Alteration Of Chromite Research Articles

Alteration of chromite during serpentinization of peridotites

Trace element (Zn, Co, Mn, Ni, Ga, V, Ti, and Sc) compositions of chromite in mafic-ultramafic rocks are widely used for addressing their petrogenesis and discriminating tectonic environments, but they can be altered by subsolidus re-equilibration and hydrothermal alteration, and the detailed dynamic processes are not clear. The Jinchang ophiolite in SW China is part of the Tethyan ophiolite, and consists mainly of harzburgite with minor lherzolite. These peridotites are strongly serpentinized, and contain variably modified chromite and magnetite. Two types of chromite are identified in harzburgite, including magmatic and altered chromite. The magmatic chromite occurs as granular or amoeboid minerals, or as cores surrounded by the altered chromite, and they have homogeneous major elemental compositions (Mg0.65Fe0.35(Cr0.46Al0.54)2O4), but their trace elements, such as Ni, Ga, V, Zn, Co, and Mn, are variably changed during subsolidus re-equilibration. The altered chromite consists of ferrous chromite (Mg0.15Fe0.85(Cr0.46Al0.11Fe0.43)2O4) and chromian magnetite (Fe(Cr0.15Fe0.85)2O4), which were formed by two stages of hydrothermal alteration. They are depleted in Ga and Sc, and enriched in Zn, Co and Mn during the alteration, but their Ti, Ni and V are increased and decreased in the first and second stage, respectively. Thermodynamic modeling reveals that: (1) Reaction between magmatic chromite and olivine at the first stage of alteration produces ferrous chromite and chlorite under high temperature (610–470 °C) and increasing oxygen fugacity (ΔFMQ increasing from −1 to 3) and water-rich conditions; (2) Chromian magnetite is generated by further alteration of the ferrous chromite in the second stage alteration under low temperature (< 470 °C) and decreasing oxygen fugacity conditions (ΔFMQ decreasing from 3 to −4). In addition, the first stage alteration significantly changed trace-element compositions of the magmatic chromite compared with the second stage alteration that slightly modified the ferrous chromite. This study clarifies trace element migration of chromite in the silicate mineral-hydrothermal fluid system, indicating that chromite microstructures and trace elemental compositions can record the detailed serpentinization processes.

Extreme enrichment of arsenic and antimony during alteration of serpentinized peridotites to form listvenite-like dolomite–quartz rocks and Ni–Cr-rich jasper and quartzites in the Highland Border Complex of Scotland

Alteration of serpentinized peridotites of the Highland Border Complex in Scotland took place in two steps. Listvenite-like dolomite–quartz rocks formed by addition of CaO, Sr and CO 2 at constant MgO and SiO 2 involving a mass increase of c. 140%. Stage two involved the dissolution of dolomite, evinced by the abundant pores and rhombohedral grains of quartz, to form Cr- and Ni-rich jasper and quartzites. Formation of the jasper–quartzites involves a mass reduction of c. 80%. The listvenite-like and jasper–quartzite rocks show enrichment in the fluid-mobile elements Ba, Sr, Cs, As and Sb. The As is present in the Aluminium–Phosphate–Sulfate group of minerals formed during alteration of Cr-spinel. Cr-spinel also alters to porous hematite and ferrihydrite with patches containing up to 5.5 wt% As 2 O 3 . Enrichment of As, related to alteration of chromite, is previously unknown from natural rocks, but strongly resembles efficient methods used for remediation of this toxic element. Formation of quartzite and jasper from peridotite and their common presence as pebbles in the Devonian Old Red conglomerates, the Highland Border Complex and Devonian basins in the Scandinavian Caledonides highlight their importance and potential for provenance and tectonostratigraphic correlations. Supplementary material: Supplementary data tables are available at https://doi.org/10.6084/m9.figshare.c.6764598 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

The Influence of the Magmatic to Postmagmatic Evolution of the Parent Rock on the Co Deportment in Lateritic Systems: The Example of the Santa Fé Ni-Co Deposit (Brazil)

