Genesis of the South Zhuguang Uranium Ore Field,South China: Pb Isotopic Compositions and Mineralization Ages

U–Pb isotopic analyses indicate that ores from the South Zhuguang uranium ore field, south China, have high common (non‐radiogenic) Pb contents, with variable and relatively radiogenic initial Pb contents. The U–Pb isochron method was used to date these ores, with plots of ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb being used to identify sample suites with similar initial Pb isotopic ratios and to normalize variable initial Pb isotopic ratios. The resulting U–Pb isochrons indicate two substages of uranium mineralization at ~57 and 52 Ma, with a later hydrothermal reformation at ~49 Ma, which homogenized Pb isotopic compositions. Initial Pb isotopic systematics indicate that the ore‐forming fluid was characterized by high ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios and low ²⁰⁸Pb/²⁰⁴Pb ratios, suggesting that the ore‐forming fluid was sourced from Cretaceous–Paleogene red‐bed basins, rather than from magma or the mantle, with consideration of mineralization ages.     

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.     

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.     

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.     

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.     

Lead isotope constraints on the genesis of Pb–Zn deposits in the Neoproterozoic Vazante Group, Minas Gerais, Brazil

The Neoproterozoic Vazante Group at the western border of the São Francisco Craton, Brazil, hosts the largest Zn–Pb district in South America. Several authors have classified this mineral district as Mississippi Valley-type (MVT), based on the intimate association with carbonates and the epigenetic character of most ore bodies. In this paper, we present 47 new lead isotope data from four deposits located along the 300 km N–S Vazante–Paracatu–Unai linear trend. Pb isotope ratios indicate sources with relatively high U/Pb and Th/Pb ratios. Considering the ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios as indicative parameters for the source, we suggest an upper crustal source for the metals. The small variation on the Pb isotope ratios compared to those observed in the classical MVT deposits, and other geological, fluid inclusion and sulphur isotopic data indicates a metallogenic event of long duration. It was characterized by focused circulation of hydrothermal fluids carrying metals from the basement rocks and from the sedimentary pile. The data obtained are more compatible with an evolution model similar to that of IRISH-type deposits. The existence of three Pb isotopic populations could be the result of regional differences in composition of the source rocks and in the fluid–rock interaction since the mineralization is a long-term process.     

Isotopes and Geochronology of the Meixian-type Pb-Zn-(Ag)Deposits,Central Fujian Rift,South China:Implications for Geological Events

Central Fujian Rift is another new and important volcanogenic massive sulfide Pb-Zn polymetallic metallogenetic belt. In order to find out the material genesis and mineralization period of Meixian-type Pb-Zn-Ag deposits, S and Pb isotope analysis and isotope geochronology of ores and wall rocks for five major deposits are discussed. It is concluded that the composition of sulfur isotope from sulfide ore vary slightly in different deposits and the mean value is close to zero with the 834S ranging from -3.5‰ to ＋5.6‰ averaging at ＋2.0‰, which indicates that the sulfur might originate from magma or possibly erupted directly from volcano or was leached from ore-hosted volcanic rock. The lead from ores in most deposits displays radioactive genesis character （206pb/204pb〉18.140, 207Pb/204pb〉15.584, 208pb/204pb〉38.569） and lead isotope values of ores are higher than those of wall rocks, which indicates that the lead was likely leached from the ore-hosted volcanic rocks. Based on isotope data, two significant Pb-Zn metallogenesis are delineated, which are Mid- and Late-Proterozoic sedimentary exhalative metailogenesis （The single zircon U-Pb, Sm-Nd isochronal and Ar-Ar dating ages of ore- hosted wall rocks are calculated to be among 933-1788 Ma.） and Yanshanian magmatic hydrothermal superimposed and alternated metallogenesis （intrusive SHRIMP zircon U-Pb and Rb-Sr isochronal ages between 127-154 Ma）.     

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.     

