Formation Of Chromitites Research Articles

The Critical Role of Fluid‐Mediated Diffusion in Anomalous Fe‐Mg‐O Isotope Fractionations in Ultramafic Rocks of Ophiolites

AbstractA number of bulk‐rock and mineral Fe and Mg isotope data have been increasingly reported from ophiolitic rocks, but their anomalous fractionations are still not well addressed. This study presents new Fe, Mg, and O isotopes and published Li isotope data of olivine and/or chromite in chromitite, dunite, and harzburgite from the Pozantı‐Karsantı ophiolite (southern Turkey) to reveal the mechanisms of inter‐ and intra‐mineral isotope fractionation during chromitite formation. In the studied samples, both olivine and chromite have highly variable δ56Fe ranging from −0.125‰ to 0.101‰ and −0.261‰ to 0.191‰, respectively, but with heavier olivine than coexisting chromite in most samples. In contrast, olivine records homogeneous Mg isotopic compositions with δ26Mg values of −0.36‰ to −0.26‰, whereas chromite grains have variable but relatively high δ26Mg values ranging from −0.02‰ to 0.45‰. The δ18O values of olivine range from 4.65‰ to 5.86‰ and co‐vary with δ7Li (−5.43‰ to 16.65‰), δ56Fe, and δ26Mg, particularly at lithological scale. These isotopic features cannot be explained solely by subsolidus diffusion between olivine and chromite, or by partial melting and fractional crystallization. Instead, they require fluids as a diffusive medium for high efficient element exchange between chromite and olivine. The fluids were likely the hydrous fluids that were collected on chromite grain surfaces during crystallization and released later from chromite piles during precipitation. The fluid‐olivine reaction preferentially transferred light Fe from olivine grains resulting in elevated δ56Fe, but caused small O and Mg isotopic variations in the olivine. The Li diffusion depends on Li concentration gradient between olivine and fluid and olivine/fluid ratio, which can account for large δ7Li variations in olivine between different rock types. As these reactions proceeded, they induced elemental and isotopic variations in the fluids. These fluids could continuously penetrate the harzburgites through the dunite envelopes around the chromitites, producing some of the observed anomalous geochemical features of the harzburgites. Thus, we conclude that the hydrous fluids acted as critical media for ion exchange between chromite and olivine and for significant modification of the host harzburgites.

Amphibole as a witness of chromitite formation and fluid metasomatism in ophiolites

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.

Platinum mineralization and geochemistry of the Matysken zoned Ural-Alaskan type complex and related placer (Far East Russia)

Ural-Alaskan type complexes are the sources for unique placer platinum deposits and contain platinum-group minerals (PGM) within lodes. Such complexes occur along modern and ancient convergent tectonic margins and are comprised of dunite, wehrlite, clinopyroxenite and gabbro, which gradually interchange towards the marginal parts of the complex. In-situ PGM are largely restricted to chromitites, which are comprised of small veinlets and schlieren of chromian spinel in the dunite unit. PGM mineralization in chromitites can be extremely rich; however, their distribution is very sporadic, without any apparent regularity. Moreover, the role of accessory chromian spinel and post-magmatic overprinting of platinum group element (PGE) concentrations is poorly defined. Detailed studies of PGM assemblages, including those in chromitites, serpentine veinlets in dunites, and wehrlites, combined with PGE geochemistry could provide insights into the distribution and processes responsible for PGE accumulation.The Matysken complex in the Koryak Highlands (Far East Russia) is comprised of dunite, wehrlite, clinopyroxenite and gabbro units, and has a classic zoned structure, rendering it an excellent example of an Ural-Alaskan type complex, ideal for this case study. This locality contains mm-scale isoferroplatinum (Pt3Fe) nuggets that cement chromitites and occur as µm-scale euhedral inclusions in chromian spinel. Placer PGM assemblages in the nearby Matysken River consist of exactly the same assemblages of minerals that are found in dunites and chromitites but with larger compositional scatter, reflecting a complex history of alluvium accumulation, which was sourced from different eroded parts of the complex. Detailed textural investigations discovered a diverse array of hydrous silicate–related mineralization, including euhedral isoferroplatinum grains in chlorite matrix, isoferroplatinum–amphibole intergrowths, and a wide range of PGE, Fe and Cu alloys, sulfarsenides and antimonides, which formed in serpentine veinlets together with awaruite (Ni3Fe) and base metal sulfides. This provides further evidence that isoferroplatinum, which is widely accepted to be a magmatic mineral, may form under a wide range of conditions, including during serpentinization.Dunites have fractioned primitive mantle-normalized PGE patterns, typical of Ural-Alaskan type complexes, with strong enrichments in Pt and depletions in Ir, Ru and Pd. PGE, with an exception of Pd, are quite uniformly distributed in dunite containing accessory chromian spinel, with good correlations between PGE and Cr. This correlation, together with data from previous studies, allowed us to calculate the concentration of Pt in accessory chromian spinel (probably hosted as minute PGM inclusions), which is of the same grade as for chromitites (ppm levels). This suggests that the formation of chromitites does not concentrate PGE over Cr, but only assembles them into larger aggregates, causing the observed unevenness in the PGM distribution.

