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1.
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.  相似文献   

2.
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.  相似文献   

3.
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.
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.

With 3 Figures  相似文献   

4.
Lead isotope determinations were made on galenas from three strata-bound sulfide ores in the early Proterozoic (Svecokarelian) Bergslagen district of central Sweden and four epigenetic deposits in the Älvdalen and Vansbro districts in the early to middle Proterozoic post-Svecokarelian belt. The leads from the strata-bound Bergslagen deposits show exceptional isotopic homogeneity over large areas. Their isotopic composition suggests the existence of a pre-Svecokarelian crust in the district and is consistent with exhalative-sedimentary ore formation in an active continental margin environment. The Vansbro and Älvdalen leads display constant compositions within each district, but marked divergence between the districts. Their compositions preclude derivation exclusively from recycled Svecokarelian lead and suggest a substantial lead contribution from a mantle-like source. The difference between model ages and geologic ages for many of the deposits, with a small but significant excess of radiogenic lead, suggests a significant deviation of the Fennoscandian Shield from conventional global lead evolution models.  相似文献   

5.
A lead isotope study of mineralization in the Saudi Arabian Shield   总被引：1，自引：0，他引：1  
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.  相似文献   

6.
Summary The Dachang Sn-polymetallic ore district is one of the largest tin producing districts in China. Its origin has long been in dispute between magmatic-hydrothermal replacement and submarine exhalative-hydrothermal origin. The Dachang ore district comprises several types of ore deposits, including the Lamo magmatogenic skarn deposit near a granite intrusion, the Changpo-Tongkeng bedded and vein-type sulfide deposit, and the Gaofeng massive sulfide deposit. Sulfide minerals from the Lamo skarn ores show δ34S values in the range between −3 and +4‰ with a mean close to zero, suggesting a major magmatic sulfur source that likely was the intrusive Longxianggai granite. Sulfide minerals from the Gaofeng massive ores show higher δ34S values between +5 and +12‰, whereas sulfide minerals from the Changpo-Tongkeng bedded ores display lighter δ34S values between −7 and −0.2‰. The difference in the sulfur isotope ranges in the two deposits can be interpreted by different degrees of inorganic thermochemcial reduction of marine sulfate using a one-step batch separation fractionation model. Sulfur isotopic compositions from the vein-type ores at Changpo-Tongkeng vary widely from −8 to +4‰, but most of the data cluster around −2.9‰, which is close to that of bedded ores (−3.6‰). The sulfur in vein-type ores might be derived from bedded ores or it represents a mixture of magmatic- and sedimentary-derived sulfur. Pb isotopic compositions of sulfide minerals in the Dachang ore district reveal a difference between massive and bedded ores, with the massive ores displaying more radiogenic Pb isotope ratios. Correlations of 206Pb/204Pb and 207Pb/204Pb or 208Pb/204Pb for the massive and bedded ores are interpreted as two-component mixing of Pb leached from sedimentary host rocks and from deep-seated Precambrian basement rocks composed of metamorphosed volcano-sedimentary rocks. Pb isotopic compositions of sulfide minerals from vein-type ores overlap with those of bedded sulfides. Similar to the sulfur, the lead in vein-type ores might be derived from bedded ores. Skarn ores at Lamo show very limited variations in Pb isotopic compositions, which may reflect a major magmatic-hydrothermal lead source. Helium isotope data of fluid inclusions trapped in sulfides indicate that He in the massive and bedded ores has a different origin than He in fluorite of granite-related veins. The 3He/4He ratios of 1.2–2.9 Ra of fluid inclusions from sulfides at Gaofeng and Changpo-Tongkeng imply a contribution of mantle-derived fluids. Overall our data support a submarine exhalative-hydrothermal origin for the massive and bedded ore types at Dachang. Supplementary material to this paper is available in electronic form at Appendix available as electronic supplementary material  相似文献   

7.
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 σ6/4 = 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.  相似文献   

