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.     

Sediment hosted lead–zinc deposits of the Neoproterozoic Bambuí Group and correlative sequences, São Francisco Craton, Brazil: A review and a possible metallogenic evolution model

The Proterozoic sediment-hosted Zn–(Pb) sulfide and non-sulfide deposits of the São Francisco Craton, Brazil, are partially syn-diagenetic and epigenetic and were probably formed during extensional events. The majority of the deposits occur within shallow water dolomites. The Pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the Plumbotectonic model. Some of the deposits are characterized by highly radiogenic lead (²⁰⁶Pb/²⁰⁴Pb ≥ 21) originating from the highly radioactive crust of the São Francisco Craton. Pb and S isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments, respectively. The relatively high temperatures of formation (100 to 250 °C) and moderate salinity (3% to 20% NaCl equiv.) of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation. The steep paleo-geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids.     

Source of metals in metamorphic ore-forming processes in the Apuane Alps (NW Tuscany,Italy): Constraints by Pb-isotope data

Summary A total of 19 samples of sulfides, barite and country rocks from three important deposits of the Apuane Alps district (Bottino, Pollone and Monte Arsiccio) were analyzed for Pb-isotopic compositions. Ore lead shows a fairly homogeneous isotopic signature (206/204: 18.2-18.4; 207/204 15.7; 208/204: 38.5-38.6), defining a high , high W province suggestive of an evolution in a crustal environment since at least the Middle Proterozoic. Pb-Pb model age of the ores is in the order of 350 Ma, in gross agreement with the Lower-Middle Paleozoic age assigned to most country rocks. The present-day Pb-isotope ratios of presumed exhalative tourmalinites associated with the Bottino deposit and of other country rocks from mineralized areas are also similar. Moreover, most of these rocks show relatively high total Pb contents (> 70 ppm). HCl-soluble lead in these samples also has a roughly similar isotopic signature. In contrast, rock samples collected at greater distances from mineralized bodies have lower Pb concentrations (< 10 ppm) and more radiogenic 206/204 and 208/204 ratios (18.6 to 18.9 and 38.9 to 39.3 respectively). HCl-soluble lead in these samples has distinctly lower 208/204, and to a lesser extent 206/204 ratios (38.6 to 38.9 and 18.5 to 18.9 respectively). In agreement with previously established genetic models, these results may be interpreted in terms of a major episode of lead extraction in the Paleozoic, possibly in association with exhalative tourmalinites, followed by remobilization and reconcentration in vein mineralization during the Apenninic orogeny. The isotopic signature of country rocks in mineralized areas may be ascribed to a Pb-isotope halo effect induced by metamorphism. When compared with Pb-isotope data from deposits of the southern Tuscany mining district, the ore lead in Apuane Alps exhibits similar isotopic patterns, but with lower 208/204 and 206/204 ratios. This fact suggests for the two districts source(s) with similar evolutions of their U/Pb and Th/Pb ratios, but distinctly different times of Pb extraction.

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Die Herkunft von Metallen bei metamorphen erzlagerstättenbildenden Prozessen in den Apuanischen Alpen (NW Toskana, Italien): Eigebnisse von Pb-Isotopendaten

Zusammenfassung In den Apuanischen Alpen (NW Toskana) treten prämetamorphe, schichtförmige Fe-Ba Erzlagerstätten in grünschieferfaziell metamorphen Serien sedimentären und vulkanosedimentären Ursprungs auf (e.g. Pollone und Monte Arsiccio), die von syn- bis spätkinematisch gebildeten, diskordanten Vererzungen begleitet werden. Ein Gangsystem mit Quarz, Karbonat und Pb-Zn-Ag (Bottino) wird von einem stratiformen Turmalinithorizont mit erhöhten Metallgehalten begleitet.Um die Frage eines metamorphogenen Ursprungs der gangförmigen Vererzungen im Gefolge der Appenninischen Metamorphose (Oligozän-Miozän) und die vermutete Herkunft der Metalle aus den schichtförmigen Lagerstätten und Metallanreicherungen zu überprüfen, wurden Pb-Isotopenuntersuchungen an Sulfiden, Bariten, Turmaliniten und Nebengesteinen durchgeführt. Das Erzblei aus stratiformen wie auch gangförmigen Lagerstätten zeigt nur geringe Variationen der Pb-isotopenverhältnisse und ist innerhalb eines Vorkommens homogen (206/204: 18.2-18.4; 207/204: 15.68; 208/204: 38.538.6). Seine relativ hohen - and W-Werte weisen auf eine Bleientwicklung in kontinentalem Krustenmilieu, spätestens seit dem mittleren Proterozoikum hin. Die 207/206 Modellalter von ca. 350 Ma stimmen größenordnungmäßig mit dem vermuteten paläozoischen Alter des Nebengesteins überein, sind aber für die gangförmigen tertiären Vorkommen zu hoch. Baryt-, Turmalinit- und Gesamtgesteinsproben aus der Nähe der Lagerstätten zeigen alle ähnliche Pb-Isotopenverhältnisse wie die Sulfide. Die Gesteine weisen hohe Pb-Gehalte von > 70 ppm auf. In größerer Entfernung zu den Lagerstätten sind die Pb-Gehalte mit < 10 ppm deutlich niedriger und die Pb-Isotopenverhältnisse sind radiogener (206/204: 18.8-18.9; 208/204: 38.9-39.3). Ihr HCl-1ösliches Blei ist hingegen weniger radiogen.Die Resultate stützen die neueren Ansichten, daß es im Paläozoikum zur Bildung von exbalativen Turmaliniten verbunden mit einer Metallanreicherung und von stratiformen Fe-Ba-Lagerstätten kam. Während der appenninischen Metamorphose wurden die Metalle remobilisiert, und es kam zur Bildung von gangförmigen Vorkommen und Lagerstätten. Die hohen Pb-Gehalte der Nebengesteine und die Isotopensignatur des HC1-löslichen Bleis deuten auf eine metamorphogene Halo-Bildung. Die Bleiisotopen von jungtertiären Erzen aus der südlichen Toskana sind im Vergleich mit dem apuanischen Erzblei radiogener. Sie weisen aber dieselben hohen - und W-Werte auf. Das heißt, das Blei stammt aus Gesteinen, die eine qualitativ wie auch quantitativ ähnliche Entwicklung bezüglich der U/Pb und Th/Pb Verhältnisse durchlaufen haben wie jene, die das Blei der apuanischen Lagerstätten geliefert haben.