Geodynamic setting of high-Cr chromite mineralization in nascent subduction zones: Li isotopic and REE constraints from the Zambales ophiolite, Philippines

The Acoje block of the Zambales ophiolite in the Northwest Philippines is a remnant of forearc lithosphere that formed during an Eocene subduction initiation event in the Western Pacific Ocean. It hosts abundant high-Cr chromitites in the uppermost part of the mantle sequence and the overlying dunitic Moho transition zone. The host harzburgites of the uppermost mantle sequence vary upward from Cpx-rich to Cpx-poor varieties. Clinopyroxene grains in both types of harzburgites and the dunites have prevalent finger-like protrusions and diopsidic compositions, indicative of melt impregnation. From the Cpx-rich harzburgites to Cpx-poor harzburgites and dunites, the clinopyroxene grains display increasingly depleted major element and rare earth element (REE) compositions, e.g., Al2O3 contents from 5.15 wt% to 0.84 wt% and Yb concentrations from ~ 0.2 ppm to 1.5 ppm. These changes are compatible with a transition from MORB to boninitic affinity, Olivine grains in the dunites and sparsely disseminated chromitites vary widely in δ7Li values, primarily from ~ +1 to +10. Such a δ7Li range implies that the parental magmas of the high-Cr chromitites were derived from mantle sources that were modified by slab fluids and had Li isotopic signatures either similar to, or heavier than, the normal mantle range (+4 ± 2). Partial melting in the mantle sources was triggered by heating of previously hydrated Cpx-rich harzburgites during slab rollback and subsequent asthenospheric upwelling. The upwelling brought asthenospheric materials and high heat flux into the lithospheric mantle, facilitating formation of high-Ca boninitic magmas and high-Cr chromitites in the nascent mantle wedge. The different Li isotopic domains in the mantle sources of the high-Cr chromitites indicate that the mantle temperatures were elevated rapidly after the hydration event in order to generate the boninitic magmas.

A new model for chromitite formation in ophiolites: Fluid immiscibility

Although the involvement of hydrous fluids has been widely invoked in formation of podiform chromitites in ophiolites, there is lack of natural evidence to signify the role and mechanism of fluids. In this study, a new model for the genesis of podiform chromitite is proposed on basis of revisits of comprehensive petrological, mineralogical and geochemical results of the well-preserved Kizildag ophiolite and the well-characterized Luobusa chromite deposit. In this model, ascending magmas intruding oceanic lithospheric mantle would presumably form a series of small magma chambers continuously connected by conduits. Tiny chromite nuclei would collect fluids dispersed in such magmas to form nascent droplets. They tend to float upward in the magma chamber and would be easily transported upward by flowing magmas. Chromite-rich droplets would be enlarged via coalescence of dispersed droplets during mingling and circulation in the magma chamber and/or transport in magma conduits. Crystallization of the chromite-rich liquid droplets would proceed from the margin of the droplet inward, leaving liquid entrapped within grains as precursor of mineral inclusions. With preferential upward transportation, immiscible chromite-rich liquids would coalesce to a large pool in a magma chamber. Large volumes of chromite would crystallize in situ , forming podiform chromitite and resulting in fluid enrichment in the chamber. The fluids would penetrate and compositionally modify ambient dunite and harzburgite, leading to significant fractionations of elemental and isotopic compositions between melts and fluids from which dunite and chromitite respectively formed. Therefore, fluid immiscibility during basaltic magma ascent plays a vital role in chromitite formation.

New Data on Platinum Group Element Bearing Chromitites from the Kurtushiba Ophiolite, Western Sayan

Abstract—Small grains of platinum group minerals (PGMs) have been found in chromitites of the Ergaksky and Kalninsky ultramafic massifs, which are marginal northeastern fragments of the Kurtushiba ophiolite belt in Western Sayan. Along with previously described PGMs, the Cu–Ru–Os solid solution, arsenic-bearing (As up to 3.6 wt %) laurite with minor Pt, Ni, and Co, platinum group element (PGE) sulfides of the MeS and Me3S4 types, sulfoarsenide with the high Rh content (12.7 wt %) in chromitites of the Ergaksky massif, and laurite and sulfoarsenides in chromitites of the Kalninsky massif were found for the first time. The PGM compositions indicate a high formation temperature of Ru–Os sulfide (1200–1250°C) in both massifs, as well as a high partial sulfur pressure during the formation of chromitites and syngenetic PGMs in the Ergaksky massif. The PGM assemblages were formed during several stages. The Os–Ir–Ru alloys and the laurite–erlichmanite series disulfides (predominantly partitioning Ru) are early; the Cu–Ru–Os alloys, sulfides (Ru,Fe)3S4, (Ru,Ni,Fe)S, and PGE sulfoarsenides are late and were crystallized under the influence of reductive fluids as a result of PGE remobilization and redeposition.

