Age determinations on lead ores

The isotopic abundances of about 150 samples of lead minerals from Africa and Europe have been measured. pb²⁰⁷/Pb²⁰⁶ model ages are calculated, the Pb²⁰⁸ content is used as a check for this model age. The usefulness and the limits of this common lead dating method are discussed.The results, including model ages higher than 3×10⁹ years are presented. It is pointed out that ages, determined by the common lead method, are in principle defined differently from ages obtained by other methods. In the case of a secondary mineral argon, lead, and strontium may be lost. Thus the Pb-Th-U, A-K, and Sr-Rb ages will give the time of the secondary formation in the idealized case. On the other hand, the common lead method principally gives the time of the first mineralization of the material involved. Thus, by means of the common lead method a geochemical important event, the first mineralization can be investigated, which is impossible by using the other methods.The common leads with anomalous isotopic composition similar to Joplin lead (J-type) are discussed. Criteria for recognizing anomalities are summarized and preliminary results in connection with these are given. Volcanism is recognized as one possible source of this anomaly, according to measurements of lead sublimates from Vesuvio and the Island of Volcano in the Mediterranean sea.

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Zusammenfassung Die Isotopenhäufigkeit von ungefähr 150 Bleimineral-Proben aus Afrika und Europa wurden untersucht.Pb²⁰⁷/Pb²⁰⁶-Modell-Alter wurden berechnet, der Pb²⁰⁸-Gehalt diente als Kontrolle dieses Modellalters. Brauchbarkeit und Grenzen der Methode werden diskutiert.Es wurden Modell-Alter bis zu mehr als 3×10⁹ Jahren gefunden. Modell-Alter von Bleimineralien sind prinzipiell anders definiert als solche, die mit anderen Methoden bestimmt werden. Im Falle eines Sekundärminerals können Argon, Blei und Strontium verloren gehen. Deshalb ergeben Pb-Th-U, A-K und Sr-Rb Alter, im idealisierten Fall, den Zeitpunkt der letzten Mineralisation. Die gewöhnliche Bleimethode ergibt das Alter der ersten Mineralisation des Materials.Bleie mit anomaler Isotopenzusammensetzung, ähnlich dem Joplin-Blei (J-Typen), werden diskutiert. Kriterien zur Identifikation der Anomalien sowie vorläufige Resultate werden aufgeführt. Auf Grund von Messungen an Bleisublimaten des Vesuves und der Insel Vulcano im Mittelmeer kann der Vulkanismus als eine mögliche Ursache dieser Anomalien betrachtet werden.

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Résumé On a mesuré la fréquence des isotopes dans environ 150 échantillons de minerai de plomb provenant d'Afrique et d'Europe. Les âges ont été calculés suivant la méthode du Pb²⁰⁷/Pb²⁰⁶; la teneur en Pb²⁰⁸ a servi de contrôle pour cette formule. Les auteurs discutent l'utilisation et les limites de la méthode. On trouve des âges de plus de 3×10⁹ années. Il est établi qu'avec des méthodes normales au plomb les âges obtenus sont différents de ceux obtenus par d'autres méthodes.Lors de déterminations d'âge sur des «minéraux secondaires» il a pu se produire une déperdition d'argon, de plomb et de strontium. La méthode ordinaire au plomb donne l'âge de la première minéralisation du matériau.L'auteur discute le cas du plomb ayant une composition isotopique anormale semblable à celle du plomb de Joplin.

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This paper has been read by one of us (J. G.) at the International Geological Congress in Mexico City 1956. We are informed only now that the publication of the proceedings of this congress had to be interrupted for technical reasons, as far as geochemistry is concerned. Though this paper has been distributed to a limited number of interested laboratories we take the opportunity to publish our paper in its original form in the Geologische Rundschau.     

