Spinels,Fe–Ti oxide minerals,apatites, and carbonates hosted in the ophiolites of Eastern Desert of Egypt: mineralogy and chemical aspects

Spinels, Fe–Ti oxide minerals, apatites, and carbonates hosted in ophiolitic serpentinites and metagabbros of Gabal Garf (southern ED) and Wadi Hammariya (central ED) of Egypt are discussed. Microscopic and electron probe studies on these minerals are made to evaluate their textural and compositional variations. Alteration of chromites led to form ferritchromite and magnetite; rutile–magnetite intergrowths and martite are common in serpentinites. Fine trillis exsolution of ilmenite–magnetite and ilmenite–hematite and intergrowth of rutile–magnetite and ilmenite–sphene are recorded. Composite intergrowth grains of titanomagnetite–ilmenite trellis lamellae are common in metagabbros. The formation of ilmenite trellis and lamellae in magnetite and titanomagnetite indicate an oxidation process due to excess of oxygen contained in titanomagnetite; trapped and external oxidizing agents. This indicates the high P _H2O and oxygen fugacity of the parental magma. The sulfides minerals include pyrrhotite, pyrite and chalcopyrite. Based on the chemical characteristics, the Fe–Ti oxide from the ophiolitic metagabbros in both areas corresponds to ilmenite. The patites from the metagabbros are identified as fluor-apatite. Carbonates are represented by dolomites in serpentinites and calcite in metagabbros. Spinel crystals in serpentinites are homogenous or zoned with unaltered cores of Al-spinel to ferritchromit and Cr-magnetite toward the altered rims. Compared to cores, the metamorphic rims are enriched in Cr# (0.87–1.00 vs. 0.83–0.86 for rims and cores, respectively) and impoverished in Mg# (0.26–0.48 vs. 0.56–0.67) due to Mg–Fe and Al (Cr)–Fe³⁺ exchange with the surrounding silicates during regional metamorphism rather than serpentinization process. The Fe–Ti oxides have been formed under temperature of ~800 °C for ilmenite. Al-spinels equilibrated below 500–550 °C, while the altered spinel rims correspond to metamorphism around 500–600 °C. Geochemical evidence of the podiform Al-spinels suggest a greenschist up to lower amphibolite facies metamorphism (at 500–600 °C), which is isofacial with the host rocks. Al-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rims and surrounding silicates, suggesting relic magmatic composition unaffected by metamorphism. The composition of Al-spinel grains suggest an ophiolitic origin and derivation by crystallization of boninitic magma that belonging to a supra-subduction setting could form either in forearcs during an incipient stage of subduction initiation or in back-arc basins.     

Origin of chromite compositional variation in the Panton Sill,Western Australia

The compositional variation of chromite and associated olivine in chromite-rich and chromitepoor cumulus layers of the Panton Sill is described and a diffusion-controlled crystallization mechanism is proposed to explain this variation. By this mechanism, chromite initially precipitates with a fairly uniform composition, irrespective of the relative proportions of coprecipitating olivine and chromite, and is modified by continued growth during the postcumulus stage. The effect of postcumulus overgrowth of chromite, K _d =(Mg/Fe²⁺)liquid/(Mg/Fe²⁺) chromite6, is to deplete the surrounding magma in chromium and decrease Fe²⁺ relative to Mg such that a chemical gradient exists between the overlying magma, through which the cumulus grains settled, and the magma in contact with settled chromite grains near the magma/crystal pile interface. Postcumulus equilibration of olivine and chromite with the surrounding magma results in higher Mg/(Mg + Fe²⁺) ratios of both olivine and chromite and higher Al content of chromite. The extent of this postcumulus modification is directly related to the proportion of chromite to olivine in a particular layer. This model can be extended to stratiform intrusions elsewhere in which chromite coprecipitates with olivine, orthopyroxene or plagioclase and displays similar compositional trends.     