Comparison of Redox States between the Ultramafic Bodies of Xigaze and Luobusha Ophiolites, Tibet, China

AbstractThe tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity (fO2) (e.g., Ballhaus, 1993; Dare et al., 2009; Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by Mössbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet.The fO2 values (−1.02∼0.04 log units against the FMQ buffer) and Cr# (22∼54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values (0.04∼2.25 log units) and Cr# (73∼84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values (−0.22∼2.19 log units) and Cr# (22∼82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values (−0.3∼0.71 log units) and Cr# (16∼63%), both of which are similar to those of MORB‐like basalts, inferring that they form in an extensional setting. Both high‐Cr and high‐Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values (e.g., Chashchukhin and Votyakov, 2009; Melcher et al., 1997; Quintiliani et al., 2006; Rollinson and Adetunji, 2015), while the distribution‐limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.

Experimental study on the effects of H2O upon crystallization in the Lower and Critical Zones of the Bushveld Complex with an emphasis on chromitite formation

Mafic–ultramafic layered rocks of the Lower and Critical Zones of the Bushveld Complex are believed to have formed from a parental magma usually labelled B1 with SiO2 content at 55 wt.%, MgO at about 12 wt.% and Cr2O3 concentration at 0.15 wt.%. Previous experimental study at 300 MPa and nominally dry conditions showed that this composition had orthopyroxene on liquidus between 1250 and 1300 °C, and the mineral was joined by plagioclase and clinopyroxene at and below 1166 °C. The objective of this study was to investigate the effects of variable additions of H2O (2, 4 and 6 wt.% in starting compositions) on liquidus and sub-liquidus phase equilibria. Main attention was given to the solubility of Cr-spinel and Cr distribution between melt and crystalline phases. Six experiments were carried out in the internally heated pressure vessel (IHPV) at 300 MPa and in the temperature interval 1100–1300 °C. Redox conditions were not controlled and thermal dissociation of the H2O component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products. The effective values of fO2 in experiments were calculated using SPINMELT 2.0 software bases on the regression of experimental data on the spinel-melt equilibrium. According to the calculations, H2O additions in isothermal series of experiments increased fO2 by 0.8–1.6 log-units from the level in a nominally dry run. Overall, the experiments showed that H2O additions suppressed crystallization of silicates, shifted olivine-orthopyroxene peritectic equilibrium towards higher olivine stability (olivine first appears in H2O-bearing compositions at 1175 °C) and increased the proportion of magnetite component in spinel solid solution. The addition of 4 wt.% H2O to the starting composition at 1200 °C resulted in complete melting of orthopyroxene, which left Cr-spinel as the only liquidus phase. Equal amount of H2O at 1125 °C leads to complete melting of plagioclase, orthopyroxene and clinopyroxene, which leaves only olivine and spinel stable and increases the liquid mass fraction from 0.62 in the nominally dry compositions to 0.9. The results of this study appear to support the idea that chromitite layers in the Critical Zone of the Bushveld Complex were formed by episodes of hydration melting of silicate cumulates.

Role of magma injection and mixing in the formation of chromitite in Archean anorthosites: Evidence from the Sittampundi Complex, southern India