Abstract The Santa Fé Ni-Co deposit is a major undeveloped lateritic deposit located in the Goiás State of Central Brazil. The deposit comprises two properties that together have indicated resources of 35.7 million tonnes (Mt), grading 1.14% Ni and 0.083% Co, and inferred resources of 104.3 Mt at 1.03% Ni and 0.054% Co. The laterite was derived from Late Cretaceous alkaline ultramafic lithologies that experienced an initial silicification from Eocene to Oligocene, followed by lateritization and partial reworking in Miocene-Pliocene. The deposit is characterized both by oxide- and phyllosilicate-dominated ore zones. In the former, Ni- and Co-bearing hematite and goethite dominate the supergene mineralogical assemblage, while ore-bearing Mn oxyhydroxides occur as minor components. In the phyllosilicate-dominated horizons the major Ni-carrying phase is chlorite. Multivariate statistical analyses (factor analysis and principal components analysis) conducted on the drill core assay database (bulk-rock chemical analyses) showed that significant differences exist between Ni and Co distributions. The Ni distribution is not controlled by any clear geochemical correlation. This is because the highest Ni concentrations have been measured in the ferruginous and in the ochre saprolite zones, where Ni-bearing minerals (chlorite and goethite) are mostly associated with reworked material and only in a limited way, with zones affected by in situ ferrugination. Cobalt has an atypical statistical distribution at Santa Fé if compared with other laterites, correlated not only with Mn but also with Cr in the majority of the laterite facies. From microchemical analyses on several potential Co-bearing minerals, it was found that the Co-Cr association is related to elevated Co contents in residual spinels, representing unweathered phases of the original parent rock now included in the laterite. This element distribution is atypical for Ni-Co laterite deposits, where Co is normally associated with Mn in supergene oxyhydroxides. In the case of the Santa Fé laterite, the Co concentration in spinels is likely related to magmatic and postmagmatic processes that affected the original parent rock before lateritization, specifically (1) orthomagmatic enrichment of Co in chromite, due to its high affinity to spinels in alkaline melts, and (2) trace elements (i.e., Co, Mn, Ni, and Zn) redistribution during the hydrothermal alteration of chromite into ferritchromite. The Santa Fé deposit represents a good example of how the prelateritic evolution of a parent rock strongly affects the efficiency of Co mobilization and enrichment during supergene alteration. Based on the interpretation of metallurgical test work, a fraction of total Co between 20 and 50% is locked in spinels.

Metamorphic fingerprints of Fe-rich chromitites from the Eastern Pampean Ranges, Argentina

Chromitites hosted in the serpentinized harzburgite bodies from Los Congos and Los Guanacos (Eastern Pampean Ranges, north Argentina) record a complex metamorphic evolution. The hydration of chromitites during the retrograde metamorphism, their subsequent dehydration during the prograde metamorphism and the later-stage cooling, have resulted in a threefold alteration of chromite: i) Type I is characterized by homogeneous Fe3+- and Cr-rich chromite; ii) Type II chromite contains exsolved textures that consist in blebs and fine lamellae of a magnetite-rich phase hosted in a spinel-rich phase; iii) Type III chromite is formed by variable proportions of magnetite-rich and spinel-rich phases with symplectitic texture. Type I chromite shows lower Ga and higher Co, Zn and Mn than magmatic chromites from chromitites in suprasubduction zone ophiolites as a consequence of the redistribution of these elements between Fe3+-rich non-porous chromite and silicates during the prograde metamorphism. Whereas, the spinel-rich phase in Type III chromite is enriched in Co, Zn, Sc, and Ga, but depleted in Mn, Ni, V and Ti with respect to the magnetite-rich phase, due to the metamorphic cooling from high-temperature conditions. The pseudosection calculated in the fluid-saturated FCrMACaSH system, and contoured for Cr# and Mg#, allows us to constrain the temperature of formation of Fe3+-rich non-porous chromite by the diffusion of magnetite in Fe2+-rich porous chromite at &lt;500 ºC and 20 kbar. The subsequent dehydration of Fe3+-rich non-porous chromite by reaction with antigorite and chlorite formed Type I chromite and Mg-rich olivine and pyroxene at &gt;800 ºC and 10 kbar. The ultimate hydration of silicates in Type I chromite and the exsolution of Type II and Type III chromites would have started at ~600 ºC. These temperatures are in the range of those estimated for ocean floor serpentinization (&lt;300 ºC and &lt;4 kbar), the regional prograde metamorphism in the granulite facies (800 ºC and &lt;10 kbar), and subsequent retrogression to the amphibolite facies (600 ºC and 4-6.2 kbar) in the host ultramafic rocks at Los Congos and Los Guanacos. A continuous and slow cooling from granulite to amphibolite facies produced the exsolution of spinel-rich and magnetite-rich phases, developing symplectitic textures in Type III chromite. However, the discontinuous and relatively fast cooling produced the exsolution of magnetite-rich phase blebs and lamellae within Type II chromite. The P-T conditions calculated in FCrMACaSH system and the complex textural and geochemical fingerprints showed by Type I, Type II and Type III chromites leads us to suggest that continent-continent collisional orogeny better records the fingerprints of prograde metamorphism in ophiolitic chromitites.

