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1.
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   

2.
Lead isotope analyses have been performed on the two major Miocene mining districts of Romania, Baia Mare and Apuseni Mountains. These two districts have different non-overlapping 206Pb/204Pb isotopic signatures ranging from 18.752 to 18.876 and 18.497 to 18.740. In the Baia Mare district, epithermal deposits are overall homogeneous in their lead isotopic compositions and have values similar to the average of the calc-alkaline volcanic rocks. These results suggest a magmatic signature for the Pb (and possibly other metals) in the hydrothermal fluids. However, magmas in this district show isotopic evidence of crustal assimilation. In the southern Apuseni Mountains, the lead isotope compositions of sulfide minerals in porphyry copper deposits are clustered, confirming that Pb, and probably other metals, were derived principally from associated porphyry stocks. On the other hand, lead isotope data on sulfides in epithermal ore deposits are much more scattered, indicating a notable contribution of Pb from local country rocks. In the Apuseni Mountains, 'fertile' volcanics are few and appear to come from a more primitive mantle-derived source. Most of the analysed volcanic rocks seem 'barren'. Differences in lead isotopic compositions between the Baia Mare district and the Apuseni Mountains are due to a different basement, and probably to variations in crustal assimilation superimposed on variations in the mantle source composition. In the Apuseni Mountains, Pb may be partly inherited from the previous Mesozoic magmatic-hydrothermal stage. From a geodynamic point of view, it seems that the nature and the source of volcanic rocks and their position related to the collision area of the Carpathian arc are not the only factors controlling the 'fertility' of a volcanic district.  相似文献   

3.
The Skellefte district in northern Sweden is host to abundant volcanogenic massive sulphide (VMS) deposits comprising pyritic, massive, semi-massive and disseminated Zn–Cu–Au ± Pb ores surrounded by disseminated pyrite and with or without stockwork mineralisation. The VMS deposits are associated with Palaeoproterozoic upper crustal extension (D1) that resulted in the development of normal faults and related transfer faults. The VMS ores formed as sub-seafloor replacement in both felsic volcaniclastic and sedimentary rocks and partly as exhalative deposits within the uppermost part of the volcanic stratigraphy. Subsequently, the district was subjected to deformation (D2) during crustal shortening. Comparing the distribution of VMS deposits with the regional fault pattern reveals a close spatial relationship of VMS deposits to the faults that formed during crustal extension (D1) utilising the syn-extensional faults as fluid conduits. Analysing the shape and orientation of VMS ore bodies shows how their deformation pattern mimics those of the hosting structures and results from the overprinting D2 deformation. Furthermore, regional structural transitions are imitated in the deformation patterns of the ore bodies. Plotting the aspect ratios of VMS ore bodies and the comparison with undeformed equivalents in the Hokuroko district, Japan allow an estimation of apparent strain and show correlation with the D2 deformation intensity of the certain structural domains. A comparison of the size of VMS deposits with their location shows that the smallest deposits are not related to known high-strain zones and the largest deposits are associated with regional-scale high-strain zones. The comparison of distribution and size with the pattern of high-strain zones provides an important tool for regional-scale mineral exploration in the Skellefte district, whereas the analysis of ore body shape and orientation can aid near-mine exploration activities.  相似文献   