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This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids and Mineral Deposits, ETH Zürich, March 21–23, 1991.     

Isotopic composition of phanerozoic ore leads from the Swedish segment of the Fennoscandian Shield

95 analyses of ore lead isotope ratios from 23 Phanerozoic ore deposits from the Swedish segment of the Fennoscandian Shield form a marked linear trend on a ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb diagram. The line may be interpreted in a two-stage model, the lead being derived from 1.8±0.15 Ga old Svecokarelian basement and mineralization occurring at 0.4±0.15 Ga. The initial composition of the Svecokarelian rock lead was similar to the lead in early Proterozoic volcanogenic sulfide ores in Sweden. — The large spread in the isotope ratios was caused by a combination of selective leaching of different minerals in the source rocks, mixing with less radiogenic Caledonian lead, and local or regional variations in the U, Th and Pb contents of the basement. As a consequence, conventional methods of identifying source rocks from lead isotopic data (e.g. mu-values, Th/U ratios) may not be directly applicable. Phanerozoic ore lead development in the Swedish section of the Fennoscandian Shield was ensialic. That is, the ore lead was almost entirely derived from the Precambrian basement, although this basement does not appear to be anomalously enriched in Pb. No juvenile or mantle lead was apparently contributed to this section of the crust after the Precambrian, except for that mechanically transported onto the western edge of the Shield by the Caledonian nappes. However, some of Europe's largest lead deposits are included in these Swedish Phanerozoic mineralizations, suggesting that it was the nature of the processes involved rather than the richness of the source, that determined their formation.     

Sandstone-hosted lead-zinc mineral deposits and their relation to the tectonic mobilization of the baltic shield during the Caledonian orogeny—a reinterpretation

Summary Sandstone-hosted lead-zinc impregnation deposits in Scandinavia occur in Vendian to Cambrian and, locally, Ordovician sandstones that rest unconformably on the deeply eroded Baltic Shield. The sandstones are overthrust by the Caledonian nappes or form part of the lowermost Caledonian thrust units. Sandstone-hosted lead-zinc deposits, that occur along the present erosional front of the Caledonides, contain galena and sphalerite cementing fractures and pore space. The deposits formed by fluid-mixing processes in the sandstones. Early ore genetic models considered the ore-hosting sandstones, because of the positive correlation between ore grade and palaeo-permeability, as main aquifer for the metalliferous fluids that were interpreted to be either ground-waters or hot basinal brines driven out from geosynclinal sediments during the Caledonian orogeny.It is suggested here that the distribution of sandstone-hosted lead-zinc deposits is controlled by Caledonian reactivated basement structures, as the ores overlay faults and lithologic discontinuities in the basement. The geographic distribution of the Scandinavian sandstone-hosted lead-zinc deposits coincides with areas that show both extensive thrust sheets of the Lower Allochthon unit at the front and basement culminations in the interior of the Caledonian orogen. These areas are characterized by deeper thrusting levels and probably more intense reactivation of basement faults, which made the basement more susceptible to large-scale fluid migration. Metalliferous fluids emerging from Caledonian reactivated basement faults mixed with fluids in the sedimentary cover, which resulted in metal precipitation. The lead-zinc deposits in sandstones that formed by these processes occur selectively in the lowermost permeable cover.

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Zusammenfassung Entlang der erosiven Front der kaledonischen Decken treten Bleiglanz- und Zinkblendeimprägnierungen in wendischen bis kambrischen, sowie teilweise ordovizischen, Sandsteinen auf, welche auf dem tief erodierten proterozoischen Baltischen Schild abgelagert und von den kaledonischen Decken überschoben worden sind. Die Bleiglanz- und Zinkblendeimprägnierungen waren infolge der Mischung von zwei hydrothermalen Lösungen in den Sandsteinen gebildet worden. ältere erzgenetische Modelle betrachteten die vererzten Sandsteine infolge der positiven Korrelation zwischen den Erzgehalten und der Faläopermeabilität als hautpsächlichen Zufuhrsweg für die vererzenden Fluide. Die Erzlösungen wurden entweder als Grundwässer und frühdiagenetische Lösungen oder als während der kaledonischen Deckenüberschiebung aus geosynklinalen Sedimenten ausgepreßte Lösungen interpretiert.Es wird hier vorgeschlagen; daß die Bildung der Vererzungen, welche Verwerfungen und Gesteinskontakte im Grundgebirge überlagern, von Strukturen im kaledonisch reaktivierten Grundgebirge kontrolliert wurde. Die geographische Verbreitung der Blei Zinkvererzungen in den Sandsteinen fällt sowohl mit mächtigen Überschiebungsdecken des Unteren Allochthons an der Gebirgsfront als auch mit Grundgebirgsaufwölbungen im Gebirgsinnern zusammen, welche beide einen tieferen Abscherungshorizont und eine möglicherweise damit zusammenhängende kräftigere Verschuppung des Grundgebirges bedingen. Eine lokal intensivere tektonische Beanspruchung des Grundgebirges resultierte in einer erhöhten großräumigen Durchlässigkeit für hydrothermale Lösungen. Aus dem Grundgebirge austretende Fluide mischten sich mit in der Überlage vorhandenen Fluiden und fällten dadurch ihren Metallgehalt aus. Das selektive Auftreten von Blei-Zinkvererzungen in den Sandsteinen ist durch deren Lage als unterste auf dem Grundgebirge abgesetzte fluidführende lithologische Einheit bedingt.