Genesis of metasomatic gold mineralization in the Pahardiha-Rungikocha gold deposit, eastern India: Constraints from trace element signatures in chromite-cored magnetite and bulk rock geochemistry

The Pahardiha- Rungikocha gold deposit is located south of the Dalma volcano-sedimentary belt within the North Singhbhum Mobile Belt (NSMB), in the eastern part of India. The rocks of this area have undergone three phases of deformation, viz. D1, D2, D3. While the D1 and D2 events signify progressive regional metamorphism, the D3 stage was a consequence of continued N-S compression resulting in E-W trending displacement fractures. This D3 deformation event was responsible for the mobilization of auriferous fluids within the host quartz-chlorite-schist and subsequent gold precipitation. Here, gold occurs as native grains either within the cracks or along grain boundaries of euhedral pyrite. Detailed mineral paragenesis reveals that euhedral pyrite associated with gold is coeval with abundant chromite-cored magnetite. The chromite-cored magnetite shows a complex paragenetic history, i.e., an initial chromite alteration event, followed by epitaxial growth of magnetite rims on the chromite cores, as revealed by LA-ICP-MS trace element data and EPMA X-ray compositional maps. The paragenetic stages of the chromite-cored magnetite is synchronous with the successive deformation events of the host rock. The final phase of deformation is responsible for the formation of the chromite-cored magnetite rims which is coeval with the gold mineralizing event. These magnetite rims formed as a result of metasomatism brought about by the interaction of hydrothermal fluids with the wall rock. The chromite-cored magnetite trace element signatures resemble the high temperature Ca-Fe alteration and high temperature K-Fe alteration facies, reported from various IOCG and IOA type deposits. There is a distinct enrichment of mafic elements such as Ni, Cr, V, Mg, V, Cu within magnetite rims. A similar pattern of enrichment in mafic elements is also seen in the bulk rock geochemical data. Major element discrimination diagrams reveal that the chlorite schists evolved from a mixed provenance of felsic and mafic/ultramafic rocks. Consequently, the D3 induced metasomatism most likely resulted in preferential stripping of the mafic elements, which is well reflected in the analyzed magnetite rims. Thus, we propound that the bulk rock chemical composition of the host chlorite-schist had a first order control on the chemistry of the chromite-cored magnetite, coeval with the gold mineralizing event. Given the fact that there are no causative igneous intrusions underlying the host chlorite schists, we infer that the gold mineralizing fluids were intrabasinal in nature and were mobilized due to reactivation of pre-existing faults and fractures during the D3 deformation event.

