Chromite Grains Research Articles

PGM in chromitites of Kraka massifs (the Southern Urals): diversity and origin

The paper provides results of study of platinum group minerals (PGMs) from 18 ore occurrences and deposits of the Kraka massifs, most of these located in ultramafic rocks of the upper mantle section (15), and several occurrences in a crust-mantle transition complex (3). It is shown that chromitites in the upper mantle section have refractory geochemical specialization (Os-Ir-Ru), while chromitites of the transition complex typically contain Pt and Pd minerals. The highest concentrations of the platinum group elements (PGE) are observed in chromitites of the transition complex (up to 2500 ppb of the total PGE). However, minor amounts of chromitites at these sites do not allow us to consider this mineralization type as promising in practical terms. chromitites in the upper mantle section are about an order lower in PGE (50–200 ppb of the total PGE). Analysis of the obtained data suggests the following explanation for various PGM types identified. PGMs occurred in chromitites of the upper mantle section at two stages: 1) disulfides of the laurite-erlichmanite series and, to a lesser extent, Os-Ir-Ru alloys were formed within chromite grains in result of subsolidus processes in the upper mantle restite during solid-phase segregation of PGEs initially incorporated in the crystal lattice of chromite; 2) sulfoarsenides and other PGE compounds with basic metals and antimony were formed by hydrothermal processing of chromitites in crustal conditions. Pt and Pd minerals were produced by differentiation of magmatic melts separated from restite; they were completely or partly transformed under the impact of hydrothermal processes.

HRTEM Study of Desulfurization of Pt- and Pd-Rich Sulfides from New Caledonia Ophiolite

Oxygen-bearing platinum group minerals (O-bearing PGMs) are intergrown with base metal sulfides (BMS, e.g., pentlandite–[NiFe]9S8) within fractures in chromite grains from chromitite bodies on Ouen Island, New Caledonia. These PGMs are hosted in chlorite and serpentine, which formed during serpentinization of olivine and pyroxene. The O-bearing PGM grains are polygonal, show microfracturing (indicating volume loss), and contain Pt-Pd-rich sulfide remnants, suggesting pseudomorphic replacement of primary (magmatic) sulfides. They display chemical zonation, with Pt(-Pd-Ni-Fe) relict sulfide cores replaced by Pt-Fe-Ni oxidized alloy mantles and Pt-Cu-Fe(-Pd) alloy rims (tulameenite), indicating desulfurization. The core and mantle show a nanoporous structure, interpreted as the result of coupled dissolution–reprecipitation reactions between magmatic sulfides and low fO2–fS2 serpentinite-related fluids, probably formed during olivine transformation to serpentine + magnetite (early stages of serpentinization). This fluid infiltrated magmatic sulfides (PGE-rich and BMS), degrading them to secondary products and releasing S and metals that were accommodated in the mantle and rim of O-bearing PGMs. Upon olivine exhaustion, an increase in fO2 might have stabilized Pt-Fe-O compounds (likely Pt0/Pt-Fe + Fe oxyhydroxides) alongside Ni-Fe alloys. Our results show that post-magmatic desulfurization of primary sulfides produces complex nano-scale intergrowths, mainly driven by changes in the fluid’s physicochemical properties during serpentinization.

Phase, Minero-Chemical and Microstructural Characterization of Chromite Ore

Chromite ore sample was collected from active mine of Heroshah (34.503041 °N, 71.904565 °E) Malakand agency, Khyber Pakhtunkhwa (KP), Pakistan. The sample was properly washed, dried, and grounded to 74 m (200 mesh) in a pestle and mortar. The phase, microstructure and chemical composition of the ore was studied. X-rays diffraction (XRD) analysis revealed the presence of Aluminium chromite [Fe(Al,Cr)2O4] and Ringwoodite (Mg2SiO4) phases. Microstructural study revealed granular morphology where the chromite grains were surrounded by veins and grains of gangue material. It also shows fractures filled with gangue minerals. Elemental analysis confirmed the presence of high content of Chromium (43% on average) and iron (18% on average) in the as mined sample along with some proportion of Mg, Al and Si. Moreover, Fourier transform infra-red (FTIR) spectroscopy revealed the characteristic bands of chromium oxides (Cr+2-O-2).