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With 3 Figures     

Differences in lead isotope composition in the stratiform McArthur zinc-lead-silver deposit

Lead isotopic composition and uranium and lead concentrations have been determined for galena, sphalerite, pyrite and acetic acid soluble material from the McArthur area in order to test the hypothesis of a dual sulphur source suggested by the sulphur isotope data of Smith and Croxford (Sulphur isotope ratios in the McArthur lead-zinc-silver deposit, Nature Phys. Sci. 245, 10–12 (1973)). Galena, sphalerite and the acetic acid washes from the McArthur deposit have uniform isotopic ratios (²⁰⁶Pb/²⁰⁴Pb, 16.07–16.15; ²⁰⁷Pb/²⁰⁴Pb, 15.37–15.47; ²⁰⁸Pb/²⁰⁴Pb, 35.57–35.89) consistent with other conformable ore deposits, whereas the ratios for pyrite are variable and quite radiogenic (²⁰⁶Pb/²⁰⁴Pb, 16.24–16.49; ²⁰⁷Pb/²⁰⁴Pb, 15.42–15.58; ²⁰⁸Pb/²⁰⁴Pb, 35.82–36.98). Acid washes where dolomite is a major dissolved phase are also radiogenic. The lead in the pyrite appears to have been derived from at least two sources: the less radiogenic lead coming from an exhalative source as for galena and sphalerite and the more radiogenic lead probably being leached from the country rocks. It is proposed that analysis of pyrite for isotopic composition and concentration of lead could be used as an indicator for similar types of deposits in this area.     

Genesis of basalt-hosted massive sulphide deposits from the Trondheim and Sulitjelma districts,Norway: ore lead isotopic considerations

Lead isotopic ratios of bulk sulphides from eleven stratigraphically equivalent deposits from the Köli Nappe sequence in the Trondheim district, and eleven from the Köli sequence at Sulitjelma Norway, have been determined. When plotted on ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagrams, the data define a linear trend extending from the mantle to the upper crustal model growth curves of Doe and Zartman (1979). Moreover, the data from both districts lie on the same trend. This isotopic trend is interpreted as resulting from the mixing of lead from a mantle source (probably the host basalts) with that of an upper-crustal end member (either sialic basement or the terrigenous sediments surrounding the host basalts). It is also concluded that the deposits in both camps formed more or less contemporaneously. The isotopic mixing line is comparable with that obtained from Besshi ore pyrites in Japan, for which an aulacogenic depositional environment, similar to that found today in the Gulf of California, has been proposed (Fox 1984). It is concluded that a similar depositional environment was responsible for the Trondheim and Sulitjelma ores, although an ensialic back-arc basin, or other possible environments, cannot be entirely ruled out.     

A lead isotope study of mineralization in the Saudi Arabian Shield

New lead isotope data are presented for some late Precambrian and early Paleozoic vein and massive sulfide deposits in the Arabian Shield. Using the Stacey Kramers (1975) model for lead isotope evolution, isochron model ages range between 720 m.y. and 420 m.y. Most of the massive sulfide deposits in the region formed before 680 m.y. ago, during evolution of the shield. Vein type mineralization of higher lead content occurred during the Pan African event about 550 m.y. ago and continued through the Najd period of extensive faulting in the shield that ended about 530 m.y. ago. Late post-tectonic metamorphism may have been responsible for vein deposits that have model ages less than 500 m.y. Alternatively some of these younger model ages may be too low due to the mineralizing fluids acquiring radiogenic lead from appreciably older local crustal rocks at the time of ore formation.The low²⁰⁷Pb/²⁰⁴Pb ratios found for the deposits in the main part of the shield and for those in north-eastern Egypt, indicate that the Arabian craton was formed in an oceanic crustal environment during the late Precambrian. Involvement of older, upper-crustal material in the formation of the ore deposits in this part of the shield is precluded by their low²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb characteristics.In the eastern part of the shield, east of longitude 44°20E towards the Al Amar-Idsas fault region, lead data are quite different. They exhibit a linear²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb relationship together with distinctly higher²⁰⁸Pb/²⁰⁴Pb characteristics. These data imply the existence of lower crustal rocks of early Proterozoic age that apparently have underthrust the shield rocks from the east. If most of the samples we have analyzed from this easterly region were mineralized 530 m.y. ago, then the age of the older continental rocks is 2,100±300 m.y. (2).The presence of upper crustal rocks, possibly also of early Proterozoic age, is indicated by galena data from Hailan in South Yemen and also from near Muscat in Oman. These data are the first to indicate such old continental material in these regions.     