Desulphurisation, chromite alteration, and bulk rock PGE redistribution in massive chromitite due to hydrothermal overprint of the Panton Intrusion, east Kimberley, Western Australia

The Central and Western Zones of the Halls Creek Orogen (HCO) of the East Kimberley region of Western Australia host various Paleoproterozoic layered mafic–ultramafic intrusions. The 1856 ± 2 Ma layered mafic–ultramafic Panton Intrusion of the Central Zone contains a reef-type platinum-group element (PGE) mineralisation associated with metamorphosed chromitite horizons. Here we investigate the compositional variation of chromite within the A chromitite layer, or Main Chromitite Layer (MCL), base metal sulphides (BMS), and the associated PGE distribution. Electron-microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data reveal complex chemical zoning of the chromite reflecting post-magmatic sub-solidus re-equilibration and fluid–rock interaction during metamorphism of the host rocks. The sulphur isotopic composition of the BMS (δ34S: −1.67 to 2.89‰) suggests negligible contamination of the parental melt by crustal sulphur and thus favours BMS formation due to fractional crystallisation. Furthermore, pentlandite compositions indicate the highest ƒS2 in the centre of the MCL, which coincides with a considerably heavier sulphur isotope composition compared to pentlandite in the surrounding ultramafic host rock. Alteration of the BMS during amphibolite facies hydration of the Panton Intrusion resulted in the formation of secondary bornite and digenite from the chalcopyrite as well as desulphurisation of pyrrhotite and pentlandite to secondary magnetite. The bulk rock PGE concentrations are controlled by the distribution of platinum-group minerals (PGM) due to the low PGE concentrations in BMS. PGM are present as inclusions in chromite, suggesting that they formed under sulphur-undersaturated conditions. Additional PGM are present within the interstitial serpentine-chlorite matrix suggesting that they precipitated during fluid-rock interaction and hydration of the silicate matrix. Likely sources for the remobilised PGE include PGM inclusions in altered rim zones of magmatic chromite and PGE released due to desulphurisation of BMS despite their low PGE concentrations.

Slab-derived origin of tremolite–antigorite veins in a supra-subduction ophiolite: the Peridotite Nappe (New Caledonia) as a case study

Hydration of mantle peridotites provides information on the exhumation history and on the fluid regime accompanying exhumation of these rocks (reservoirs involved, fluid/rock ratios, temperature of interaction). Highly depleted harzburgites and dunites of the Peridotite Nappe of New Caledonia are crosscut by fractures, which have been pervasively serpentinized, producing lizardite, brucite, magnetite and minor chrysotile in a near-static environment, probably by sea water circulating in cooling joints. This event is generally referred to as “primary serpentinization”. In a next step, already serpentinized joints were re-opened to produce tension and shear cracks sealed by higher-temperature synkinematic fibrous minerals. Locally, tremolite-bearing veins and pockets, which do not display evidence for void infill, were generated by metasomatic replacement of the wall rock peridotite. Most veins only contain fibrous antigorite but some display tremolite-antigorite intergrowths or even pure tremolite. The latter rocks yield high contents of Ca and incompatible elements, which contrast with the overall depletion of the peridotite host rock and suggest contribution of an external source. Whole-rock geochemical and isotopic features (87Sr/86Sr, 18O and H) suggest that antigorite veins, which bear highly radiogenic Sr isotope signatures, were strongly influenced by fluids emitted by the subducted slab and associated sediments. In contrast, the geochemical and isotopic signatures of tremolite-bearing rocks suggest a genetic link with Early Eocene supra-subduction dykes and fluids that leached them. Calcium, strontium and REE-bearing oxidized fluids reacted with already serpentinized fracture wall rock as shown by fibre nucleation, chromite alteration and high Cr contents in tremolite. The bulk of syn-tectonic fluid-rock interaction associated with shear and tensile fracture development probably occurred at the onset of Eocene intra-oceanic subduction, when the buoyant lower plate (the South Loyalty Basin) obliquely forced its way beneath the nascent Loyalty fore-arc. Mobile elements extracted from supra-subduction dykes temporarily enriched the circulating fluids and generated tremolite, antigorite, Mg-chlorite and magnetite instead of antigorite and magnetite solely. Tremolite crystallization probably ceased due to the exhaustion of Ca-rich fluids in a globally cooling fore-arc environment.

