Minerals In Chromitites Research Articles

PGE geochemistry and platinum-group minerals in chromitites from Indus Suture Zone ophiolite, northwest Himalaya, India

PGE geochemistry and platinum-group minerals in chromitites from Indus Suture Zone ophiolite, northwest Himalaya, India

Chromitites from the Vavdos ophiolite (Chalkidiki, Greece): Petrogenesis and geotectonic settings; constrains from spinel, olivine composition, PGE mineralogy and geochemistry

Podiform chromitites of the Vavdos ophiolite are associated with dunite bodies hosted within residual mantle harzburgite. The mantle source is highly depleted and apparently developed at fore-arc settings as inferred from the presence of boninitic melts. The ascendance of these melts through dunite pathways and their subsequent interaction with the depleted harzburgite instigated chromitite mineralization within dunite. Spinel occurs in a) massive accumulations (Mg# = 0.56–0.68 and Cr# = 0.69–0.77), b) in non-massive disseminated textures (Mg# = 0.52–0.61 and Cr# = 0.80–0.81) and c) in schlieren chromitite deposits within dunite bodies (Mg# = 0.43–0.48 and Cr# = 0.79–0.82). Olivine is the main silicate inclusion in chromitites, whereas the absence of hydrous mineral phases is linked with crystallization at relatively deeper parts of the mantle.Olivine inclusions in spinel are enriched in Fo, Ni and Ca compared to the interstitial olivine grains. No compositional gaps were noted between the mineral analyses of chromitites and dunites; the calculated parental melts are also homogeneous, reinforcing the view for production of melt batches by similar processes. Platinum-group elements (PGE) concentrations and platinum-group minerals (PGM) demonstrate high (Os + Ir + Ru)/ (Rh + Pt + Pd) ratios, typical of many Tethyan ophiolitic chromitites. The main PGM phase is laurite that forms an extended solid solution series with substitution of Ru for Os + Ir, linked to the cooling of the system. Massive chromitites contain Ru-rich laurite and Os-Ir alloys whereas non-massive ores contain Ru-poor laurite. The PGM were included in fresh magnesiochromite favoring their magmatic origin and initial entrapment at ~ 1200 °C and low sulfur fugacity.

Ultramagnesian Olivine in the Monchepluton (Fo96) and Pados-Tundra (Fo93) Layered Intrusions (Kola Peninsula)

Abstract —The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25–30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91–92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20–25 vol.% magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5–90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 (n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 (n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)–chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity (fO2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91–92), may be caused by shortage of ferrous iron.

High-Cr chromitites of the Nidar Ophiolite Complex, northern India: Petrogenesis and tectonic implications

The Nidar Ophiolite Complex (NOC) in the Ladakh Himalaya region of northern India is well-preserved and exposed along the Indus Yarlung–Zangbo suture zone. This sequence has been thrusted onto the Indus Formation to the north and to the Zildat ophiolitic mélange in the south. Chromitite mineralization in the NOC occurs as massive, semi-massive, and disseminated ore bodies hosted by dunite. The chromite in these ore bodies contains inclusions of isolated silicates, base metal sulfides, and platinum-group minerals. Little variation in the [Cr/(Cr + Al)] (i.e., Cr#) (0.78–0.86) is observed, in contrast to Mg# ([Mg/(Mg + Fe2+)]), which is more variable (0.56–0.73). The NOC chromitites are high in Cr (Cr# > 0.78), suggesting a possible boninitic parent magma. The calculated (Al2O3)melt and (FeO/MgO)melt range from 9.29 to 11.50 and 0.49–0.10, respectively. The trace element compositions of these chromitites (Ga: 14–27 ppm; Ni: 440–1301 ppm; V: 612–802 ppm; Sc: 2–6 ppm; Co: 207–542 ppm) are comparable to those hosted in the mantle sections of other ophiolite complexes. The overall chemical composition, deduced from the Al2O3 content and FeO/MgO ratio of the primitive parent magma of high-Cr chromitites, is consistent with island arc tholeiitic melts of boninitic affinity. This parent magma would have been in equilibrium with podiform chromitites and associated ultramafic rocks. Further, the distributions of minor and trace elements in NOC chromites also indicate a boninitic magma source. The field relations, petrographic observations, and major and trace element compositions of the studied chromitites suggest that they have undergone melt–rock and melt–melt interactions, which led to the precipitation of high-Cr chromitites from Cr-rich and Al- and Ti-poor boninitic melts. The occurrence of high-Cr chromitites and the boninitic nature of the parent magma indicate that the chromitite layers of the NOC likely formed in an island arc supra-subduction zone.