Exploring Optical and Geochemical Zoning Variation in Chromite: Metasedimentary vs. Orthomagmatic Origins in Singhbhum Craton, Eastern India

The Banded Chromite-Quartzite (BCQ) deposits of Ghutrigaon, Singhbhum Craton, exhibit a metasedimentary geneology, diverging significantly from the world's well-established orthomagmatic deposits of Sukinda located in the southern fringe of the Singhbhum Craton, Eastern India. This study explores the zoning characteristics of chromite grains from Sukinda and Ghutrigaon in the Singhbhum Craton, revealing insights into their genetic evolution. It aims to elucidate the connection between their development and the physico-chemical changes that occurred during post-protolithic processes. Petrography and mineral chemistry analysis reveal two distinct features i.e., Ghutrigaon chromite displays a brighter core zone and is in sharp contrast with Sukinda chromite grains that show greater marginal reflectivity. The zoning of Ghutrigaon chromite follows an aluminium trend, with a core enrichment of Cr and Fe and a rim concentration of Al and Mg. The iron trend is evident in the zoned chromite of Sukinda, with concentrations of Mg, Al, and Ni dominating at the core, while Cr, Fe, Ti, and Mn are enriched at the periphery. In Ghutrigaon chromite, there is a preferential migration of Mg ions from the core to the altered rim, while trace elements such as Sc, Nb, and U move from rim to core, and Sr, Hf, and Zr increase from core to rim. In contrast, Sukinda chromite experiences leaching of Cr and Fe from the core, with subsequent precipitation in the rim. Trace elements in Sukinda chromite show a decreasing trend for Cu, Sr, Zr, Hf, and La with Mg#, indicating that Mg variations also influence these trace and rare earth elements during alteration. Ghutrigaon chromite likely originated in a Supra-Subduction Zone environment and zoning in its Cr-spinel is attributed to solid-state diffusion during metamorphism. In contrast, the Sukinda chromite is associated with boninitic magma in a back-arc rifting environment and its chromite zoning is the product of secondary changes that occurred during serpentinisation.

Flexibilizing Mechanisms of Spinel-, Hercynite-, Galaxite-, and Chromite-Containing Basic Refractories for Cement Rotary Kilns

Background: Refractory linings of cement rotary kilns are subjected to severe thermomechanical stresses of cranking, ovality, tyre hooping/migration, and uneven thermal distribution. An insistent demand is to discover the significance of spinel, hercynite, galaxite, and chromite in magnesia refractories, as well as their flexibilizing mechanisms. Objectives: The objectives are to compare the fracture behavior of magnesia–spinel, – hercynite, -galaxite, and -chromite refractories and to unveil the flexibilizing mechanisms of different spinels. Methods: The wedge-splitting test is carried out to produce various fracture parameters. Their flexibilizing mechanisms are unveiled by performing microstructural observations and analyses. Results: Various fracture parameters are obtained, including specific fracture energy, brittleness number, characteristic crack length, and thermal-shock resistance parameter. Generally, magnesia–hercynite bricks and magnesia–galaxite bricks have demonstrated the obvious advantages of fracture resistance, which are more flexible than magnesia–spinel bricks and magnesia–chromite bricks. Conclusion: The flexibility of magnesia–spinel bricks is attributed to microcracks generated from the thermal mismatch between spinel grains and surrounding periclase, which is recognized as the thermal-expansion mismatch mechanism. In magnesia–hercynite and magnesia-galaxite refractories, the active-ion-diffusion mechanism is predominant beyond similar microcracks, to drive the flexibility by the continuous diffusion of Fe2+, Mn2+, and Mg2+ during high-temperature processes. In magnesia–chromite bricks, the pore rims contribute to the flexibility as a silicate-migration mechanism, after silicate envelopes first arise around chromite grains and then vanish into the surrounding magnesia by the suction of capillary force during the burning process.

Platinum-Group Element and Trace Element Contents of Chromite Grains from the Alexo Komatiite Flows Compared with Chromites from Other Volcanic Rocks: Implications for the Use of Chromite Compositions in Petrogenesis

Platinum-Group Element and Trace Element Contents of Chromite Grains from the Alexo Komatiite Flows Compared with Chromites from Other Volcanic Rocks: Implications for the Use of Chromite Compositions in Petrogenesis

The Genesis of Ultramafic Rock Mass on the Northern Slope of Lüliang Mountain in North Qaidam, China