Platinum-group minerals and Fe–Ni minerals in the Sartohay podiform chromitite (west Junggar, China): Implications for T–pH–fO2–fS2 conditions during hydrothermal alteration

Complex assemblages consisting of native iron + pentlandite, pentlandite + heazlewoodite + magnetite, heazlewoodite + magnetite + platinum-group minerals (PGM), and millerite ± heazlewoodite are associated with three types of altered chromites (partly altered chromite, double-rimmed chromite, and porous chromite) discovered in the Sartohay podiform chromitite hosted by the Sartohay ophiolitic mélange in west Junggar of China. We propose a two-stage genetic model to explain the chromite alteration processes based on detailed petrographic observations and thermodynamic calculations. Reactions between primary chromite and olivine formed porous Fe2+-rich chromite and chlorite patches under low-temperature, reducing conditions. Afterwards, oxidizing Fe3+- and CO2-rich fluids derived from host serpentinite were incorporated into the Fe2+-rich chromite, shifting its composition to heterogeneous Fe3+-rich chromite (ferrian chromite). Thermodynamic calculations suggest that the co-evolution of fO2 and fS2 played a crucial role in controlling the formation and/or transformation of the Fe–Ni minerals coevally with the sulfides, antimonides, and alloys of platinum-group elements (PGE). Pd and Pt precipitated as sulfides, while Ir precipitated as an alloy when log fS2 ranged from –16.0 to –14.2 within the heazlewoodite stability field. Compared with the host serpentinite, the fO2 in the Sb-bearing fluid equilibrated with chromitite was low enough to decrease the Pd solubility, leading to co-precipitation of Sb and Pd.

Garnierite-bearing serpentinite from the Central Eastern Desert of Egypt: A signature of paleo-weathering in the Arabian Nubian Shield?

This paper records the garnierite in Wadi Dubur Neoproterozoic serpentinites from the Central Eastern Desert of Egypt. Garnierite (Ni-Mg hydrous silicates) occurs in two types (Type I and Type II) of infillings in the weathered serpentinite that was displaced sinisterly by NNW-SSE trending strike-slip faulting and dextrally with NE-trending shearing. All recognized garnierites are serpentine-like (népouite/pecoraite) existing in mixtures with the hosting serpentines. The Type I garnierite is Fe-richer [Mg0.26-0.71 Ni1.22-1.79 Fe1.28-3.05 Al 0.0-0.02 Si1.11-1.73 O5(OH)4] showing zoning in a collomorph texture, while the Type II garnierite is homogenous with more Si and Mg [Mg0.54-0.94 Ni1.38-2.04 Fe0.38-1.29 Al 0.0-0.01 Si1.66-2.10 O5(OH)4]. Raman spectra indicate that the substitution of Ni for Fe formed a poorly crystalline goethite. The serpentinization of the Dubur peridotite protolith and the associated alteration of chromite and Ni-sulfides were important for the early individualization of Ni. It is proposed that the unweathered serpentinites represent the bedrock, while the altered garnierite-bearing ones represent the saprock. The early weathering of serpentinites contemporaneous with a faulting activation caused in situ precipitation of Ni and Fe in veins (Type I garnierite). With the faulting repetition and progressive alteration maintenance Ni, Si and Mg were transferred to the percolating meteoric solutions, and moved downward through the weathering profile precipitating the Type II garnierite in gouges. Although the studied Ni-rich rocks represent a narrow mineralogical occurrence, they are remnants of a former lateritic regolith most probably formed by tropical or sub-tropical weathering during Cretaceous at the expense of Wadi Dubur Neoproterozoic serpentinites. The formed regolith has been subsequently largely eroded, buried and recently exhumed in arid climate.

Mineralogy and chemical aspects of some ophiolitic metaultramafics, central Eastern Desert, Egypt: Evidences from chromites, sulphides and gangues