Ore petrology of the V-Ti magnetite (lodestone) layers of the Kurihundi area of Sargur schist belt,Dharwar craton

The V-Ti magnetite layers (lodestone) occur within the layered gabbro-anorthosites-ultramafic rocks emplaced into the migmatitic gneisses close to the high grade Archeaen Sargur supracrustal rocks in the Kurihundi area. The ore petrographic studies of the lodestone reveal the presence of primary Ti-magnetite, ilmenite, ulvospinel, pleonaste, hematite and pyrite, chalcopyrite, pyrrhotite and secondary Ti-maghemite, martite and goethite as well as secondary covellite. These layers contain Ti-magnetite (60%) and ilmenite (30%) with silicates (<5%) exhibiting granular mosaic texture with well-defined triple junctions and are classified as adcumulus rocks. The grain-boundary relationships in the ores indicate considerable postcumulus growth and readjustment due to combined effects of sintering and adcumulus growth. Intergrowth textures (ulvospinel, ilmenite and pleonaste in Ti-magnetite and hematite in ilmenite) reflects exsolution features crystallized from solid-solutions compositions under different conditions of oxygen fugacities. Larger bodies of pleonaste and ilmenite in Ti-magnetite become lensoid or rounded in outline and these morphological modifications took place during the regional upper amphibolite to lower granulite facies metamorphism at 2.6 Ga ago. The lodestone contains high TiO₂ (20 to 22.59 wt%), with V₂O₅ (0.85 to 1.15%) and Fe₂O₃ ^t (72.03 to 74.25%). Ti-magnetite shows alteration to Ti-maghemite, martite and goethite due to low temperature oxidation and hydration during weathering.     

Effects of hydrothermal alteration on the compositions of chromian spinels

Chromite spinels in hydrothermally altered rocks from fracture-zone ultramafic rocks and from both ultramafic cumulate pods and sheeted dikes in the Josephine ophiolite, California, display a wide variety of compositions. Alteration of the spinel may not be visible in thin section. The primary composition changes accompanying hydrothermal alteration are increase in Cr/(Cr+Al) and/or Fe²⁺/(Fe²⁺+Mg). In general, altered spinel grains associated with hornblende and chlorite show an increase in Cr/(Cr+Al) from core to rim. Altered spinel grains associated with serpentine show an increase in Fe²⁺/(Fe²⁺+Mg) from core to rim but may not show an increase in Cr/(Cr+Al). The compositional zoning in some altered spinel grains appears to result both from reaction of clinopyroxene plus spinel to form hornblende, and from reaction of hornblende to form chlorite. These observations suggest that subsolidus hydrothermal metamorphic effects need to be considered when interpreting spinel compositions and the compositions should not be interpreted solely in terms of igneous processes. Further, the presence of highly altered spinels may be indicative of hydrothermal alteration in rocks where other evidence of such alteration is absent.     

Mechanisms of retrograde changes in oxide minerals from the Proterozoic Serrote da Laje deposit,northeastern Brazil

Retrograde textural and chemical changes in oxide minerals from the Proterozoic Serrote da Laje deposit, northeastern Brazil, have been investigated. The deposit is situated in a mafic-ultramafic layered sill. Oxidation and cooling leading to successively decreasing diffusion rates resulted in disequilibrium on the microscale. Pleonaste in particular shows a rapid change in composition between (a) coarse grains in a granoblastic magnetite host, indicating metamorphic peak conditions, (b) coarse lamellae in magnetite, indicating commencement of exsolution, and (c) composite pleonaste — ilmenite lamellae in magnetite, which indicate oxidation exsolution. Barren rock layers cooled under more oxidized conditions compared with oxide-rich layers. Formation of pleonaste- and ilmenite lamellae in magnetite and ilmenite — hematite relations are discussed.     

Ilmenite composition in the Tellnes Fe–Ti deposit,SW Norway: fractional crystallization,postcumulus evolution and ilmenite–zircon relation

