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1.
This study firstly presents chemical and initial Os-isotopic compositions of Os-Ir-Ru minerals of two ultramafic formations of Polar Siberia, which are exemplified by Guli clinopyroxene-dunite massif of the Maimecha-Kotui Province and the Kunar dunite-harzburgite massif from the Chelyuskin ultramafic belt of the Taimyr Peninsula. The study employed a range of methods, including electron microprobe analysis, negative thermal ionization mass spectrometry (N-TIMS) and laser ablation attached to an inductively coupled plasma mass spectrometry (LA MC-ICP-MS). The majority of platinum-group minerals (PGM) from the Guli massif are Os-(Ir-Ru) solid solutions or Os-rich minerals. At Kunar, minerals of Ru-Os-Ir system (i.e., osmium, ruthenium, iridium and rutheniridosmine) dominate the PGM assemblage. The ruthenium trend in the mineral compositions is due to the formation of these minerals under high pressures and temperatures at considerable depths. The 187Os/188Os values of Os-rich minerals from the Guli massif range from 0.12309 ± 0.00002 to 0.12606 ± 0.00003 (n = 168). The initial Os-isotopic composition of PGM from the central block of the Guli massif is characterized by the 187Os/188Os values, varying in the range 0.12404–0.12606. Osmiumrich minerals from the southwestern block of the Guli massif are characterized by the least “radiogenic” 187Os/188Os values (i.e., 0.12309–0.12341). Low relative to the chondritic universal reservoir (CHUR) 187Os/188Os values are indicative of a near-to-chondritic source of platinum-group elements (PGE). The most “productive” stage of PGM formation at Guli (n = 121) is recorded in the time interval of 545–615 Ma. The older model 187Os/188Os ages of osmium minerals are characteristic of the southwestern block of the Guli massif (e.g., 745–760 Ma). The results of the initial Os-isotopic composition for Os-rich alloys are consistent with a model, in which PGM were formed during multi-stage melt depletion events in the mantle. This agrees well with the suggestion that the Guli massif consists of heterogeneous blocks of ultramafic rocks. The 187Os/188Os ratio in the investigated PGM from the Kunar massif varies in a wider range (0.1094–0.1241, n = 28). For the dominant set of PGM samples (n = 25), regardless of their chemical composition, four groups of the initial osmium isotopic compositions can be estimated, with average 187Os/188Os values of 0.1217 ± 0.0002 (n = 7), 0.1223 ± 0.0002 (n = 7), 0.1230 ± 0.0002 (n = 6) and 0.1238 ± 0.0003 (n = 6), respectively. The average model Re-Os ages for the defined groups of the Kunar massif are consistent with Late Riphean age interval (e.g., 975 ± 42 Ma, 892 ± 42 Ma, 791 ± 28 Ma and 681 ± 42 Ma, respectively). Significant variations in the 187Os/188Os values and model ages for Ru-Os-Ir alloys at Kunar are close to those from other duniteharzburgite massifs of the Earth, pointing out for their prolonged multi-stage evolution within the upper mantle.  相似文献   