Diverse metal sources of Archaean gold deposits: evidence from in situ lead-isotope analysis of individual grains of galena and altaite in the Ross and Kirkland Lake deposits,Abitibi Greenstone belt,Canada

Lead isotope compositions for individual grains of galena and altaite (PbTe) were determined in situ using a secondary ion mass spectrometer (SIMS). Galena was collected from the Ross deposit and altaite from the Kirkland Lake (KL) deposits in the southern Abitibi greenstone belt, Superior Province of Canada. The samples from KL are more radiogenic than those from the Ross deposit. Isotopic compositions vary significantly between different grains in each deposit and form broad linear arrays in ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagrams. The linear arrays of Pb-isotope data are attributed to mixing of Pb from different sources. At least two sources are required for individual deposits: one with low U/Pb and Th/Pb ratios and the other with high ratios. Lead minerals occurring with Au are less radiogenic than those that are not obviously associated with Au, suggesting that Au was supplied from low U/Pb sources such as sulphides or older ultramafic-mafic rocks. While most data are consistent with the derivation from local rocks, highly radiogenic Pb with relatively low ²⁰⁷Pb/²⁰⁶Pb ratios recorded at KL require post-Archaean mineralization or derivation of the Pb from an unusual crustal source with low . The latter interpretation is favored because of the lack of textural evidence and because it is difficult to dissolve and precipitate altaite at low temperatures. The presence of a Pb reservoir with low is also inferred from the data of Archaean banded iron formations and volcanogenic massive sulphide deposits. Different isotopic patterns of the two deposits suggest different sources of metals in the two deposits. While this conclusion does not reject the derivation of fluids from the lower crust or mantle, the data are not in accord with conceptual models invoking a common source reservoir for metals. The study suggests that fluids, which may have a common origin, leached metals and other constituents from the upper crustal rocks during their ascent. The proposed model, different origins for different constituents, explains much of the conflicting evidence presented by Archaean Au deposits, including provinciality of mineralogy and relatively uniform fluid inclusion and C-isotope data from many Au deposits.     

A re-assessment of the Upper Mississippi valley lead isotope data

Corrections for systematic bias, and a better regression method, are tested on the lead isotope data of Heyl, Delevaux, Zartman and Brock (1966). The new calculations suggest the time of mineralization in the Upper Mississippi zinc-lead district is more likely to lie in the range Mid Devonian (370 my) to Upper Pennsylvanian (280 my) on any reasonable assumption for the age of the basement. It is argued from the isotopic trends that the ore lead in this district was simply derived by radiogenic addition to lead originally derived from rocks of basement age, although the data do not exclude residence in the Lower Paleozoic sediments as an intermediate step. An isotopically different magmatic component is not required. Correlation with the geology suggests that genesis of the deposits probably was related to regional tectonic deformation in Middle to Late Paleozoic time.

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Zusammenfassung Korrekturen systematischer Fehler und eine bessere Regressionsmethode wurden anhand der Blei-Isotopendaten von Heyl, Delevaux, Zartman und Brock (1966) getestet. Diese neuen Berechnungen unter Annahme eines vernünftigen Alters des Grundgebirges zeigen, daß die Zeit der Mineralisation im Oberen Mississippi Zink-Blei Distrikt eher zwischen Mitteldevon (370 Millionen Jahre) und Oberkarbon (280 Millionen Jahre) zu setzen ist. Auf Grund der Verteilung der Isotopenverhältnisse wird auf eine einfache Formierung (durch Addition radioaktiver Zerfallsprodukte zum ursprünglichen Blei im Gestein mit demselben Alter wie das Grundgebirge) des Bleies im Bleierz geschlossen. Die Meßresultate schließen jedoch eine Ablagerung im Unterpaläozoikum als Übergangsstufe nicht aus. Magma mit andern Isotopenverhältnissen wird nicht angenommen. Vergleiche mit der Geologie deuten darauf hin, daß die Bildung der Ablagerungen im Zusammenhang mit tektonischen Deformationen im Mittleren bis Oberen Paläozoikum erfolgte.

     

Lead isotopes of the carbonate-hosted Kabwe, Tsumeb, and Kipushi Pb-Zn-Cu sulphide deposits in relation to Pan African orogenesis in the Damaran-Lufilian Fold Belt of Central Africa