The ultramafic rock located on the northern slope of Lüliang Mountain in the northwestern region of North Qaidam Orogen is altered to serpentinite. The occurrence of disseminated chromite within the serpentinite holds significant implications for understanding the petrogenesis of the protolith. This work provides strong evidence of a distinct zonal texture in the chromite found in the ultramafic rock, using petrographic microstructure and electron probe composition analysis. The core of the chromite is characterized by high contents of Cr#, with enrichment in Fe3+# (Fe3+/(Cr + Al + Fe3+)) and depletion in Al2O3 and TiO2. The Cr2O3 content ranges from 51.64% to 53.72%, while the Cr# values range from 0.80 to 0.84. The FeO content varies from 24.9% to 27.8%, while the Fe2O3 content ranges from 5.19% to 8.74%. The Al2O3 content ranges from 6.70% to 9.20%, and the TiO2 content is below the detection limit (<0.1%). Furthermore, the rocks exhibit Mg# values ranging from 0.13 to 0.25 and Fe3+# values ranging from 0.07 to 0.12. The mineral chemistry of the chromite core in the ultramafic rock suggests it to be from an ophiolite. This ophiolite originated from the fore-arc deficit asthenosphere in a supra-subduction zone. The estimated average crystallization temperature and pressure of the chromite are 1306.02 °C and 3.41 GPa, respectively. These values suggest that the chromite formed at a depth of approximately 110 km, which is comparable to that of the asthenosphere. The chromite grains are surrounded by thick rims composed of Cr-rich magnetite characterized by enrichment in Fe3+# contents and depletions in Cr2O3, Al2O3, TiO2, and Cr#. The FeO content ranges from 28.25% to 31.15%, while the Fe2O3 content ranges from 44.94% to 68.92%. The Cr2O3 content ranges from 0.18% to 23.59%, and the Al2O3 and TiO2 contents are below the detection limit (<0.1%). Moreover, the rim of the Cr-rich magnetite exhibits Cr# values ranging from 0.90 to 1.00, Mg# values ranging from 0.01 to 0.06, and Fe3+# values ranging from 0.64 to 1.00, indicating late-stage alteration processes. The LA-ICP-MS zircon U-Pb dating of the ultramafic rock yielded an age of 480.6 ± 2.4 Ma (MSWD = 0.46, n = 18), representing the crystallization age of the ultramafic rock. This evidence suggests that the host rock of chromite is an ultramafic cumulate, which is part of the ophiolite suite. It originated from the fore-arc deficit asthenosphere in a supra-subduction zone during the northward subduction of the North Qaidam Ocean in the Ordovician period. Furthermore, clear evidence of Fe-hydrothermal alteration during the post-uplift-denudation stage is observed.

How Does Nodular Chromite Nucleate and Grow? An Integrated Microstructural and Petrological Approach

Abstract Nodular chromite ore deposits are found in ophiolites and crop out in structures interpreted as former dykes. Most of them are transposed parallel to the plastic foliation of the host peridotite. Many studies have been conducted in order to decipher the origin and evolution of nodular chromite but the outstanding lack of consensus paves the way for an integrated field, geochemical and microstructural approach to be carried out. We sampled the well-characterized Maqsad chromitite dyke that crops out at the top of the mantle–crust dunitic transition zone in the Oman ophiolite and that was not affected by transposition. The spectacular variations in nodule size and texture and their distribution within the dyke have been perfectly preserved which is a rather unique situation. We selected about 40 nodules representative of the shapes and size variability of this ore deposit. Nodules have been classified in three categories: Type-1 nodules are large skeletal chromite grains associated with amphibole and olivine filling their former porosity; Type-3 nodules have a central nucleus of chromite/silicate surrounded by a mantle of close-packed chromite grains. Their shape is best described as almond-like, and they may reach 3 cm in length; Type-2 includes all of the intermediate nodules shapes and sizes between type-1 and type-3 varieties. Electron Probe MicroAnalyser (EPMA) transects and maps show that mineral chemical variations in type-1 nodules and in the nuclei of type-2 and type-3 nodules record out of equilibrium crystal growth. They exhibit high XCr and relatively low-TiO2 and likely resulted from transient interactions between hydrothermal fluids and basaltic melts. Contrary to type-1 and nuclei, the mantles of type-2 and -3 nodules have lower Cr2O3 contents, decreasing toward the nodule edges. They are richer in TiO2 than type-1 nodules but concentration patterns of this element along edge to edge transects are quite variable. The chromite mantles of type-2 and -3 are best understood as the fractional crystallization products of a parent melt of type-1 nodules in different pFluids and/or redox conditions. The analysis of the distribution of the misorientation axis between the skeletal grains and the adjacent chromite grains in the mantle (N = 222 in 28 nodules) revealed a clustering around a [111] axis across the whole range of misorientation angles. Accordingly, we suggest that the growth of the Maqsad nodules is achieved by accretion of finer euhedral chromite grains onto a skeletal chromite grain by juxtaposition of their flat crystal facets. Combined EDS-EBSD mapping together with EPMA transects revealed that a ~1-mm-thick rim of high XCr, displaying a homothetic shape to its corresponding nuclei, is a common feature in the mantle when the nodule sizes exceed about 1 cm. We interpret this observation as the result of the accretion of grains with higher XCr compositions during the nodule construction. Overall, our new study of the Maqsad nodular chromitite highlights the peculiar and transient conditions needed to give rise to the textural and chemical complexity that is preserved in these enigmatic rocks.

