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1.
The Nizhny Tagil and Guli clinopyroxenite-dunite massifs, located in the Middle Urals and Maimecha-Kotui Province, respectively, are associated with world-class platinum-group elements (PGE) placer deposits. Both massifs contain small bodies of schlieren to massive chromitite associated with dunite. The predominance of Pt-Fe alloys at Nizhny Tagil is consistnt with the whole-rock “M”-shaped mantle-normalized PGE pattern of the chromitite. In contrast, the preponderance of laurite and Os-Ir alloys at Guli is consistent with a negatively sloped PGE pattern, the latter being characteristic of ophiolite-type podiform chromitites. The ‘unradiogenic’ 187Os/188Os values obtained for both platinum-group minerals (PGM) and chromitite are indicative of a common near-to-chondritic source for the PGE and implies that the osmium isotope budget of chromitite is largely controlled by laurite and Os-rich alloy. Average model 187Os/188Os ages calculated for the Nizhny Tagil and Guli massifs correspond to the late Riphean (e.g., 862 ± 48 Ma and 616 ± 8 Ma, respectively). The compositional and isotope-geochemical results provide new constraints on the temporal evolution of ultramafic rocks of the Uralian Platinum Belt and northern segment of the Siberian Platform.  相似文献   

2.
Summary ?Geological, mineralogical and Os isotopic data for detrital PGE-mineralization derived from the Guli and Bor-Uryah ultramafic massifs, within the Maimecha-Kotui Province (the northern part of the Siberian Platform, Russia), are presented for the first time. The detrital platinum-group minerals (PGM) are dominated by Os–Ir–(Ru) species, which is typical for ophiolites or Alpine-type complexes. However, the PGM assemblage in the placers investigated is similar to that derived from zoned platiniferous clinopyroxenite–dunite massifs (also known as Uralian-, Alaskan-type and Aldan-type massifs). The unique features of the Au-PGE placers at Guli are (1) the dominance of Os-rich alloys over other PGM and Au, and (2) the considerable predicted resources of noble metals, particularly osmium. Dominant chromite, olivine and clinopyroxene inclusions recorded in Os–Ir–(Ru) alloys imply that they were derived from ultramafic sources (e.g., chromitite, dunite and clinopyroxenite). The first in situ osmium-isotope measurements by laser ablation - multiple collector - inductively coupled plasma mass spectrometry of different, intimately intergrown, PGM (e.g., laurite and Os-rich alloys) in various nuggets from Guli have revealed low 187Os/188Os and γOs values. They yield a very narrow range of 187Os/188Os (0.12432 to 0.12472) and γOs (− 2.39 to − 2.07). These values are indicative of a common chondritic or subchondritic mantle source of PGE. 187Os/188Os and γOs values of Os-rich alloys, derived from the Bor-Uryah massif, are different (i.e., γOs ranges from − 2.67 to − 1.30). The mineral-isotopic data obtained are consistent with the conclusion that the PGM were derived from parent ultramafic source rocks. Os-isotope model ages in the range of 495 to 240 Ma constrain the age of ultramafic protoliths in the northern part of the Siberian Craton. The variation in 187Os/188Os values for detrital PGM, where the provenance source is unknown, is considered to be a useful technique for distinguishing parent bedrock sources. Received July 12, 2001; revised version accepted December 27, 2001  相似文献   