Major and trace element XRF and in situ LA-ICP-MS analyses of ilmenite in the Tellnes ilmenite deposit, Rogaland Anorthosite Province, SW Norway, constrains a two stage fractional crystallization model of a ferrodioritic Fe-Ti-P rich melt. Stage 1 is characterized by ilmenite-plagioclase cumulates, partly stored in the lower part of the ore body (Lower Central Zone, LCZ), and stage 2 by ilmenite-plagioclase-orthopyroxene-olivine cumulates (Upper Central Zone, UCZ). The concentration of V and Cr in ilmenite, corrected for the trapped liquid effect, (1) defines the cotectic proportion of ilmenite to be 17.5 wt% during stage 1, and (2) implies an increase of D _V^Ilm during stage 2, most likely related to a shift in fO₂. The proportion of 17.5 wt% is lower than the modal proportion of ilmenite (ca. 50 wt%) in the ore body, implying accumulation of ilmenite and flotation of plagioclase. The fraction of residual liquid left after crystallization of Tellnes cumulates is estimated at 0.6 and the flotation of plagioclase at 26 wt% of the initial melt mass. The increasing content of intercumulus magnetite with stratigraphic height, from 0 to ca. 3 wt%, results from differentiation of the trapped liquid towards magnetite saturation. The MgO content of ilmenite (1.4–4.4 wt%) is much lower than the expected cumulus composition. It shows extensive postcumulus re-equilibration with trapped liquid and ferromagnesian silicates, correlated with distance to the host anorthosite. The Zr content of ilmenite, provided by in situ analyses, is low (<114 ppm) and uncorrelated with stratigraphy or Cr content. The data demonstrate that zircon coronas observed around ilmenite formed by subsolidus exsolution of ZrO₂ from ilmenite. The U-Pb zircon age of 920 ± 3 Ma probably records this exsolution process. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

长江河流沉积物磁铁矿化学组成及其物源示踪

运用电子探针分析了长江干流和主要支流河漫滩沉积物中磁铁矿的元素组成.磁铁矿中的FeO平均含量稍高于其标准组成,而Fe2O3平均含量则明显低于标准组成;Ti、Al、Cr、V、Mn、Mg、Co和Zn等元素在磁铁矿中含量变化大,不同支流的磁铁矿的元素组成不同,同一取样点不同样品磁铁矿的元素组成变化也较大.金沙江、湘江、汉江及长江干流磁铁矿与钛磁铁矿、钛尖晶石、钒钛磁铁矿和铬铁矿等出溶交生,TiO2、Cr2O3和V2O3等元素含量高且变化大.金沙江磁铁矿富Mg、Al和Cr;大渡河、雅砻江和岷江磁铁矿中微量元素含量大多低于0.5%;涪江、汉江磁铁矿富Ti和V,而湘江磁铁矿富Ti和Al;总体上,长江干流上游磁铁矿富Ti,而下游磁铁矿中Ti、Al、Cr、V、Mg和Mn含量低于0.15%.干流磁铁矿的元素组成变化反映主要支流源岩组成及对干流影响程度的差异.     

Magnetites and ilmenites in the Somerset dam layered basic intrusion,southeastern Queensland

16 chemical analyses and 71 microprobe analyses of magnetites and ilmenitesin the Somerset Dam layered basic intrusion demonstrate that the chemistry of these oxides is a sensitive indicator of crystallisation and subsolidus conditions. Microprobe analyses prove the existence of two chemically distinct types of ilmenite — secondary ilmenite formed by subsolidus oxidation of magnetite-ulvöspinel_ss, and primary ilmenite, which has been only partially affected by the subsolidus oxidation.Application of experimental Tf_OaX relations shows that the inferred crystallisation conditions approximately the NNO buffer. Subsolidus cooling appears to have occurred approximately parallel to the NNO buffer, and was probably buffered partly an H₂O-rich fluid, and partly by the Fe-Ti oxides. The proportions of H₂O and of sulphur compounds appear to have determined the temperatures at which subsolidus oxidation of magnetite ceased.     

Magnetite, ilmenite and ulvite in rocks and ore deposits: petrography, microprobe analyses and genetic implications