Geological Evidence does not Support a Shallow Origin for Diamonds in Ophiolite

Geological Evidence does not Support a Shallow Origin for Diamonds in Ophiolite

Characteristics of chromitite mineralization in Sebuku Island based on thin section, polished section, and geochemical data

Sebuku Island is located in Kotabaru Regency, South Kalimantan and is known as one of the main sources of laterite iron in Indonesia. Based on its tectonic setting, Sebuku Island is located within the suture zone that connects Southwest Borneo Block (SWB) and East Java West Sulawesi Block (EJWB). Due to its tectonic setting, Sebuku Island is composed of various rocks associated with suture zone, such as ophiolite rocks. The ophiolite rocks could host mineralization that occurred in Sebuku Island. The aim of this study is to determine the characteristics of lithology and mineralization as determinants of geological processes that influence the formation of rocks and mineral deposits in Sebuku Island. This study was carried out through petrological, petrographic, ore microscopy analysis, and geochemical analysis (x-ray fluorescence). 22 rock samples from Sebuku Island were collected and the result of this research shows that the chromite host rocks are composed of serpentinized dunite and serpentinite. Chromite minerals (FeCr2O4) were found in massive forms with cataclastic, brecciated texture and disseminated with pull-apart texture. Based on observation of the polished sections, it is known that the associated mineral of chromite is magnetite which is an alteration of chromite minerals. Chromitite chemical data shows the chromite composition is Al-rich (Cr# = 0.6) and classified as podiform chromitite formed by fractional crystallization. There are two magma series of igneous rock in the study area i.e. tholeiitic series consists of ultramafic-mafic rocks and calc-alkaline consists of micro-diorite. The abundance of Al2O3 and TiO2 in chromitite shows that Al-rich chromitite has formed in Supra Subduction Zone (SSZ) and has occurred near or above the Moho-transition zone.

Morphology, Composition, and Ontogenesis of Platinum-Group Minerals in Chromitites of Zoned Clinopyroxenite–Dunite Massifs of the Middle Urals

Abstract —We present results of analysis of the morphology and chemical composition of platinum-group minerals from chromite– platinum orebodies of zoned clinopyroxenite–dunite massifs in the Middle Urals (Nizhnii Tagil, Svetlyi Bor, Veresovyi Bor, and Kamenushenskii). Study of more than 500 grains has given an insight into the sequence of formation of platinum-group minerals in chromitites of the studied massifs. Three assemblages of platinum-group minerals have been revealed: magmatic (Os–Ir–Ru intermetallic compounds, isoferroplatinum, ferroplatinum, sulfides of the isomorphous series erlichmanite–laurite and kashinite–bowieite, and thiospinels of the series cuproiridsite–cuprorhodsite–malanite); postmagmatic (with a predominance of tulameenite, tetraferroplatinum, and ferronickelplatinum, resulted from serpentinization of dunites); and latest secondary (minerals with a predominance of PGE sulfides, arsenides, sulfoantimonides, sulfoarsenides, plumbides, and amalgams).

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).