4.
The Bajiazi deposits of western Liaoning Province, northeastern China, are aligned approximately west-east in an 8-× 15-km district. The Zn-Pb-Ag-FeS2 sulfide ores are hosted in intensively folded and faulted Early Proterozoic carbonates and minor clastic rocks, near their contact with Late Jurassic-Early Cretaceous Bajiazi granite. The ore-bearing strata on both limbs of the Bajiazi syncline can be correlated throughout the district. Orebodies are stratigraphically conformable with the host rocks and mostly parallel the contact metamorphic halo of the granite, in the skarn mineralizing zone. The ores are composed of stratabound and stratiform sulfide lenses and layers, are associated with sediments formed in local depressions of a tectonically active, shallow-marine environment, and are limited to dolomitized tidal flat and lagoonal facies with cryptalgal lamination and to adjacent oolitic grainstones of barrier facies. The ores consist of fine- to medium-grained intergrowths of sulfides in dolomite and locally skarn gangues. Sulfides are intensely recrystallized and annealed in the contact metamorphic halo. Isotopes of sulfur, carbon, oxygen, and lead were examined for genetic interpretation of the sulfides and host rocks. Sulfur isotope values range from ?9.1 to 13.0 per mil for sulfides, suggesting sedimentary exhalative origin, though they were, to some extent, homogenized during late overprinting events. The δ18O values range between ?6 and ?15 per mil PDB, and the δ13C values between +1 and ?5 per mil PDB. Late crystallization generations are enriched in the light isotopes of oxygen and carbon compared to primary ones. Lead isotope ratios in galenas from all Bajiazi ores have means of 206Pb/204Pb = 16.23, 207Pb/204Pb = 15.23, and 208Pb/204Pb = 36.51. These isotope ratios, characterized by a model age of about 1350 Ma, suggest an early Proterozoic, crustal source for the lead in the ores, unrelated to the lead in the Mesozoic granite. All evidence identified in this study suggests that the Bajiazi polymetallic sulfide deposits formed as sedimentary exhalative accumulations in a Lower Proterozoic, shallow-marine sedimentary basin. The present distribution of the ores and their distinctive textures indicate overprinting by. contemporaneous and/or post-depositional tectonic processes, as well as contact metamorphism during the emplacement of the Mesozoic Bajiazi granite.  相似文献   

5.
Galena and other sulphides are widespread in the Old Red Sandstone (Devonian) Orcadian Basin. Syndiagenetic occurrences are associated with organic-rich sedimentary rocks, especially algal limestones. Epigenetic galenas are often spatially related to these syndiagenetic occurrences, particularly in the vicinity of Permo-Carboniferous igneous intrusions. Mineralization is concentrated about the granite-gneiss basement inlier in southwest Orkney.Orcadian Basin galenas have lead isotope compositions which show a linear trend related to Lewisian granulite facies metamorphism at about 2,700 Ma. The most radiogenic leads must include a component of average orogene or upper crustal lead. The isotopic compositions are notably similar to those of Caledonian granite feldspar leads measured by Blaxland et al. (1979). Granite was probably a source for lead in galena deposits about the basement in south-west Orkney, but away from the basement an average orogene component would have become increasingly important. There was no major input of magmatic lead to the basin during Permo-Carboniferous times, only a remobilization of existing lead.  相似文献   

6.
The Boliden deposit (8.3 Mt at 15.9 g/t Au) is interpreted to have been formed between ca. 1894 and 1891 Ma, based on two new U–Pb ID-TIMS ages: a maximum age of 1893.9?+?2.0/?1.9 Ma obtained from an altered quartz and feldspar porphyritic rhyolite in the deposit footwall in the volcanic Skellefte group and a minimum age of 1890.8?±?1 Ma obtained from a felsic mass-flow deposit in the lowermost part of the volcano-sedimentary Vargfors group, which forms the stratigraphic hanging wall to the deposit. These ages are in agreement with the alteration and mineralization being formed at or near the sea floor in the volcanogenic massive sulfide environment. These two ages and the geologic relationships imply that: (1) volcanism and hydrothermal activity in the Skellefte group were initiated earlier than 1.89 Ga which was previously considered to be the onset of volcanism in the Skellefte group; (2) the volcano-sedimentary succession of the Vargfors group is perhaps as old as 1892 Ma in the eastern part of the Skellefte district; and (3) an early (synvolcanic) deformation event in the Skellefte group is evidenced by the unconformity between the ≤1893.9?+?2.0/?1.9 Ma Skellefte group upper volcanic rocks and the ≤1890.8?±?1 Ma Vargfors sedimentary and volcanic rocks in the Boliden domain. Differential block tilting, uplift, and subsidence controlled by synvolcanic faults in an extensional environment is likely, perhaps explaining some hybrid VMS-epithermal characteristics shown by the VMS deposits of the district.  相似文献   