Chromitite occurs in many Archean anorthosite complexes. The Neoarchean Sittampundi Anorthosite Complex (SAC) in southern India is composed of mainly (amphibole-rich) anorthosite, minor gabbro, two pyroxene granulite, and many anorthosite-related chromitite layers and lenses. High-Mg# orthopyroxene megacrysts are present in the chromitite but absent in the anorthosite. These megacrysts are disturbed and/or corroded by Cr-spinel and interstitial amphibole into fragment, but are still largely optically continuous. The absence of orthopyroxene in the anorthosite indicates that the mineral was formed in an earlier stage in the deep magma chamber(s), and then partially remelted when it was entered into the extant magma chamber. Two types of amphiboles are recognized based on textural and trace elemental features. Amphibole-1 in the chromitites is coarse-grained, and commonly surrounded by Cr-spinel and amphibole-2, which forms resorption texture along amphibole-1 rims. Chondrite-normalized REE patterns of the amphibole-1 in chromitite and that in anorthosite have similarly positive Eu* anomalies (1.53–3.04) and low total REE contents (ΣREE = 2.90–4.42 ppm). This implies that amphibole-1 in the chromitite was captured from the anorthosite. Meanwhile, amphibole-2 is fine-grained and occurs as interstitial infill among Cr-spinel, plagioclase, and amphibole-1. Amphibole-2 shows negative to weakly positive Eu anomalies (Eu/Eu* = 0.67–1.17), and slightly elevated total REE contents (ΣREE = 5.42–14.20 ppm). The resorption textures on orthopyroxene, amphibole-1, and plagioclase, and the close paragenetic relations between Cr-spinel and amphibole-2, suggest that Cr-spinel was saturated through the mixing of the anorthositic mush with a replenished, more primitive magma. The amphibole-2 REE patterns indicate that the injected magma had partially corroded the pre-existing plagioclase and amphibole-1, and mixed with the interstitial melt in the mush. Addition of Cr2O3 from the replenished magma and SiO2 from the anorthositic crystal mush had likely pushed the melt toward the Cr-spinel saturation and triggered chromitite formation. The high Al2O3 content of Cr-spinel is likely led by plagioclase remelting, which increased the Al3+ content in the mixed magma. Comparable Cr-spinel compositions of the Archean anorthosites worldwide suggest that their chromitites were possibly formed via similar magma injection and mixing processes.

Experimental Constraints on Formation of Low-Cr# Chromitite: Effect of Variable H2O and Cr2O3 on Boninitic-Magma and Harzburgite Reactions

Reactions between a boninitic or basaltic magma and harzburgite at shallow mantle depths are thought to be closely related to the formation of podiform chromitites, but little experimental data is available on these reactions. In this study, a series of experiments were conducted at 1.5 GPa and 1 000–1 400 °C to investigate the interactions between boninitic magma and harzburgite in homogenous mixed systems with varied bulk concentrations of water (∼0.7 wt.%–10 wt.%) and Cr2O3 (∼0.2 wt.%–4 wt.%). In the experimental charges, chromite grains can be observed coexisting with orthopyroxene, clinopyroxene±olivine, and quenched melt in the Cr-bearing systems. The bulk concentration of Cr2O3 in the starting material has a slight effect on compositional changes in the chromites generated. However, the Cr# (Cr#=100×Cr/(Cr+Al)) and Mg# (Mg#=100×Mg/(Mg+Fe)) values for the chromites exhibit positive and negative correlations, respectively, with the bulk H2O concentrations. At 1 100 °C, chromite Cr# values range from ∼33–35 to ∼58–65, and chromite Mg# values range from ∼70–73 to ∼55–58 when bulk H2O contents in the starting material are increased from ∼0.7 wt.% to ∼10 wt.%. The experimentally produced chromites have compositions (as expressed by Cr#, Mg#, and NiO and MnO contents) similar to natural chromites from low-Cr# chromitite bodies. We suggest that the interactions between boninitic magmas with varied H2O contents and harzburgite in a shallow mantle wedge could be a possible mechanism that forms the low-Cr# chromitites found in ophiolites. We emphasize here that H2O may play an important role in the compositional evolutions of natural chromitites.

Hydrated Peridotite–Basaltic Melt Interaction Part II: Fast Assimilation of Serpentinized Mantle by Basaltic Magma

The most abundant terrestrial lavas, mid-ocean ridge and ocean island basalt (MORB and OIB), are commonly considered to be derived from a depleted MORB-mantle component and more specific, variably enriched mantle plume sources. However, findings of oceanic lavas and mafic cumulates issued from melts, enriched in chlorine and having a radiogenic Sr ratio, can be attributed to an interaction between the asthenosphere-derived melts and lithospheric peridotite variably hydrated due to penetration of hydrothermal water down to and below Moho level. To constrain mechanisms and rates responsible for the interaction, we report results of experiments of reaction between serpentinite and tholeiitic basaltic melt. Results show that the reaction proceeds via a multi-stage mechanism: (i) transformation of serpentinite into Cr-rich spinel-bearing harzburgite containing pore fluid, (ii) partial melting and dissolution of the harzburgite assemblage with formation of interstitial hydrous melts, and (iii) final assimilation of the Cr-rich spinel-bearing harzburgite/dunite and formation of hybrid basaltic melts with high MgO and elevated Cr and Ni contents. Assimilation of serpentinite by basaltic melt may occur under elevated melt/rock ratios and may lead to chromitite formation. Our experiments provide evidence that MORB and high-Mg-Cr orthopyroxene-rich cumulates depleted in incompatible elements can be produced from common mid-ocean ridge basaltic melts modified by reaction with hydrated lithospheric peridotite. We established that the rate of assimilation of serpentinized peridotite is controlled by silica diffusion in the reacting hydrous basaltic melt.