Anomalous lead isotope ratios and provenance of offshore sediments,Gulf of Carpentaria,northern Australia

Anomalous Pb isotope ratios measured by Inductively Coupled Plasma Mass Spectrometry in terrigenous marine sediments (<63 μm fraction) from the Gulf of Carpentaria originated from depositional mixing of clay/silt with average modern crustal Pb isotope ratios and detrital monazite with high ²⁰⁸Pb/²⁰⁶Pb and low ²⁰⁷Pb/²⁰⁶Pb. This interpretation is supported by strong correlations between Pb isotope ratio and Th, U and light rare‐earth element concentrations in the sediments as well as by monazite compositional data. A likely source of the detrital monazite is the western portion of the Georgetown Inlier of mainly Proterozoic S‐type granitic rocks. A clear distinction between Pb isotope ratios in sediments deposited from the Norman and Bynoe Rivers in the southeast Gulf of Carpentaria and the persistence of catchment‐specific Pb isotope ratios 45 km offshore suggest that Pb isotope data are useful in tracing the provenance of terrigenous offshore sediments when the source rocks of catchments show sufficient chemical and/or mineralogical variation.     

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.     

Pb isotopic constraints on the formation of the Dikulushi Cu–Pb–Zn–Ag mineralisation, Kundelungu Plateau (Democratic Republic of Congo)

Base metal–Ag mineralisation at Dikulushi and in other deposits on the Kundelungu Plateau (Democratic Republic of Congo) developed during two episodes. Subeconomic Cu–Pb–Zn–Fe polysulphide ores were generated during the Lufilian Orogeny (c. 520 Ma ago) in a set of E–W- and NE–SW-oriented faults. Their lead has a relatively unradiogenic and internally inhomogeneous isotopic composition (²⁰⁶Pb/²⁰⁴Pb = 18.07–18.49), most likely generated by mixing of Pb from isotopically heterogeneous clastic sources. These sulphides were remobilised and enriched after the Lufilian Orogeny, along reactivated and newly formed NE–SW-oriented faults into a chalcocite-dominated Cu–Ag mineralisation of high economic interest. The chalcocite samples contain only trace amounts of lead and show mostly radiogenic Pb isotope signatures that fall along a linear trend in the ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagram (²⁰⁶Pb/²⁰⁴Pb = 18.66–23.65; ²⁰⁷Pb/²⁰⁴Pb = 15.72–16.02). These anomalous characteristics reflect a two-stage evolution involving admixture of both radiogenic lead and uranium during a young fluid event possibly c. 100 Ma ago. The Pb isotope systematics of local host rocks to mineralisation also indicate some comparable young disturbance of their U–Th–Pb systems, related to the same event. They could have provided Pb with sufficiently radiogenic compositions that was added to less radiogenic Pb remobilised from precursor Cu–Pb–Zn–Fe polysulphides, whereas the U most likely originated from external sources. Local metal sources are also suggested by the ²⁰⁸Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb systematics of combined ore and rock lead, which indicate a pronounced and diversified lithological control of the immediate host rocks on the chalcocite-dominated Cu–Ag ores. The Pb isotope systematics of polysulphide mineralisation on the Kundelungu Plateau clearly record a diachronous evolution.     

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     

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.     

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.     

Metal sources of the large Nezhdaninsky orogenic gold deposit, Yakutia, Russia: Results of high-precision MC-ICP-MS analysis of lead isotopic composition supplemented by data on strontium isotopes

A collection of galena from the Nezhdaninsky gold deposit (62 samples), as well as galena from the Menkeche silver-base-metal deposit and the Sentyabr occurrence and K-feldspar from intrusive rocks of the Tyry-Dyby ore cluster have been studied using the high-precision (±0.02%) MC-ICP-MS method. Particular ore zones are characterized by relatively narrow variations of isotope ratios (no wider than σ_6/4 = 0.26%). Vertical zoning of Pb isotopic composition is not detected. Variation in Pb isotope ratios mainly depends on the type of mineral assemblage. Galena of the gold-sulfide assemblage dominating at the Nezhdaninsky deposit is characterized by the following average isotope ratios: ²⁰⁶Pb/²⁰⁴Pb = 18.472, ²⁰⁷Pb/²⁰⁴Pb = 15.586, and ²⁰⁸Pb/²⁰⁴Pb = 38.605. Galena from the regenerated silver-base-metal assemblage is distinguished by less radiogenic lead isotope ratios: 18.420, 15.575, and 38.518, respectively. In lead from the Nezhdaninsky deposit, the component, whose source is identified as Permian host terrigenous rocks, is predominant. The data points of isotopic composition of lode lead make up a linear trend within the range of μ₂ = 9.5-9.6. K-feldspar of granitic rocks has less radiogenic and widely varying lead isotopic composition compared to that of galena. The isotopic data on Pb and Sr constrain the contribution of Late Cretaceous granitic rocks as a source of gold mineralization at the Nezhdaninsky deposit. The matter from the Early Cretaceous fluid-generating magma chamber participated in the ore-forming system of the Nezhdaninsky deposit. The existence of such a chamber is confirmed by the occurrence of Early Cretaceous granitoid intrusions on the flanks of the Nezhdaninsky ore field. The greatest contribution of magmatic lead (～30%) is noted in galena from the silver-base-metal mineral assemblage. This component has isotopic marks characteristic of lower crustal lead: the elevated ²⁰⁸Pb/²⁰⁶Pb ratio relative to the mean crustal value and the lower ²⁰⁷Pb/²⁰⁴Pb ratio. Taken together, they determine a high Th/U ～ 4.0 in the source and μ₂ = 9.37–9.50. This conclusion is consistent with the contemporary tectonic model describing evolution of the South Verkhoyansk sector of the Verkhoyansk Foldbelt and the Okhotsk Terrane.     