7.
The El Faldeo is the southernmost Andean mineral district and consists of Paleozoic polymetamorphic basement unconformably overlain by a 200 m thick homoclinal sequence of sedimentary and felsic volcanic beds of the Ibáñez formation. In addition, two epizonal intrusive units and related subvolcanic porphyries, sills and dikes which intrude both the metamorphic basement and the Ibáñez formation, are recognized: the Quebrada Colorada Granodiorite (QCG) and the Cerro Esmeralda Tonalite (CET). The polymetallic mineralization was formed in four stages and includes pyrite, arsenopyrite, gold, silver, sphalerite, galena and chalcopyrite hosted in felsic tuffs of the Ibáñez formation, hydrothermal breccias and epizonal intrusions. Field observations and geochronological data (U-Pb, Ar-Ar and fission track) on CET rocks suggest: (1) the development of subsiding pull-apart basins and coeval formation of spaces for magmatic emplacement and mineralization in an extensional tectonic regime in mid-Jurassic time; (2) a tectonic exhumation of the rocks of the district, probably triggered by the Chile ridge–trench interactions that started in Late Miocene time. Sr-Nd isotopic data from CET samples, and the Pb isotope characteristics of galenas from the second and third mineralization stages, are indicative of crustal contribution to the origin of the magmas and galena mineralization. Rocks of the Paleozoic metamorphic basement are prime candidates for the crustal source components of the analyzed galenas.  相似文献   

8.
The Jiapigou gold belt (>150 t Au), one of the most important gold-producing districts in China, is located at the northeastern margin of the North China Craton. It is composed of 17 gold deposits with an average grade around 10 g/t Au. The deposits are hosted in Archean gneiss and TTG rocks, and are all in shear zones or fractures of varying orientations and magnitudes. The δ34S values of sulfide from ores are mainly between 2.7?‰ and 10?‰. The Pb isotope characteristics of ore sulfides are different from those of the Archean metamorphic rocks and Mesozoic granites and dikes, and indicate that they have different lead sources. The sulfur and lead isotope compositions imply that the ore-forming materials might originate from multiple, mainly deep sources. Fluid inclusions in pyrite have 3He/4He ratios of 0.6 to 2.5 Ra, whereas their 40Ar/36Ar ratios range from 1,444 to 9,805, indicating a dominantly mantle fluid with a negligible crustal component. δ18O values calculated from hydrothermal quartz are between ?0.2?‰ and +5.9?‰, and δD values of the fluids in the fluid inclusions in quartz are from ?70?‰ to ?96?‰. These ranges suggest dominantly magmatic water with a minor meteoric component. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore-forming fluids have a dominantly mantle source with minor crustal addition.  相似文献   

9.
Major elements and Re–Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Quaternary intraplate basalts from the Tariat volcanic field, Central Mongolia. The sulfides are dominantly high-temperature (>900 °C) Fe-rich monosulfide solid solution (MSS). Some sulfides with low Ni contents may be residual MSS, whereas other sulfides defining a negative Ni–Cu correlation may be crystallization products of fractionated sulfide melts. The subchondritic 187Re/188Os and 187Os/188Os of some sulfides also indicate they are residual MSS. Os isotope compositions of sulfides reveal the presence of Archean to Proterozoic lithospheric mantle beneath the region. The sulfides have TRD model ages ranging from 3.0 to 0.2 Ga, with peaks at 1.5–1.3, 1 and 0.7–0.5 Ga. The peak ages are indicative of significant events in the lithospheric mantle at those times. The timing of these events is remarkably consistent with those of the major crust-building events within the Tarvagatay Terrane where the Tariat volcanic field is located. The similarity in the ranges of crustal U–Pb ages and Nd model ages, and our sulfide Os model ages, suggests that the sulfide ages may date metasomatic events in the underlying lithospheric mantle, which were related to tectonothermal events that affected the overlying crust. Radiometric ages from the Tarvagatay Terrane appear to correspond to the Archean model ages from its SCLM counterpart. The last two events (1.1 and 0.7–0.5 Ga) recorded in the Tarvagatay Terrane suggest involvement of the “CAOB mantle” and development of significant juvenile crustal growth in the orogeny.  相似文献   