Hydrothermal Chromitites from the Oman Ophiolite: The Role of Water in Chromitite Genesis

The role of water-rich solutions in the formation of chromitites has been the matter of controversy. We found small chromite concentrations (chromitites) in diopsidites, precipitated from high-temperature hydrothermal fluids, in the mantle to the crust of the Oman ophiolite. Here, we present petrologic characteristics of the hydrothermal chromitites to understand their genesis. In the chromitites, the chromite is associated with uvarovite in the crust and diopside + grossular in the mantle. They are discriminated from the magmatic podiform chromitite by dominance of the Ca-Al silicates in the matrix. The fluids responsible for chromite precipitation are possibly saline, being derived from the seawater circulated into the mantle through the crust. The saline fluids precipitate chromite to form chromite upon decompression and cooling, and transport platinum-group elements (especially Pt and Pd). The fluids obtain Ca and Al from the crustal rocks and Cr from the mantle rocks during circulation. Saline fluids are also supplied from the slab to the mantle wedge, and can metasomatically precipitate chromite and pyroxenes within peridotites. They re-distribute Cr and chromite in peridotites along with circulation of saline fluids in the mantle wedge.

Formation of banded chromitites and associated sulphides in the Neoproterozoic subarc deep-crustal magma inferred from the Alaskan-type complex, Egypt

The concentrically zoned Dahanib Alaskan-type complex in the Southern Eastern Desert (SED) of Egypt contains cumulate dunites associated with banded chromitites in the core, through harzburgites, wehrlites, and gabbroic rocks to the rim. Primary mineral inclusions hosted in chromian spinels of the banded chromitites are usually pargasite, orthopyroxene (Opx), aspidolite, phlogopite, edenite, olivine and PGE-bearing sulphides. The chromian spinel in Dahanib chromitites has a narrow range of Cr# (0.53–0.63) along with high Ti and Fe+3, like that in the host clinopyroxene-rich cumulate dunites. It resembles in chemistry to spinels in the Alaskan-type complex formed in an island arc setting. The early cumulus olivine in disseminated chromitites and host dunites (Fo88-91) shows a typical fractional crystallization trend. The interstitial clinopyroxene (Cpx) in chromitites is similar in chemical properties to Cpx crystallized from tholeiitic basalt melts, and traces a cumulate trend of arc-related Alaskan complexes. The peridotitic and gabbroic rocks contain primary amphiboles, which are pargasite and edenite with high Ti, Na and Al. The sulphide minerals are usually chalcopyrite, pentlandite and horomanite with subordinate Cu-pentlandite, cubanite, samaniite, heazlewoodite and pyrite. Pentlandite, horomanite and cubanite at temperatures between 900 and 1040 °C are rich in some platinum group elements (PGE: Os, Pt, Ir). They are classified into primary and secondary origins, which were crystallized from segregated sulphide droplets and from hydrothermal fluids at temperature < 760 °C during postmagmatic stage, respectively. The Dahanib banded chromitites and their mineral inclusions crystallized from hybrid melts, consisting of hydrous tholeiitic basalt melts mixed with slab-derived melts/fluids. They possibly formed by gravitational settling of semi-consolidated crystal mushes in a sub-arc lower-crustal magma chamber. Chemistry of primary minerals suggests that the Dahanib subarc deep-crustal magma is rich in Mg, Cr, Al, Ti, V, Ca, Sc and Na during the early magmatic stage, followed by increasing Fe, Mn, Ni, Co, Cu, S, Cs, B, Sr, and Pb with a progress of fractional crystallization.

Petrogeochemical characteristics of chrome-bearing ultramafites in the Kurtushibinsky ophiolite belt (Western Sayan)