Platinum-group minerals and their host chromitites in Macedonian ophiolites

Several ophiolite bodies that have been a significant source of chromium ore are located in the Vardar Zone of Macedonia. Three relatively small (maximum 15 × 4 km) bodies have been studied in detail. In the Radusa and Lojane complexes, the mantle series consisting of harzburgite and rare dunite are well preserved, whereas minor pyroxenite and gabbro occurrences belong to a poorly preserved cumulate series. The Rabrovo massif corresponds to the basal part of the cumulate sequence. In all three complexes, chromitite mineralization occurs as pods and irregular layered bodies and exhibits all the characteristics of typical ophiolite mineralization with nodular, orbicular, net, schlieren or massive texture, and an Mg–Cr-rich composition. Platinum-group minerals (PGM) are associated with the chromitite concentrations. Described for the first time in Macedonia, they are typical of ophiolitic chromitite, dominated by the laurite–erlichmanite solid solution, and rare Ru–Os–Ir alloy, cuprorhodsite and cuproiridsite. One of the characteristics of the Macedonian PGM is a relative Cu enrichment, marked by Cu-PGM, but also by the presence of Cu in solid solution in laurite, and the occurrence of a Cu-sulfide rim around the PGM trapped in chromite crystals, suggesting that Cu was present in the PGE–S system.

Diamonds, Super‐Reduced and Crustal Minerals in Chromitites of the Hegenshan and Sartohay Ophiolites, Central Asian Orogenic Belt, China

The Central Asian Orogenic Belt (CAOB) is a huge tectonic mélange that lies between the North China Craton and the Siberian Block. It is composed of multiple orogenic belts, continental fragments, magmatic and metamorphic rocks, suture zones and discontinuous ophiolite belts. Although the Hegenshan and Sartohay ophiolites are separated by nearly 3000 km and lie in completely different parts of the CAOB, they are remarkably similar in many respects. Both are composed mainly of serpentinized peridotite and dunite, with minor gabbro and sparse basalt. They both host significant podiform chromitites that consist of high‐Al, refractory magnesiochromite with Cr#s [100Cr/(Cr+Al)] averaging >60. The Sartohay ophiolite has a zircon U‐Pb age of ca. 300 Ma and has been intruded by granitic plutons of similar age, resulting in intense hydrothermal activity and the formation of gold‐bearing listwanites. The age of the Hegenshan is not firmly established but is thought to have formed in the Carboniferous.Like many other ophiolites that we have investigated in other orogenic belts, the chromitites in these two bodies have abundant diamonds, as well as numerous super‐reduced and crustal minerals. The diamonds are mostly, colorless to pale yellow, 200–300 μm across and have euhedral to anhedral shapes. They all have low carbon isotopes (δ14C = −18 to −29) and some have visible inclusions. These are accompanied by numerous super‐reduced minerals such as moissanite, native elements (Fe, Cr, Si, Al, Mn), and alloys (e.g., Ni‐Mn‐Fe, Ni‐Fe‐Al, Ni‐Mn‐Co, Cr‐Ni‐Fe, Cr‐Fe, Cr‐Fe‐Mn), as well as a wide range of oxides, sulfides and silicates. Grains of zircon are abundant in the chromitites of both ophiolites and range in age from Precambrian to Cretaceous, reflecting both incorporation of old zircons and modification of grains by hydrothermal alteration.Our investigation confirms that high‐Al, refractory chromitites in these two ophiolites have the same range of exotic minerals as high‐Cr metallurgical chromitites such as those in the Luobusa ophiolite of Tibet. These collections of exotic minerals in ophiolitic chromitites indicate complex, multi‐stage recycling of oceanic and continental crustal material at least to the mantle transition zone, followed by uprise and emplacement of the peridotites into relatively shallow ophiolites.

Distribution of Platinum-Group Elements in Chromite Ores of the Sorkhband Ultramafic Complex, Kerman, Southeastern Iran