2.
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   

3.
The Mayarí-Baracoa ophiolitic belt in eastern Cuba hosts abundant chromite deposits of historical economic importance. Among these deposits, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43–0.55) to high Cr (Cr#=0.60–0.83) compositions. Platinum-group element (PGE) contents are also variable (from 33 ppb to 1.88 ppm) and correlate positively with the Cr# of the ore. Bulk PGE abundances correlate negatively with the Pd/Ir ratio showing that chromite concentrates mainly Os, Ir and Ru which gives rise to the characteristic negatively sloped, chrondrite-normalized PGE patterns in many chromitites. This is consistent with the mineralogy of PGEs, which is dominated by members of the laurite–erlichmanite solid solution series (RuS2–OsS2), with minor amounts of irarsite (IrAsS), Os–Ir alloys, Ru–Os–Ir–Fe–Ni alloys, Ni–Rh–As, and sulfides of Ir, Os, Rh, Cu, Ni, and/or Pd. Measured 187Os/188Os ratios (from 0.1304 to 0.1230) are among the lower values reported for podiform chromitites. The 187Os/188Os ratios decrease with increasing whole-rock PGE contents and Cr# of chromite. Furthermore, γOs values of all but one of the chromitite samples are negative indicating a subchondiritc mantle source. γOs decrease with increasing bulk Os content and decreasing 187Re/188Os ratios. These mineralogical and geochemical features are interpreted in terms of chromite crystallization from melts varying in composition from back-arc basalts (Al-rich chromite) to boninites (Cr-rich chromite) in a suprasubduction zone setting. Chromite crystallization occurs as a consequence of magma mixing and assimilation of preexisting gabbro sills at the mantle–crust transition zone. Cr#, PGE abundances, and bulk Os isotopic composition of chromitites are determined by the combined effects of mantle source heterogeneity, the degree of partial melting, the extent of melt-rock interactions, and the local sulfur fugacity. Small-scale (μm to cm) chemical and isotopic heterogeneities in the platinum-group minerals are controlled by the mechanism(s) of chromite crystallization in a heterogeneous environment created by the turbulent regime generated by successive inputs of different batches of melt.  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.
Ultramafic rocks around the city of Muğla in SW Turkey are represented by mantle peridotites depleted to various degrees, ranging from cpx-rich harzburgites to depleted harzburgite and dunite. Cpx-rich harzburgites are thought to be the residua left after extraction of MORB-type basalt, from which high-Al chromitite [49.2 < Cr# = 100 × Cr/(Cr + Al) < 53.5] crystallised with a higher proportion of 187Os/188Os (average of 0.1361). However, depleted harzburgites are assumed to be the residua left after extraction of hydrous boninitic melt produced by second stage partial melting of already depleted mantle due to a subducting slab, from which high-Cr chromitites (64.2 < Cr# < 85.9) with lower and heterogeneous 187Os/188Os ratio (average of 0.1324) were crystallised as a result of melt–rock interaction in a supra-subduction environment. Dunites around the chromite deposits are considered to be the product of melt–peridotite interaction. Most of the chromitites contain high-Cr chromite and display enrichment in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), with PGE concentrations between 61 and 1,305 ppb. Consistently, laurite-erlichmanite series minerals with various Os concentrations are found to be the most abundant PGM inclusions in chromite. Os–Ir–Ru alloy, irarsite, and kashinite, as well as Pt–Fe alloy and Pt-oxide, which are not common in ophiolitic chromitites, were also detected as magmatic PGM inclusions. Pentlandite, millerite, and, rarely heazlewoodite form the magmatic inclusions of base-metal sulphide. The presence of olivine and clinopyroxene, as well as hydrous silicate inclusions such as amphibole and phlogopite, in high-Cr chromitite supports the idea that high-Cr chromitites were formed in a supra-subduction environment.  相似文献   

6.
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   

7.
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   

8.
Separation of a metal-rich core strongly depleted the silicate portion of the Earth in highly siderophile elements (HSE), including Pt, Re, and Os. To address the issues of how early differentiation, partial melting, and enrichment processes may have affected the relative abundances of the HSE in the upper mantle, 187Os/188Os and 186Os/188Os data for chondrites are compared with data for Os-rich alloys from upper mantle peridotites. Given that 187Os and 186Os are decay products of 187Re and 190Pt, respectively, these ratios can be used to constrain the long-term Re/Os and Pt/Os of mantle reservoirs in comparison to chondrites. Because of isotopic homogeneity, H-group ordinary and other equilibrated chondrites may be most suitable for defining the initial 186Os/188Os of the solar system. The 186Os/188Os ratios for five H-group ordinary chondrites range only from 0.1198384 to 0.1198408, with an average of 0.1198398 ± 0.0000016 (2σ). Using the measured Pt/Os and 186Os/188Os for each chondrite, the calculated initial 186Os/188Os at 4.567 Ga is 0.1198269 ± 0.0000014 (2σ). This is the current best estimate for the initial 186Os/188Os of the bulk solar system. The mantle evolution of 186Os/188Os can be defined via examination of mantle-derived materials with well-constrained ages and low Pt/Os. Two types of mantle-derived materials that can be used for this task are komatiites and Os-rich alloys. The alloys are particularly valuable in that they have little or no Re or Pt, thus, when formed, evolution of both 187Os/188Os and 186Os/188Os ceases. Previously published results for an Archean komatiite and new results for Os-rich alloys indicate that the terrestrial mantle evolved with Pt-Os isotopic systematics that were indistinguishable from the H-group ordinary and some enstatite chondrites. This corresponds to a Pt/Os of 2.0 ± 0.2 for the primitive upper mantle evolution curve. This similarity is consistent with previous arguments, based on the 187Os/188Os systematics and HSE abundances in the mantle, for a late veneer of materials with chondritic bulk compositions controlling the HSE budget of the upper mantle. It is very unlikely that high pressure metal-silicate segregation leading to core formation can account for the elemental and isotopic compositions of HSE in the upper mantle.  相似文献   