3.
The Suwałki anorthosite massif, located in extreme northeast Poland beneath more than a kilometer of Phanerozoic cover, hosts major Fe-Ti-V deposits. These deposits, discovered in 1962, are contained in Fe and Ti oxide minerals that coexist with subordinate quantities of Fe, Cu, Ni, and Co sulfides in massif-style anorthosites, norites, and gabbronorites. Accessibility and other considerations preclude development of this natural resource in the present economic climate. Detailed work by Polish geologists during the last 35 years provides a sound geologic framework for this Re-Os study of the age and origin of oxide and sulfide deposits associated with a major, but lesser known anorthosite massif. Rhenium and osmium abundances and Os isotopic compositions were measured for nine sulfides and four titanomagnetites from the Suwałki anorthosite massif. The titanomagnetites are over an order of magnitude lower in Re (0.4–1.5 ppb) and Os (0.036–0.144 ppb) concentrations than co-precipitated pyrrhotite, pyrite, and chalcopyrite that yield consistent concentrations for Re (30–55 ppb) and Os (1–6 ppb). Parallel lines connecting co-existing titanomagnetite and sulfides have slopes of ∼1 on Re versus common Os concentration plots, indicating that both Re and Os behave similarly during crystallization in their high preference for any sulfide phase over magnetite. Samples from three deposits within the anorthosite massif were analyzed. An age of 1559 ± 37 Ma (n=10) with an initial 187Os/188Os of 1.16 ± 0.06 for the Jezioro Okrągłe and Krzemianka deposits is essentially identical to an age of 1556 ± 94 Ma (n=3) for the Udryń deposit. Udryń, however, yielded a marginally lower initial 187Os/188Os of 0.87 ± 0.20. The high initial 187Os/188Os combined with the Proterozoic Re-Os age indicates that the source for Suwałki oxides-sulfides is older crust, and hypothetically, could involve Archean rocks. An average crustal value of 50 for 187Re/188Os yields a 2777 Ma age for Suwałki source rocks. Widespread Phanerozoic cover severely limits knowledge of basement rocks in Poland, however, and no Archean rocks are known in the immediate region. More likely, 187Re/188Os ratios may be higher than average continental crust, reflecting mafic crust in the source, and may move the source age for Suwałki anorthosite and mineral deposits toward younger values that easily include ∼2.0 Ga Proterozoic rocks. This more favorable case also accommodates Paleoproterozoic Nd model ages. Regardless of Archean or Proterozoic source age, the high initial 187Os/188Os ratios derived from the Re-Os isochron indicate that the source for the oxide-sulfide mineral deposits is more likely the crust and not the mantle. Given that these deposits are clearly magmatic, the Re-Os results add a new dimension to the long-standing “origin of anorthosite” problem, implying a crustal source for the anorthosite as well. The 1559 Ma Suwałki age is compatible with a well-exposed east-west band of 1530-1660 Ma rapakivi granite-anorthosite magmatism to the immediate north, transecting western Russia, southern Finland, Estonia and Latvia, and central Sweden. In particular, the age and isotopic character of Suwałki are not unlike those of the well-studied Salmi rapakivi granite-anorthosite batholith in western Russia (Karelia). Received: 4 December 1998 / Accepted: 11 November 1999  相似文献   

4.
Chromitite pods in the Mayarí-Cristal ophiolitic massif (eastern Cuba) were formed in the Late Cretaceous when island arc tholeiites and MORB-like back-arc basin basalts reacted with residual mantle peridotites and generated chromite-rich bodies enclosed in dunite envelopes. Platinum-group minerals (PGM) in the podiform chromitites exhibit important Os-isotope heterogeneities at the kilometric, hand sample and thin section scales. 187Os/188Os calculated at the time of chromitite crystallization (~90 Ma) ranges between 0.1185 and 0.1295 (γOs = −7.1 to +1.6, relative to enstatite chondrite), and all but one PGM have subchondritic 187Os/188Os. Grains in a single hand sample have initial 187Os/188Os that spans from 0.1185 to 0.1274, and in one thin section it varies between 0.1185 and 0.1232 in two PGM included in chromite which are only several millimeters apart. As the Os budget of a single micrometric grain derives from a mantle region that was at least several m3 in size, the variable Os isotopic composition of PGM in the Mayarí-Cristal chromitites probably reflects the heterogeneity of their mantle sources on the 10–100 m scale. Our results show that this heterogeneity was not erased by pooling and mingling of individual melt batches during chromitite crystallization but was transferred to the ore deposits on mineral scale. The distribution of the Os model ages calculated for PGM shows four main peaks, at ~100, 500, 750 and 1,000 Ma. These variable Os model ages reflect the presence of different depleted domains in the oceanic (Pacific-related) upper mantle of the Greater Antilles paleo-subduction zone. The concordance between the age of crystallization of the Mayarí-Cristal chromitites and the most recent peak of the Os model age distribution in PGM supports that Os in several grains was derived from fertile domains of the upper mantle, whose bulk Os isotopic composition is best approximated by that of enstatite chondrites; on the other hand, most PGM are crystallized by melts that tapped highly refractory mantle sources.  相似文献   