Chromites and sulphides are the most common ore minerals in the ophiolitic metasomatized ultramafics (~790 Ma) of Um Halham, Fawakhir and Barramiya, central Eastern Desert, Egypt. These ultramafics exhibit variable degrees of alteration and metasomatism and include massive serpentinites (serpentinized peridotite and serpentinite), tremolite‐talc rocks, talc‐carbonate rocks, listwaenite‐like rocks and typical listwaenite.The alteration of chromite to Cr‐magnetite was accompanied by the formation of chloritic aureoles due to the release of Al and Mg from chromite. Textural and compositional features of the chromites suggest a greenschist up to lower amphibolite facies metamorphism (at 500–600 °C), which is facial with the host ultramafics. The chromites exhibit a boninitic affinity and a forearc suprasubduction setting of the present ophiolite assemblages. The variability of chromite chemistry indicates a melt‐rock reaction, together with the water‐bearing melt which is necessary for crystallizing chromite. Pentlandite, pyrrhotite, arsenopyrite, gersdorffite and pyrite are the most common sulphide minerals. Gold occurs either as an invisible phase associated with gersdorffite, arsenopyrite, and As‐rich pyrite or as native nuggets in listwaenites of Barramiya area. Gangue metasomatized phases associated with these ore minerals are identified as sericite, carbonate and chlorite. The investigated sericite ranges in composition from mariposite (&gt;0.5% Cr) to fuchsite (&gt;1% Cr), which indicates typical listwaenites. Five varieties of carbonate minerals are identified; magnesite, breunnerite, dolomite, calcite and ankerite.The CO2‐rich fluids are released during progressive decarbonation reactions in carbonate‐bearing metasediments, whereas Ni and As‐rich fluids are attributed to nearby granitic intrusions affected on serpentinites and leached Ni and precious metals like gold forming As‐Ni‐rich fluids. These metasomatic fluids are thought to reflect a range of melts derived from a compositionally evolving source during subduction initiation in a forearc environment.

Metasomatic alteration of chromite from parts of the late Archaean Sittampundi Layered Magmatic Complex (SLC), Tamil Nadu, India

Chromite deposits associated with layered anorthosite complexes in the Archaean high-grade terranes are rare in the world. The late Archaean Sittampundi Layered Magmatic Complex, Tamil Nadu, India is one of the few such deposits in the world where layers of Fe-Al rich chromites are associated with extremely calcic (An>95) anorthosite. ‘Frozen in’ magmatic mineralogy of the chromitite and the enclosing anorthosite suggest successive crystallization of chromite+clinopyroxene and chromite+clinopyroxene+anorthite from a hydrous Al-rich basaltic melt that was emplaced in a suprasubduction zone setting. Intense deformation and upper amphibolite facies metamorphism at ∼2.45Ga converted the magmatic assemblages to hitherto unreported hornblende+gedrite+Mg-Al rich spinel±chlorite bearing assemblages. During metamorphic reconstitution, chromite was pseudomorphically replaced by green spinel in the domains rich in secondary amphiboles. Mass-balance calculation and algebraic analyses of the observed mineralogy suggest that a number of chemical species including chromium became mobile during the formation of spinel pseudomorph in response to infiltration driven metamorphism. Aluminium became mobile in the length scale of chromite grain but remained immobile in the length scale of a thin section.

Platinum-Group Minerals in Chromitites of the Niquelândia Layered Intrusion (Central Goias, Brazil): Their Magmatic Origin and Low-Temperature Reworking during Serpentinization and Lateritic Weathering

A variety of platinum-group-minerals (PGM) have been found to occur associated with the chromitite and dunite layers in the Niquelândia igneous complex. Two genetically distinct populations of PGM have been identified corresponding to phases crystallized at high temperatures (primary), and others formed or modified during post-magmatic serpentinization and lateritic weathering (secondary). Primary PGM have been found in moderately serpentinized chromitite and dunite, usually included in fresh chromite grains or partially oxidized interstitial sulfides. Due to topographically controlled lateritic weathering, the silicate rocks are totally transformed to a smectite-kaolinite-garnierite-amorphous silica assemblage, while the chromite is changed into a massive aggregate of a spinel phase having low-Mg and a low Fe3+/Fe2+ ratio, intimately associated with Ti-minerals, amorphous Fe-hydroxides, goethite, hematite and magnetite. The PGM in part survive alteration, and in part are corroded as a result of deep chemical weathering. Laurite is altered to Ru-oxides or re-crystallizes together with secondary Mg-ilmenite. Other PGM, especially the Pt-Fe alloys, re-precipitate within the altered chromite together with kaolinite and Fe-hydroxides. Textural evidence suggests that re-deposition of secondary PGM took place during chromite alteration, controlled by variation of the redox conditions on a microscopic scale.