The Ordovician Sorkhband ultramafic complex lies in southern Kerman Province of Iran. The wedge shape complex covers an area of more than 100 km2 and is divided into: lower part comprises of dunites, largest podiform chromitite deposits in Iran (Faryab mine), olivine clinopyroxenite dykes and massive's, wehrlite and olivine websterite dykes; and upper part comprises of clinopyroxene bearing harzburgites, with subordinate lenses and dykes of dunite, massive and dyke like olivine clinopyroxenite and minor orthopyroxenite dykes with no significant chromitite mineralization. Chromitite orebodies exhibit variable sizes and shapes, forming pods, lenses, bands, vein-like bodies and rich dissemination. Podiform chromitites in dunite form tabular to lenticular bodies although may occur also as pencil-like masses. The chromitites occur in four distinct textural modes. Massive, disseminated, banded and nodular chromitites are the most common textural types and commonly grade into one other. Massive chromitites have sharp contacts with the enclosing dunite whereas disseminated bodies grade outward into dunite and occasionally pass into interbanded chromitite and dunite. A detailed electron microprobe study reveals very high Cr#, Mg# and very low TiO2 contents for chromian spinels in chromitites. The Sorkhband chromitites contain up to 440 ppb total PGE, and display a systematic enrichment in IPGE relative to PPGE, with a steep negative slope in the PGE spidergrams and very low PPGE/IPGE ratios, a feature typical of ophiolitic podiform chromitites worldwide. The mineral chemistry data and PGE geochemistry of the chromitites indicates that the Sorkhband ultramafic complex was generated from an arc-related magma with boninitic affinity in a supra-subduction zone setting.

Discovery of in situ super-reducing, ultrahigh-pressure phases in the Luobusa ophiolitic chromitites, Tibet: New insights into the deep upper mantle and mantle transition zone

![][1] Previous research on super-reducing ultrahigh-pressure (SuR UHP) phases from the Tibetan ophiolitic chromitites were mainly conducted on isolated grains extracted from extremely large samples. This approach has been questioned because of possible contamination. To elucidate the occurrence and origin of these SuR UHP minerals, we studied 33 thin sections and rock chips of three ophiolitic chromitites from the Yarlung Zangbo suture zone. Here we report and analyze unambiguously in situ SuR UHP assemblages from the ophiolitic chromitites by electron probe micro-analyzer, scanning microscope and Laser Raman spectroscope. The SuR UHP and associated phases include: (1) blue moissanite as inclusions in olivine (Fo96–98), and in olivine domains between disseminated chromite grains; (2) multiple inclusions of moissanite + wustite + native Fe in olivine; (3) FeNi and FeCr alloys in olivine and chromite; and (4) native Fe and Si in chromite. Crustal asphaltum and h-BN also occur as inclusions in chromite. Our documented in situ SuR UHP phases, combined with the previously inferred existence of ringwoodite + stishovite, all indicate that these assemblages formed under a highly reducing environment (oxygen fugacities several orders of magnitude lower than that of the iron-wustite buffer) in the mantle transition zone (MTZ) and in the deep upper mantle. Diamond + moissanite with distinct 13C-depleted compositions from chromitites have a metasedimentary carbon source. Associations with existing crustal minerals in chromitites demonstrate that carbon-bearing metasedimentary rocks were recycled into the mantle through subduction, and locally modified its composition. Finally we propose a three-stage model to explain the formation of SuR UHP phase-bearing chromitite. Discoveries of SuR UHP phases in Luobusa and other ophiolitic podiform chromitites from the polar Ural Mountains and from Myanmar imply existence of a new type of ophiolitic chromitite. Such occurrences provide an additional window to explore the physical-chemical conditions of the MTZ, mantle dynamics, and the profound recycling of crustal materials. [1]: /embed/graphic-1.gif

Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance

Several chromite deposits located in the in the South and Southeast Arms of Sulawesi, Indonesia, have been investigated by electron microprobe. According to the variation of the Cr# = Cr/(Cr + Fe3+), the chromite composition varies from Cr-rich to Al-rich. Small platinum-group minerals (PGM), 1–10 μm in size, occur in the chromitites. The most abundant PGM is laurite, which has been found included in fresh chromite or in contact with chlorite along cracks in the chromite. Laurite forms polygonal crystals, and it occurs as a single phase or in association with amphibole, chlorite, Co-pentlandite and apatite. Small blebs of irarsite (less than 2 μm across) have been found associated with grains of awaruite and Co-pentlandite in the chlorite gangue of the chromitites. Grains of olivine, occurring in the silicate matrix or included in fresh chromite, have been analyzed. They show a composition typical of mantle-hosted olivine. The bimodal composition and the slight enrichment in TiO2 observed in some chromitites suggest a vertical zonation due to the fractionation of a single batch magma with an initial boninitic composition during its ascent, in a supra-subduction zone. This observation implies the accumulation of Cr-rich chromitites at deep mantle levels and the formation of the Al-rich chromitites close or above the Moho-transition zone. All of the laurites are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at temperature of around 1200 °C and a sulfur fugacity below the sulfur saturation. Irarsite possibly represents a low temperature, less than 400 °C, exsolution product.