5.
The isotopic and geochemical characteristics of PGE mineralization in high-Mg chromitite from the banded dunite–wehrlite–clinopyroxenite complex of the Nurali lherzolite massif, the South Urals, Russia is characterized for the first time. Electron microprobe analysis and LA MC-ICP-MS mass spectrometry are used for studying Cr-spinel and platinum-group minerals (PGM). Two processes synchronously develop in high-Mg chromitite subject to metamorphism: (1) the replacement of Mg–Al-rich Cr-spinel, orthopyroxene, and diopside by chromite, Cr-amphibole, chlorite, and garnet; (2) the formation of a secondary mineral assemblage consisting of finely dispersed ruthenium or Ru-hexaferrum aggregate and silicate–oxide or silicate matter on the location of primary Ru–Os-sulfides of the laurite–erlichmanite solid solution series. Similar variations of Os-isotopic composition in both primary and secondary PGM assemblages are evidence for the high stability of the Os isotope system in PGM and for the possibility of using model 187Os/188Os ages in geodynamic reconstructions.  相似文献   

6.
This study evaluates in detail the mineral chemistry, whole-rock and mineral separate Os-isotope compositions of distinct platinum-group mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Os-rich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of ‘unradiogenic’ 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from ‘primary’ PGM assemblage (0.12473–0.12488) and similar ‘unradiogenic’ 187Os/188Os values for both ‘primary’ and ‘secondary’ PGM assemblages (0.1242 ± 0.0008 and 0.1245 ± 0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240 ± 0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re–Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a supra-subduction zone setting shortly before obduction.  相似文献   

7.
Malitch  K. N.  Kogarko  L. N. 《Doklady Earth Sciences》2011,440(2):1455-1459
This contribution firstly presents particularities of mineral chemistry of platinum-group elements (PGE) mineralization from placer deposits linked to the Bor-Uryakh massif of the Maimecha-Kotui Province, northern part of the Siberian Craton. The chemical composition of PGE mineralization has been studied by electron microprobe analysis. At Bor-Uryakh, main platinum-group minerals (PGM) comprise Os-Ir and Pt-Fe alloys represented by individual crystals, and polyphase PGM assemblages. The majority (e.g., 12 out of 19) of the Os-rich nuggets are iridian osmium, with subordinate amounts of native osmium (Os) and chengdeite (Ir3Fe). Pt-Fe alloys have a stoichiometric composition close to Pt2Fe. According to the nomen-clature by L. Cabri and C. Feather [1975] these minerals correspond to ferroan platinum. Based on geological position and geochemical features of investigated PGE mineralization the particular rock sources have been established. This study has demonstrated the similarity of chemical characteristics of Os-Ir and Pt-Fe alloys of the Bor-Uryakh massif to those of PGM from the Guli massif (Maimecha-Kotui Province), platiniferous zoned-type ultramafic massifs (e.g., Kondyor, Inagli and Chad) of the Aldan Province and Platinum belt of the Urals (Nizhny Tagil, Kytlym, etc.).  相似文献   

8.
The Yaogangxian deposit in the central Nanling region, South China consists of vein-type ore bodies hosted in Cambrian to Jurassic strata and Mesozoic granitic intrusions. Wolframite and molybdenite are the dominant ore minerals intergrown with gangue minerals of quartz, feldspar, phlogopite, and muscovite. We have carried out molybdenite Re–Os and phlogopite and muscovite 40Ar/39Ar dating to better understand the timing and genesis of mineralization. Re–Os dating of eight molybdenite samples yielded model ages ranging from 152.0±3.5 to 161.1±4.5 Ma, with an average of 156.0 Ma. The Re–Os analyses give a well-defined 187Re/187Os isochron with an age of 154.9±2.6 Ma (MSWD=2.4). Hydrothermal phlogopite and muscovite display extremely flat 40Ar/39Ar age spectra. Phlogopite yields a 40Ar/39Ar plateau age of 153.0±1.1 Ma, whereas muscovite yields a plateau age of 155.1±1.1 Ma. Both 40Ar/39Ar ages are in good agreement with the Re–Os ages, placing the timing of tungsten mineralization at about 154 Ma. This age is consistent with the field relationships. Our new data, when combined with published geochronological results from other major deposits in this region, suggest that large scale W–Sn mineralization occurred throughout the central Nanling region in the Late Jurassic.  相似文献   