10.
The isotopic composition of lead from galenas and feldspars of granitic rocks covering a time span of 3600 m.y. is reviewed. Hew data are combined with data from the literature, all ratios being normalized to a fixed value for the isotopic composition of lead in an inter-laboratory reference sample. Comparison of feldspar and galena results show that the isotopic composition of initial lead in granitic rocks may be identical to, or more radiogenic than, that in coeval stratiform (or ‘single stage’) ore deposits. The isotopic evolution of terrestrial lead over the past 3600 m.y. deduced from the stratiform ore data does not fit the closed system source. The data are best fitted to evolution models in which the value of μ (U238/Pb204 normalized to the present day) in the source has increased either over approximately the past 3.6 billion years or since the time of formation of the Earth. A limited number of high precision data for strontium likewise indicate open system isotopic evolution. In this system the Rb/Sr ratio appears to have decreased as a function of time. Physical models that explain the data are presented. Either mantle differentiation or crustal contamination processes could account for the change in μ in the source for the lead. The strontium data and some detailed studies of the isotopic composition of lead in oceanic volcanic rocks support mantle differentiation rather than crustal contamination. The exact nature of the mantle differentiation process cannot be specified, although empirical data favoring the existence of such a process are given. The best estimate of the age of the Earth, calculated with the data from 2750 and 3290 m.y.-old lead ores, is 4660 m.y.  相似文献   

11.
The Lince–Estefanía stratabound copper deposit in the Michilla district is one of the most important deposits in the Coastal Cordillera of northern Chile and is one of the most representative of this type of deposit. Chalcocite and bornite characterize the main stage of hypogene copper sulfide mineralization. Rhenium and osmium isotopes are used here to constrain the age of hypogene mineralization and the source of osmium contained in these ore minerals. A Re–Os isochron yielded an age of 160±16 Ma (2σ), with an associated initial 187Os/188Os ratio of 1.06±0.09 (mean square of weighted deviates=1.8). This age is consistent with available geochronological data from volcanic rocks that host the mineralization and associated alteration phases. The high initial 187Os/188Os ratio indicates a lower crustal component for the source of Os and, by inference, the Cu sulfides that contain this Os. Late hematite occurs as an isolated phase or, more commonly, is associated with the chalcocite–bornite and supergene chalcocite–covellite associations. Analyses performed on pure hematite indicate a disturbance of the Re–Os system, and hence, this mineral phase is not useful as a Re–Os geochronometer.  相似文献   

12.
The Skellefte District in northern Sweden is a roughly 150 by 50 km2 large early Proterozoic massive sulphide belt. Based on high-resolution reflection seismic data along two parallel seismic profiles, potential field modeling has been carried out and two geologic cross sections have been constructed that are consistent with the available geophysical data as well as surface geologic observations. The combined modelling suggests that the Kristineberg deposit occurs on the northern limb of a regional E–W striking syncline. The interpretations help to identify new prospective areas, both down-plunge from known ores, and on the ore-bearing horizon on the southern limb of the syncline. The new results suggest that the post-orogenic Revsund granites can be divided into two major types of intrusives, those which are intruded as domes/stokes with a maximum present day thickness of about 3–3.5 km and those which are intruded as thin sheets, with a maximum thickness of a few hundred meters. The margins of the intrusions are generally inclined inwards, suggesting that the current erosion level is near the middle, or toward the base, of the granites. The contact between the Skellefte volcanic rocks and the Bothnian Basin has been interpreted as a thrust fault. We also suggest that crustal thickening predates the Skellefte volcanism and that the interpreted Bothnian Basin rocks are either a structural basement or a separate terrane to the Skellefte volcanism. Diffraction patterns in the reflection seismic data can be interpreted as originating from either a mafic–ultramafic intrusion or a mineralization zone, similar to observations elsewhere in the world. The results obtained in this study have greatly improved our understanding of the tectonostratigraphic framework and architecture of the poly-deformed c. 1.9 Ga Skellefte VHMS belt and is a key step towards building a 3D geological model in the area.  相似文献   