Research subject. The metamorphic ultramafites of the Kalninsky and Ergaksky massifs located in the northeastern part of Western Sayan (Russia) constitute the Kurtushibinsky ophiolite belt. These rocks are considered to be potentially interesting as bearing chromium and noble metals, which fact determines the relevance of this research.Materials and methods. Thin and polished sections of silicate and ore minerals from the ultramafic rocks under study were investigated using a polarizing microscope AxioScop, Carl Zeiss. The chemical composition of minerals was determined by the method of X-ray spectrum microanalysis using a scanning electron microscope Tescan Vega II LMU equipped with an energy-dispersive spectrometer INCA Energy 350 and a wave-dispersive spectrometer INCA Wave 700. The petrochemical composition of the rocks was studied by the methods of XRF analysis using an X-ray fluorescence energy-dispersive spectrometer Oxford ED-2000. A quantitative ICP-MS analysis was carried out using an Agilent 7500 spectrometer.Results. The massifs are found to be composed of dunites and harzburgites, which were formed during an uneven depletion of the mantle. As a result of depyroxenization, the ultramafites were enriched with chromospinelides. Subsequent intense plastic deformations contributed to their segregation into ore bodies. Regenerated olivinites predominate in the northern part of the Ergaksky massif. The microstructural features of ultramafites and the composition of minerals indicate them to have been subjected to inhomogeneous high-temperature plastic deformations, which occurred during their migration in the upper mantle and crust.Conclusions. The petrochemical studies have shown that the ultramafites of the Ergaksky massif are least depleted, frequently featuring clinopyroxene. The most depleted ultramafites of the Kalninsky massif are characterized by a more extensive distribution of dunites, the absence of clinopyroxene and lower contents of REE and rare elements in comparison with the ultramafites of the Ergaksky massif. The obtained geochemical data indicate a fluid-magmatic effect of boninite melts on depleted ultramafites, which apparently occurred under mantle conditions over the subduction zone and consequently led to their enrichment with incompatible LREE (La, Ce) and Sr, Zr and Hf, as well as to the formation of high-chromium chromitites.

Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems: Insights from the Rum layered intrusion, Scotland

The cycling of the halogens in sub-volcanic systems may play an important role in magmatic ore deposit formation and the emission of climate-altering gases via volcanic outgassing. However, the mobile and reactive nature of the halogens, as well as their typically low abundances in most intrusive igneous rocks, means that their behaviour in shallow crustal level magmatic systems is difficult to constrain. We present data from the ∼60 Ma Rum Layered Suite, NW Scotland, utilising the neutron irradiation-noble gas mass spectrometry (NI-NGMS) technique, to provide insight into the provenance and behaviour of Cl, Br and I in magma chamber and precious metal enrichment processes. As a well-characterised and superbly exposed example of an open system sub-volcanic magmatic body, the Rum intrusion is an ideal locality to investigate halogen behaviour in this context. Our data reveal concentrations of <1 ppm to 124 ppm for Cl, <7 ppb to 484 ppb for Br and <1 ppb to 363 ppb for I in the Rum rocks. Halogen ratios span a range of 2.3 × 10−4 to 1.5 × 10−2 for Br/Cl and 4.0 × 10−5 to 7.4 × 10−2 for I/Cl, defining an array from MORB-like to sedimentary-like compositions. Chlorine and Br behave sympathetically in all lithologies, and in the rocks considered to be primary magmatic differentiates (i.e., peridotites, troctolites and gabbros), the variation in abundances is likely controlled by accessory quantities of primary hydrous magmatic phases crystallising from the mush interstitial liquid. Iodine behaviour is decoupled from that of Br and Cl, and when combined with noble gas isotope data, suggests variable but pervasive contamination by a sedimentary source or sources during crystallisation of the Rum intrusion. In PGE-rich chromitites, high I/Cl ratios relative to their putative picritic parental melts point to the involvement of a crustal I source and hydrous fluids during chromitite formation. Our combined halogen and noble gas isotope datasets therefore reveal a record of high temperature processes associated with magmatic-crustal interactions and precious metal mineralisation, and provide new insights into volatile evolution during solidification of the Rum intrusion. A key implication of this work is that magma-crust interactions in mafic magmatic systems may lead to I enrichment (i.e., from 12% to up to two orders of magnitude greater than MORB, in the Rum intrusion), before potential outgassing to the atmosphere.

Solid-state redistribution of mineral particles in the upwelling mantle flow as a mechanism of chromite concentration in the ophiolite ultramafic rocks (on the example of Kraka ophiolite, the Southern Urals)

The main regularities of the structure of chromitite-bearing zones of ultramafic rock of the ophiolitic association are considered on the example of Kraka massifs. In all studied chromitite-bearing zones, olivine demonstrates a strong preferably crystallographic orientation, indicating that plastic flow was one of the main factors of petrogenesis and ore formation. A critical review of existing ideas about the origin of ophiolitic chromitites has been carried out. It is shown that for models involving the reaction and magmatic formation of dunites and chromitites, there are a number of difficulties. In particular, the application of the magma mixing model to the mantle ultramafiс rocks for the formation of chrome ores is faced with the problem of “free space”. Free space is necessary for the deposition of large volumes of ores, which is absent in a very low-porous crystalline upper mantle. In the “melt-mantle” interaction model, it is difficult to explain the often observed abrupt contacts of dunites and harzburgites, as well as an increase in the content of orthopyroxene in the near-contact parts of harzburgites, which is very often observed in ophiolite massifs. In addition, there is no mechanism for the formation of chromitites as geological bodies in this model. We have shown that the main trend in the composition and structure of the mantle section of ophiolites is stratification, accompanied by the separation of the rheologically most “weak” aggregates of polycrystalline olivine (dunites), which are host rocks for chrome ores. The stratification of the mantle material occurred during the solid-phase redistribution of minerals in the rocks, which are a dispersion system. In this work, a thermodynamic model is substantiated, which demonstrates the possibility of the emergence of solid-state flows in the conditions of the upper mantle and which makes it possible to eliminate some of the difficulties and contradictions characteristic of the magmatic and reaction-magmatic hypotheses.

