Amphibole as a witness of chromitite formation and fluid metasomatism in ophiolites
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Abstract Here we present new occurrences of amphibole in a suite of chromitites, dunites, and harzburgites from the mantle sequence of the Lycian ophiolite in the Tauride Belt, southwest Turkey. The amphibole occurs both as interstitial grains among the major constituent minerals and as inclusions in chromite grains. The interstitial amphibole shows generally decreasing trends in Na2O and Al2O3 contents from the chromitites (0.14–1.54 wt% and 0.04–6.67 wt%, respectively) and the dunites (0.09–2.37 wt%; 0.12–11.9 wt%) to the host harzburgites (<0.61 wt%; 0.02–5.41 wt%). Amphibole inclusions in chromite of the amphibole-bearing harzburgites are poorer in Al2O3 (1.12–8.86 wt%), CaO (8.47–13.2 wt%), and Na2O (b.d.l.–1.38 wt%) than their counterparts in the amphibole-bearing chromitites (Al2O3 = 6.13–10.0 wt%; CaO = 12.1–12.9 wt%; Na2O = 1.11–1.91 wt%). Estimated crystallization temperatures for the interstitial amphibole grains and amphibole inclusions range from 706 to 974 °C, with the higher values in the latter. A comparison of amphibole inclusions in chromite with interstitial grains provides direct evidence for the involvement of water in chromitite formation and the presence of hydrous melt/fluid metasomatism in the peridotites during initial subduction of Neo-Tethyan oceanic lithosphere. The hydrous melts/fluids were released from the chromitites after being collected on chromite surfaces during crystallization. Different fluid/wall rock ratios are thought to have controlled the crystallization and composition of the Lycian amphibole and the extent of modification of the chromite and pyroxene grains in the peridotites. Considering the wide distribution of podiform chromitites in this ophiolite, the link between chromitite formation and melt/fluid metasomatism defined in our study may be applicable to other ophiolites worldwide.Keywords:
Amphibole
Chromite
Chromitite
Metasomatism
Peridotite
The texture, mineralogy and composition of chromite in the upper chromitite of the Muskox intrusion, in the Northwest Territories, have been studied in two 0.5-meter sections of drill core. The principal rock-type is an orthopyroxenite that contains cumulus olivine, orthopyroxene and chromite, and the intercumulus minerals clinopyroxene and plagioclase. The minor minerals ilmenite and biotite are found, together with a number of accessory minerals, in pockets that are interpreted as sites of late intercumulus melt. The chromitite seam is up to 10 cm thick and contains chromite with a narrow range in composition: 0.64 2 ) = -9.1. The disseminated chromite in the orthopyroxenite shows a much greater range in composition, and increases in Fe (super 2+) /(Fe (super 2+) +Mg), Fe (super 3+) /(Fe (super 3+) +Al+Cr), Ti and Ni with stratigraphic height above the massive chromitite. The chromite in the Muskox chromitite is significantly higher in Fe (super 3+) , Ti and Fe (super 2+) /(Fe (super 2+) +Mg) than chromite in the Bushveld, Stillwater and Great Dyke chromitites; furthermore, the Muskox chromitites formed much higher in the stratigraphic section of the layered series than in these other intrusions. The Muskox chromitites are considered to have formed late in the magmatic history of the intrusion as a result of mixing of a fractionated magma with a more primitive magma and a component due to wall-rock assimilation.
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Ultramafic massif of Bulqiza belongs to Eastern Jurassic Albanian ophiolite belt of IAT-BSV- type. This massif is the most important chromite-bearing ore. The mantle ultramafics have extremely refractory nature. This is due to the high partial fusion of upper mantle which is depleted in CaO and Al2 O3 . The chromitite is situated to different parts of ultramafic pile, from bottom Cpx harzburgites up to massive dunites and cumulate ultramafic but the mainly chromite potential belongs to mantle harzburgite –dunite level and to transition dunites partly. The chromite is chiefly of Cr-rich metallurgical type. The atomic ratios of chromite , Fo of olivine and some physical properties of them vary according to the chromitite setting and reflects the evolution of Ol-Sp equilibrium process depended of the chromite concentration, from baren dunitic lenses towards dunite envelops of the ore bodies and the interstitial and inclusions of olivine within chromite grains. Two particular chromite deposits are the Bulqiza- Batra tabular folded ore body and Shkalla, pencil –like ore body.