9.
A Re–Os isochron age is reported for massive sulfides from near the basal contact of the Radio Hill layered mafic‐ultramafic intrusion in the west Pilbara Craton, Western Australia. The isochron age is 2892 ± 34 Ma (mean square of weighted deviates = 1.06) with an initial 187Os/188Os = 0.1265 ± 0.0028. This age is in agreement with the ages of other nearby layered mafic intrusions that are considered to have a similar geological evolution to the Radio Hill Intrusion.  相似文献   

10.
会泽超大型铅锌矿是滇东北铅锌多金属成矿域中典型的密西西比河谷型(MVT)或会泽型(HZT)矿床,因其独特的成矿系统以及矿床中富锗而被地质学者熟知,由于该类型矿床成矿温度较低且缺少合适的定年矿物,其成矿时代一直存在较大的争议。本文在会泽铅锌矿麒麟厂矿区1584中段0-11号穿脉坑道块状铅锌硫化物矿石中挑选了9件硫化物样品(黄铁矿、方铅矿和闪锌矿),采用负离子热表面电离质谱法进行Re-Os同位素分析,获得Re-Os等时线年龄为40.7±2.6 Ma(n=9),与模式年龄加权平均值40.0±2.6Ma(n=8)在误差内完全一致,闪锌矿和方铅矿模式年龄分别为38.24±0.41 Ma和36.57±0.40 Ma。上述同位素年龄揭示了会泽超大型铅锌矿的成矿时代可能为始新世。结合滇东北铅锌矿集区NE向逆冲断层和冲断褶皱控矿构造区域构造解析以及断裂、矿体构造-岩相蚀变特征,提出会泽超大型铅锌矿经历了燕山期、喜山期两阶段构造-流体贯入的成矿作用模型。  相似文献   

11.
Mineralogical studies of the heavy fraction from a Holocene pyrope-rich garnet placer deposit at Vestřev (Krkonoše Piedmont Basin, Bohemian Massif) have identified the presence of very rare grains of platinum group minerals (PGM). Pt–Fe alloy grains are accompanied by Os–Ir–Ru minerals (native osmium, iridium, and ruthenium) with inclusions of Pt–Fe alloy and hongshiite (PtCu). This mineral assemblage is typical for several mantle settings including ophiolites. The chemistry of the Os–Ir–Ru minerals shows an enrichment of the PGM in Ru, which is typical of ophiolites. The grain morphology of PGM and pyrope-rich garnet (mostly rounded with numerous euhedral/subhedral grains) does not exclude a common source. In-situ laser-ablation MC-ICP-MS was used to measure the Re–Os isotopic compositions of single Os-rich grains, which show heterogeneous subchondritic Os isotopic compositions (187Os/188Os = 0.12082–0.12505 ± 0.00003). This precludes their low-temperature origin and indicates derivation of platinum-group elements (PGEs) essentially from mantle-derived rocks without a significant contribution of crustal Os. The mantle model age (TMA) and Re-depletion model age (TRD) model ages range from ~ 0.4 to ~ 1.0 Ga and most likely reflect a long history of melt depletion that affected the mantle sources of PGM.  相似文献   