Summary ?Detailed petrographic studies and microchemical analyses of titanomagnetite from igneous and metamorphic rocks and ore deposits form the basis of this investigation. Its aim is to compare the data obtained and their interpretations with the experimentally deduced subsolidus oxidation-exsolution model of Buddington and Lindsley (1964). The results are also considered relevant for the interpretation of compositional variations in black sands which are recovered for titanium production. The arrangement of the samples investigated is in accordance with textural stages C1 to C5 caused by subsolidus exsolution with increasing degrees of oxidation (Haggerty, 1991). Stage 1 is represented by two types of optically homogeneous TiO₂-rich magnetite: a. An isotropic type considered to represent solid solutions of magnetite and ulvite containing between 5.2 to 27.5 wt% TiO₂ corresponding to about 14.7 to 77.7 mol% Fe₂TiO₄ in solid solution with magnetite. The general formula of this type is Fe²⁺ _1+x Fe³⁺ _2−2x Ti _x O₄ (x = 0.0–1.0). b. The second type which has not been reported so far is anisotropic and shows complex internal twinning resembling inversion textures. It is thus attributed to inversion of a high-temperature ilmenite modification (with statistical distribution of the cations) which forms solid solutions with magnetite. TiO₂ varies between 9.3 and 24.5 wt% corresponding to about 17.2 to 43.6 mol% ilmenite in solid solution with magnetite. This type is interpreted as a cation-deficient spinel with the general formula Fe²⁺ _{12/12 + 1/4x}Fe³⁺ _{24/12 − 3/2x} □ _{0 + 1/4x} Ti _x O₄ (x = 0.0–16/12). Isotropic and anisotropic homogeneous magnetites occur in volcanic rocks only; the homogeneity of the solid solutions was explained by fast cooling which prevented the development of exsolution textures. Stages 2 and 3 are represented by magnetite with or without ulvite. The magnetite host contains ilmenite lamellae forming trellis and sandwich textures. In contrast to the requirement of the oxidation-exsolution model, the ilmenite lamellae are concentrated exclusively in the cores of the host crystals. The reverse host-guest relationship may also occur. Stages 4 and 5 are identical with thermally generated martite (= martite due to heating). The textures are characterized by very broad lamellae of ferrian ilmenite or titanohematite dominantly concentrated along the margins of the host crystals. Thermally generated martite is restricted to subsolidus-oxidation reactions. The ilmenite lamellae of trellis and sandwich textures contain low Fe₂O₃-concentrations (average 4.8 mol%; to a maximum of 8.3), whereas the Fe₂O₃-content of thermally generated martite is between 32 to 71 mol%. With respect to the Fe₂O₃-concentrations in the ilmenite lamellae, no transition between the two types was observed. The results of this paper show that the widely accepted oxy-exsolution model of Buddington and Lindsley (1964) which is based on experimental results can – with the exception of thermally generated martite – not explain the tremendous variety of magnetite–ilmenite–ulvite relationships in natural rocks and ore deposits. Received October 16, 2001; accepted May 2, 2002     

Economic minerals of the Kovdor baddeleyite-apatite-magnetite deposit,Russia: mineralogy,spatial distribution and ore processing optimization

The comprehensive petrographical, petrochemical and mineralogical study of the Kovdor magnetite-apatite-baddeleyite deposit in the phoscorite–carbonatite complex (Murmansk Region, Russia) revealed a spatial distribution of grain size and chemical composition of three economically extractable minerals — magnetite, apatite, and baddeleyite, showing that zonal distribution of mineral properties mimics both concentric and vertical zonation of the carbonatite-phoscorite pipe.The marginal zone of the pipe consists of (apatite)-forsterite phoscorite carrying fine grains of Ti–Mn–Si–rich magnetite with ilmenite exsolution lamellae, fine grains of Fe–Mg-rich apatite and finest grains of baddeleyite, enriched in Mg, Fe, Si and Mn. The intermediate zone accommodates carbonate-free magnetite-rich phoscorites that carry medium to coarse grains of Mg–Al-rich magnetite with exsolution inclusions of spinel, medium-grained pure apatite and baddeleyite. The axial zone hosts carbonate-rich phoscorites and phoscorite-related carbonatites bearing medium-grained Ti–V–Ca-rich magnetite with exsolution inclusions of geikielite–ilmenite, fine grains of Ba–Sr–Ln-rich apatite and comparatively large grains of baddeleyite, enriched in Hf, Ta, Nb and Sc. The collected data enable us to predict such important mineralogical characteristics of the multicomponent ore as chemical composition and grain size of economic and associated minerals, presence of contaminating inclusions, etc. We have identified potential areas of maximum concentration of such by-products as scandium, niobium and hafnium in baddeleyite and REEs in apatite.     