8.
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  相似文献   

9.
The Bainiuchang deposit in Yunnan Province,China,is located geographically between the Gejiu ore field and the Dulong ore field.In addition to the>7000 t Ag reserves,the deposit also boasts of large-scale Pb,Zn and Sn reserves with a lot of dispersed elements(In,Cd,Ge,Ga,etc.).We have determined systematically the Pb isotope composition of the deposit.The Pb isotope ratios of the ores that are of sea-floor exhalative sedimentary origin in the northwest of the mining district,are ~(206)Pb/~(204)Pb=17.758-18.537,~(207)pb/~(204)pb=15.175-15.862 and ~(208)pb/~(204)pb=37.289-39.424,while those of ores that are of magmatic hydrothermal superimposition origin in the southeast of the mining district, are ~(206)Pb/~(204)Pb=17.264-18.359,~(207)Pb/~(204)Pb=14.843-15.683 and ~(208)Pb/~(204)Pb=36.481-38.838, respectively.In terms of the Pb isotope composition of feldspar in magmatic rocks or magmatic whole- rock samples from the mining district,we have determined the Pb isotope composition and acquired the Pb isotope ratios as:~(206)Pb/~(204)Pb=18.224-18.700,~(207)Tpb/~(204)Pb=15.595-15.797 and ~(208)Pb/~(204)Pb= 38.193-39.608.Then,in the light of the Pb isotope composition of metamorphic rock samples from the Proterozoic basement exposed in the Dulong ore field,we have determined the Pb isotope composition and obtained the isotope ratios as:~(206)Pb/~(204)Pb=18.434-19.119,~(207)Pb/~(204)Pb=15.644-15.693,and ~(208)Pb/~(204)Pb=38.514-38.832.And the Pb isotope ratios of Cambrian sedimentary rocks,which are exposed in the Bainiuchang mining district,are ~(206)Pb/~(204)Pb=18.307-19.206,~(207)Pb/~(204)Pb= 15.622-15.809,and ~(206)Pb/~(204)Pb=38.436-39.932.By comparing the two types of ores with respect to their Pb isotope compositions,it is indicated that lead in the Bainiuchang deposit was derived largely from the lower-crust granulite which is earlier than Neoproterozoic in age,but the Yanshanian magmatic hydrothermal fluids probably provided a part of ore-forming elements such as Sn for the ore blocks in the south of the mining district.  相似文献   

10.
The Nanmushu Zn‐Pb deposit, hosted by the Neoproterozoic Dengying Formation dolostone, is located in the eastern part of the Micangshan tectonic belt at the northern margin of the Yangtze Craton, China. This study involves a systematic field investigation, detailed mineralogical study, and Rb‐Sr and Pb isotopic analyses of the deposit. The results of Rb‐Sr isotopic dating of coexisting sphalerite and galena yield an isochron age of 486.7 ± 3.1 Ma, indicating the deposit was formed during the Late Cambrian to Early Ordovician. This mineralization age is interpreted to be related to the timing of destruction of the paleo‐oil reservoir in the Micangshan tectonic belt. All initial 87Sr/86Sr ratios of sphalerite and galena (0.70955–0.71212) fall into the range of the Mesoproterozoic Huodiya Group basement rocks (0.70877–0.71997) and Dengying Formation sandstone (0.70927–0.71282), which are significantly higher than those of Cambrian Guojiaba Formation limestone (0.70750–0.70980), Cambrian Guojiaba Formation carbonaceous slate (0.70766–0.71012), and Neoproterozoic Dengying Formation dolostone (0.70835–0.70876). Such Sr isotope signatures suggest that the ore strontium was mainly derived from a mixed source, and both of the Huodiya Group basement rocks and Dengying Formation sandstone were involved in ore formation. Both sphalerite and galena are characterized by an upper‐crustal source of lead (206Pb/204Pb = 17.849–18.022, 207Pb/204Pb = 15.604–15.809, and 208Pb/204Pb = 37.735–38.402), and their Pb isotopes are higher than, but partly overlap with, those of the Huodiya Group basement rocks, but differ from those of the Guojiaba and Dengying Formations. This suggests that the lead also originated from a mixed source, and the Huodiya Group basement rocks played a significant role. The Sr and Pb isotopic results suggest that the Huodiya Group basement rocks were one of the most important sources of metallogenic material. The geological and geochemical characteristics show that the Nanmushu Zn‐Pb deposit is similar to typical Mississippi Valley type, and the fluid mixing may be a reasonable metallogenic mechanism for Nanmushu Zn‐Pb deposit.  相似文献   

11.
《International Geology Review》2012,54(13):1478-1507
The Central and Eastern Taurides contain numerous carbonate-hosted Pb–Zn deposits, mainly in Devonian and Permian dolomitized reefal–stramatolitic limestones, and in massive Jurassic limestones. We present and compare new fluid inclusion and isotopic data from these ore deposits, and propose for the first time a Mississippi Valley-type (MVT) mode of origin for them.