13.
费利东  肖晓牛  肖娥  刘军  白涛 《现代地质》2020,34(3):579-587
滇中播卡铜矿床是著名“东川式”铜矿床的典型代表,但对其成矿物质来源和矿床成因认识存在争议。对矿区典型铜矿发育的金属硫化物进行硫和铅同位素组成分析,探讨其成矿物质来源。硫同位素测试结果表明,人占石铜矿金属硫化物的δ34S值为1.6‰~10.7‰,指示硫以岩浆(火山喷发)作用为主要来源,并受到沉积作用影响。铅同位素测试结果则表明人占石铜矿、天生塘铜矿、竹箐凹子铜矿和白石岩铜矿中的铅主要来源于壳幔混合物质。综合前人研究和本次硫、铅同位素分析结果,认为播卡铜矿床成矿物质主要来自地幔,且受到地壳物质的混染。  相似文献   

14.
Most attention has been given to the geology of the extensive VMS and subordinate precious metals mineralization in the Skellefte district. Less attention has been given to indications of deep-seated origins of felsic and mafic/ultramafic volcanic rocks; of VMS and precious metals mineralizing fluids; and the primary origins of these metals. A holistic view of the significance of mafic/ultramafic volcanic rocks to both the geotectonic evolution of the area and the existence of its important base and precious metals deposits has never been presented. These subjects are discussed in this investigation.Primitive mantle normalized spider diagrams of rare-earth-elements (REE) distinguish two groups of mafic/ultramafic volcanic rocks, each with distinct geochemical characteristics: a mid-ocean-ridge “MORB”-type, and a geochemically unusual and problematic calc–alkaline–basalt “CAB”-type which is the main subject of this investigation. The “MORB”-type mafic volcanic rocks are mostly older than the Skellefte Group felsic volcanic rocks hosting the VMS deposits, whereas the more primitive “CAB”-type mafic/ultramafic volcanic rocks are mostly younger.A common source for these “CAB”-type, mafic-(MgO wt.% < 14%) and ultramafic-(MgO wt.% > 14%) volcanic rocks is suggested by their similar and distinctive geochemical features. These are near-chondritic (Al-undepleted) Al2O3/TiO2 ratios; moderate to strong high-field-strength-element (HFSE) depletion; light-rare-earth-element (LREE) enrichment and moderate heavy-rare-earth-element (HREE) depletion. They outcrop throughout an area of at least 100 × 100 km. Gold mineralization is spatially associated with ultramafic volcanic rocks.Zr and Hf depletion has been shown to be associated with Al-depletion in mafic/ultramafic volcanic rocks elsewhere, and has been attributed to deep-seated partial melting in ascending mantle plumes. Zr and Hf depletion in “CAB”-type Al-undepleted mafic/ultramafic volcanic rocks is therefore unusual. The solution to this dilemma is suggested to be contamination of an Al-depleted mantle plume by felsic crustal rocks whereby Al-depleted ultramafic magmas become Al-undepleted. It will be argued that this model has the potential to explain previous observations of deep-seated origins; the spatial association of ultramafic volcanic rocks with occurrences of gold mineralization; and even the primary origin of metals in VMS deposits.  相似文献   

15.
The Lovozero alkaline massif—an agpaitic nepheline syenite layered intrusion—is located in the central part of the Kola Peninsula, Russia, and belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Associated loparite and eudialyte deposits, which contain immense resources of REE, Nb, Ta, and Zr, constitute a world class mineral district. Previous Sr, Nd, and Hf isotope investigations demonstrated that these rocks and mineral deposits were derived from a depleted mantle source. However, because the Sr, Nd, and Hf abundances in the Kola alkaline rocks are significantly elevated, their isotopic compositions were relatively insensitive to contamination by the underlying crustal rocks through which the intruding magmas passed. Pb occurring in relatively lower abundance in the KACP rocks, by contrast, would have been a more sensitive indicator of an acquired crustal component. Here, we investigate the lead isotopic signature of representative types of Lovozero rocks in order to further characterize their sources. The measured Pb isotopic composition was corrected using the determined U and Th concentrations to the age of the crystallization of the intrusion (376?±?28 Ma, based on a 206Pb/204Pb versus 238U/204Pb isochron and 373?±?9 Ma, from a 208Pb/204Pb versus 232Th/204Pb isochron). Unlike the previously investigated Sr, Nd, and Hf isotopes, the lead isotopic composition plot was well outside the FOZO field. The 206Pb/204Pb values fall within the depleted MORB field, with some rocks having lower 207Pb/204Pb but higher 208Pb/204Pb values. Together with other related carbonatites having both lower and higher 206Pb/204Pb values, the combined KACP rocks form an extended linear array defining either a?~2.5-Ga secondary isochron or a mixing line. The projection of this isotopic array toward the very unradiogenic composition of underlying 2.4–2.5-Ga basaltic rocks of the Matachewan superplume and associated Archean granulite facies country rock provides strong evidence that this old lower crust was the contaminant responsible for the deviation of the Lovozero rocks from a presumed original FOZO lead isotopic composition. Evaluating the presence of such a lower crustal component in the Lovozero rock samples suggests a 5–10% contamination by such rocks. Contamination by upper crustal rock is limited to only a negligible amount.  相似文献   