9.
Summary ?The PGE contents of chromite separated from peridotite layers of Archaean mafic–ultramafic flows, Abitibi belt (Canada), indicate enrichment in Os–Ir–Ru (600 ppb) relative to Pd–Au (<5 ppb). Evidently, chromite was a sink for Ir–Os–Ru during melt-chromite fractionation in each of the flows. However, an additional phase, probably olivine, is required to explain the bulk Ir content of the sulphide-poor peridotites. In contrast, the chromite Pt contents range from <10 ppb to 400 ppb, with large variation in Pt/Ru (0.02–2.76) and Pt/Pd (5–400) ratios. The Pt enrichment may be related to the presence of Pt spinel structure compounds in oxidised melt, reflecting Fe–Ti spinel-related mineralisation in higher pyroxenite-gabbro layers. Received December 5, 2002; revised version accepted January 7, 2003  相似文献   

10.
Summary The study of platinum-group minerals (PGM) concentrates from the Nizhni Tagil placers related to the Soloviev Mountain (Gora Solovieva) Uralian-Alaskan-type intrusion revealed a predominance of (Pt, Fe) alloys over Ir-, and Os-bearing alloys. (Pt, Fe) alloys (“isoferroplatinum-type”) are interstitial with respect to chromite and show important variations in their chemical compositions, which are, however, falling within the experimentally determined stability field of isoferroplatinum. Tetraferroplatinum, enriched in Cu and Ni and tulameenite represent low-temperature mineral phases replacing (Pt, Fe) alloys. Alloys belonging to the Os–Ir–Ru ternary system have compositions corresponding to native osmium, iridium and ruthenium, respectively, and to rutheniridosmine. Osmium exsolutions appear in Ir-, and (Pt, Fe) alloys, and iridium exsolutions in (Pt, Fe) alloys. Laurite is a high-temperature phase included in native iridium and (Pt, Fe) alloys. Low-temperature PGM association comprises Ir-bearing sulpharsenides, including a phase (Ir, Os, Fe, Pt, Ru, Ni)3(As, Sb)0.85S, and a palladium antimonide Pd20Sb7. These two phases were previously unknown in nature. Furthermore, native palladium occurs in the studied concentrates. This low-temperature paragenesis indicates an interaction of Pt-, Os-, Ir- and Ru-bearing alloys with late fluids enriched in volatiles, As and Sb. The chromite composition is characterized by the predominance of Cr3+ → Fe3+ substitution like in other Uralian-Alaskan-type intrusions; that indicates a fO2 variation during the chromite precipitation. Monomineralic inclusions of euhedral clinopyroxene and chromite crystals in (Pt, Fe) alloys were observed. Furthermore, (Pt, Fe) alloys contain polyphase silicate inclusions, which occupy the alloy negative crystals. Two types of silicate inclusions were recognized: (1) Low-pressure inclusions composed of amphibole, biotite, Jd-poor clinopyroxene, magnetite, apatite and glass; (2) High-pressure inclusions include: omphacitic clinopyroxene (up to 56 mol.% Jd), tremolite, muscovite, apatite, titanite and glass. In this case, the clinopyroxene is strongly zoned, revealing a pressure drop from about 25 to 5 kbar. The chemical composition of glass is corundum-normative and its H2O content varies from about 12 to 15 wt.%. The composition of magmatic melts, from which the silicate inclusions have originated was estimated using EPMA and image analysis interpreted by stereology. Their compositions are close to those obtained experimentally by hydrous partial melting of upper mantle rocks. The interpretation of analytical data shows that magmatic melts entrapped by (Pt, Fe) alloys crystallized from about 1100 to 700 °C. The (Pt, Fe) alloys formed after the crystallization of chromite, clinopyroxene and albite. Consequently, the precipitation temperature of (Pt, Fe) alloys is estimated at about 900 °C. The significant pressure drop implies a decrease of volatile concentrations in the magmatic melt and the possible formation of a fluid phase, which might have generated, the precipitation of chromite and PGM.  相似文献   

