential source of lead.The second population includes quartz-barite and fluorite veins. It shows much more isotopic variation (18,215<²⁰⁶Pb/²⁰⁴Pb<18,472) correlating with the geographical and geological location of the veins. Because the veins have a contemporaneous age, the isotopic dispersion therefore depends on two complementary phenomena: the leaching of a large volume of crust, including metamorphic basement, with a progressive contribution of the uraniferous granite of Margeride; and the partial remobilization of lead from Permian veins. The latter inheritance phenomena result in the genesis of major lead-sulfosalt vein deposits.

     

First evidence of Campanian radiolarians in Turkey and implications for the tectonic setting of the Upper Antalya Nappes

The definition and inventory of the upper units of the Antalya Nappes or “Calcareous Antalya Nappes” (CAN) are still a matter of controversies and often conflicting interpretations. In the Gedeller type locality, we logged a new succession that sheds light on the detailed stratigraphy of the Upper Antalya Nappes. The lower part of the series corresponds to the uppermost part of the Kemer Gorge Nappe and is overthrust by the Ordovician Seydişehir Formation of the Tahtalı Dağ Nappe. The newly described Gedeller Formation belongs to the Kemer Gorge Nappe and is represented by Campanian (Upper Cretaceous) Scaglia-type pelagic limestones, which yielded radiolarians of the Amphipyndax pseudoconulus Zone. It is demonstrated that the “Calcareous Antalya Nappes” are composed of three different nappes, the Kemer Gorge, Bakırlı and the Tahtalı Dağ nappes, all of them belonging to the Upper Antalya Nappes system.     

Geometry and genesis of feeder zones of massive sulphide deposits: constraints from the Rio Tinto ore deposit (Spain)

Vein-related data have been collected around the giant Rio Tinto orebody in southern Spain within the root zones of the massive sulphide deposits. Here, we report the main results of this study, concerning the geometry of the stockwork and the conditions of formation. Although field and thin-section studies have shown that a wide range of vein configurations exist, from micro cracks (fluid-inclusion planes) to large paleo-flow channels, two groups seem to dominate. The first corresponds to small, constricted micro cracks and capillary-flow channels, now mainly filled with quartz, whereas the veins of the second group have large widths, are continuous over several meters and are filled with quartz and sulphides. Most are tension veins and only very few (<0.1%) show evidence of shearing. The pyrite-dominated variety (i.e., pyrite?>?quartz) tends to post-date the quartz-dominated veins (quartz?>?pyrite). The vein-thickness and -spacing distribution is modal rather than logarithmic, and their densities are not fractal, but are characterized by a Poisson distribution. From the immediate sub-surface zone to more than 100?m below the base of the massive sulphide deposits, most hydrothermal quartz-sulphide stockwork veins are sub-parallel to the base of the massive sulphide deposit. The assumption that the base of this deposit corresponds to a paleo-horizontal plane, implies that most veins were sub-horizontal. This is particularly evident for small veins, but the larger ones can be strongly oblique to the base of the deposit. The hydrothermal fluids that generated the massive sulphide deposits and underlying stockworks, were very saline and probably underwent sub- or super-critical phase separation in the root zones of the system. This phase separation was the probable mechanism producing the periodic over-pressures of at least 20 MPa that were necessary to generate the sub-horizontal veins of the stockworks.     

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.     

Lead-isotope evidence for a Hercynian origin of the Salsigne gold deposit (Southern Massif Central,France)

The Salsigne gold deposit contains a complex association of sulphide layers, gold-rich disseminations, quartz-bearing veins and flat reefs, which are hosted by folded and slightly metamorphosed Paleozoic sediments on both sides of a major thrust zone. It is demonstrated that these various Au-As ore types have similar lead-isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.22–18.56), which are characteristic of Hercynian ore deposits in the southern Massif Central. The Lower Cambrian to Devonian host rocks and associated Pb-Zn-Ba occurrences display distinctly less radiogenic corrected isotopic signatures (²⁰⁶Pb/²⁰⁴Pb = 17.83–17.98), which are characteristic of Cambrian lead in the Montagne-Noire. Concerning the controversial origin of the Salsigne gold mineralization, these results disagree with the former syngenetic hypothesis and support a new model of Hercynian syntectonic gold concentration.This work was supported by BRGM'S scientific program: Le gisement de Salsigne: caractérisation du modèle et évaluation du potentiel aurifère du district     

From sandstone- to carbonate-hosted stratabound deposits: an isotope study of galena in the Upper-Moulouya District (Morocco)

The Upper Moulouya district displays three main types of lead ores in the Hercynian domain: (1) Aouli vein-type deposits hosted in the Hercynian basement, (2) Zeïda Triassic sandstone-hosted, and (3) Mibladén carbonate-hosted stratabound and karstic mineralisations in the Mesozoic cover. Lead and sulfur isotope studies indicate that the Aouli vein-type and Zeïda sandstone-hosted stratabound mineralisations are formed by a mixing of two fluids, one formed by the leaching of the Aouli granite massif, and the other by leaching of the Saharian basement. These deposits may correspond to the same Triassic metallogenic event, focused along fault systems or within permeable sandstone. Deep fluids were mobilised during the early extensional movements associated with the opening of the Atlas rifting basin. The Mississippi Valley type Mibladén mineralisation is related to a distinct metallogenic event superimposed on the earlier one, and represents a remobilization of earlier concentrations, or a more recent leaching of the same sources, but with a more pronounced contribution of the local organic matter. The Upper Moulouya area demonstrates the close relationship between vein type and sandstone-hosted lead-zinc mineralisations, and their major differences with Mississippi Valley type deposits.     

Les événements de la limite Permien–Trias : derniers survivants et/ou premiers re-colonisateurs parmi les ostracodes du Taurus (Sud-Ouest de la Turquie)The events of the Permian–Trias boundary: last survivors and/or first colonisers among the ostracods of the Taurides (southwestern Turkey)

Very few data on Early Triassic marine ostracods are available. In most of cases, the data concern the upper part of the Early Triassic. The Çürük da? section, located in the Western Taurides (Turkey), well stratigraphically constrained by conodonts and foraminifera, has been re-sampled for ostracod study. A significant fauna has been discovered in the Late Permian and the levels of the Earliest Triassic (Hindeodus parvus and Isarcicella isarcica staeschei zones). The ostracods of the basis of the Kokarkuyu Fm. are the oldest Triassic forms ever discovered. The occurrence of Palaeocopes in the earliest Triassic and similarity between the Permian and Induan assemblages suggest that the Lower Triassic Çürük da? ostracods represent a survival assemblage after the Permo-Triassic mass-extinction. To cite this article: S. Crasquin-Soleau et al., C. R. Geoscience 334 (2002) 489–495.