16.
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.  相似文献   

17.
Many Archaean mesothermal gold deposits are spatially associated with felsic to lamprophyric minor intrusions and it has been suggested that magmatic processes related to such intrusions may be important in the genesis of these deposits. A comparison of the Pb-isotopic signature of gold-related galenas from Kambalda and Norseman with that of spatially associated minor intrusions (at the time of mineralization) indicates that the ore-fluid Pb cannot have been derived solely from the intrusions or their source regions. For both study areas, the galena Pb-isotopic compositions are bracketed by those of local volcanic (mafic) and intrusive (largely felsic) rock types. This is consistent with the ore fluid having derived metallic components from the crust (or crustally derived granitic rocks) and the mantle (or mantle-derived rocks of the greenstone succession) via metamorphic dewatering or mantle/crustal degassing. Interaction of granite-derived magmatic fluids with greenstone lithologies could plausibly produce a similar array of Pb-isotopic signatures. The Norseman data, as a whole, are more radiogenic than the Kambalda data for broadly synchronous mineralization, reflecting the greater abundance of older granitic rocks with respect to mafic/ultramafic rocks in the Norseman district. The provinciality exhibited by the Pb-isotopic composition of the ore fluid indicates that the gold-mineralizing process formed galena whose Pb-isotopic composition was very sensitive to local variations in crustal Pb-isotopic composition, either within the source region of the fluid or along fluid conduits.  相似文献   

18.
The Pb and Sr isotope ratios of Plio-Pleistocene volcanic rocks from the Aleutian volcanic arc are used as tracers of the lithospheric subduction process at the converging Pacific and Bering plates. Aleutian arc lavas do not have the same Pb isotopic compositions as volcanic rocks of the subducted Pacific ocean crust or the nearby Pribilof Islands, but appear to contain an ‘old continental crustal component’ with high 207Pb/204Pb ratio, as has been found in some other volcanic arcs.87Sr/86Sr ratios in the Aleutian volcanic arc rocks average 0.70322, slightly higher than fresh volcanic rocks from normal ridge segments, but within the range of values from ‘Icelandic’ ridge segments, oceanic islands and the Pribolof Islands. The Pb and Sr isotopic compositions of Aleutian lavas show a positive correlation and the range of values does not change for volcanoes distributed along strike in the arc, even though the crustal type in the hanging wall of the Benioff zone changes from oceanic in the west to continental in the east. Since the basement of the continental arc segment is older than the basement of the oceanic segment, and probably has a different isotopic character, the constancy of isotopic ratios along the arc argues against contamination by wall rocks of the type exposed in the arc.A sufficient explanation for the isotopic data is the mixture of several per cent of continent-derived sediment with melt derived from the underthrust oceanic crust and overlying mantle. This small amount of contaminant is difficult to document by geophysical observations. Such a model implies extensive recycling of Ba, Pb, K and Rb through volcanism at convergent plate margins like the Aleutians.  相似文献   