Textural re-equilibration,hydrothermal alteration and element redistribution in Fe-Ti oxide pods,Singhbhum Shear Zone,eastern India

This study describes textures and mineral chemistry of magnetite-ilmenite-bearing pods/pockets in mineralogically diverse feldspathic schist near Pathargora in the Singhbhum Shear Zone, eastern India. The textural and geochemical characteristics of the magnetite-ilmenite assemblage are the results of a protracted geological history involving magmatic crystallization and oxidation-exsolution of titanomagnetite, deformation-induced recrystallization and textural re-equilibration and hydrothermal fluid-induced hematitization of magmatic magnetite. The magnetite grains contain characteristic trellis and sandwich ilmenite lamella, which are interpreted to be the products of oxidation-exsolution of ulvöspinel component of magnetite-ulvöspinel solid solution. The exsolution process was accompanied by preferential partitioning of spinel elements such as Cr, Al and V in magnetite and Ti, Mn, Mg, HFS elements (Nb, Ta), transition elements (Sc, Co, Cu and Zn) and granitophile elements (Mo, Sn and W) in ilmenite. The deformed sandwich lamella is locally recrystallized and transformed into granular ilmenite close to fractures, micro-shear planes and magnetite grain boundaries. Coarse granules of ilmenite, within or associated with magnetite, are of two textural types: one invariably contains Fe-rich exsolved phase and may be of magmatic origin, while the other mostly formed by strain-induced, fluid-mediated expulsion (from the interior of magnetite to its boundary) and dynamic recrystallization of existing ilmenite lamella in magnetite, and dynamic recrystallization of primary ilmenite containing Fe-rich exsolved phases. Magnetite is variably hematitized. The highly porous nature and trace element geochemistry of hematite and mass-balance calculations suggest the hematitization was mostly redox-independent and was caused by infiltration of metal-rich, reduced and acidic fluid. The hematitization process was associated with significant enrichment and immobilization of U, Th, Pb, REEs, Cu, Mo and W and depletion of Ni, Cr, V in hematite.     

Mineralogical Characteristics of Exsolved Spinel in the Panzhihua V-Ti Magnetite Deposit, Sichuan: Implications for the Mineralization Process

Spinel exsolution is widespread in titanomagnetite from the Fe-Ti oxide gabbro of the Panzhihua intrusion, Emeishan Large Igneous Province, SW China. However, little research has been conducted into the implications of patterns in the mineralogical characteristics of the spinel for spatial variation in the controls on the exsolution mechanism and, hence, the formation process of the ore deposit. This study selected the Lanjiahuoshan Ore Block in the Panzhihua V-Ti magnetite deposit to explore this issue, systematically studying exsolution textures in the titanomagnetite through petrographic observation and the integrated use of in-situ microanalysis. The results show that the exsolved spinel gradually becomes finer-grained and less abundant from the center to edge and the bottom to top of the ore bodies. Compositionally, there is an inverse correlation between the size of exsolved spinel grains and their Mg# value. In addition, there is compositional zonation in the spinel interiors, with a gradual increase in the Mg content and decrease in Fe content from the core to the rim. The analysis suggests that fractional crystallization of ferrotitanium magma with a high oxygen fugacity in a shallow magma chamber caused compositional differences in the primary magnetite solid solution in different parts of the Panzhihua intrusion. Additionally, the thermal evolution of the magnetite solid solution differed in different parts of orebody, bringing about variations in spinel development. Together, these effects resulted in spatial variation in the abundance, grain size, and morphology of spinel in different parts of the orebody and intrusion that follows an identifiable distribution law. Furthermore, the compositional zonation of exsolved spinels reflects the rapid growth of exsolution features in a high-temperature environment. Thus, the size, morphology, abundance, and composition of spinel exsolution features in titanomagnetite provide a valuable petrogenetic tool for estimating the maturity and formational environment of the deposit.     