12.
The Raobazhai ultramafic massif of the ultrahigh pressure Sulu–Dabie orogenic belt, central China, is thought to be a segment of subcontinental lithospheric mantle that was subducted and exhumed during the Triassic collision of the North China and Yangtze cratons. We performed a Re–Os isotopic study of peridotites from the massif, associated with major and trace element analysis and textural examination. Os (1.02 to 6.28 ppb) and Re (0.004 to 0.376 ppb) concentrations are typical of orogenic lherzolite values, and 187Os/188Os ratios (0.1157 to 0.1283) are all similar to or lower than the proposed primitive upper mantle value. 187Os/188Os is roughly correlated with 187Re/188Os, and strongly correlated with Al2O3. These correlations can be explained by radiogenic ingrowth of 187Os since an ancient partial melting event. TMA model ages (1.7 to 2.0 Ga) of refractory peridotites from the lower massif are consistent with the model age (1.8 Ga) obtained from the 187Os/188Os vs. Al2O3 correlation at ~1% Al2O3. This age cannot distinguish the cratonic provenance of the Raobazhai massif, since similar Re–Os model ages have been obtained from both the North China and the Yangtze cratons. The poor quality of the 187Os/188Os vs. 187Re/188Os correlation indicates that the Re/Os ratios were disturbed, perhaps during Triassic subduction. The mainly lherzolitic samples of the upper massif, which were most strongly affected by this process, have porphyroclastic textures with fine-grained olivine, pyroxene and amphibole neoblasts, suggesting Re mobility during recrystallization in the presence of fluids.Previous studies of ultramafic xenoliths from arc volcanics demonstrate that slab-derived melts or fluids can both scavenge mantle Os and add substantial amounts of radiogenic Os to the suprasubduction mantle. In Raobazhai, both trace element patterns and the abundance of hydrous phases provide evidence for extensive interaction with fluids during subduction and/or exhumation. Nevertheless, the strong correlation between 187Os/188Os and Al2O3, and the high Os concentrations of these rocks indicate that Os isotopic ratios, and probably even Os concentrations, were essentially unaffected by this process. Assuming that the arguments favoring a suprasubduction setting for the Raobazhai massif are valid, these data provide evidence that Os systematics are sometimes surprisingly robust, even above subduction zones.  相似文献   

13.
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   

14.
Re-Os isotopes were used to constrain the source of the ore-forming elements of the Tharsis and Rio Tinto mines of the Iberian Pyrite Belt, and the timing of mineralization. The pyrite from both mines has simila]r Os and Re concentrations, ranging between 0.05–0.7 and 0.6–66 ppb, respectively. 187Re/188Os ratios range from about 14 to 5161. Pyrite-rich ore samples from the massive ore of Tharsis and two samples of stockwork ore from Rio Tinto yield an isochron with an age of 346 ± 26 Ma, and an initial 187Os/188Os ratio of about 0.69. Five samples from Tharsis yield an age of 353 ± 44 Ma with an initial 187Os/188Os ratio of about 0.37. A sample of massive sulfide ore from Tharsis and one from Rio Tinto lie well above both isochrons and could represent Re mobilization after mineralization. The pyrite Re-Os ages agree with the paleontological age of 350 Ma of the black shales in which the ores are disseminated. Our data do not permit us to determine whether the Re-Os isochron yields the original age of ore deposition or the age of the Hercynian metamorphism that affected the ores. However, the reasonable Re-Os age reported here indicates that the complex history of the ores that occurred after the severe metamorphic event that affected the Iberian Pyrite Belt massive sulfide deposits did not fundamentally disturb the Re-Os geochronologic system. The highly radiogenic initial Os isotopic ratio agrees with previous Pb isotopic studies. If the initial ratio is recording the initial and not the metamorphic conditions, then the data indicate that the source of the metals was largely crustal. The continental margin sediments that underlie the deposits (phyllite-quartzite group) or the volcanic rocks (volcanogenic-sedimentary complex) in which the ores occur are plausible sources for the ore-forming metals and should constrain the models for the genesis of these deposits. Received: 15 March 1999 / Accepted: 26 July 1999  相似文献   