Lead isotope ratios of galena from the carbonate-hosted massive sulphide deposits of Kabwe (Pb-Zn) and Tsumeb (Pb-Zn-Cu) in Zambia and Namibia, respectively, have been measured and found to be homogeneous and characteristic of upper crustal source rocks. Kabwe galena has average isotope ratios of ^206/204Pb = 17.997 ± 0.007, ^207/204Pb = 15.713 ± 0.010 and ^208/204Pb = 38.410 ± 0.033. Tsumeb galena has slightly higher ^206/204Pb (18.112 ± 0.035) and slightly lower ^207/204Pb (15.674 ± 0.016) and ^208/204Pb (38.276 ± 0.073) ratios than Kabwe galena. The isotopic differences are attributed to local differences in the age and composition of the respective source rocks for Kabwe and Tsumeb. The homogeneity of the ore lead in the two epigenetic deposits suggests lead sources of uniform isotopic composition or, alternatively, thorough mixing of lead derived from sources with relatively similar isotopic compositions. Both deposits have relatively high ²³⁸U/²⁰⁴Pb ratios of 10.31 and 10.09 for Kabwe and Tsumeb galenas, respectively. These isotope ratios are considered to be typical of the upper continental crust in the Damaran-Lufilian orogenic belt, as also indicated by basement rocks and Cu-Co sulphides in stratiform Katangan metasediments which have a mean μ-value of 10.25 ± 0.12 in the Copperbelt region of Zambia and the Democratic Republic of Congo (formerly Zaire). The ²³²Th/²⁰⁴Pb isotope ratios of 43.08 and 40.42 for Kabwe and Tsumeb suggest Th-enriched source regions with ²³²Th/²³⁵U (κ-values) of 4.18 and 4.01, respectively. Model isotopic ages determined for the Kabwe (680 Ma) and Tsumeb (530 Ma) deposits indicate that the timing of the mineralisation was probably related to phases of orogenic activity associated with the Pan-African Lufilian and Damaran orogenies, respectively. Galena from the carbonate-hosted Kipushi Cu-Pb-Zn massive sulphide deposit in the Congo also has homogeneous lead isotope ratios, but its isotopic composition is comparable to that of the average global lead evolution curve for conformable massive sulphide deposits. The μ (9.84) and κ (3.69) values indicate a significant mantle component, and the isotopic age of the Kipushi deposit (456 Ma) suggests that the emplacement of the mineralisation was related to a post-tectonic phase of igneous activity in the Lufilian belt. The isotope ratios (^206/204Pb, ^207/204Pb, ^208/204Pb) of the three deposits are markedly different from the heterogeneous lead ratios of the Katangan Cu-Co stratiform mineralisation of the Copperbelt as well as those of the volcanogenic Nampundwe massive pyrite deposit in the Zambezi belt which typically define radiogenic linear trends on lead-lead plots. The host-rock dolomite of the Kabwe deposit also has homogeneous lead isotope ratios identical to the ore galena. This observation indicates contamination of the Kabwe Dolomite Formation with ore lead during mineralisation. Received: 8 September 1997 / Accepted: 21 August 1998     

Mobile Pb-isotopes in Proterozoic sedimentary basins as guides for exploration of uranium deposits

Lead isotope ratios and associated trace element concentrations (U, Th and Pb) extracted by partial-leaching with 2% nitric acid from Proterozoic sandstones and basement rocks reveal much about the fluid evolution of sedimentary basins hosting unconformity-type uranium deposits. In addition, these techniques have great potential as a guide for exploration of uranium and other types of deposits in basins of any age. Isotope ratios of Pb in Proterozoic sandstones from basins known to contain high-grade uranium deposits are radiogenic at key geological localities and settings distal to known mineralization and particularly in altered zones proximal to mineralization. Sandstones completely cemented by quartz overgrowths typically have non-radiogenic Pb isotope ratios, indicating early closure of porosity and isolation of these rocks from later fluid events. Alternatively, the unconformity served as both a source of uranium and radiogenic Pb as well as an avenue for late-stage (<250–900 Ma) fluid flow. The mafic volcanic units, which are relatively reducing lithologies and therefore have removed uranium from basinal brines, have uranium-supported radiogenic Pb isotope ratios. Comparison of ²³⁸U/²⁰⁶Pb and ²⁰⁶Pb/²⁰⁴Pb ratios is useful in determining the timing and nature of U and Pb migration before, during and after mineralization in these basins. This comparison can be used to delineate the presence of radiogenic Pb isotope ratios that are not internally supported by uranium and thorium in rocks, eventually providing the explorationist with geochemical vectors that point toward sites of high potential for economic uranium mineralization.     

Caledonian formation of gold-bearing sulfide depositions in Early Proterozoic gabbroids in the northern Ladoga region

We have studied Pb isotopic systems of K-feldspar, pyrite, and pyrrhotine from gabbroids and ore of the Velimyaki Early Proterozoic massif in the northern Ladoga region in the southeastern part of the Fennoscandian Shield. The isochronous Pb–Pb age of sulfides has been determined as ～450 Ma, which corresponds to intersection of the regression line with the lead accumulation curve with μ = 10.4–10.8; the model Pb age of sulfides is close to isochronous under the condition that the composition of lead evolved from a geochemical reservoir with an age of 1.9 Ga. The isotopic parameters of the lead in sulfides and K-feldspar indicate their formation in upper crust conditions (μ = ²³⁸U/²⁰⁴Pb > 10). From the obtained data, it follows that the isotopic composition of lead in K-feldspar corresponds to a Proterozoic age (1890 Ma) of magmatic crystallization of the rocks in the massif, and strongly radiogenic lead sulfides testify, with the greatest probability, to the later (Caledonian) formation of sulfide ores.     