Chromite alteration processes within Vourinos ophiolite

The renewed interest in chromite ore deposits is directly related to the increase in Cr price ruled by international market trends. Chromite, an accessory mineral in peridotites, is considered to be a petrogenetic indicator because its composition reflects the degree of partial melting that the mantle experienced while producing the chromium spinel-bearing rock (Burkhard in Geochim Cosmochim Acta 57:1297–1306, 1993). However, the understanding of chromite alteration and metamorphic modification is still controversial (e.g. Evans and Frost in Geochim Cosmochim Acta 39:959–972, 1975; Burkhard in Geochim Cosmochim Acta 57:1297–1306, 1993; Oze et al. in Am J Sci 304:67–101, 2004). Metamorphic alteration leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chlorite. In this study, we investigate the Vourinos complex chromitites (from the mines of Rizo, Aetoraches, Xerolivado and Potamia) with respect to textural and chemical analyses in order to highlight the most important trend of alteration related to chromite transformation. The present study has been partially funded by the Aliakmon project in collaboration between the Public Power Corporation of Greece and Institute of Geology and Mineral Exploration of Kozani.

Petrogenetic and geotectonic significance of Neoproterozoic suprasubduction mantle as revealed by the Wizer ophiolite complex, Central Eastern Desert, Egypt

Ophiolite complexes, formed in a suprasubduction zone environment during Neoproterozoic time, are widely distributed in the Eastern Desert of Egypt. Their mantle sections provide important information on the origin and tectonic history of ocean basins these complexes represent. The geochemistry and mineralogy of the mantle section of the Wizer ophiolite complex, represented by serpentinites after harzburgite containing minor dunite bodies, are presented. Presence of antigorite together with the incipient alteration of chromite and absence of chlorite suggests that serpentinization occurred in the mantle wedge above a Neoproterozoic subduction zone. Wizer peridotites have a wide range of spinel compositions. Spinel Cr# [100Cr/(Cr + Al)] decrease gradually from dunite bodies (Cr# = 81–87) and their host highly depleted harzburgites (Cr# = 67–79) to the less depleted harzburgites (Cr# = 57–63). Such decreases in mantle refractory character are accompanied by higher Al and Ti contents in bulk compositions. Estimated parental melt compositions point to an equilibration with melts of boninitic composition for the dunite bodies (TiO2 = ~<0.07–0.22 wt%; Al2O3 = 9.4–10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO2 = <0.09–0.28 wt%; Al2O3 = 11.2–14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO2 = ~<0.38–0.51 wt%; Al2O3 = 14.5–15.3 wt%). Estimated equilibrium melts are found in the overlying volcanic sequence, which shows a transitional MORB–island arc geochemical signature with a few boninitic samples. Enrichment of some chromites in TiO2 and identification of sulfides in highly depleted peridotites imply interaction with an impregnating melt. A two-stage partial melting/melt–rock reaction model is advocated, whereby, melting of a depleted mantle source by reaction with MORB-like melts is followed by a second stage melting by interaction with melts of IAT–boninitic affinities in a suprasubduction zone environment to generate the highly depleted harzburgites and dunite bodies. The shift from MORB to island arc/boninitic affinities within the mantle lithosphere of the Wizer ophiolite sequence suggests generation in a protoarc-forearc environment. This, together with the systematic latitudinal change in composition of ophiolitic lavas in the Central Eastern Desert (CED) of Egypt from IAT–boninitic affinities to more MORB-like signature, implies that the CED could represent a disrupted forearc-arc-backarc system above a southeast-dipping subduction zone.