Platinum mineralization of the Svetly Bor and Nizhny Tagil intrusions, Ural Platinum Belt

This paper presents the results of a detailed mineralogical and micro analytical study of two Ural-Alaskan type intrusions in the Ural Platinum Belt: (1) the dunite-hosted mineralization of the Svetly Bor intrusion, and (2) the chromitite mineralization of the Nizhny Tagil intrusion. Two generations of platinum minerals are typical of both intrusions: magmatic Pt–Fe(Ni) alloys, and post-magmatic Pt(Fe,Ni,Cu) alloys. A trend from ferroan platinum to isoferroplatinum (Pt,Fe→Pt3Fe) is shown for magmatic alloys of the Svetly Bor intrusion. Magmatic alloys of the Nizhny Tagil intrusion are represented by ferroan platinum (with Ni) only, varying in Fe. The magmatic Pt–Fe alloys of both intrusions were depleted in Fe during the evolution of ore-forming systems and crystallized during the entire magmatic process, generally as fine cubic crystals and anhedral grains hosted by dunite at the magmatic stage during and after the crystallization of dunite (platinum–dunite type of the Svetly Bor intrusion). The evolution of mineral paragenesis was accompanied by a temperature drop, as well as increases in fO2 and fS2.Most of the platinum was concentrated in the residual melts together with chromium, and crystallized in the final stage of the magmatic process (platinum–chromite ore of the Nizhny Tagil intrusion). Post-magmatic Pt(Fe,Cu,Ni) alloys formed during the serpentinization of dunite evolved according to a general compositional trend from tetraferroplatinum and ferronickelplatinum to tulameenite and later to Pt–Cu alloy and Pt-oxide (PtFe)→Pt(Fe,Cu,Ni)→Cu3Pt→Pt–O. Platinum-rich mineralization of both intrusions was formed in the late magmatic stage from a melt rich in volatiles, regardless of their location in dunite or chromitites. Most probably, the rock structure (fracture systems) was the determining factor in the migration of PGE-rich residual melts and ore accumulation.

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.

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.

Composition and textures of chromite and platinum-group minerals in chromitites of the western ophiolitic belt from Pampean Ranges of Córdoba, Argentina

Chromitite bodies hosted in the Neoproterozoic western ophiolitic belt of Pampean Ranges of Córdoba (Argentina) were studied at Los Congos and Los Guanacos ultramafic bodies, with regard to the composition and textures of the chromite and platinum group minerals. Primary chromite composition is only preserved in some massive chromitites from the Los Guanacos ultramafic body, and is similar to Al-rich ophiolitic chromitites, suggesting that they crystallized from melts with back arc basin basalts (BABB) affinity in the suprasubduction mantle. Subsequently, these chromitites underwent a prograde metamorphism. Chromites from chromitites and associated metamorphosed ultramafic rocks show complex replacement and exsolution textures. Mineral chemistry and texture indicate that the chromite composition records two main metamorphic trends. A first trend defined by chromite from massive chromitite, in which there is an enrichment in Fe 3+ and Fe 2+, Cr remain relatively constant, and slightly depleted in Al, Mg. A second trend is defined by chromite from disseminated chromitite and metamorphosed dunite and harzburgite, in which a Fe-rich phase is replacing the Al-rich chromite. This alteration trend is characterized by enrichment in the total iron content (Fe 3+ + Fe 2+) and a strong depletion in Al and Mg. The chemical composition of all analyzed spinels from Los Guanacos and Los Congos, as plotted on the ternary Fe 3+–Cr–Al diagram, correlates well with the Cr-spinels from the upper amphibolite to granulite-facies metamorphism. Platinum group minerals (PGM) identified include native osmium, laurite, erlichmanite, irarsite, platinum and a number of inadequately identified phases such as an oxide or hydroxide of Ru, Pt and Ir–Ru, Pt telluride, Ir–Ru–As–Se and Ir–Ru–Ti compounds. Native osmium was the only PGM which remained unaltered; other PGM underwent mineralogical reworking during metamorphism. Although it is difficult to establish the extent of platinum group element mobilization based on mineralogical observation, our results suggest that the Ru–Os–Ir PGM in the Los Guanacos and Los Congos chromitites were modified in situ, producing re-distribution of these PGE on a small scale. The presence of rare Pt and PGE–As–Se minerals was possibly related to remobilization of Pt, As and Se by fluids during the alteration processes.