Strontium and Lead Isotopic Study of the Carbonate‐hosted Xujiashan Antimony Deposit from Hubei Province,South China: Implications for its Origin

The Xujiashan antimony deposit is hosted by marine carbonates of the Upper Sinian Doushantuo and Dengying Formations in Hubei Province, South China. Our Sr isotopic data from pre‐ and syn‐mineralization calcites that host the mineralization show that the pre‐mineralization calcite displays a narrow range of ⁸⁷Sr/⁸⁶Sr ratios (0.7096 to 0.7097), similar to the ratios of the Sinian seawater, and high Sr concentrations (2645 to 8174 ppm). In contrast, the syn‐mineralization calcite exhibits low Sr concentrations (785 to 2563 ppm) and high ⁸⁷Sr/⁸⁶Sr ratios (0.7109 to 0.7154), which is interpreted as the result of addition of radiogenic strontium during the antimony mineralization. The study of Sr isotopes suggests that their Sr component to the pre‐mineralization calcite derived directly from the host rocks (i.e. the Sinian marine carbonates), while radiogenic ⁸⁷Sr for the syn‐mineralization calcite derived from the underlying Mesoproterozoic Lengjiaxi Group basement through hydrothermal fluid circulation along the major fault that hosts the mineralization. The Pb isotopic ratios of stibnite are subdivided into two groups (Group A and Group B), Group A is characterized by higher radiogenic lead, with ²⁰⁶Pb/²⁰⁴Pb = 18.874 to 19.288, ²⁰⁷Pb/²⁰⁴Pb = 15.708 to 15.805, and ²⁰⁸Pb/²⁰⁴Pb = 38.642 to 39.001. Group B shows lower lead isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 17.882 to 18.171, ²⁰⁷Pb/²⁰⁴Pb = 15.555 to 15.686, and ²⁰⁸Pb/²⁰⁴Pb = 37.950 to 38.340). The single‐stage model ages of Group A are mainly negative or slightly positive values (‐258 to 3 Ma), while those of Group B range from 636 to 392 Ma, with an average of 495 ± 65 Ma. In addition, there are positive linear correlations among Pb isotopic ratios. These results suggest that the lead of Group A stibnite was mainly derived from the Sinian marine carbonates, and that of Group B stibnite from the underlying Lengjiaxi Group basement. This conclusion is consistent with the results of the Sr isotopes. These results indicate that the Xujiashan deposit is not syngenetic sedimentary and in situ reworked origin as previously considered. The metal (mainly Sb) of this deposit was not only derived from the Sinian host rocks, but also partly derived from the underlying Mesoproterozoic Lengjiaxi Group basement.     

Isotope characterization of lead in galena from ore deposits of the Aysén Region, southern Chile

Lead isotope analyses of galena from five ore deposits and six prospects in the Aysén region of southern Chile are reported. Most of the deposits are either low sulfidation epithermal gold–silver veins or skarn and manto deposits; the majority are either suspected to be, or dated as, Late Jurassic to mid-Cretaceous. Galena lead isotope data for most of the deposits from southern Chile cluster near the “orogene” within a “plumbotectonic” model framework. Average values (²⁰⁶Pb/²⁰⁴Pb=18.53, ²⁰⁷Pb/²⁰⁴Pb=15.63, and ²⁰⁸Pb/²⁰⁴Pb=38.50) are near Jurassic to Cretaceous model ages on the “orogene” curve of Zartman and Doe (1981) and the second-stage curve of Stacey and Kramers (1975) on a ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb plot. These model ages are compatible with absolute ages as currently known. The elongate trends in the general cluster indicate mainly an orogenic model fit, suggesting variable mixing of lead from different sources, mainly model upper crust and lesser model mantle and lower crust reservoirs. Galena lead associated with one deposit (El Faldeo) is relatively radiogenic, and lies near a Jurassic age on the “upper crustal” curve of Zartman and Doe (1981), which is compatible with the Ar/Ar age of the deposit. Galena lead isotope clusters define three main groups of deposits. These three groups appear to be related to three mineralizing events, dated by K–Ar and Ar/Ar, in the Late Jurassic (group 3), and in the Early and mid-Cretaceous (groups 1 and 2 respectively). Averages for group 1, the northern group including El Toqui and Katerfeld, are ²⁰⁶Pb/²⁰⁴Pb=18.51, ²⁰⁷Pb/²⁰⁴Pb=15.62, ²⁰⁸Pb/²⁰⁴Pb=38.48. Averages for group 2, the southern group with Fachinal and Mina Silva, are ²⁰⁶Pb/²⁰⁴Pb = 18.56, ²⁰⁷Pb/²⁰⁴Pb=15.63, ²⁰⁸Pb/²⁰⁴Pb=38.52. Averages for group 3, the southernmost group with the El Faldeo, Lago Chacabuco and Lago Cochrane prospects, are ²⁰⁶Pb/²⁰⁴Pb=18.83, ²⁰⁷Pb/²⁰⁴Pb=15.65, ²⁰⁸Pb/²⁰⁴Pb=38.63. The Cretaceous deposits (groups 1 and 2) contain orogene-type lead that becomes increasingly radiogenic southward. Lead from the Late Jurassic deposits (group 3) appears to reflect mixing of orogene lead with highly radiogenic lead. The observed linear array of lead in group 3 probably reflects mixing of orogene lead with highly radiogenic lead, which was likely extracted by selective leaching of mineralizing hydrothermal solutions from the metamorphic basement. Received: 10 July 1999 / Accepted: 15 July 2000     

Nd,Sr and Pb isotopic and REE geochemical study of some Miocene submarine hydrothermal deposits (Kuroko deposits) in Japan

Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (_Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, ⁸⁷Sr/⁸⁶Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (²⁰⁶Pb/²⁰⁴Pb=18.35–18.84, ²⁰⁷Pb/²⁰⁴Pb=15.59–15.97 and ²⁰⁸Pb/²⁰⁴Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.     