New lead isotope determination from five Proterozoic sulfide deposits,Skellefte district,Sweden

Lead isotope analyses of 25 sulfide samples (galenas, iron sulfides, and sulfosalts) from five different mines of the Skellefte district, northern Sweden, demonstrate that the Pb-isotopic composition of galenas and other sulfides rich in lead varies between individual deposits within the district. This contrasts with many other base-metal districts, where ore lead is isotopically homogeneous on a regional basis. Although all of the Skellefte leads are depleted in ²⁰⁷Pb relative to average global lead evolution models, thus suggesting a large mantle-derived component in their sources, the Nasliden deposit lying at the contact of the host volcanic rocks and the overlying metasediments contains a significant component of crustal lead. It is concluded that while the Pb-isotope data are consistent with a volcanic exhalative origin of the ores of the Skellefte district, they also demonstrate that older crustal lead was incorporated into the sulfides during their emplacement and the subsequent period of magmatic and metamorphic activity which followed their deposition.     

Arsenopyrite and sphalerite as T-P indicators in sulfide ores from northern Sweden

The compositions of arsenopyrite and sphalerite from five Swedish metamorphosed Cu, Zn-sulfide deposits are related to T and P, according to the methods described by Kretschmar and Scott (1976) and Scott (1973). The As/S ratio of arsenopyrite indicates an equilibrium temperature around 400°C for all the deposits studied, whereas the sphalerite barometer shows pressures between 5 and 7 kb. The mineral assemblages of the bedrocks indicate a similar temperature but a lower pressure. A constant fs2 is probably only effective over distances of millimetres or a few centimetres in the samples studied. The fs2 has always been lower in the surrounding rocks than in the ores as indicated by a higher As/S in arsenopyrite, more FeS in sphalerites and the absence of pyrite.     

Lead isotope and Pb-Pb model age determinations of ores from Central Europe and their metallogenetic interpretation

The paper presents lead isotope data from 211 samples from Phanerozoic lead-bearing ore occurrences in Central Europe, particularly from the Southern part of the former German Democratic Republic. The data are interpreted in terms of Amov's dynamic model of continuous lead isotope evolution. The relationships between thoro-genic and urano-genic model ages and the source of lead in different regional units are discussed. We observed differences in lead isotope evolution in the Hercynian internides and externides. Within the Moldanubian and Saxothuringian zones we distinguish five main lead-bearing ore associations: (1) Cambrian, stratiform base metal (Hermsdorf-Waldsassen; ²⁰⁶Pb/ ²⁰⁴Pb=17.50–17.70), (2) Devonian, vein type Sb-bearing, metamorphogene (neumühle-Hartmannsdorf; 17.80–18.00), (3) Upper Carboniferous-Permian, polymetallic, including tin, vein type (Kutna hora-Freiberg-Altenberg; 18.00–18.20), (4) Triassic (-Jurassic), Pb–Ba, vein type (Stibro-Halsbrücke; 18.20–18.60), (5) Cenozoic, polymetallic, vein type, riftogene (Roztoky-Banska tiavnica; 18.80–19.10). Pb isotope characteristics from ores of the Montagne Noire and the Brioude-Massiac district correspond to this subdivision. Ore associations from the Rheno-Hercynian zone display higher ²⁰⁷Pb/²⁰⁴Pb ratios which can be explained by more evolved and less metamorphosed source rocks. Mineralizations of the eastern Harz (Straßberg-Neudorf) belong to the Permian association, those from the western Harz (Clausthal-Bad Grund) to the Triassic-Jurassic. Because of Pb isotope agreement the stratabound Rammelsberg and the vein bound Ramsbeck-I mineralization are presumed to be isogenetic. Pb isotope identity of distinct mineralizations in the basement zone (Halsbrücke-Bad Grund) and in the Triassic sediments (Gorny Slask-Mechernich-Bleiglanzbänke) suggests a strong genetic coherence. Pb isotope conformity between the Upper Carboniferous-Permian-Triassic ore associations and Hercynian postkinematic granitoids, and lamprophyric rocks, also favours a close relationship. Pb isotope and other data indicate crustal sources. As the age of the ore associations decreases, crustal influences generally increase, apart from the Roztoky mineralization.     