11.
The syn-tectonic breccia-hosted Mount Isa Cu deposit in northwest Queensland is the largest sediment-hosted Cu deposit in Australia. Whole-rock samples of chalcopyrite-rich Cu ore form an isochron with a Re–Os age of 1,372 ± 41 Ma. This age is more than 100 Ma younger than the previously accepted age of Cu ore formation, an Ar–Ar mineral age for biotite separated from the host rocks within the alteration envelope to the Cu orebody. This discrepancy cannot be unequivocally resolved due to a lack of other absolute geochronological constraints for Cu mineralisation or the deformation event associated with Cu emplacement. The 1,372 ± 41 Ma date may reflect (a) the time of Cu deposition, (b) the time of a hydrothermal event that reset the Re–Os signature of the Cu ore or (c) mixing of the Re–Os isotope systematics between the host rocks and Cu-bearing fluids. However, a range of published Ar–Ar and Rb–Sr dates for potassic alteration associated with Cu mineralisation also records an event between 1,350 and 1,400 Ma and these are consistent with the 1,372 Ma Re–Os age. The 1.8 Ga Eastern Creek Volcanics are a series of tholeiitic basalts with a primary magmatic Cu enrichment which occur adjacent to the Mount Isa Cu deposit. The whole-rock Os isotopic signature of the Eastern Creek Volcanics ranges from mantle-like values for the upper Pickwick Member, to more radiogenic/crustal values for the lower Cromwell Member. The Re–Os isotope signature of the Cu ores overlaps with those calculated for the two volcanic members at 1,372 Ma; hence, the Os isotope data are supportive of the concept that the Os in the Cu ores was sourced from the Eastern Creek Volcanics. By inference, it is therefore postulated that the Eastern Creek Volcanics are the source of Cu in the Mount Isa deposit, as both Os and Cu are readily transported by oxidised hydrothermal fluids, such as those that are thought to have formed the Cu orebody. The Pickwick Member yields a Re–Os isochron age of 1,833 ± 51 Ma, which is within error of previously reported age constraints. The initial 187Os/188Os isotopic ratio of 0.114 ± 0.067 (γOs = −0.7) is slightly subchondritic, and together with other trace element geochemical constraints, is consistent with a subcontinental lithospheric mantle source. The Pickwick Member records a minimum age of ca. 1.95 Ga for melt depletion in the subcontinental lithospheric mantle beneath the Mount Isa Inlier prior to the extraction of the magmas which formed the Eastern Creek Volcanics. This corresponds with the end of subduction-related magmatism along the eastern margin of the Northern Australian Craton, which included the Mount Isa Inlier.  相似文献   

12.
We report petrological, chemical and Os–Nd–Sr isotopic data for the Gaositai ultramafic complex from northern North China craton (NCC) to reveal its petrogenesis. The complex shows features of Alaskan-type intrusions, including (1) the concentric zoning from dunite core, to clinopyroxenite and hornblendite in the rim, and the common cumulative textures; (2) the abundance of olivine, clinopyroxene and hornblende, and the scarcity of orthopyroxene and plagioclase, and (3) the systematic decrease in Mg# of ferromagnesian phases from core to rim, accompanied by the Fe-enrichment trend of accessory spinel. The different rock types show highly varied, radiogenic Os isotopic ratios (0.129–5.2), and unradiogenic Nd isotopic composition (εNd(t) = −8 to −15), but are homogeneous in ISr ratios (0.7054–0.7066). The (187Os/188Os)i ratios are found to be anti-correlated with εNd(t) values and whole-rock Mg# as well. These data suggest significant crustal contamination during magma evolution. The crustal contaminants are dominantly Archean mafic rocks in the lower crust, and subordinate TTG gneisses at shallower crustal levels. The parental magma was hydrous picritic in composition, derived from an enriched lithospheric mantle source above a subduction zone. The zoned pattern of the complex formed probably through “flow differentiation” of a rapidly rising crystal mush along a fracture zone that was developed as a result of lithospheric extension in a back-arc setting in the northern margin of the NCC at ca. 280 Ma.  相似文献   