15.
The absolute timing of epigenetic mineralization, including most types of gold deposits, is difficult to resolve due to the absence of suitable minerals in veins and replacement zones. However, gold is commonly closely associated with pyrite and arsenopyrite, which may be amenable to Re–Os geochronology, providing sufficient Re and Os are present within them. This short paper outlines the use of this method to date two gold deposits in Newfoundland using pyrite. Although the Os contents of the pyrites are extremely low (≪0.1 ppb), the Os is almost exclusively radiogenic 187Os, and data are amenable to model age calculations, as used in Re–Os molybdenite dating. The pyrites from these deposits correspond to low-level highly radiogenic sulphides, as defined by other studies. The Stog’er Tight and Pine Cove gold deposits yield mean Re–Os model ages of 411 ± 7 Ma (n = 4) and 420 ± 7 Ma (n = 5), respectively, which agree with isochron regression of 187Os against 187Re. The Re–Os age for Stog’er Tight is within uncertainty of a previous U–Pb age from ‘hydrothermal’ zircon (420 ± 5 Ma) in spatially related alteration. A latest Silurian–earliest Devonian age for the mineralization is consistent with indirect age constraints from some other gold deposits in central Newfoundland and suggests a broad temporal link to the mid-Silurian Salinic Orogeny. However, the gold mineralization appears to be younger than most plutonic activity associated with this event. The results illustrate the potential value of Re–Os pyrite geochronology in understanding the temporal framework of epigenetic mineralization, especially if future improvements in analytical precision and reductions in procedural blanks allow wider application to material with similarly low Re and Os concentrations.  相似文献   

16.
《International Geology Review》2012,54(14):1783-1791
The Chibaisong magmatic Cu–Ni sulphide deposit is located in Tonghua City, Jilin Province, in the eastern part of the northern margin of the North China Craton. The geological characteristics of the deposit have been investigated, and pyrrhotite Re–Os isotope dating has been utilized to constrain the age. Five pyrrhotite samples separated from the Chibaisong Cu–Ni sulphide deposit yielded a Re–Os isotopic isochron age of 2237 ± 62 Ma (mean squared weighted deviation = 1.13, n = 5), indicating that the only Palaeoproterozoic magmatic Cu–Ni sulphide deposit in China is the Chibaisong Cu–Ni sulphide deposit. The geodynamic setting during ore formation was related to the Liaoning–Jilin Palaeoproterozoic rift split. The Re–Os isotope analyses showed an initial 187Os/188Os ratio of 0.778 ± 0.033, and (187Os/188Os)i and γOs(t) values ranged from 0.7531 to 0.8013 (average 0.7734) and from 574 to 617 (average 592), respectively, indicating that abundant crustal material (5–10%) was mixed with the Cu–Ni sulphide ore system during magma ascent and ore formation.  相似文献   

17.
We report the first Re-Os data on gold-associated arsenopyrite from mesothermal gold-quartz veins in the ancient Egyptian Fawakhir–El Sid gold mining district in the central Eastern Desert. This mining district has an ~5000-year-old history and is displayed in the Turin Papyrus Map (about 1150 BC), which is widely acclaimed as the world’s oldest geographic map, as well as the oldest geologic and mine map. The Fawakhir–El Sid district is part of a regional NNW-trending shear corridor (15 km wide) that hosts several other historic gold mines associated with left-lateral wrench structures and related granite intrusions. Vein-style gold mineralization is hosted within and at the margin of an I-type and magnetite-series monzogranite, the Fawakhir granite intrusion, and a Pan-African (~740 Ma) ophiolite sequence. The ore mineralogy of the mineralized quartz veins includes pyrite-arsenopyrite-pyrrhotite-sphalerite-galena-chalcopyrite-electrum plus a number of tellurides of Ag, Au, and Bi. The 187Re/188Os versus 187Os/188Os regression on 5 points of arsenopyrite gives an age of 601 ± 17 Ma with an initial 187Os/188Os of 0.24 ± 0.07 (2 σ; MSWD = 17). This age coincides within error with the U-Pb age on zircon from the Fawakhir monzogranite (598 ± 3 Ma). The age coincidence and the hydrothermal Te and Bi metal signature suggest a foremost role of granite-related fluids in the quartz-vein system.  相似文献   