Petrology, chronology and isotope geochemistry of the proterozoic amphibolites from Xiangshan, central Jiangxi province, China

On the basis of a comprehensive study on the petrology,trace elements and isotopic geochemistry of the Xiangshan amphibolites,we suggest that the protoliths of the amphibolites were basalts formed in an island-arc tectonic setting.The basaltic magma was derived from a slightly depleted mantle source with a small amount of crustal contamination.Assemblage of the rock-froming minerals indicates that these amphibolites underwent a low-grade metamorphism of amphibolite facies.According to the formation age(1113Ma) and subsequent metamophic age(726.6Ma) of the basalts aw well as the geological and gochemical features of these amphibolites,a tectonic model of Proterozoic oceanic island-arc setting is proposed for central Jiangxi.     

PGE distribution in sulfide ores from ultramafic massifs of the central East Sayan Mountains,Southern Siberia,Russia

Data on the composition of sulfide ores from ultramafic massifs in the central East Sayan Mountains and on the regularities of platinum group elements (PGE) in these ores are presented. It is found that the highest PGE contents are characteristic for net-textured and massive ores from the Zhelos massif: total PGE content there is up to 15 ppm, with Pd/Pt = 3–8, for Ni and Cu contents of 1.5–2.8 and 0.5–2.7 wt%, respectively. In the disseminated ores of the Zhelos massif, PGE contents vary from 1 to 7 ppm, at Ni and Cu contents varying in the ranges of 0.5–1.0 and 0.2–0.4 wt %, respectively. In the Tokty-Oi massif, disseminated ores are characterized by higher absolute PGE contents (1.6 to 3.3 ppm) at similar Ni content. PGE tenor of disseminated ores is higher compared to that of massive and net-textured ones. In the cross-sections of both massifs, net-textured and massive ores of an essentially pyrrhotine composition are found at the contact between ultramafic and host rocks. Total PGE in these ores is up to 12 ppm. The obtained data on sulfur isotopes indicate the common, well-homogenized sources, and close physical–chemical depositional conditions of all ore types.     

A model for the formation of the iron,manganese and sulphide ores of central Sweden

The non-apatitic iron ores, manganese ores and sulphide ores of central Sweden, which occur in a volcano-sedimentary complex of Early Proterozoic age, have a common volcanogenic origin. The close association between specific ore types and metavolcanics suggests a stratigraphic control of the ores on a regional scale. Deposited in basins, the ores formed with increasing water depth (increasing pH and decreasing Eh) in the following order: quartz-banded iron ores mostly lacking sulphides; non-man-ganiferous skarn iron ores with Cu-Fe sulphides; manganiferous skarn iron ores with Zn-Pb sulphides, and Fe²+-Mn²+ silicates (eulysites). The latter two ore types occur adjacent to argillites/greywackes indicating a reducing environment. The manganese oxide ores of the Långban type represent a formation intermediate between the quartzbanded iron ores and the manganiferous skarn iron ores.The main structural pattern of the ore-bearing district is made up of NE-NNW trending isoclinal folds, in which the non-manganiferous skarn iron ores occur in antiforms, in contrast to the quartz-banded iron ores, the manganiferous skarn iron ores and the eulysites which occur in synforms. The sulphide ores often occur close to the iron-bearing horizons. Associated magnesia-rich alterations, paracontemporaneous with the volcanism, follow the iron ore-bearing horizons within the synforms.