Occurrence of Graphite-Like Carbon in Podiform Chromitites of Greece and Its Genetic Significance

The role of post-magmatic processes in the composition of chromitites hosted in ophiolite complexes, the origin of super-reduced phases, and factors controlling the carbon recycling in a supra-subduction zone environment are still unclear. The present contribution compiles the first scanning electron microscope/energy-dispersive (SEM/EDS) data on graphite-like amorphous carbon, with geochemical and mineral chemistry data, from chromitites of the Skyros, Othrys, Pindos, and Veria ophiolites (Greece). The aim of this study was the delineation of potential relationships between the modified composition of chromite and the role of redox conditions, during the long-term evolution of chromitites in a supra-subduction zone environment. Chromitites are characterized by a strong brittle (cataclastic) texture and the presence of phases indicative of super-reducing phases, such as Fe–Ni–Cr-alloys, awaruite (Ni3Fe), and heazlewoodite (Ni3S2). Carbon-bearing assemblages are better revealed on Au-coated unpolished sections. Graphite occurs in association with hydrous silicates (chlorite, serpentine) and Fe2+-chromite, as inclusions in chromite, filling cracks within chromite, or as nodule-like graphite aggregates. X-ray spectra of graphite–silicate aggregates showed the presence of C, Si, Mg, Al, O in variable proportions, and occasionally K and Ca. The extremely low fO2 during serpentinization facilitated the occurrence of methane in microfractures of chromitites, the precipitation of super-reducing phases (metal alloys, awaruite, heazlewoodite), and graphite. In addition, although the origin of Fe–Cu–Ni-sulfides in ultramafic parts of ophiolite complexes is still unclear, in the case of the Othrys chromitites, potential reduction-induced sulfide and/or carbon saturation may drive formation of sulfide ores and graphite-bearing chromitites. The presented data on chromitites covering a wide range in platinum-group element (PGE) content, from less than 100 ppb in the Othrys to 25 ppm ΣPGE in the Veria ores, showed similarity in the abundance of graphite-like carbon. The lack of any relationship between graphite (and probably methane) and the PGE content may be related to the occurrence of the (Ru–Os–Ir) minerals in chromitites, which occur mostly as oxides/hydroxides, and to lesser amounts of laurite, with pure Ru instead activating the stable CO2 molecule and reducing it to methane (experimental data from literature).

Formation of chromitites and ferrogabbros in ultramafic and mafic members of the Variscan Ślęża ophiolite (SW Poland)

Ultramafic and mafic rocks of the Variscan Ślęża ophiolite (Bohemian Massif, Sudetes, SW Poland) contain abundant Cr and Fe-Ti mineralizations. The occurrence of chromitites is related with serpentinites of refractory harzburgitic protoliths. Chromite I from chromitites has variable Cr and Mg numbers (Cr# = 0.41–0.58, Mg# = 0.62–0.83) and low TiO2 contents (<0.20 wt%). Chromite II surrounds chromite I or fills fissures in it and is enriched in Cr and depleted in Al relative to chromite I (Cr# = 0.54–0.68). Chromite I grains contain spherical grains of diopside, phlogopite, pargasite, pyrite and chalcopyrite, occurring as mono- or polyphase solid inclusions. Metagabbros with isotropic fabric contain locally up to 14.6 vol% of magnetite and ilmenite. Magnetite has variable TiO2, Al2O3 and V2O3 contents (0.19–9.82 wt%, 0.13–1.67 wt% and 1.08–1.61 wt%, respectively). It forms xenomorphic grains with trellis or sandwich exsolutions of ilmenite, as well as aggregates with individual ilmenite grains.The moderate Cr# of chromite I points to its origin from a MORB-type melt. The occurrence of hydrous inclusions and nodular textures indicates chromitite formation and alteration under low pressure conditions. Primary sulfide, phlogopite and pargasite inclusions suggest that the parental melt was moderately hydrous and sulphur-rich. Water in the magmatic system originated supposedly from interaction of primary MORB melt with hydrothermal fluids and a significant influence of fluids on chromite composition is expressed by its very low Ti content.Metagabbros containing abundant Fe-Ti oxide mineralization may be classified as ferrogabbros. High V and Ti contents in magnetite indicate a magmatic origin. Its xenomorphic and intragrain mode of occurrence suggest crystallization between pre-existing gabbroic minerals like pyroxene and plagioclase. Ilmenite exsolutions indicate its growth at the expense of magnetite under oxidizing conditions. Occurrence of chromitites in harzburgites and ferrogabbros related to isotropic gabbros may suggest various spreading conditions during formation of the mafic and ultramafic members of the Ślęża ophiolite.