13.
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   

14.
Summary Polyphase, penetrative hydrothermal metasomatism in chromitites of the Campo Formoso layered intrusion produced spectacular chromite – ferrian chromite zoning and transformed the primary intercumulus silicates into a chlorite – serpentine – carbonate – talc assemblage. Alteration did not substantially modify the composition of chromite cores and the distribution of platinum-group elements (PGE) through the sequence of chromitite layers, which still are consistent with magmatic fractionation processes. Texture and composition of laurite and Os–Ir–Ru alloys included in chromite cores indicate that these PGM were not altered, and are probably magmaticin origin. In contrast, the PGM located in the intergranular chlorite matrix (laurite, Ir–Ru–Rh sulfarsenides and Pt–Pd compounds with Sb, Bi and Te) display evidence of hydrothermal reworking. These PGM are intimately intergrown with low-temperature Ni-sulfides. The paragenesis suggests that the Ni-sulfides-PGM assemblage formed at the expenses of unknown PGM precursors, which must have been originally present in the intercumulus silicate matrix. Mechanism of formation involves a sequence of dissolution-precipitation events controlled by variation of redox conditions during chromite alteration. The presence of a secondary ore mineral assemblage consisting of galena, bismuthinite, native antimony, and various Pb–Sb compounds suggests a possible contribution of fluids derived from the adjacent granite.  相似文献   

15.
The Gabbro Akarem (Late Precambrian) intrusion is concentrically zoned with a dunite core surrounded by lherzolite–clinopyroxenite enveloped by olivine–plagioclase hornblendite and plagioclase hornblendite. Cu–Ni–PGE mineralization is closely associated with peridotite, especially in the inner, olivine-rich core (dunite pipes) where net-textured and massive sulfides (pyrrhotite, pentlandite, chalcopyrite) are found in association with Al–Mg-rich spinel and Cr-magnetite. Primary magmatic textures are well preserved; however, deformation and mobilization due to shearing are locally observed. Platinum-group minerals (PGM) documented from the deposit are: merenskyite (PdTe2) and michenerite (PdTeBi), as well as palladian bismuthian melonite (Ni,Pd) (Te,Bi)2. These minerals occur in intimate association with hessite (Ag2Te) and electrum (Au0.65Ag0.31Bi0.04) in two distinct textural positions: (1) as inclusions in pyrrhotite, pentlandite, and rarely chalcopyrite and (2) at sulfide–silicate grain boundaries and on microfractures in base-metal sulfides (BMS) and olivine associated with serpentine and secondary magnetite. Textural features suggest that PGM were exsolved from monosulfide solid solution over a wide range of temperatures. Late-stage, low-temperature hydrothermal solutions led to redistribution of PGE. Mineralized samples show Ni/Cu ratios ranging from 0.2 to 2 with an average of 1.0. The (Pt + Pd + Rh)/(Os + Ir + Ru) ratio is generally >6 in most samples, and Os, Ru, and Ir are below the detection limit (2 ppb). The PGE contents show positive correlation with S only at low sulfur contents. The PGE patterns of Gabbro Akarem are similar to those of Alaskan-type deposits. Compared with stratiform deposits, Gabbro Akarem is depleted in PGE. The consistently low PGE contents of the mineralization and their uniform distribution in the ultramafic rocks despite the high sulfur content of the rock is attributed to rapid crystallization of sulfides in a highly dynamic environment. Received: 3 November 1999 / Accepted: 29 July 2000  相似文献   

16.
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   

17.
Sulfide inclusions in diamonds from the 90-Ma Jagersfontein kimberlite, intruded into the southern margin of the Kaapvaal Craton, were analyzed for their Re–Os isotope systematics to constrain the ages and petrogenesis of their host diamonds. The latter have δ13C ranging between −3.5 and −9.8‰ and nitrogen aggregation states (from pure Type IaA up to 51% total N as B centers) corresponding to time/temperature history deep within the subcontinental lithospheric mantle. Most sulfides are Ni-poor ([Ni + Co]/Fe = 0.05–0.25 for 15 of 17 inclusions), have elevated Cu/[Fe + Ni + Co] ratios (0.02–0.36) and elemental Re–Os ratios between 0.5 and 46 (12 of 14 inclusions) typical of eclogitic to more pyroxenitic mantle sources. Re–Os isotope systematics indicate two generations of diamonds: (1) those on a 1.7 Ga age array with initial 187Os/188Os (187Os/188Osi) of 0.46 ± 0.07 and (2) those on a 1.1 Ga array with 187Os/188Osi of 0.30 ± 0.11. The radiogenic initial Os isotopic composition for both generations of diamond suggests that components with high time-integrated Re–Os are involved, potentially by remobilization of ancient subducted oceanic crust and hybridization of peridotite. A single sulfide with higher Os and Ni content but significantly lower 187Os/188Os hosted in a diamond with less aggregated N may represent part of a late generation of peridotitic diamonds. The paucity of peridotitic sulfide inclusions in diamonds from Jagersfontein and other kimberlites from the Kaapvaal craton contrasts with an overall high relative abundance of diamonds with peridotitic silicate inclusions. This may relate to extreme depletion and sulfur exhaustion during formation of the Kaapvaal cratonic root, with the consequence that in peridotites, sulfide-included diamonds could only form during later re-introduction of sulfur.  相似文献   