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Zusammenfassung Die zentralschwedischen Eisen-, Mangan- und Sulfidlagerstätten, die in einem vulkanisch-sedimentären Komplex Früh-Proterozoischen Alters liegen, haben einen gemeinsamen vulkanogenen Ursprung. Die nahe Assoziation zwischen bestimmten Erztypen und Meta-Vulkaniten spricht für eine stratigraphische Anlegung der Erze im regionalen Sinne. Abgesetz in Becken, erfolgte die Bildung der Erze mit zunehmender Wassertiefe (steigendes pH und abnehmendes Eh) in der folgenden Ordnung: quarzgebänderte Eisenerze häufig ohne Sulfide, manganfreie Skarneisenerze mit Cu-Fe-Sulfiden, manganführende Skarneisenerze mit Zn-Pb-Sulfiden sowie Fe²+-Mn²+-Silicate (Eulysite). Die letztgenannten Erztypen treten in Nähe von Meta-Argilliten und Meta-Grauwacken auf, was ein reduzierendes Milieu andeutet. Die Manganoxiderze vom Typus Långban stellen eine Bildung zwischen den quarzgebänderten Eisenerzen und den manganführenden Skarneisenerzen dar.Die hauptsächlichen Strukturen der erzführenden Provinz sind NO-NNW isoklinale Falten, in welchen sich die manganfreien Skarneisenerze in den Antiformen befinden und die quarzgebänderten Eisenerze, die manganführende Skarneisenerze und die Eulysite in den Synformen. Die Sulfiden finden sich oft nahe zu den Horizonten mit Eisenerzen. Assozierte magnesiumreiche Umwandlungen, die zu dem Vulkanismus gehören, folgen den eisenführenden Horizonten in den Synformen.

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Résumé Les minerais de fer, de manganèse et de sulfures, exempts d'apatite, de la Suède centrale, qui se présentent dans un complexe volcano-sédimentaire d'âge protérozoïque ancien, ont une origine volcanique commune. L'association étroite entre certains types de minerai et les méta-volcanites, indique un contrôle stratigraphique à l'échelle régionale. Déposés dans des bassins, les minerais se sont formés sous une profondeur d'eau croissante (pH augmentant et Eh diminuant) dans l'ordre suivant: minerais de fer siliceux-rubanés le plus souvent sans sulfures, minerais de fer skarnique non manganésifères avec sulfures de Cu et Fe, minerais de fer en skarn, manganésifères avec des sulfures de Zn et Pb, et finalement des silicates à base de Fe²⁺–Mn²⁺ (eulysites). Les deux derniers types sont à proximité des méta-argillites et méta-grauwackes, indiquant un milieu réducteur. Les minerais d'oxydes manganeux du type Långban représentent une formation intermédiaire entre les minerais de fer siliceux-rubanés et les minerais de fer en skarns manganésifères.Les structures principales de la région métallifère consistent en plis isoclinaux NE-NNW, les minerais de fer en skarns sont dans les antiformes, et les minerais de fer siliceux-rubanés, les minerais de fer en skarns manganésifères et les eulysites dans les synformes. Les sulfures se trouvent à proximité des horizons ferrifères. Les transformations associées riches en magnesium, qui sont liées à l'activité volcanique, se produisent suivant les horizons ferrifères dans les synformes.

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Genesis of proterozoic iron-formation: sulphur isotope evidence

Seven units of carbonaceous shale or sulphide-facies iron-formation have been sampled. They are associated with Proterozoic iron-formations that range in age from ~ 1.9 to ~2.5 Ga: Sokoman and Gunflint (Canada), Riverton (United States), Penge (South Africa) and Brockman (Australia). Sulphur isotope ratios have been determined on the sulphides removed from these shales by both physical and chemical means.The mean δ³⁴S composition of the seven units varies between ?4.9%. and +6.6%. and the sample variance is low within each unit. These distributions are more characteristic of hydrothermal sulphide than sulphide produced by biogenic reduction. This hydrothermal sulphide is believed to have originated from high temperature reduction of seawater sulphate and from magmatic sulphide. A model is suggested whereby this sulphide was exhaled into stratified anoxic/oxic basins. The sulphide and associated base metals were deposited in the reduced sediments beneath the anoxic waters, while some iron and manganese was deposited on oxygenated shelves.The data support, but do not prove, a hydrothermal exhalative origin for lower Proterozoic iron-formation.     