Fluid inclusion studies reveal that the ore fluids were highly saline (13–26% NaCl equiv.), chloride-rich (CaCl2) brines, and have average homogenization temperatures of 112°C, 174.5°C, and 211°C for the Celal Da?, Delikkaya, and Ayrakl? deposits, respectively. Furthermore, the δ34S values of carbonate-hosted Pb–Zn deposits in the Central and Eastern Taurides vary between –5.4‰ and?+13.70‰. This indicates a possible source of sulphur from both organic compounds and crustal materials. In contrast, stable sulphur isotope data (average δ34S –0.15‰) for the Çad?rkaya deposit, which is related to a late Eocene–Oligocene (?) granodioritic intrusion, indicates a magmatic source. The lead isotope ratios of galena for all investigated deposits are heterogeneous. In particular, with the exception of the Suçat? district, all deposits in the Eastern (Delikkaya, Ayrakl?, Denizovas?, Çad?rkaya) and Central (Katranba??, Küçüksu) Taurides have high radiogenic lead isotope values (206Pb/204Pb between 19.058 and 18.622; 207Pb/204Pb between 16.058 and 15.568; and 208Pb/204Pb between 39.869 and 38.748), typical of the upper continental crust and orogenic belts.

Fluid inclusion, stable sulphur, and radiogenic lead isotope studies indicate that carbonate-hosted metal deposits in the Eastern (except for the Çad?rkaya deposit) and the Central Taurides are similar to MVT Pb–Zn deposits described elsewhere. The primary MVT deposits are associated with the Late Cretaceous–Palaeocene closure of the Tethyan Ocean, and formed during the transition from an extensional to a compressional regime. Palaeogene nappes that typically limit the exposure of ore bodies indicate a pre-Palaeocene age of ore formation. Host rock lithology, ore mineralogy, fluid inclusion, and sulphur?+?lead isotope data indicate that the metals were most probably leached from a crustal source such as clastic rocks or a crystalline massif, and transported by chloride-rich hydrothermal solutions to the site of deposition. Localization of the ore deposits on autochthonous basement highs indicates long-term basinal fluid migration, characteristic of MVT depositional processes. The primary MVT ores were oxidized in the Miocene, resulting in deposition of Zn-carbonate and Pb-sulphate–carbonate during karstification. The ores underwent multiple cycles of oxidation and, in places, were re-deposited to form clastic deposits. Modified deposits resemble the ‘wall-rock replacement’ and the ‘residual and karst fill’ of non-sulphide zinc deposits and are predominantly composed of smithsonite.  相似文献   

12.
Abstract The Dajiangping pyrite deposit located in the middle sector of the Yunkai uplift in western Guangdong is a stratiform sulphide deposit occurring in Sinian marine clastic and fine clastic rocks. The formation of the deposit was related to submarine exhalation and hot brine deposition. A part of it was reformed by late-stage hydro thermal solution. The δ34S values of pyrite vary from — 25.55‰ to + 21.07‰, which are inversely proportional to the content of organic carbon in ore and pyrite. Passing from striped fine-grained pyrite ore to massive coarse-grained pyrite ore, i.e. from south to north, the sulphur isotopic composition changes from the light sulphur-enriched one to the heavy sulphur-enriched one. The lead isotopic composition of striped ore is consistent with that of the country locks of orebodies and the lead is radiogenic lead derived from the upper crust. The lead isotopic composition of massive ore is relatively homogeneous and its 206/Pb204Pb, 207/Pb204Pb and 208/Pb204Pb ratios are a bit lower than those of striped ore; the lead result from mixing of synsedimentary ore lead with that derived from basement migmatite brought by late-stage hydrothermal solutions.  相似文献   