Petrological characteristics and geochemical compositions of the Neotethyan Mersin ophiolite (southern Turkey): Processes of melt depletion, refertilization, chromitite formation and oceanic crust generation

The Mersin ophiolite of the Central Taurides in southern Turkey represents an incomplete section of ocean lithosphere consisting from bottom to top of chromitite-bearing mantle peridotites, cumulate dunites, clinopyroxenites, and gabbros with rare pillow lavas covered by bathypelagic sedimentary rocks. The tectonized peridotites are harzburgites with minor dunites. They are strongly melt-depleted peridotites, comprising chromian spinel (Cr-spinel) with medium to high Cr-numbers (Cr#) [100 × Cr/(Cr + Al) = 48–82] and low Al2O3 concentrations (0.05–2.2 wt%). They display concave chondrite-normalized rare earth element (REE) patterns, which we interpret as evidence for their refertilization by boninitic melts in a mantle wedge. Geochemical evidence suggests that the parental magmas of the high-Cr podiform chromitites (Cr#Sp = 71–83) were of boninitic composition. The chromitites host a platinum-group mineral (PGM) assemblage of laurite [Ru# = 64.0–90.7] and Os-Ir alloys, indicating crystallization within a range of low fS2 conditions (logfS2 ∼ −2 to −1.3) and a Tmax of ∼1200 °C.Mineral compositions and geochemical data of the basalts show tholeiitic to alkali compositions. The tholeiites show mildly light REE depleted patterns and high Th/Yb and La/Nb ratios indicate the presence of a sediment component, implying a subduction-related origin. Petrogenetic modeling indicates that the tholeiitic basalts resulted from 20% melting of a spinel-bearing asthenospheric mantle. The initial εNd values of 7.2 and 4.9 for two tholeiite samples indicate depleted asthenosphere and more enriched mantle sources probably due to the input of older sediment. A comparison with the data of other ophiolite complexes from southern Turkey suggests subduction-overprinted mantle sources as seen in immature island arcs. The trace element patterns of the alkali basalts are similar to OIB implying an origin unrelated to that of the ophiolite assemblage. The composition of the OIB can be modeled by mixing of magmas from low-degree (1–2%) melting of garnet-bearing lherzolite (GLS) with melt fractions from a spinel-bearing lherzolite (SLS) at variable proportions (GLS/SLS = 7/10–1/10).We propose the following tectono-magmatic model for the petrogenetic evolution of the Mersin ophiolite: i) oceanic ridge spreading generated MORB type magmas (not observed in our sample collection), (ii) melting of deep mantle sources beneath MORB type crust generated OIB-like basalts in Neo-Tethyan oceanic basin, (iii) subduction initiation and formation of an infant forearc basin formed island arc tholeiites and boninites.

Платиноносность хромититов Куртушибинского офиолитового пояса, Западный Саян: новые данные

Small scattered grains of PGE minerals have been found in chromitites of Ergaksky and Kalninsky ultramafic massifs, which are extreme northeastern fragments of the Kurtushibinsky Ophiolite Belt of Western Sayan. For the first time, together with previously known PGE minerals, there were found the Cu—Ru—Os solid solution, arsenic-bearing (As up to 3.6 wt %) laurite with minor Pt, Ni, and Co, PGE sulfides of MeS and Me 3 S 4 types, sulfoarsenide with the high content of Rh (~12.7 wt %) in chromitites of the Ergaksky massif, and laurite and sulfoarsenides in chromitites of the Kalninsky massif. Compositions of PGE minerals indicate high temperature of the Ru—Os sulfide formation (~1200—1250 oC) in both massifs, as well as high partial pressure of sulfur under the formation of chromitites and syngenetic PGE minerals in the Ergaksky massif. PGE mineral assemblages were formed in several stages. Os—Ir—Ru solid solutions and disulfides of the laurite—erlichmanite series (mainly fractionating ruthenium) formed earlier than Cu—Ru—Os solid solutions, sulfides (Ru,Fe) 3 S 4 , (Ru,Ni,Fe)S, and PGE sulfoarsenides, which crystallized under influence of reduced fluids as a result of remobilization and redeposition of PGE.