19.
The Kristineberg volcanic-hosted massive sulphide (VMS) deposit, located in the westernmost part of the Palaeoproterozoic Skellefte district, northern Sweden, has yielded 22.4 Mt of ore, grading 1.0% Cu, 3.64% Zn, 0.24% Pb, 1.24 g/t Au, 36 g/t Ag and 25.9% S, since the mine opened in 1941, and is the largest past and present VMS mine in the district. The deposit is hosted in a thick pile of felsic to intermediate and minor mafic metavolcanic rocks of the Skellefte Group, which forms the lowest stratigraphic unit in the district and hosts more than 85 known massive sulphide deposits. The Kristineberg deposit is situated lower in the Skellefte Group than most other deposits. It comprises three main ore zones: (1) massive sulphide lenses of the A-ore (historically the main ore), having a strike length of about 1,400 m, and extending from surface to about 1,200 m depth, (2) massive sulphide lenses of the B-ore, situated 100–150 m structurally above the A-ore, and extending from surface to about 1,000 m depth, (3) the recently discovered Einarsson zone, which occurs in the vicinity of the B-ore at about 1,000 m depth, and consists mainly of Au–Cu-rich veins and heavily disseminated sulphides, together with massive sulphide lenses. On a regional scale the Kristineberg deposit is flanked by two major felsic rock units: massive rhyolite A to the south and the mine porphyry to the north. The three main ore zones lie within a schistose, deformed and metamorphosed package of hydrothermally altered, dominantly felsic volcanic rocks, which contain varying proportions of quartz, muscovite, chlorite, phlogopite, pyrite, cordierite and andalusite. The strongest alteration occurs within 5–10 m of the ore lenses. Stratigraphic younging within the mine area is uncertain as primary bedding and volcanic textures are absent due to strong alteration, and tectonic folding and shearing. In the vicinity of the ore lenses, hydrothermal alteration has produced both Mg-rich assemblages (Mg-chlorite, cordierite, phlogopite and locally talc) and quartz–muscovite–andalusite assemblages. Both types of assemblages commonly contain disseminated pyrite. The sequence of volcanic and ore-forming events at Kristineberg is poorly constrained, as the ages of the massive rhyolite and mine porphyry are unknown, and younging indicators are absent apart from local metal zoning in the A-ores. Regional structural trends, however, suggest that the sequence youngs to the south. The A- and B-ores are interpreted to have formed as synvolcanic sulphide sheets that were originally separated by some 100–150 m of volcanic rocks. The Einarsson zone, which is developed close to the 1,000 m level, is interpreted to have resulted in part from folding and dislocation of the B-ore sulphide sheet, and in part from remobilisation of sulphides into small Zn-rich massive sulphide lenses and late Au–Cu-rich veins. However, the abundance of strongly altered, andalusite-bearing rocks in the Einarsson zone, coupled with the occurrence of Au–Cu-rich disseminated sulphides in these rocks, suggests that some of the mineralisation was synvolcanic and formed from strongly acidic hydrothermal fluids. Editorial handling: P. Weihed  相似文献   

20.
The lead isotopic compositions of galena in Early Proterozoic gold deposits have been determined for three districts in northern Sweden and central Finland. The deposits are hosted by a variety of 1870–1890 Ma Svecofennian host rocks including the volcanosedimentary succession within the Skellefte District island arc in Sweden as well as I-type tonalites at Jörn (Sweden) and Pohjanmaa (Finland). The deposits are epigenetic in relation to these Svecofennian rocks and are part of a goldbearing metallogenetic belt, which can be followed for 600 km parallel to the southwestern margin of the Archaean Domain. In spite of these epigenetic relationships, the lead isotopic data indicate that the deposits are not dramatically younger than the 1870–1890 Ma Svecofennian host rocks (probably not exceeding 10–20 million years). Two principal lead sources were activated when the gold deposits were formed. The most significant source is represented by the I-type tonalites, which constitute a relatively primitive ( = 9.3) and widely distributed source in the entire metallogenic belt. In addition, the volcanic components in the westernmost part of the Skellefte District constitute an extremely primitive ( <9.0) source, which only locally was an important contributor to the epigenetic deposits in this metallogenetic belt. The significantly different lead isotopic composition estimated for these sources indicates that the volcanic rocks in the western part of the Skellefte District were not comagmatic with the I-type tonalites recognized at Jörn and central Finland.  相似文献   

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