18.
The Ransko gabbro–peridotite massif in Eastern Bohemia is a strongly differentiated intrusive complex, which hosts low-grade Ni–Cu ores mainly developed close to the contact of olivine-rich rocks with gabbros, in troctolites, and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase-rich peridotites. Gabbro, troctolite, peridotite and Ni–Cu ores from the Jezírka Ni–Cu (PGE) deposit, considered to be a typical example of the liquid segregation style of mineralization, were analyzed for Re–Os concentrations and isotopic ratios. Seven barren and mineralized samples from the Jezírka deposit yielded a Re–Os regression of 341.5?±?7.9 Ma (MSWD?=?69). Strongly mineralized peridotite with mantle-like initial 187Os/188Os ratio of 0.125 suggests that Os as well as other PGE present in the Ni–Cu mineralization are predominantly of mantle origin. On the other hand, barren and low-mineralized samples have radiogenic initial 187Os/188Os ratios of 0.14–0.16 suggesting some import of Re and/or radiogenic 187Os most likely through contamination by continental crust during magma emplacement. The Re–Os age of the Ransko Massif is significantly younger than the previously suggested Lower Cambrian age, but it is similar to and/or younger than the age of metamorphism of the adjacent Kutná Hora crystalline complex and the Moldanubian unit. Therefore, it is likely that the emplacement of the Ransko massif and its Ni–Cu mineralization was closely connected with the late-stage evolution of the Kutná Hora crystalline complex.  相似文献   

19.
《Geochimica et cosmochimica acta》1999,63(23-24):4005-4012
Previous studies have shown that 187Os/188Os in seawater has become increasingly radiogenic over the last 40 Ma in a manner analogous to strontium. This rapid rise in the marine 187Os/188Os over the last 17 Ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the Himalayas and/or selective weathering and erosion of highly radiogenic organic rich ancient sediments. The key test of this hypothesis is the 187Os/188Os and the total osmium concentration of the Himalayan rivers. We report the concentration and isotopic composition of osmium in the Ganges, the Brahmaputra, and the Indus rivers. The 187Os/188Os of the Ganges close to its source (at Kaudiyal, 30°05′N, 78°50′E) is 2.65 and [Os] = 45 fM/kg. A second sample of the lower reaches of the Ganges at Patna (25°30′N, 85°10′E) gives 187Os/188Os =1.59 and [Os] = 171 fM/kg. The 187Os/188Os of the Brahmaputra at Guwahati (26°10′N, 91°58′E) is 1.07 and [Os] = 52 fM/kg. A sample of the Indus (Besham, 34°55′N, 72°51′E) has a 187Os/188Os of 1.2 and [Os] = 59 fM/kg. We infer that the Himalayas do not provide either a high flow of osmium or a highly radiogenic osmium component to the oceans. The overall trend for osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the Himalayas themselves appear not to be the dominant source.  相似文献   

20.
Rhenium and osmium isotopes in sulfide minerals from the Bagdad porphyry Cu–Mo deposit have been used to determine timing of mineralization and the source of osmium and, by inference, ore metals. Molybdenite, chalcopyrite and pyrite were analyzed mainly from the quartz monzonite and porphyritic quartz monzonite units, which are characterized by moderate to strong potassic alteration (secondary biotite and K-feldspar). Rhenium concentrations in molybdenite are between 330 and 642 ppm. Four Re–Os analyses of two molybdenite samples from the quartz monzonite and porphyritic quartz monzonite yield a weighted average age of 71.8±0.2 Ma (2s). Analyses of a third sample from a molybdenite vein in Precambrian rocks, outside of the main ore zone, yield a weighted average age of 75.9±0.2 Ma (2s), and provide evidence of two separate mineralization episodes. Chalcopyrite samples contain 6 to 12 ppt Os and 1.7 to 4.1 ppb Re; 187Os/ 188Os initial ratios are between 0.1 and 0.8. Pyrite samples have osmium and rhenium concentrations varying in the range 8–17 ppt and 3.9–6.8 ppb, respectively. Analyses from these pyrite samples yield an eight-point isochron with an age of 77±15 Ma (2s) and an initial 187Os/ 188Os ratio of 2.1±0.8 (MSWD=0.90). The results presented here add to the growing body of work indicating that porphyry-type mineralization is produced by long-term, multiple episodes of magmatism and associated mineralization. The data also support the hypothesis that a significant part of the metals and magmas may have a crustal source, as has been suggested for other copper deposits and districts in Arizona.  相似文献   

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