Chromite-PGM Mineralization in the Lherzolite Mantle Tectonite of the Kraka Ophiolite Complex (Southern Urals, Russia)

The mantle tectonite of the Kraka ophiolite contains several chromite deposits. Two of them consisting of high-Cr podiform chromitite—the Bolshoi Bashart located within harzburgite of the upper mantle transition zone and Prospect 33 located in the deep lherzolitic mantle—have been investigated. Both deposits are enveloped in dunite, and were formed by reaction between the mantle protolith and high-Mg, anhydrous magma, enriched in Al2O3, TiO2, and Na2O compared with boninite. The PGE mineralization is very poor (<100 ppb) in both deposits. Laurite (RuS2) is the most common PGM inclusion in chromite, although it is accompanied by erlichmanite (OsS2) and (Ir,Ni) sulfides in Prospect 33. Precipitation of PGM occurred at sulfur fugacity and temperatures of logƒS2 = (−3.0), 1300–1100 °C in Bolshoi Bashart, and logƒS2 = (−3.0/+1.0), 1100–800 °C in Prospect 33, respectively. The paucity of chromite-PGM mineralization compared with giant chromite deposits in the mantle tectonite in supra-subduction zones (SSZ) of the Urals (Ray-Iz, Kempirsai) is ascribed to the peculiar petrologic nature (low depleted lherzolite) and geodynamic setting (rifted continental margin?) of the Kraka ophiolite, which did not enable drainage of the upper mantle with a large volume of mafic magma.

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Compositionally heterogeneous podiform chromitite in the Shetland Ophiolite Complex (Scotland): Implications for chromitite petrogenesis and late-stage alteration in the upper mantle portion of a supra-subduction zone ophiolite

Lithos ◽

10.1016/j.lithos.2012.11.013 ◽

2013 ◽

Vol 162-163 ◽

pp. 279-300 ◽

Cited By ~ 36

Author(s):

E.J. Derbyshire ◽

B. O'Driscoll ◽

D. Lenaz ◽

R. Gertisser ◽

A. Kronz

Keyword(s):

Subduction Zone ◽

Upper Mantle ◽

Late Stage ◽

Ophiolite Complex ◽

Podiform Chromitite

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Early Permian granitoids of the East Magnitogorsk zone (South- ern Urals): petrology, geochemistry, and geodynamic formation environment

News of the Ural State Mining University ◽

10.21440/2307-2091-2020-1-47-62 ◽

2020 ◽

Vol 1 (1) ◽

pp. 47-62

Author(s):

Timofey Nikolaevich SURIN ◽

Keyword(s):

Water Pressure ◽

Southern Urals ◽

Geodynamic Setting ◽

Type I ◽

Early Permian ◽

The Southern Urals ◽

Quartz Syenite ◽

Crystallization Differentiation ◽

Geochemical Study

The relevance of the problem. The Early Permian magmatism of the Southern Urals is poorly studied with the help of modern methods. The granitoid massifs of this age locally developed in the East Magnitogorsk zone contain important information about the geodynamic conditions of their formation. Clarification of this issue makes an important contribution to the understanding of the geodynamic development of the Urals. The nature of granitoids is still debatable. The connection with the massifs combined in the Balkan complex of gold-tungsten mineralization indicates the need for a comprehensive study. The purpose of the study is to determine the petrological and geochemical features of the rocks of the Balkan complex, to identify the mechanism of their petrogenesis and to establish the geodynamic conditions of their formation. Results. The petrological and geochemical study of the formations of the Balkan complex was carried out and their place in the typical taxonomy of granitoids was determined. Their belonging to the I-type is shown. Mineralogical and petrogeochemical methods were first studied for shonkinite xenoliths in granitoids. The mechanism of petrogenesis of rocks is proposed and the geodynamic setting of their formation is determined. It is shown that the monzonitemonzodiorite-quartz syenite-granosyenite-leucogranite series of rocks was formed as a result of crystallization differentiation of a single parental melting, and it was also concluded that the massifs of the complex are formed under conditions of early collision conditions with the important role of the subduction process. The mechanism of formation of the massifs of the complex is largely similar to mechanism for granitoids in other conflict areas, although it has its own specifics. Conclusions. 1). The Early Permian granitoids of the Balkan complex relates to type I. 2). All rocks of the complex, from monzonites to quartz syenites and leucogranites, including xenolith shonkinites, form a petrogenetic series formed as a result of crystallization differentiation of a single parent alkaline-gabbroic melting with increased water pressure. 3). The Balkan complex was formed in an early collisional setting under the action of deep subduction. 4). Transpression in the upper part of the crust induced formation of the massifs of the complex. 5). The Balkan complex is a kind of indicator of the growth of the newly formed crust as a result of collision and accretion processes.

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The Ferguson Lake deposit: an example of Ni–Cu–Co–PGE mineralization emplaced in a back-arc basin setting?