Metamorphism on Chromite Ores from the Dobromirtsi Ultramafic Massif, Rhodope Mountains (SE Bulgaria)

Podiform chromitite bodies occur in highly serpentinized peridotites at Dobromirtsi Ultramafic Massif (Rhodope Mountains, southeastern Bulgaria). The ultramafic body is believed to represent a fragment of Palaeozoic ophiolite mantle. The ophiolite sequence is associated with greenschist - lower-temperature amphibolite facies metamorphosed rocks (biotitic gneisses hosting amphibolite). This association suggests that peridotites, chromitites and metamorphic rocks underwent a common metamorphic evolution. Chromitites at Dobromirtsi have been strongly altered. Their degree of alteration depends on the chromite/silicate ratio and to a lesser extent, on the size of chromitite bodies. Alteration is recorded in individual chromite grains in the form of optical and chemical zoning. Core to rim chemical trends are expressed by MgO- and Al2O3- impoverishment, mainly compensated by FeO and/or Fe2O3 increases. Such chemical variations correspond with three main alteration events. The first one was associated with ocean-floor metamorphism and was characterized by a lizardite replacement of olivine and the absence of chromite alteration. The second event took place during greenchist facies metamorphism. During this event, MgO- and SiO2-rich fluids (derived from low temperatura serpentinization of olivine and pyroxenes) reacted with chromite to form chlorite; as a consequence, chromite became altered to a FeO- and Cr2O3-rich, Al2O3-poor chromite. The third event, mainly developed during lower temperature amphibolite facies metamorphism, caused the replacement of the primary and previously altered chromite by Fe2O3-rich chromite (ferritchromite).

Ferritchromite and chromian-chlorite formation in melange-hosted Kalkan chromitite (Southern Urals, Russia)

Spinel is often used as a magmatic indicator of crystallization processes, without considering the effects of metamorphic alteration on spinel geochemical features. Serpentinized mélanges in the southern Urals host different kinds of disseminated to massive chromitite mineralization. In mélange environments, intense metamorphic alteration above 300 °C leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chromian-chlorite. Based on textural and chemical analyses, mélange-hosted Kalkan chromitites exhibit a hydration and oxidation reaction that can explain the formation of ferritchromite and chromian-chlorite from chromite and serpentine: Textural analyses fit well with the proposed reaction and show that it usually proceeds very close to completion. The degree of alteration of chromite into ferritchromite is controlled by the initial chromite to serpentine ratio. In chromitites, high ratios prevent complete transformation of chromite into ferritchromite. The most likely environment for such reaction is a prograde metamorphic event post-dating serpentinization of the Kalkan ophiolite, possibly related to emplacement within an accretionary wedge.

Platinum group minerals in podiform chromitites of the Bou Azzer ophiolite, Anti Atlas, Central Morocco

The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite. Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr2O3) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios [(Mg/(Mg + Fe2+)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel. Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides, arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides, sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As ores.

Distribution and mineralogy of platinum-group elements in altered chromitites of the Campo Formoso layered intrusion (Bahia State, Brazil): control by magmatic and hydrothermal processes

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

Nickeliferous opaque minerals associated with chromite alteration in the Sakhakot-Qila complex, Pakistan, and their compositional variation

Chemical compositions of the awaruite-heazlewoodite-pentlandite assemblage from the Sakhakot-Qila ultramafic complex, Pakistan, indicate its genesis in highly reducing conditions with f o 2 and f s 2 much below those yielding nickel sulphide ores. The compositions from various parts of the complex exhibit larger variations than those reported so far from a single complex, and include unusually high Ni contents. Awaruite compositions extend beyond the previously known limit of Ni content, and are unique in containing Cu and not Co as the third constituent after Ni and Fe; with sympathetic relation between Cu and Ni. Previous reports indicate genesis of such minerals by serpeatinization only, but the present example links them to chloritization and the formation of ‘ferritchromit’ as well. Their petrographic distribution is related to that of chromite grains and segregations and not to that of serpentine.

Chromite alteration in the Panton Sill, East Kimberley Region, Western Australia

SummaryIn addition to the normal type of chromite alteration, an antipathetic form of alteration has been observed in the ultramafic zone of the Panton Sill, Western Australia. The chromites display margins of lower reflectivity related to a marked enrichment in MgO and Al2O3and depletion in Cr2O3, Fe2O3, and FeO, such that a spinellid of composition lying between the end members MgAl2O4and FeAlaO4is formed. The alteration is related to the formation of secondary spinel in the ground-mass during the hornblendization of primary silicates under metasomatic conditions accompanied by low oxygen activity.