13.
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.  相似文献   

14.
The isotopic composition of Pb in pyrite of the Mindyak orogenic gold deposit located in the Main Ural Fault Zone, the Southern Urals, has been studied by the high-precision MC-ICP-MS method. Orebodies at the deposit are composed of early pyrite and late polysulfide–carbonate–quartz mineral assemblages. The orebodies are localized in olistostrome with carbonaceous clayey-cherty cement. Pyrites from early and late mineral assemblages are close in Pb isotope ratios. For early pyrite 206Pb/204Pb = 18.250–18.336, 207Pb/204Pb = 15.645–15.653, 208Pb/204Pb = 38.179–38.461; while for late pyrite 206Pb/204Pb = 18.102–18.378, 207Pb/204Pb = 15.635–15.646, 208Pb/204Pb = 38.149–38.320. The model parameters μ2 (238U/204Pb = 9.91 ± 2), ω2 (232Th/204Pb = 38.5 ± 4), and 232Th/238U = 3.88 ± 3 indicate that an upper crustal Pb source played a leading role in ore formation. Carbonaceous shale as an olistostrome cement and syngenetic sulfide mineralization are considered to be the main Pb sources of both early and late mineral assemblages. An additional recept in apparently magmatic lead is suggested for the late veinlet mineralization. The involvement of lead from several sources in ore formation is consistent with the genetic model, which assumes a two-stage formation of orebodies at the Mindyak deposit.  相似文献   

15.
Lead (and U-Pb) isotope data for sulphides and whole rocks, U-3Pb data for zircons, and Rb-Sr data for whole rocks have been determined in an attempt to elucidate the processes by which the volcanic Pb-Zn-Cu deposit at Woodlawn, southeastern N.S.W., was formed, and to relate this information to current theories of crustal effects in the genesis of volcanic Pb-Zn deposits.The lead isotope compositions of pyrite, galena, sphalerite, and chalcopyrite from the ore horizon are the same, and identical to the initial lead isotope compositions of pyrite in the host volcanics.Linear relations are obtained for plots of 207Pb/ 204Pb vs 206Pb/204Pb, 208Pb/204Pb vs 204Pb/204Pb, and 238U/204Pb vs 206Pb/204Pb for least altered whole rock volcanics, indicating no loss or gain of U to these rocks since their formation. The similar initial 206Pb/204Pb ratios in the sulphides and host volcanics suggest a common source for the lead. However, acid leach experiments, carried out to remove the ore lead influence, suggest the lead to be a complex mixture of ore and rock lead. The differences observed for high and low lead samples in the acid leaching experiments suggest that these tests may provide an additional tool in prospecting for base-metal sulphides of this type.U-Pb data for zircons from the whole rocks give a spectrum of apparent ages ranging from 428 to 477 m.y. reflecting varying amounts of older zircons or resetting of the U-Pb systems in the volcanics. The older zircons are present either as discrete rounded crystals or cores surrounded by new euhedral growth. The populations and U-Pb data suggest the rocks were pre-existing volcanics with a possible detrital component, whose ages have not been fully reset during remelting.In contrast to the zircon data, U-Pb and Rb-Sr whole rock data define lines proportional to ages of 413±6 m.y. and 409±4 m.y. (1.39×10–11/yr) respectively and the strontium has a relatively high initial ratio of 0.710.The complex zircon population, high initial Sr ratio, Th/U ratios and rare earth data suggest the Woodlawn volcanics were formed by multistage remelting of material of similar chemical composition. Consequently, conformable deposits of this type should not be discussed in terms of the Growth Curve and single stage models of lead development.Mineralization is thought to have occurred at 420 m.y., either from solutions associated with the volcanism and/or concentrated from the volcanics and sediments by circulating sea water, in a shallow convective cell and soon after, or during, the formation of the pile. Alternatively, the multistage processes operating during formation of the Woodlawn volcanics acted as an effective homogeniser of lead isotopes.  相似文献   

16.
Soil O and C horizon samples (N = 752) were collected at a sample density of 1 site/36 km2 in Nord-Trøndelag and parts of Sør-Trøndelag (c. 25,000 km2), and analysed for Pb and three of the four naturally occurring Pb isotopes (206Pb, 207Pb and 208Pb) in a HNO3/HCl extraction. Soil O and C horizons are decoupled in terms of both Pb concentrations and Pb isotope ratios. In the soil C horizon the Grong-Olden Culmination, a continuous exposure of the Precambrian crystalline basement across the general grain of the Caledonian orogen, is marked by a distinct 206Pb/207Pb isotope ratio anomaly. No clear regional or even local patterns are detected when mapping the Pb isotope ratios in the soil O horizon samples. Variation in the isotope ratios declines significantly from the soil C to the O horizon. On average, Pb concentrations in the O horizon are four times higher and the 206Pb/207Pb isotope ratio is shifted towards a median of 1.15 in comparison to 1.27 in the C horizon. It is demonstrated that natural processes like weathering in combination with plant uptake need to be taken into account in order to distinguish anthropogenic input from natural influences on Pb concentration and the 206Pb/207Pb isotope ratio in the soil O horizon.  相似文献   