Trends in the Unit Cell Parameter for Kimberlitic Versus Non-Kimberlitic and Diamond-Indicating Chromite

Abstract Spinel-group minerals are among the best-known and widely used minerals in diamond exploration due to their ubiquity, resistance to weathering, and utility as petrogenetic indicators. The kimberlite indicator mineral chromite is investigated in this study using micro-X-ray diffraction (µXRD) to measure chromite unit cell parameter ao. We used epoxy-mounted chromium-rich spinel (henceforward called ‘chromite’) mineral separates with known chemical composition from kimberlitic and non-kimberlitic sources to evaluate structural-chemical correlations for potential use in diamond exploration. Chromite grains of <300 µm size from the Koala, Misery, and Sheiba kimberlites in the Ekati property (Northwest Territories, Canada), as well as from exploration programs in Botswana and Gabon, Africa, were examined in situ, as mounted for standard electron probe microanalysis (EPMA). Unit cell parameter ao was measured by µXRD for several natural kimberlitic and non-kimberlitic chromite grains, and these data have been correlated with chemical composition as determined by EPMA on a grain-by-grain basis. Conventional chemical discrimination plots with unit cell size denoted by color demonstrate clearly discernable unit cell trends that are useful for classification. Two kimberlitic chromite compositional trends can be discriminated by chromite unit cell size. The kimberlitic phenocryst trend is delineated by a distinct increase in unit cell size (ao > 8.336 Å), whereas the kimberlitic xenocryst trend is delineated by a distinct decrease in the unit cell (ao < 8.322 Å). The latter trend is also followed by the Gabon non-kimberlitic samples. Notably, the unit cell parameters for chromite in the diamond-indicating field have a tightly determined value of ao = 8.329 (± 0.007) (or 8.322–8.336 Å). This field partially overlaps with the unit cell values for some non-kimberlitic chromites (e.g., Botswana). Unit cell values of chromite grains recovered from heavy mineral concentrates could serve as a preliminary screening technique for identifying diamond-indicating chromites prior to chemical analysis if their kimberlitic provenance is known. More broadly, the μ-XRD unit cell technique is a useful, non-destructive tool that shows promise for application to other kimberlite indicator minerals.

To the transition zone and back: Records from diopside exsolution lamellae in chromite from Archean ophiolitic podiform chromitites

Plate tectonics and deep subduction provide the conveyor belt for elemental cycling between Earth's surface and deep interior. Here, we report Archean diopside exsolution lamellae in chromite grains from 2.5 billion years old (Ga) subducted then exhumed Archean oceanic crustal fragments in ophiolitic mélange. Diopside exsolution lamellae are a key indicator of a precursor ultra-high pressure (UHP) phase of chromite, known previously only from Phanerozoic subduction zones, indicating deep subduction and elemental cycling between the surface and deep Earth. These lamellae, along with trapped dolomite, and UHP TiO2(II)-bearing multi-phase solid crustal mineral inclusions in the host chromite, suggest that a precursor chromite polymorph (chenmingite) with a crystal structure stable only at conditions in the mantle transition zone (> 413 to 660 km) preserve the history of subduction, entrapment of UHP crustal mineral inclusions, and return flow to the surface in an Archean subduction/forearc system. These nano-scale diopside lamellae and UHP TiO2(II)-bearing multi-phase solid inclusions are strong evidence for deep and steep subduction in the Neoarchean, and early operation of the deep crustal material recycling, linking surface and deep environments, forging a habitable world.

FTIR study of H2O in silicate minerals and mineral inclusions in chromite from the peridotite zone of the Stillwater complex (Montana, USA): Evidence for chromitite formation in an H2O-rich environment

Although the involvement of hydrous fluids has been frequently invoked in the formation of stratiform chromitites in layered intrusions, there is a lack of natural evidence to signify their presence and mechanism. Here, Fourier-transform infrared spectroscopy (FTIR) of H2O in silicate minerals in the lowermost layer and G chromitite layer of the Stillwater complex, Montana, USA, shows that olivine grains have 20−55 ppm H2O, orthopyroxene has 30−45 ppm H2O, and clinopyroxene has 144−489 ppm H2O. The jointly increasing H2O contents of olivine and orthopyroxene in silicate cumulates along with magma differentiation record a negative correlation in chromitites. On the basis of poikilitic clinopyroxene, we calculated that the interstitial melts had averages of 1.3 wt% and 2.3 wt% H2O in dunite and chromitite, respectively, showing significant differences between chromitites and silicate cumulates. More than 10% of the chromite grains contained polymineralic inclusions up to 100 μm in size that were composed mainly of orthopyroxene, hornblende, plagioclase, and phlogopite. Most of these minerals were characterized by higher MgO and fluid-mobile element contents, such as Na and K, than minerals in associated silicates. Based on the mineral modes of the hydrous phases and their compositions, the trapped fluids contained ∼2.6 wt% H2O, consistent with the FTIR estimates, indicating the inclusion compositions represent interstitial melts instead of parental magmas. These observations indicate that the chromite microlites collected fluids during early crystallization, leading to a heterogeneous fluid redistribution in the melt. The fluids were collected on the surface of chromite grains during crystallization and then dissolved into poikilitic pyroxene. Chromite grains could also be efficiently floated by these fluids, causing them to migrate away from the silicate minerals in the magma channel and leading to the formation of nearly monomineralic chromitite seams. This process serves as a kinetic model indicating that chromite could be completely separated from silicates during mechanical sorting in layered intrusions.