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Published and unpublished compositions of chromite in 333 chromitite samples from 14 ultramafic complexes of the Urals are overviewed. The chromitites occur in the mantle unit and/or the supra-Moho cumulate sequence of ophiolite complexes, as well as in Alaskan-type intrusions. They vary in size from giant ore deposits associated with ophiolites (e.g., Kempirsai, Ray-Iz, Voykar-Syninsky) to sub-economic mineralization in the Alaskan-type complexes (e.g., Svetly Bor, Kachkanar). Mantle-hosted chromitites occur either as discordant, podiform, high-Cr ore bodies and sub-concordant elongated lenses of high-Al chromite. In the supra-Moho sequences of ophiolites, chromitite is mainly of the high-Al variety, and occurs as concordant layers alternated with peridotite and pyroxenite cumulates. In the Alaskan-type intrusions of the Urals, chromitite occurs as centimeter to meter-size pods and lenses having syngenetic or epigenetic relationship with the host dunite. Calculated melt compositions in equilibrium with chromite and comparison of chromite composition with those from various volcanic suites, and chromitites from different plutonic complexes, allow division of the Urals chromitites into four different compositional groups, corresponding to different geodynamic environments of formation: 1) The high-Al, low-Ti suite (Al 2 O 3 > 20 wt%, Cr# 20 wt%, Cr# 0.70, Al 2 O 3 0.70, Al 2 O 3 < 20 wt%, TiO 2 = 0.38-1.30 wt%, Fe 3+ # = 0.20-1.29, δ logf(O 2 ) = +0.9 ÷ +5.9) is represented by chromitites from the Urals Alaskan-type intrusions and the East-Khabarny complex. They have crystallized from Fe-rich magma (av. FeO/MgO = 1.35) under oxygen- fugacity conditions well above the FMQ buffer. The melt is characterized by high-Ti, high-K, calc-alkaline composition, having many geochemical characteristics in common with ankaramites. It was generated by partial melting of a fluid-metasomatized mantle source, in a subduction-influenced arc setting. However, the close similarity with the zoned complexes emplaced in the Russian-Far-East craton suggests that formation of Alaskan-type melts may be not restricted to SSZ, island arc settings.
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The Voskhod podiform chromitite is one of more than 30 chromitite deposits that collectively form the Main Ore Field (MOF) within the Kempirsai Massif, in Kazakhstan. The MOF is the largest podiform chromitite ore-field in the world. The Voskhod deposit, encased in a serpentinised dunite halo, is situated within harzburgite units that comprise the mantle sequence of the Kempirsai ophiolite. This study arose from a unique opportunity to work on drill core samples through an un-mined podiform chromite deposit and investigate its internal structure, composition and genesis.
The 18Mt ore-body has a strike of 600 m, is 170 m to 360 m wide and has an average thickness of 39 m. It has an immediate dunite halo between 1 m and 5 m thick. The ore body is made up of multiple stacked chromitite layers. Mineralised layers are separated by barren dunite or by weakly disseminated dunite lenses ranging from 5 – 45 m) units of massive chromite (>80% chromite), with progression towards the south west disseminated chromite (10 – 40% chromite) becomes increasingly abundant. Drill core logging and cross-section profiling of the internal structure of the ore body has identified an intricately connected network of what appear to be chromite-filled channel-ways.
Outside of the halo the host rocks are inter-layered harzburgite and dunite. Accessory chromite in harzburgite has an average Cr# of 0.31 compared to Cr# 0.49 in the dunite. The harzburgites are depleted, having formed from intermediate degrees of partial melting (~15 – 18 %) of a fertile mantle source at a mid-ocean ridge (MOR) setting. The dunite units have transitional geochemical fingerprints that imply they formed from the interaction of MOR mantle harzburgite with both mid ocean ridge baslt-melt and an arc derived-melt. They are not the products of extremely high degrees of partial melting.
The encasing dunite halo is extensively serpentinised (>80%). Chromite is only present as an accessory phase having an average Cr# of 0.62. The dunite has a geochemical signature indicating that it formed by reaction between residual harzburgite and a boninite melt in supra-subduction zone (SSZ) tectonic setting.
A variety of geochemical fingerprints have been identified; residual MOR harzburgite, reacted-MOR dunite, reacted-SSZ dunite and harzburgite, indicating that the mantle section has had a
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polygenetic tectonic evolution, recording both ocean basin opening (MOR setting) and closing (SSZ setting) events.
Trace element and REE whole rock geochemistry of the chromitites and associated host rocks provide evidence of depletion and a later-stage LREE-enrichment event. LREE-enrichment is most intense within and immediately adjacent to the chromitite.
Chromites from the ore zone are at the Cr-rich extreme for podiform chromites (Cr# ave. 0.80-0.85) and are TiO2 poor (ave. 0.16 wt%), similar to chromite in boninite worldwide and nearby. Al/Ti ratios have been used to calculate the composition of the parent melts from which the Voskhod podiform chromitite crystallised: compositions that are synonymous with a boninite melt composition.
Chemical variation in chromite is systematic and on a much smaller scale than was anticipated. Even variations in a single thin section provide key evidence for different magmatic processes. An apparent melt-rock reaction in harzburgite has been examined in freeze-frame.