17.
Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (206Pb/204Pb), 15.622 (207Pb/204Pb) and 38.191 (208Pb/204Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (206Pb/204Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.  相似文献   

18.
论“华夏古大陆”—铅同位素研究证据   总被引：10，自引：1，他引：9  
张理刚  吴克隆 《地质论评》1994,40(3):200-208
我国东南部地区一批中生代花岗岩类岩石中长石铅同位素数据表明,至少可将该地区地壳基底岩石划分三大块体。一是武夷块体,其范围包括江－绍断裂带以南,赣江－河源断裂以东和政和一大埔断裂中,南段以西地区,它们的中生代花岗岩长石铅同位素＾２０６Ｐｂ／＾２０４Ｐｂ,＾２０７Ｐｂ／＾２０４Ｐｂ和＾２０４Ｐｂ／＾２０４Ｐｂ三组比值平均分别为１８．２２６;１５．６２０和３８．７２５;另一块体为闽台块体,即政和一大埔断  相似文献   

19.
More than 20 sediment-hosted massive sulphide deposits occur in Late Palaeozoic basins in South China. These deposits are accompanied by a certain amount of volcanic rocks in the host sequence and are economically important for their Cu, Pb, Zn, Au and Ag reserves. The deposits and their host strata were commonly intruded by Mesozoic granitoids. Remobilisation of sedimentary ores and magmatic hydrothermal overprinting processes resulted in the coexistence of massive sulphides with vein-, skarn- and porphyry-type orebodies in the same region or within a single deposit. The ore-containing basins occur in different tectonic settings. The Lower Yangtze basin occurs on a passive continental margin, where the deposits are high in Cu and Au with minor Pb and Zn and recoverable Ag, Co and Mo. The ores have a lower concentration of radiogenic lead, and δ34S values close to zero. Fluid inclusions are highly saline and Na-rich. Fluids and metals of the Lower Yangtze Region are interpreted to have been derived essentially from deep sources including the Precambrian basement. By contrast, basins of the Nanling Region formed in an intracontinental setting developed on a folded Caledonian basement. These deposits are higher in Pb, Zn, Sn and W, as well as Cu, with recoverable Ag, Sb, Hg, U, Bi, Tl and Mo. The ores are characterised by a higher concentration of radiogenic lead and a wide variation of δ34S composition. Fluid inclusions have lower salinities and higher K+/Na+ ratios. Fluids are considered to have been sourced substantially from seawater by convection. Metals for the Nanling deposits were essentially derived from the Caledonian basement by leaching. The contrast in ore composition between these two regions appears to have been controlled by differences in basement composition of the ore-forming basins.  相似文献   

20.
Ore lead isotope ratios have been analysed in galenas and sulphosalts from nineteen massive sulphide deposits hosted by Svecofennian (1.9 Ga) supracrustals in the Skellefte ore district, northern Sweden. The ore lead data can be grouped on the basis of their geographical distribution. Most probably, this feature is reflecting a number of lead isotopic provinces which correspond to crustal blocks. The obtained ore lead data define linear trends in conventional Pb-Pb plots. The ore lead signatures are due to synvolcanic mixing processes as lead was leached from mafic and acid volcanic sources. The relationships inferred for initial ratios in source rocks at 1.89 Ga require a pre-Svecofennian crustal history. It is suggested that magmatic processes at c. 2.0 Ga involved recycling of Archean sedimentary material into the mantle and the formation of a crystalline, felsic basement. The metasomatized mantle and the basement melted at the time of Svecofennian magmatism (1.89 Ga) which created mafic and acid magma sources. Subsequently, basalts and rhyolites were extruded onto the sea floor. Ore was formed as hot solutions penetrated isotopically different levels of the volcanic pile.  相似文献   

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