18.
The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 103–3.2 × 103) are lower than mantle (6.2 × 103) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 103; komatiite: Cu/Pd ratio ∼8 × 103) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.  相似文献   

19.
Elevated 187Os/188Os ratios compared to ambient oceanic mantle, i.e.,187Os/188Os>0.13, have been reported for both arc lavas and mantle wedge xenoliths, which have been ascribed to the addition of crustal Os through slab dehydration or melting. By contrast, much lower 187Os/188Os ratios of spinels from Izu‐Bonin‐Mariana boninites indicate slight or no crustal Os was transferred from the slab to the forearc mantle. Here we report Os isotopic compositions of peridotites from New Caledonia ophiolites, which represent relics of a forearc mantle. Some New Caledonia peridotites are characterized by Os concentrations of <1 ppb, yet have187Os/188Os ratios comparable to the ambient oceanic mantle (i.e., 187Os/188Os<0.13). This confirms that little crustal Os was transported to the forearc mantle via slab dehydration. Contrasting Os isotopes between forearc peridotites and mantle wedge xenoliths may reflect the changing behavior of Os in diverse agents released from the descending slab as a function of depth, which is mainly controlled by the stability of sulfides in the slabs. During dehydration at shallow depths, sulfides keep stable and thus little Os is transported to the overlying mantle. In comparison, sulfides become unstable and tend to break down at deeper depths where slab melting or supercritical fluid generation occurs, and thus Os behaves like a mobile element.  相似文献   

20.
Podiform chromite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone (SSZ) settings. We have measured the Re-Os isotopic compositions of the major chromite deposits and associated mantle peridotites of the Dongqiao Ophiolite in the Bangong-Nujiang suture, Tibet, to investigate the petrogenesis of these rocks and their genetic relationships.The 187Os/188Os ratios of the chromite separates define a narrow range from 0.12318 to 0.12354, less variable than those of the associated peridotites. Previously-reported 187Os/188Os ratios of the Os-rich alloys enclosed in the chromitites define two clusters: 0.12645 ± 0.00004 (2 s; n = 145) and 0.12003 to 0.12194. The ultra-depleted dunites have much lower 187Os/188Os (0.11754, 0.11815), and the harzburgites show a wider range from 0.12107 to 0.12612. The average isotopic composition of the chromitites (187Os/188Os: 0.12337 ± 0.00001) is low compared with the carbonaceous chondrite value (187Os/188Os: 0.1260 ± 0.0013) and lower than the average value measured for podiform chromitites worldwide (0.12809 ± 0.00085). In contrast, the basalts have higher 187Os/188Os, ranging from 0.20414 to 0.38067, while the plagioclase-bearing harzburgite and cumulates show intermediate values of 187Os/188Os (0.12979 ~ 0.14206). Correspondingly, the basalts have the highest 187Re/188Os ratios, up to 45.4 ± 3.2, and the chromites have the lowest 187Re/188Os ratios, down to 0.00113 ± 0.00008. We suggest that melts/fluids, derived from the subducting slab, triggered partial melting in the overlying mantle wedge and added significant amounts of radiogenic Os to the peridotites. Mass-balance calculations indicate that a melt/mantle ratio of approximately 15:1 (melt: 187Re/188Os: 45.4, 187Os/188Os: 0.34484; mantle peridotite: 187Re/188Os: 0.0029, 187Os/188Os: 0.11754) is necessary to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/mantle ratio during the formation of the chromitite deposits. The percolating melts probably were of variable isotopic composition. However, in the chromitite pods the Os from many melts was pooled and homogenized, which is why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the 187Os/188Os ratios of chromitites may not be representative of the DMM, but only reflect an upper limit. Importantly, the Os-isotope compositions of chromitites strongly suggest that such deposits can be formed by melt/mantle mixing processes.  相似文献   

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