Plumbotectonic aspects of polymetallic vein mineralization in Paleozoic sediments and Proterozoic basement of Moravia (Czech Republic)

A regional isotopic study of Pb and S in hydrothermal galenas and U–Pb and S in potential source rocks was carried out for part of Moravia, Czech Republic. Two major generations of veins, (syn-) Variscan and post-Variscan, are defined based on the Pb-isotope system together with structural constraints (local structures and regional trends). The Pb-isotopic compositions of galena plot in two distinct populations with outliers in ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb space. Galena from veins hosted in greywackes provides a cluster with the lowest Pb–Pb ratios: ²⁰⁶Pb/²⁰⁴Pb = 18.15–18.27, ²⁰⁷Pb/²⁰⁴Pb = 15.59–15.61, ²⁰⁸Pb/²⁰⁴Pb = 38.11–38.23. Those hosted in both limestones and greywackes provide the second cluster: ²⁰⁶Pb/²⁰⁴Pb = 18.37–18.44, ²⁰⁷Pb/²⁰⁴Pb = 15.60–15.63, ²⁰⁸Pb/²⁰⁴Pb = 38.14–38.32. These clusters suggest model Pb ages as Early Carboniferous and Triassic–Jurassic, the latter associated with MVT-like deposits. Two samples from veins hosted in Proterozoic rocks lie outside the two clusters: in metagranitoid (²⁰⁶Pb/²⁰⁴Pb = 18.55, ²⁰⁷Pb/²⁰⁴Pb = 15.64, ²⁰⁸Pb/²⁰⁴Pb = 38.29) and in orthogneiss (²⁰⁶Pb/²⁰⁴Pb = 18.79, ²⁰⁷Pb/²⁰⁴Pb = 15.73, ²⁰⁸Pb/²⁰⁴Pb = 38.54). The results from these two samples suggest an interaction of mineralizing fluids with the radiogenic Pb-rich source (basement?). The values of δ³⁴S suggest the Paleozoic host rocks (mostly ?6.7 to +5.2‰ CDT) as the source of S for hydrothermal sulfides (mostly ?4.8 to +2.5‰ CDT). U–Pb data and Pb isotope evolutionary curves indicate that Late Devonian and Early Carboniferous sediments, especially siliciclastics, are the general dominant contributor of Pb for galena mineralization developed in sedimentary rocks. Plumbotectonic mixing occurred, it is deduced, only between the lower and the upper crust (the latter involving Proterozoic basement containing heterogeneous radiogenic Pb), without any significant input from the mantle. It is concluded that in the Moravo–Silesian and Rhenohercynian zones (including proximal districts in Poland) lead and sulfur have been mobilized from the adjacent rocks during multiple hydrothermal events in processes that are remarkably comparable in timing, geochemistry of fluids and nature of sources.     

Ore-forming material sources of the Jurassic Cu–Pb–Zn mineralization in the Qin–Hang ore belt,South China: Constraints from S–Pb isotopes

The Qin–Hang ore belt in South China, which serves as the boundary between the Yangtze and Cathaysia blocks, is marked by extensive Jurassic porphyry-skarn-metasomatic Cu–Pb–Zn polymetallic mineralization. In this contribution, S and Pb isotopic compositions of the Baoshan Cu–Pb–Zn deposit in the western portion of the Qin–Hang ore belt were analyzed to determine the ore-forming material sources in the area. This is coupled by the first systematic collection, compilation and interpretation of previously published S and Pb isotopic data of multiple sulfide minerals to reveal the metal origin and accumulation mechanism of the Cu–Pb–Zn mineralization from the significant deposits in the region (i.e., Dexing, Qibaoshan, Shuikoushan, Baoshan, Huangshaping, Tongshanling and Dabaoshan). The results show that Cu mineralization is characterized by low and narrow δ³⁴S (‰) range of values (–5 to 6) and Pb isotopic ratios (²⁰⁸Pb/²⁰⁴Pb = 38.0–39.0, ²⁰⁷Pb/²⁰⁴Pb = 15.4–15.8, and ²⁰⁶Pb/²⁰⁴Pb = 17.7–18.7), which are consistent with those of local porphyries. In contrast, the Pb–Zn mineralization reveals higher and more variable δ³⁴S (‰) values (–4 to 18) and Pb isotopic ratios (²⁰⁸Pb/²⁰⁴Pb = 38.0–39.5, ²⁰⁷Pb/²⁰⁴Pb = 15.3–16.0, and ²⁰⁶Pb/²⁰⁴Pb = 18.0–19.0) that correspond to wall-rock and basement rock compositions in the region. This indicates that the sulfur and lead that formed the Cu mineralization in the Qin–Hang ore belt was mainly sourced from regional magmatism with mantle contributions, whereas the sulfur and lead for the Pb–Zn mineralization was likely derived from the host sedimentary rocks and Proterozoic metamorphic basement rocks, respectively. The S and Pb isotopic data, combined with the geochemical signatures of mineralization-related porphyries, suggest that the Cu was sourced from the deeper levels along with mantle-derived magmas. In contrast, the Pb–Zn probably originated from the crust, with partial melting of the crystalline basement in the Cathaysia Block. Consequently, a three-stage genetic model is proposed to explain the ore-forming processes of the Qin–Hang Cu-polymetallic belt in South China.     