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2017-0185 ◽

2018 ◽

Vol 55 (8) ◽

pp. 958-979 ◽

Cited By ~ 1

Author(s):

P. Acosta-Góngora ◽

S.J. Pehrsson ◽

H. Sandeman ◽

E. Martel ◽

T. Peterson

Keyword(s):

Mineral Exploration ◽

Ultramafic Rocks ◽

Geodynamic Setting ◽

Ocean Ridges ◽

Tectonic Environment ◽

Pge Mineralization ◽

Extensional Tectonic ◽

Basaltic Composition ◽

Greenstone Belts ◽

Back Arc

The world’s largest Ni–Cu–Platinum group element (PGE) deposits are dominantly hosted by ultramafic rocks within continental extensional settings (e.g., Raglan, Voisey’s Bay), resulting in a focus on exploration in similar geodynamic settings. Consequently, the economic potential of other extensional tectonic environments, such as ocean ridges and back-arc basins, may be underestimated. In the northeastern portion of the ca. 2.7 Ga Yathkyed greenstone belt of the Chesterfield block (western Churchill Province, Canada), the Ni–Cu–Co–PGE Ferguson Lake deposit is hosted by >2.6 Ga hornblenditic to gabbroic rocks of the Ferguson Lake Igneous Complex (FLIC), which is metamorphosed up to amphibolitic facies. The FLIC has a basaltic composition (Mg# = 31–72), flat to slightly negatively sloped normalized trace element patterns (La/YbPM = 0.7–3.5), and negative Zr, Ti, and Nb anomalies. The FLIC rocks are geochemically similar to the 2.7 Ga back-arc basin tholeiitic basalts from the adjacent Yathkyed and MacQuoid greenstone belts (Mg# = 30–67; La/YbPM = 0.3–3.0), but the Ferguson Lake intrusions appear to be more crustally contaminated. We interpret the FLIC to have formed in an equivalent back-arc basin setting. This geodynamic setting is rare for the formation of Ni–Cu–PGE occurrences, and only few examples of this tectonic environment (or variations of it, e.g., rifted back-arc) are found in other Proterozoic and Archean sequences (e.g., Lorraine deposit, Quebec). We suggest that back-arc basin-derived mafic rocks within the Yathkyed and other Neoarchean greenstone belts of the Chesterfield block (MacQuoid and Angikuni) could represent important targets for future mineral exploration.

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The distribution and abundance of halogens in eclogites: An in situ SIMS perspective of the Raspas Complex (Ecuador)

American Mineralogist ◽

10.2138/am-2020-6994 ◽

2020 ◽

Vol 105 (3) ◽

pp. 307-318 ◽

Cited By ~ 1

Author(s):

Benjamin M. Urann ◽

Véronique Le Roux ◽

Timm John ◽

Grace M. Beaudoin ◽

Jaime D. Barnes

Keyword(s):

Upper Mantle ◽

Subduction Zones ◽

Secondary Ion Mass Spectrometry ◽

Bulk Rock ◽

Ocean Island Basalts ◽

High Pressure Metamorphism ◽

Southern Ecuador ◽

And Behavior ◽

Secondary Ion

Abstract We present in situ secondary ion mass spectrometry (SIMS) and electron microprobe analyses of coexisting garnet, omphacite, phengite, amphibole, and apatite, combined with pyrohydrolysis bulk-rock analyses to constrain the distribution, abundance, and behavior of halogens (F and Cl) in six MORB-like eclogites from the Raspas Complex (Southern Ecuador). In all cases concerning lattice-hosted halogens, F compatibility decreases from apatite (1.47–3.25 wt%), to amphibole (563–4727 μg/g), phengite (610–1822 μg/g), omphacite (6.5–54.1 μg/g), and garnet (1.7–8.9 μg/g). The relative compatibility of Cl in the assemblage is greatest for apatite (192–515 μg/g), followed by amphibole (0.64–82.7 μg/g), phengite (1.2–2.1 μg/g), omphacite (<0.05–1.0 μg/g), and garnet (<0.05 μg/g). Congruence between SIMS-reconstructed F bulk abundances and yield-corrected bulk pyrohydrolysis analyses indicates that F is primarily hosted within the crystal lattice of eclogitic minerals. However, SIMS-reconstructed Cl abundances are a factor of five lower, on average, than pyrohydrolysis-derived bulk concentrations. This discrepancy results from the contribution of fluid inclusions, which may host at least 80% of the bulk rock Cl. The combination of SIMS and pyrohydrolysis is highly complementary. Whereas SIMS is well suited to determine bulk F abundances, pyrohydrolysis better quantifies bulk Cl concentrations, which include the contribution of fluid inclusion-hosted Cl. Raspas eclogites contain 145–258 μg/g F and at least 7–11 μg/g Cl. We estimate that ~95% of F is retained in the slab through eclogitization and returned to the upper mantle during subduction, whereas at least 95% of subducted Cl is removed from the rock by the time the slab equilibrates at eclogite facies conditions. Our calculations provide further evidence for the fractionation of F from Cl during high-pressure metamorphism in subduction zones. Although the HIMU (high U/Pb) mantle source (dehydrated oceanic crust) is often associated with enrichments in Cl/K and F/Nd, Raspas eclogites show relatively low halogen ratios identical within uncertainty to depleted MORB mantle (DMM). Thus, the observed halogen enrichments in HIMU ocean island basalts require either further fractionation during mantle processing or recycling of a halogen-enriched carrier lithology such as serpentinite into the mantle.

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