The chromite chemistry has been investigated at 50 cm, 1 cm and 1 mm scales. Compositional differences were identified on the basis of MgO% and FeO(t)% compositions. Diagrams FeO-Fe2O3 and Cr# - Mg# were used to demonstrate the variations and identify relationships. Broad cryptic layering on a 50 cm scale has been found as well as fine-cryptic layering on a 1 – 8 cm scale. The variations are interpreted to reflect differences in the mineral phases crystallised from the melt; periods when on chromite only crystallised are distinguished from periods when both chromite with olivine crystallised. It seems likely that the deposit is made up of thousands of episodes of chromite accumulation that formed in an intermittently replenished open-system.
It also seems likely that the conduit was never a single melt-filled cavity; instead melt flow was focused through the mantle over an extended period. The conduit appears to be comprised of multiple branches, as chromite (± olivine) crystallised from the melt the channel-way became blocked and the melt was forced to deviate and make a new pathway through the mantle. As time elapsed the process resulted in the formation of stacked chromitite lenses, creating an orebody that has an internal arrangement of chromitite and dunite unites which resemble a stacked braided 'delta'.
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The Pozantı-Karsantı Ophiolite located in the middle Taurus has significant chromitite reserves in Turkey. The chromitite ore bodies belong to concordant and subconcordant form and are located within mantle harzburgite surrounded by bodies of dunite. Chromitite ore types are small or medium bodies of massive, disseminated, banded and nodular, also the result of a combination of at least two of these types of ores consist of mixed-type ore. Chromite ores bear traces of plastic deformation under the influence of lateral forces developing in the lateral direction. Plastic deformation is also observed at the micro scale. This study presents the concentrations of a complete suite of major (SiO2, Cr2O3, MgO, Al2O3, and FeO(t)) and trace elements (Ni, Ti, Co, V, Zn, S, Ca, Ga and Cl) in podiform chromitites of the Pozantı-Karsantı Ophiolite. According to the chromite ore whole-rock geochemical analysis, there was a positive relationship between the amount of Cr2O3 and Zn, V, Ti, and Co, while a negative relationship was found between Ni, S and Ca. This situation is opposite with the dunites in which the chromite ore is located.
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The Bushveld Igneous Complex (BIC) is known for its laterally extensive platinum group element-bearing layers, the most famous being the Merensky Reef and the UG-2 chromitite in the eastern and western limbs of the complex. In the northern limb, the Platreef mineralization and a thick chromitite seam below it (referred to as the "UG-2 equivalent" or UG-2E) have been proposed to be the stratigraphic equivalents of the Merensky Reef and the UG-2, respectively. In this study, we compare a suite of UG-2E samples from the Turfspruit project with a UG-2 reference suite from the western limb using petrography, electron probe microanalysis, laser ablation-inductively coupled plasma-mass spectrometry, and Mössbauer spectroscopy. The results show that (a) in Mg# vs. Cr# diagrams, UG-2E chromites have a distinct compositional field; however, when samples of similar chromite modal abundance (≥ 80%) are used, the UG-2E chromites overlap the field that characterizes UG-2 chromites; (b) the UG-2E is more variable in chromite modal abundance than the UG-2; and (c) variations in Mg# and Fe3+/ΣFe in the UG-2E indicate contamination of the magma by metasedimentary rocks of the Duitschland Formation (Transvaal Supergroup) during emplacement, followed by partial re-equilibration of chromite grains with a trapped melt. Thus, we conclude that for chromite modes higher than 80%, the chromite composition retains enough information to allow correlation and that the UG-2E in the northern limb is very likely the UG-2 chromitite.
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ABSTRACT A fundamental difference exists between the textures of chromite crystals in chromitites in layered complexes and ophiolites. Those in layered complexes generally have euhedral octahedral shapes except where sintered, whereas those in ophiolites generally have rounded shapes accompanied commonly by nodular and more rarely dendritic chromite. Here we describe another texture characteristic of ophiolitic chromitite. The analysis of high-resolution X-ray computed tomography images of chromitite from Harold's Grave in the Shetland ophiolite has revealed 3D hopper structures on chromite grains. In 2D, these hopper structures appear at the surface of the chromite grain as stepped inward facing edges. A study of chromitites in 2D from ten ophiolite complexes has shown that all commonly contain chromite grains displaying these stepped edges. They occur mainly in protected enclaves surrounded by chromite grains that otherwise have rounded edges. The hopper crystals and the often associated clusters of inclusions represent periods of chromite crystal growth in a chromite supersaturated magma due to the presence of a more supercooled and more volatile-rich magma than that present in most layered complexes. Subsequent exposure of chromite crystals to chromite-undersaturated magma caused corrosion, resulting in the characteristic rounded shape of the ophiolitic chromite grains.
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