Petrogenesis of isotopically unusual Pliocene olivine leucitites from Deep Springs Valley, California

High-K mafic alkalic lavas (5.4 to 3.2 wt% K₂O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; ⁸⁷Sr/⁸⁶Sr ratios decrease and ɛ_Nd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7121 to 0.7105 and ɛ_Nd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, ²⁰⁶Pb/²⁰⁴Pb ∼17.2, ²⁰⁷Pb/²⁰⁴Pb ∼15.5, and ²⁰⁸Pb/²⁰⁴Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛ_Nd values and unradiogenic ²⁰⁶Pb/²⁰⁴Pb ratios have relatively low ⁸⁷Sr/⁸⁶Sr ratios (the EM I array), whereas basalts with low ɛ_Nd values and high ⁸⁷Sr/⁸⁶Sr ratios have radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios and low ɛ_Nd values and ²⁰⁶Pb/²⁰⁴Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998     

A precise U–Pb age on cassiterite from the Xianghualing tin-polymetallic deposit (Hunan,South China)

We report the first precise U–Pb isotope data on cassiterite from the large Xianghualing tin-polymetallic deposit in the central Nanling district, South China. The results show that four separates from sample XF-51 have a relatively narrow range of ²⁰⁶Pb/²³⁸U apparent ages, varying from 152 to 157 Ma, and the three ²⁰⁶Pb/²³⁸U apparent ages yield a weighted average value of 156 ± 4 Ma (MSWD = 0.32). Separates from two other cassiterite samples do not have sufficient radiogenic Pb to generate a reliable ²⁰⁶Pb/²³⁸U age. Seven separates from the above three cassiterite samples define a well-constrained ²³⁸U–²⁰⁶Pb isochron corresponding to an age of 157 ± 6 Ma (MSWD = 34). A comparison of the U–Pb cassiterite ages with published Ar–Ar dates on muscovite from this deposit and K–Ar age data on biotite from the pluton genetically related to the tin mineralization in this area demonstrates that the U–Pb isotope system of cassiterite is a potential geochronometer. Combined with the Ar–Ar dates of muscovite from this deposit, we can constrain the absolute age of tin-polymetallic mineralization in Xianghualing at 154–157 Ma. The dates obtained in this study, consistent with the published geochronological results from other important deposits in this region, reveal that the large-scale tungsten–tin mineralization in the central Nanling region was predominantly emplaced during 150–161 Ma.     

Caledonian upgrading of Proterozoic low-grade base-metal deposits in Northern Sweden

Summary Proterozoic sulfide deposits within the basement of northern Sweden have lead isotopic compositions that fall on a mixing line in the²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb diagram. These deposits contain a highly radiogenic Phanerozoic lead component that was leached from the Proterozoic basement at around 0.4 Ga during the Caledonian orogeny. Within the Proterozoic deposits, the less radiogenic lead isotopic compositions occur in undeformed and little deformed sections, while the more radiogenic lead isotopic compositions are observed along fault, fracture, and shear zones. These zones with radiogenic Phanerozoic lead also have higher contents of lead, zinc, and gold, respectively, than the other parts of the deposits, which suggests that these metals were introduced together with the radiogenic lead at a much later event than the metals in the unaltered Proterozoic deposit. The Proterozoic deposits acted as traps for metal additions along Caledonian reactivated fault and shear zones in the Proterozoic basement.