Microstructural Insights into the Evolution of Ophiolitic Chromite from Luobusha

The podiform chromitite found within the Luobusha ophiolite comprises characteristic nodules and massive chromitites. However, the exact origin of these formations remains a topic of ongoing debate. In this study, the microstructures of olivine and chromite are investigated to unravel their formation processes and shed light on the associated geodynamic mechanisms. EBSD analysis provides insights into chromitite and host peridotite deformation mechanisms. Olivine grains in the host dunite and nodular chromite exhibit crystallographic preferred orientations (CPOs) with D-type fabrics, which show a girdle distribution in the [010] and [001] axes, normal to the foliation plane of the sample. The massive and disseminated chromitite displays B-type and C-type olivine fabric, with a concentration of [001] axes parallel to the lineation of the sample. Crystal plastic deformation can be observed in the Luobusha chromite grains, highlighting intercrystalline deformation processes. Small grains lacking misorientation observed in the massive chromitite are likely attributed to heterogeneous nucleation. Chromite nodules are found to be a patchwork of subgrains with various orientations and high-angle boundary misorientation. The formation of Luobusha chromitite involves deep-seated crystallization, followed by amalgamation, and subsequent deformation within the mantle peridotite. These findings distinguish Luobusha chromitite from other ophiolitic chromite deposits, offering valuable insights into the deformation history and formation processes.

Origin of chromitites in the Norilsk-1 intrusion (Siberian LIP) triggered by assimilation of argillaceous rocks by Cr-rich basic magma

Сhromite-rich rocks and monomineralic chromitites occur in mafic-ultramafic layered intrusions worldwide, and are economically important as they host major resources of chromium and platinum group elements. One of the acknowledged genetic concepts for such mineralization advocates bulk chromite crystallization in a response to hybridization of Cr-rich basic magma with felsic melts, derived from partial melting of the wall rocks. However, there has been lack of direct insights into this process and its details have been remaining largely obscured. In this study, we report data on chromite-rich assemblages of the Noril'sk-1 intrusion (Siberian LIP), with a particular focus on chromite-hosted multiphase inclusions. Composition of the latter is different from the rocks of the Noril'sk-1 intrusion and inherits geochemical fingerprints of the wall rock argillites and, therefore, represent snapshots of the heavily-contaminated medium of the magma-wall rock reaction front. Mineral relationships in chromite-rich breccias with fragments of the wall rocks also provide valuable insight into chromite mineralization mechanisms. Our results, along with geological evidence and data on other Noril'sk-type intrusions, indicate that bulk crystallization of chromite, aided by partial suppression of silicate crystallization, occurred in the hybrid medium during digestion of the wall rocks by ascending and emplacing basic magma. Continuous flow of the magma around outshoots of the wall rocks or through a “magmatic karst” in the wall rocks, allowed for a continuous precipitation of chromite. Concentration of chromite grains to form dense mineralization apparently occurred due to formation of chromite-rich blobs around the wall rock fragments, and selective collection and transport of chromite by fluid bubbles, which formed via degassing of wall rocks and then carried chromite and relics of the wall rocks (xenoliths) to the upper parts of the pluton. We propose that assimilation-driven formation of chromite-rich lithologies in intrusions requires: (1) sufficient Cr contents in magma, which allow for its oversaturation in Cr-spinel only by a sudden cooling and addition of SiO2, K2O or/and H2O; (2) efficient disintegration and digestion of the host rocks, releasing considerable amounts of these components to the magma, and (3) a mechanism ensuring accumulation of excess chromite within a small volume (e.g. continuous reaction of Cr-rich magma with the products of the host rocks' assimilation or/and mechanical concentration of chromite).