Marine conditions in central Sweden during the early Preboreal as inferred from a stable oxygen isotope gradient

The marine benthic fauna and the δ¹⁸O_c of foraminifers and ostracods from six sites situated on a west–east transect through central Sweden have been analysed in order to estimate the palaeosalinity and palaeocirculation in this shallow‐marine environment. The measurements have been undertaken on material from the early Preboreal, when the Baltic Basin was in contact with the North Sea through straits in central Sweden. The δ¹⁸O_c values have a more negative value towards the east, indicating decreasing salinity. This was the result of limited possibilities for marine water to penetrate into the Baltic Basin and the mixing with freshwater from the melting Fennoscandian ice‐sheet. Four water masses existed in the area: a surface layer of freshwater, marine water from the North Sea, brackish–marine intermediate water on the Swedish west coast and brackish Yoldia Sea water in the Baltic Basin. The chronology is based on radiocarbon dates of marine fossils and, at one site, on the occurrence of the Vedde Ash (10 400–10 300 ¹⁴C yr BP). This is the first record from marine settings in Sweden. Copyright © 2001 John Wiley & Sons, Ltd.     

Distribution of silver in massive sulfide ores

The distribution of silver in connection with the formation of massive sulfide ores is discussed. It is proposed: 1) The affinity between silver and galena-rich ores is only pronounced if antimony and/or bismuth are also present in significant amounts, and 2) The degree of correlation between silver and lead is partly a function of the Ag/Sb+Bi ratio in the ore and increasing with decrease in ratio at least up to Ag/Sb+Bi=1. Observations of correlated Cu/Ag ratios in complex ores indicate that some silver was solved in chalcopyrite at the time of deposition. This is often masked by the much higher silver contents associated with galena. The Bi/Sb ratio is expected to increase downwards stratigraphically in galena-rich ores, due to a larger solubility of silver-bismuth in galena as compared with silver-antimony, at a realistic deposition temperature (200–300 °C). However, such an effect will only rarely be seen, due to incomplete separation of the ore-constituents at the time of deposition, as well as to post-depositional changes.     

Specific-lon electrode determinations of sulfide preconcentrated from San Francisco Bay waters

Measurements of low-level dissolved-sulfide concentrations in estuarine water from San Francisco Bay have been made using the sulfide-specific electrode after preservation, separation, and preconcentration of the sulfide species. The separation and preconcentration were acheived by coprecipitation of ZnS with Zn(OH)₂ followed by collection and dissolution of the precipitate, giving concentration factors up to 160-fold Preconcentration provided sulfide solutions that were adequately measurable within the practical working range of the specific-ion electrode The sulfide detection limit with the preconcentration step is 0 02 μg/l Spike recoveries in the range of 81 to 10 1% have been achieved for laboratory-prepared samples having S²⁻ concentrations as low as 0 6 μg/l and 84 to 100% for an estuarine sample spiked in the field with 2 μg/l (S(−II) Positive correlations have been found between dissolved S(−II) concentrations and concentrations of dissolved Cd, Cu, and Ni, negative correlations have been found between bisulfide (HS⁻) activity and activities of Cd²⁺, Cu²⁺, and Ag⁺ species     

Metal sources in the Early Proterozoic Svecofennian terrain of central Sweden: Pb isotope evidence

The lead isotope composition of three Cu-Zn-Pb sulfide deposits and one Au deposit in the Early Proterozoic Svecofennian of central Sweden reveals the existence of two distinct lead sources in the region. Rockliden and Enstern contain lead of upper crustal affinity and are isotopically homogeneous. Lead in these deposits probably originates from Archean-derived detritus which was well homogenized by sedimentary processes and deposited over large areas as a component in Early Proterozoic sedimentary and tuffaceous rocks. Lead isotopic homogeneity within the Tjärnberget and Enåsen deposits is less pronounced, and the less radiogenic character of the lead suggests a source of mantle affinity. In areas where metasedimentary rocks are absent or contain very little Archean-derived material the isotopic signature of mantle-derived volcanic rocks is reflected in the ore lead. This approach is also applicable to other Svecofennian deposits. Together with deposits in west-central Finland the Rockliden deposit constitutes the northern boundary for lead of upper-crustal affinity, i.e., Archean-derived lead, in Svecofennian sulfide deposits.Despite these differences the isotope ratios of the four deposits indicate that they formed roughly contemporaneously with other Svecofennian deposits in Sweden and Finland.     