Exotic accessory minerals in layered chromitites of the Campo Formoso complex (Brazil)

The Campo Formoso stratiform intrusive complex, in Bahia State, Brazil, considered to be of Paleoproterozoic age, consists of a tabular body of ultramafic rocks about 40 km long and 100-1100 m wide. Thick horizons of chromitite are exploited and the deposits are the richest in Brazil. The complex was intruded by the Campo Formoso calc-alkaline batholith, emplaced by the result of the Transamazonian collision-related orogeny. The peridotite was firstly thoroughly serpentinized, then affected by a renewed cycle of hydrothermal alteration as the batholith cooled, leading in the roof zone to emerald mineralization around roof pendants. An even later influx of fluid led to the formation of talc, silica and carbonates, such that the ultramafic rocks were locally converted to listwanite. The chromitite sequences are highly unusual in containing rather exotic minerals, such as monazite-(La), monazite-(Ce), apatite, galena, bismuthinite, antimony, and three unknown minerals of stoichiometry PbSb2 ,P b 6Sb and PbSb4, all associated with the clinochlore. The latter phases may have formed during hydrothermal activity in the system Pb-Sb. The presence of these exotic minerals in chromitite, which makes this occurrence unique in the world, strongly support the hypothesis that the La, Ce, P, Pb, Bi and Sb were metasomatically added to the Campo Formoso chromitite horizons by hydrothermal fluids emanating from the nearby Campo Formoso calc-alkaline batholith as it cooled.

The platinum-group minerals in chromitites of the Akanvaara deposit, Northern Finland and in their processing products

The Akanvaara deposit, earlier known mostly for chromitites with proven metallurgical properties and substantial vanadium content, is now reported to contain a range of small (3–10 μm) platinum-group minerals (PGMs). The mineralogy, mineral chemistry and mineral assemblages of PGM indicate primary platinum-group elements (PGEs) fractionation and, probably, late-stage PGE redistribution. The investigations of chromite concentrates prepared earlier for chromite reduction experiments have shown that the amount of PGM in the concentrates is less than in the ore samples. A detailed study on reduced chromite pellets has led to the conclusion that the PGM may have been oxidized during sintering process. This paper suggests further steps for re-evaluation of the PGE-potential of the Akanvaara chromite deposit, including detailed investigations of the position of PGE-enriched horizons, compositional and technological properties of the ores.

Os-isotope study of platinum-group minerals in chromitites in Alpine-type ultramafic intrusions and the associated placers in Borneo

Abstract187Os/186Os ratios were determined for in-situ laurite grains in Alpine-type chromitites and platinumgroup minerals (PGM) in the associated alluvial placers in Borneo, Indonesia/Malaysia. The Osisotope ratios of laurite grains in chromite defne an 187Os/186Os ratio for the 100 Ma mantle source of c. 1.04. Thelow 187Os/186Os ratios in all grains confirm the essential derivation of these platinum-group elements (PGE) from the mantle. A minor variation in 187Os/186Os ratios was detected among PGM from placers, but no variation was found within individual grains, including a grain with chemical inhomogeneity. The values are similar to those for PGM in the associated chromitites. The data are consistent with a detrital origin of PGM in placers: the placer PGM originated in the ultramafic section of ophiolities and the release of these grains from igneous rocks and their deposition in placers was almost entirely by mechanical processes.