Composition and formation conditions of nephrite, Nyrdvomenshor deposit, Polar Urals

Research subject. Nephrite and related rocks from the Nyrdvomenshor deposit in the Polar Urals were studied. The Nyrdvomenshor deposit is located in the exocontact of the Rai-Iz ultramafic massif, confined to the Main Ural Fault. The deposit was developed in the process of geological exploration; a license has been issued for a part of the deposit. Aim. To study the nephrite and related rocks from alluvial of the deposit, to formulate a model of its origin. Methods. Qualitative characteristics were assessed visually using a binocular microscope and a special flashlight. The chemical composition was determined by the X-ray fluorescence method. The contents of trace elements were determined by ICP-MS analysis. The mineral composition was studied on a scanning electron microscope with an energy dispersive microanalysis system. Measurements of the isotopic composition of oxygen were carried out. Results. In addition to vesuvianite rodingite, hydrogarnet rodingite was found to be common at the deposit. The studied nephrite is substandard. Tremolite predominates in nephrite, diopside forms relic grains. Uvarovite is widespread, forming both idiomorphic grains, sometimes sheath, less often elongated xenomorphic, and replacing chromite. Omphacite overgrows grains of chromite and uvarovite. Grains of the Fe-dominant mineral of the shuiskite group are noted. Conclusions. Nephrite was formed through both metamorphic and metasomatic processes. Serpentinite was replaced by diopside, which was then replaced by nephrite. Metamorphism enhanced the metasomatism of the serpentinite melange and provided the cryptocrystalline tangled fibrous structure of the nephrite. Then metamorphism and metasomatism led to the formation of omphacite and cracking of the nephrite, which reduced its quality. As these processes progressed, the contribution of the crustal fluid increased, which is confirmed by the results of studying the oxygen isotopic composition of nephrite and other rocks of the deposit.

Sluggish Slab Rollback at the Early Stage of Flux Melting During Subduction Initiation: Li Isotopic Evidence From the Coto High‐Al Chromite Deposit, Zambales Ophiolite, Philippines

AbstractThe compositions of chromitites and dunites from Moho transition zone (MTZ) of the Coto block of the Zambales ophiolite, Philippines, are used to investigate the geodynamic transition from anhydrous to hydrous magmatism during subduction initiation (SI). Chromite grains in the chromitites have Cr# values [100 × Cr/(Cr + Al)] and TiO2 contents ∼35–50 and 0.05–0.30 wt.%, respectively, intermediate between those of chromite in typical MORB‐like lavas (Cr#, ∼20–60; TiO2, ∼0.6–1.7 wt.%) and boninites (Cr#, ∼70–85; TiO2, <0.4 wt.%). Olivine grains in the dunites have δ7Li values varying from ∼−2‰ to +21‰ with most between +10‰ and +15‰, beyond that of normal mantle (+4 ± 2‰) but comparable to those of some arc lavas (up to +12‰). The data set indicates that parental magmas of the high‐Al chromitites originated from hydrated harzburgitic mantle sources and formed temporally between MORB‐like and boninitic magmatism during SI, resulting from the early stage of flux melting in the Zambales proto‐forearc mantle. Modeling of Li diffusion reveals that the MTZ cooled down at a minimum rate of 0.1°C/yr in order to preserve the large δ7Li variation of olivine in the dunites, comparable to the thermal conditions below ultra‐slow to slow spreading ridges. Such a stage of transitional magmatism, although displaying notable slab contributions, took place at a sluggish period of slab rollback and asthenospheric upwelling, leading to a trough level of heat flow and magma production during the entire course of SI.

Hydrothermal alteration of chromitite-dunite pairs from the Sabzevar ophiolite (NE Iran): Chemical and nano-textural evolution of Cr-spinel

Hydrothermal alteration of chromitite bodies in the form of pods (from a few centimeters to a few tens of meters in thickness) from the central sector of the Sabzevar ophiolite belt (NE Iran) was examined down to the nanoscale. The main Cr-spinel alteration feature corresponds to homogeneous cores (primary Cr-spinel) either rimmed by or locally replaced by heterogeneous domains composed of spinel grains with little or no inclusions / pores. The chemical evolution of spinel in response to hydrothermal alteration is found to follow two distinct chemical trends: (i) in the massive and semi-massive chromitites, the trend consists in the progressive increase of Cr without Fe3+ incorporation; (ii) in serpentinized dunites containing disseminated Cr-spinel grains, the increase in Cr occurs in conjunction with Fe3+ incorporation. These two trends involve a decrease in the spinel Mg/(Mg + Fe2+) ratio. Nanoscale observation using transmission electron microscopy (TEM) shows that porous ferrian Cr-spinel which are poorer in Al and Mg and richer in Cr and Fe than the primary Cr-spinel, contains relicts of the primary Cr-spinel as well as intercalated platelets of newly formed magnetite and lizardite/chlorite. Automated crystal orientation mapping reveals that the (001) and (111) planes of newly formed lizardite/chlorite and Cr-spinel/magnetite, respectively, are parallel. The replacement of the primary Cr-spinel by porous ferrian Cr-spinel and magnetite is found to be isomorphic. The pores, ranging from 2 to 15 μm in size, may act as pathways for the aqueous solutions involved in the serpentinization process, which fed the crystallization of alteration minerals in the pores. The two different compositional trends in chromite grains from massive/semi-massive chromitite and hosted in serpentinized dunite are best explained, using mass-balance calculation, by considering the influence of the Cr-spinel/(Cr-spinel+olivine) initial ratio in the rock (XChr) and the dissolution of Mg at a water-to-rock ratio (w/r) above one. In general, at the cm-to-meter scale, Al and Fe exchange between chromitites and their serpentinized dunite envelope was not observed in the central sector of the Sabzevar ophiolite belt; however, such Al and Fe transfer is clearly observed in other parts of this ophiolite belt, where hydrothermal alteration of podiform chromitites produced magnetite ores. Comparatively, the small extend of chromitite alteration observed here, likely reflects the product of lesser fluid-rock interaction with w/r ratio close to one instead of >103 estimated for the magnetite ores based on the magnitude of the Al transfer to their dunite envelope.