Metallogeny and lead isotopic composition of base metal sulfide deposits in the Rappen area,northern Sweden

Early Proterozoic volcanic and sedimentary rocks of the Rappen district in northern Sweden were deposited at a destructive plate margin to the south of the Archaean craton of the western Baltic Shield. The volcano-sedimentary suite was intruded by two generations of early Proterozoic granites at ca. 1.89–1.85 Ga and ca.1.82–1.78 Ga, respectively, and metamorphosed at upper amphibolite facies conditions. Small stratabound iron, copper, and zinc deposits occur in felsic to mafic tuffs and arkosic sediments. Small deposits of molybdenum, tungsten, and uranium formed during the emplacement of the younger granites. The lead isotopic compositions of sulfide trace lead from the various deposits are highly heterogeneous. In the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram they fall on mixing arrays between little evolved early Proterozoic lead and highly radiogenic Caledonian lead. The least radiogenic lead isotopic compositions from the various deposits have a wide range of ²⁰⁷Pb/²⁰⁴Pb ratios and thus indicate variable involvement of Archaean crustal lead in the Proterozoic deposits. Deposits hosted by siliciclastic rocks have higher ²⁰⁷Pb/²⁰⁴Pb ratios than deposits hosted in mafic to felsic tuffites. The lead isotopic heterogeneity suggests that the lead in the various deposits was locally derived and, furthermore, that the sedimentary rocks in part originated from the Archaean craton to the north. Lead mixing arrays in the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram demonstrate that in Paleozoic time radiogenic lead was mobilized and transported in the basement. Source ages calculated from the mixing arrays (ca.1.9 Ga and ca.1.8 Ga) correspond to the age of the Early Proterozoic volcanism and metamorphism respectively. One group of deposits includes lead from at least three sources and illustrates that radiogenic lead was multiply mobilized and transported in the Proterozoic basement. It occurs in deposits that occur in zones that became permeable during the reactivations of the basement.     

Interpretation of lead isotope data from the uraniferous Cu-Fe-sulfide mineralizations in the Proterozoic greenstone belt at Kopparåsen,northern Sweden

The polymetamorphic Early Proterozoic basic metavolcanites of the Kopparåsen greenstone belt, northern Sweden, contain U mineralizations which are confined to Cu-Fe-sulfide mineralizations in mylonitized zones within basic metatuffs and graphite-bearing mica schists. Trace-lead isotope data from sulfides indicate contamination of the sulfide lead with two different lead components at ca. 430 Ma. One lead component was leached from rocks with a ²³²Th/²³⁸U ratio of 3.0–3.2, while the other lead component had evolved in a environment with a lower ²³²Th/²³⁸U ratio (0.0–0.05). The source for this latter lead component underwent a U-Th separation, probably in relation to the formation of the U mineralizations. If this lead component was leached from the U mineralizations, these mineralizations have a ²⁰⁷Pb/²⁰⁶Pb model age of ca. 1,780 Ma, which is less than the least radiogenic lead model age of the supracrustal belt (ca. 2,050 Ma). A possible genetic model for the uranium mineralizations includes the transport of U with an oxidized metamorphic fluid, which was channelled into the permeable zones, and the local reduction of the fluid by sulfides, which caused the precipitation of U.     

金属硫化物矿石中同沉积组构的成因鉴别

同沉积组构是特定地质环境的产物，在矿化地层中有一定的空间分布特点。对组构进行成因鉴别时，必须注意野外地质产状的调查。在室内，既要对组构中硫化物颗粒的形态特征、排布形式及其与胶结物的关系进行放大观察，还要对一些可能是多解性的组构形态进一步进行测试研究。电子探针可以对其中的矿物颗粒进行成分对比分析，Ｘ光岩组可以测定硫化物晶体的定向特征及其与层理的方位关系。