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Kaledonische Metallanreicherung in niedrighaltigen proterozoischen Buntmetallerzen in Nordschweden

Zusammenfassung Proterozoische Sulfidvererzungen im proterozoischen Grundgebirge weisen Bleiisotopenzusammensetzungen auf, die auf eine Mischungslinie im²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb Diagramm fallen. Die proterozoischen Vererzungen enthalten eine Komponente radiogenen Bleis, welches im Verlaufe der kaledonischen Orogenese aus dem proterozoischen Grundgebirge ausgelaugt wurde. Wenig oder nicht deformierte Abschnitte in den proterozoischen Sulfidvererzungen weisen weniger radiogene Bleiisotopenzusammensetzungen auf, als Bruch- und Scherzonen in denselben Vererzungen. Diese Zonen mit radiogenem, kaledonischem Blei weisen auch höhere Blei-, Zink- und Gold-Gehalte auf als die übrigen Teile der Vererzung, was andeutet, daß diese Metalle zusammen mit dem radiogenen Blei zu einem viel späteren Zeitpunkt in die Vererzung eingebracht worden sind. Die proterozoischen Vererzungen bewirkten die Metallausfällung aus Fluiden, die entlang von kaledonisch mobilisierten Verwerfungen und Scherzonen flossen.

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With 3 Figures     

Chemical and Sr isotopic characterization of North America uranium ores: Nuclear forensic applications

This study reports major, minor, and trace element data and Sr isotope ratios for 11 uranium ore (uraninite, UO_2+x) samples and one processed uranium ore concentrate (UOC) from various U.S. deposits. The uraninite investigated represent ores formed via different modes of mineralization (e.g., high- and low-temperature) and within various geological contexts, which include magmatic pegmatites, metamorphic rocks, sandstone-hosted, and roll front deposits. In situ trace element data obtained by laser ablation-ICP-MS and bulk sample Sr isotopic ratios for uraninite samples investigated here indicate distinct signatures that are highly dependent on the mode of mineralization and host rock geology. Relative to their high-temperature counterparts, low-temperature uranium ores record high U/Th ratios (>1000), low total rare earth element (REE) abundances (<1 wt%), high contents (>300 ppm) of first row transition metals (Sc, Ti, V, Cr, Mn, Co, Ni), and radiogenic ⁸⁷Sr/⁸⁶Sr ratios (>0.7200). Comparison of chondrite normalized REE patterns between uraninite and corresponding processed UOC from the same locality indicates identical patterns at different absolute concentrations. This result ultimately confirms the importance of establishing geochemical signatures of raw, uranium ore materials for attribution purposes in the forensic analysis of intercepted nuclear materials.     

Lead isotope heterogeneity in sulfides from different assemblages at the Verninskoe gold deposit (Baikal-Patom Highland,Russia)

Using the high-precision technique of MC-ICP mass spectrometry, the isotope composition of lead was studied for the first time in sulfides of different mineral associations at the Verninskoe deposit that belong to large gold deposits of the Lena Province. In 23 monofractions of sulfides (pyrite, arsenopyrite, galena, and sphalerite), the Pb-Pb data showed a pronounced heterogeneity of the isotope composition of ore lead (²⁰⁶Pb/²⁰⁴Pb = 18.21–18.69, ²⁰⁷Pb/²⁰⁴Pb = 15.59–15.67, and ²⁰⁸Pb/²⁰⁴Pb = 37.98–38.63) for the deposit as a whole. This heterogeneity is also seen to a lesser degree within individual samples. In this case, a correlation takes place between the isotope composition of ore Pb and the type of mineral association: the sulfides in earlier associations are characterized by lower contents of the ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb radiogenic isotopes compared to the minerals of later parageneses. The comparison of Pb-Pb isotope characteristics of ore mineralization of the Verninskoe deposit to those of the Sukhoi Log deposit (the greatest in the Lena Province) testifies to the geochemical similarity of the sources of ore Pb involved in the formation of these deposits. The sources as such were terrigenous rocks of the Bodaibo synclinorium formed mainly as a result of the disintegration of Precambrian rocks of the Siberian craton.