Origin of Alteration Patterns in Accessory Chromites from the Kudada Metaperidotites, East Singhbhum District (Jharkhand, India)

Abstract The metamorphosed ultramafic-mafic bodies of the Kudada area are located close to the Singhbhum Shear Zone (SSZ) in eastern India, where the major rock types are talc-magnesite schist and serpentinite with accessory chromite and magnetite veins. The ultramafic bodies and associated metavolcanic rocks are part of the northern extension of the Early Archean Gorumahisani greenstone belt and belong to the Iron Ore Group (IOG) supracrustal sequence. This study reveals intense compositional variability in accessory chromites of serpentinite with core composition of chromites are characterized by the variable Cr# [Cr/(Cr+Al)] = 0.53 - 0.82 and Mg# [Mg/(Mg+Fe2+)] = 0.01 - 0.17. Compositional variability on the scale of a single chromite grain occurs in the form of multi-stage zoning. To identify the patterns of compositional zoning, chromites of serpentinite are subdivided into four types depending on their grain size, reflectivity of different rims, intensity of fracture, and porosity, and supported by in-situ mineral chemistry. The type-I chromites are less fractured and non-porous variety showing the outermost chrome magnetite rim envelops the inner ferritchromit rim. Porosity is mainly developed in the type-II grains where the inner ferritchromit is formed surrounding the pore spaces. The type-III chromites are small clustered grains having ferritchromit core and chrome magnetite rim while the type-IV grains are completely altered to chrome magnetite. Textural relations and mineral chemistry indicate that metamorphism and activities of H2O and CO2-rich hydrothermal fluids during tectonic evolution of the Singhbhum Shear Zone (SSZ) might have caused these zoning patterns and compositional variabilities in accessory chromites of the Kudada area. Cation exchange between chromite and silicate minerals along with intra-grain cation diffusion within different Cr-spinel zones further intensified these processes.

Origin of high-Cr podiform chromitites from Kabaena Island, Southeast Sulawesi, Indonesia: constraints from mineralogy and geochemistry

ABSTRACT The East Sulawesi Ophiolite, one of the three largest ophiolites in the world, contains important podiform chromitites. However, the origin of these chromite deposits is not well constrained. Here, we present the detailed mineralogy and in situ chemistry of chromite and solid inclusions within chromite for podiform chromitites from the Kabaena Island, Southeast Sulawesi. Chromite grains have low TiO2 (0.17–0.27 wt%) and Al2O3 (13.4–15.1 wt%) contents and high Cr# [Cr/(Cr+Al), molar] of 0.70–0.74 and Mg# [Mg/(Mg+Fe2+), molar] of 0.69–0.75, which resemble those of other high-Cr podiform chromitites worldwide. The MORB-normalized patterns for some selected major-trace elements of chromite show slightly positive slopes from Al2O3 to Mn. Chromite-hosted silicate inclusions include olivine, clinopyroxene, orthopyroxene and amphibole and are characterized by higher Mg/Fe ratios than those of host peridotites. The geothermobarometry of chromite-hosted clinopyroxene-orthopyroxene pairs indicates that the estimated T-P conditions of the parental magma of the Kabaena chromitites are 950–1010°C and 7.0–8.4 Kbar. The extreme-Mg-rich olivine inclusions (Fo >96) with extremely high Ni (5300–8200 ppm) and Cr (1500–6700 ppm) contents are interpreted to have crystallized from high-Mg and Cr boninitic melts, and the Fo contents were then elevated by subsolidus re-equilibration (Fe–Mg and Fe–Ni exchange) with chromite. Sulfide inclusions contain base metal minerals containing millerite with rare pentlandite and chalcopyrite, and platinum-group minerals that include the laurite-erlichmanite series, Ir–Ni monosulfide, irarsite and cuproiridsite. This sulfide assemblage reveals that their parental magmas experienced the evolution of high T and low f(S2) to low T and high f(S2) from the early to late stage. Finally, combined with the palaeogeographic reconstruction in Sulawesi, we propose that the high-Cr chromitites in this region were formed by the reaction of depleted mantle and boninitic magma beneath a juvenile island arc, implying the initiation of a new subduction in the Miocene.