Pb Isotopic Composition and Metal Sources of Au and Ag Deposits of the South Verkhoyansk Region (Yakutia,Russia) According to High-Precision MC-ICP-MS Data

The paper considers the results of high-precision Pb–Pb isotopic analysis of 120 galena samples from 27 Au and Ag deposits of the South Verkhoyansk Synclinorium (SVS) including large Nezhdaninsky deposit (628.8 t Au). The Pb isotopic composition is analyzed on a MC-ICP-MS NEPTUNE mass-spectrometer from solutions with an error of no more than ±0.02% (2σ). Four types of deposits are studied: (i) stratified vein gold–quartz deposits (type 1) hosted in metamorphosed Upper Carboniferous–Lower Permian terrigenous rocks and formed during accretion of the Okhotsk Block to the North Asian Craton synchronously with dislocation metamorphism and related granitic magmatism; (ii) vein gold–quartz (Nezhdaninsky type) deposits also hosted in Lower Permian metasedimentary rocks; (iii) Au–Bi deposits localized at the contact zones of the Late Cretaceous granitic plutons; and (iv) Sn–Ag polymetallic deposits related to granitic and subvolcanic rocks of the Okhotsk Zone of the SVS. The deposits of types 2, 3, and 4 are postaccretionary. The general range of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios is 18.1516–18.5903 (2.4%), 15.5175–15.6155 (0.63%), and 38.3010–39.0481 (2.0%), respectively. In ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb diagrams, the data points of Pb isotopic compositions of all deposits occupy restricted, partly overlapping areas along a general elongated trend. The various SVS Au–Ag deposits can be classified according to the Pb isotopic composition in accordance with all three Pb ratios. Deposits of the same type show distinct Pb isotopic compositions that strongly exceed the scale of analytical error (±0.02%). The differences in Pb isotopic composition within specific deposits are low and subordinate and have little effect on variations in the Pb isotopic composition of the SVS deposits. The μ₂ values (Stacey–Kramers model), which characterize the ²³⁸U/²⁰⁴Pb ratios of ore lead sources of the SVS deposits, widely vary from 9.7 to 9.38. The ω₂ values (²³²Th/²⁰⁴Pb) are 39.82–36.61, whereas the Th/U ratios are 4.04–3.86. The content of all three radiogenic Pb isotopes and μ₂ values of feldspars from SVS intrusive rocks are strongly distinct from those of galena of stratified gold–quartz and vein gold–quartz deposits and are identical to Pb of galena from Au–Bi and Sn–Ag polymetallic deposits, indicating a mostly magmatic origin for the Pb of these deposits. Detailed isotopic study of the Nezhdaninsky deposit shows different Pb isotopic composition of two consecutive mineral assemblages (gold–sulfide and Ag polymetallic): ~0.30, ~0.07, and ~0.22% for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios, respectively. These differences are interpreted as a result of involvement of at least two metal sources during the evolution of an ore-forming system: (i) host Lower Permian terrigenous rocks and (ii) a magmatic source similar in Pb isotopic composition to that of Sn–Ag polymetallic deposits. The Pb isotopic composition and μ₂ and Th/U values show that lead of stratified gold–quartz deposits combines isotopic tracers of lower and upper crustal sources (Upper Carboniferous–Lower Permian terrigenous rocks), lead of which was mobilized by ore-bearing fluids. The high ²⁰⁸Pb/²⁰⁶Pb ratios and Th/U evolutionary parameter are common to all Pb isotopic composition of all studied Au–Ag deposits and SVS Cretaceous intrusive rocks and indicate that Pb sources were depleted in U relative to Th. Taking into account the structure of the region and conceptions on its evolution, we can suggest that the magma source was related to lower crustal subducted rocks of the Archean (~2.6 Ga) North Asian Craton and the Okhotsk terrane.     

Lead isotope study of Zn-Pb ore deposits associated with the Basque-Cantabrian basin and Paleozoic basement, Northern Spain

A total of forty-three galena samples from syngenetic and epigenetic Pb-Zn mineralizations emplaced in the Lower Cretaceous Basque-Cantabrian basin and Paleozoic basement of the Cinco Villas massif in the western Pyrenees, have been analyzed for Pb-isotopic composition. Galena from sedex mineralizations hosted in Carboniferous clastic rocks in the Cinco Villas massif display an homogeneous lead isotopic signature (²⁰⁶Pb/²⁰⁴4Pb 18.43, ²⁰⁷Pb/²⁰⁴Pb 15.66, ²⁰⁸Pb/ ²⁰⁴Pb 38.69) suggesting a single lead reservoir. These values are slightly more radiogenic than lead from other European Hercynian deposits, possibly reflecting the influence of a more evolved upper crustal source. Underlying Paleozoic sediments are proposed as lead source for the Cinco Villas massif ores. Analyses from twenty-six galena samples from the four strata-bound ore districts hosted in Mesozoic rocks reveal the existence of two populations regarding their lead isotopic composition. Galena from the western Santander districts (e.g., Reocin) is characterized by more radiogenic isotope values (²⁰⁶Pb/²⁰⁴Pb 18.74, ²⁰⁷Pb/²⁰⁴Pb 15.67, ²⁰⁸Pb/ ²⁰⁴Pb 38.73) than those from the central and eastern districts (Troya-Legorreta, Central and Western Vizcaya, ²⁰⁶Pb/²⁰⁴Pb 18.59, ²⁰⁷Pb/²⁰⁴Pb 15.66, ²⁰⁸Pb/ ²⁰⁴Pb 38.73). In all districts, the most likely source for these mineralizations was the thick sequence of Lower Cretaceous clastic sediments. The existence of two separate lead isotopic populations could be the result of regional difference in the composition of the basement rocks and the clastic sediments derived of it or different evolution histories. In both sub-basins, isotopic ratios indicate an increase in crustal influence as the age of the ores decreases.