Post-magmatic hydrothermal activities in the Bursa ophiolite (NW Turkey): Implications for unusual minerals recovered from ophiolites

Low-temperature hydrothermal activities in ophiolites and their modification of chromite have been less studied compared to the magmatic processes. Here we present evidence of hydrothermal modification of chromitites from the mantle sequence of the Bursa ophiolite (NW Turkey). The chromitites are composed mainly of chromite with variable amounts of magnesite and quartz in the matrix. Many chromite grains contain various inclusions such as monophase magnesite or quartz, as well as multi-phase inclusions of quartz + chlorite ± magnesite associated with or without ferrian chromite, which are unusual in ophiolitic chromitites worldwide. Ferrian chromite intergrown with chlorite and quartz inclusions, has higher Cr# values (88.3–94.1) but lower MgO (6.11–9.74 wt%), and Fe3+/(Fe3++Fe2+) ratios (0.13–0.25) than primary chromite (Cr#: 78.3–79.1); MgO: 15.4–15.6 wt%; Fe3+/(Fe3++Fe2+): 0.36–0.40), indicating the infiltration of reducing and SiO2-rich serpentinizing fluids. The pyroxene and olivine were likely broken down by the pre-existing fluids and contributed to the SiO2-rich characteristic of serpentinizing fluid. Fluxing by CO2-rich fluids produced carbonation of the previous serpentine matrix and formed interstitial magnesite and quartz. Lower MgO contents and overall higher Cr# values of small chromite grains hosted in magnesite matrix compared to those hosted in quartz matrix (MgO: 13.2–15.0 wt% vs. 14.8–15.4 wt%; Cr#: 78.7–79.8 vs. 78.2–78.8) suggest that the carbonated fluids redistributed Mg2+ into magnesite and subtly increased Cr solubility in the fluids. Recrystallization may have prompted fracture healing in chromite grains and preservation of quartz when not associated with ferrian chromite. The carbon and oxygen isotope compositions of magnesite (δ13C: −3.35‰; δ18O: 17.29‰) indicate that the CO2-rich fluids were likely derived by decarbonation of marble and metaclastic rocks from the Tavşanlı zone metamorphic rocks in the Bursa ophiolite.

Platinum‐group mineral and silicate inclusions in the low‐Al chromitites of the Manipur ophiolite, northeast India: Implications on Cr‐PGE mineralization in nascent subduction zone settings

Low‐Al chromitites from the Manipur ophiolite contain a wide range of mineral inclusions, including hydrous and anhydrous silicates, sulphides, and platinum group minerals (PGM). The common inclusions are primary silicates, such as clinopyroxene, orthopyroxene and secondary silicate inclusions, such as chlorite, serpentine, and garnet. These inclusions are small (<30 μm in PGM, <100 μm in silicates) and randomly distributed throughout their host chromite grains. The shape and texture indicate some inclusions (clinopyroxene, orthopyroxene, PGMs) trapped during the magmatic stage and most inclusions (chlorite, serpentine, and uvarovite) trapped during recrystallization of chromite. Clinopyroxene was discrete inside the chromite, with Cr2O3 content ranging from 0.62 to 1.77 wt.%, and Al2O3 varying between 0.96 to 1.63 wt.%. Orthopyroxene is enstatite (En93Wo2Fs4), Cr2O3 varies from 0.58 to 1.8 wt.% and the minor amount of CaO (0.41–0.81 wt.%), and Al2O3 (0.58–0.83 wt.%). Uvarovite is present in two forms: discrete crystal inclusions (anhydrous) in chromites and fractures (hydrous) as fillings. The chlorite occurs as lath shaped, and patchy phase is Cr‐rich variety ‐ kammererite (clinochlore) and penninite with variable Cr2O3 (0.22–7.25 wt.%) and FeO (2.5–3.9 wt.%). The PGM in the studied chromite are laurite (Ru, Os, Ir)S2. The parental melts of chromitites had low TiO2 (0.16–0.48 wt.%) and Al2O3 (10.18–14.58 wt.%) contents, and primary mineral inclusions, suggesting an arc‐like geochemical affinity, and the estimated melt composition is comparable with that of boninitic magma.