High pressure crystallization of the Ewarara,Kalka and Gosse Pile intrusions,Giles Complex,central Australia

Evidence is presented for the primary high pressure crystallization of the Ewarara, Kalka and Gosse Pile layered intrusions which form part of the Giles Complex in central Australia. These pressures are estimated at 10 to 12 kb. The high pressure characteristics include subsolidus reactions between olivine and plagioclase, orthopyroxene and plagioclase, and orthopyroxene and spinel; spinel and rutile exsolution in both ortho- and clino-pyroxene; spinel exsolution in plagioclase; high Al₂O₃ and Cr₂O₃ contents of both ortho- and clinopyroxene; high Al^VI in clinopyroxene; dominance of orthopyroxene as an early crystallizing phase; high distribution coefficients for co-existing pyroxene pairs; and thin chilled margins. Such phenomena are rare in documented layered basic intrusions.     

Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya

Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg–Aland Ca–Al granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.Mg–Al granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas Ca–Al granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the Mg–Algranulites, the first generation of orthopyroxene and some spinelmay be of igneous origin. In the Ca–Al granulites, hibonite(and possibly some spinel) are the earliest, possibly igneous,minerals in the crystallization sequence. Most pyroxene, spineland corundum in Mg–Al and Ca–Al granulites formedby subsolidus reactions. The qualitative P–T path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T 600–740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The Ca–Alhibonite-bearing granulites are unique. Both types enlarge thespectrum of known Ca–Al–Mg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift     

The Eocene corundum-bearing rocks in the Gangdese arc,south Tibet:Implications for tectonic evolution of the Himalayan orogen

The genesis of Liangguo corundum deposit in the southern Gangdese magmatic arc, east-central Himalaya, remains unknown. The present study shows that the corundum-bearing rocks occur as lenses with variable sizes in the Eocene gabbro that intruded into marble. These corundum-bearing rocks have highly variable mineral assemblage and mode. The corundum-rich rocks are characterized by containing abundant corundum, and minor spinel, ilmenite and magnetite, whereas the corundum-poor and corundum-free rocks have variable contents of spinel, plagioclase, sillimanite, cordierite, ilmenite and magnetite. The host gabbro shows variable degrees of hydration and carbonization. The corundum grains are mostly black, and rarely blue, and have minor Fe O and TiO_2. The spinel is hercynite, with high Fe O and low Mg O contents. The corundum-bearing rocks have variable but high Al_2O_3, FeO and TiO_2, and low SiO_2 contents. Inherited magmatic and altered zircons of the corundum-bearing rocks have similar U e Pb ages(~47 Ma) to the magmatic zircons of the host gabbro, indicating corundum-bearing rock formation immediately after the gabbro intrusion. We considered that emplacement of gabbro induced the contact metamorphism of the country-rock marble and the formation of silica-poor fluid. The channeled infiltration of generated fluid in turn resulted in the hydrothermal metasomatism of the gabbro, which characterized by considerable loss of Si from the gabbro and strong residual enrichment of Al. The metasomatic alteration probably formed under Pe T conditions of ~2.2 -2.8 kbar and ~650 -700℃. We speculate that SiO_2, CaO and Na_2O were mobile, and Al_2O_3, FeO, TiO_2 and high field strength elements remained immobile during the metasomatic process of the gabbro. The Liangguo corundum deposit, together with metamorphic corundum deposits in Central and Southeast Asia, were related to the Cenozoic Himalayan orogeny, and therefore are plate tectonic indicators.     

Geochemistry and iron isotope systematics of coexisting Fe-bearing minerals in magmatic FeTi deposits: A case study of the Damiao titanomagnetite ore deposit,North China Craton

Geochemical and iron isotopic compositions of magnetite, ilmenite and pyrite separates from the FeTi oxide ores hosted in the Damiao anorthosite-type FeTi ore deposit were analyzed to investigate sub-solidus cooling history of the titanomagnetite. The FeTi oxides form two series of solid solutions, namely, ulvöspinel-magnetite (Usp-Mt_ss) and hematite-ilmenite (Hem-Ilm_ss) solid solutions. The magnetite separates have 14–27 mol% ulvöspinel, while the ilmenite separates have 5–8 mol% hematite. Major element compositions of the mineral separates suggest that the ilmenites were mainly exsolved from the Usp-Mt_ss by oxidation of ulvöspinel in the temperature range of ~820–600 °C and experienced inter-oxide re-equilibration with the magnetites. Associated with the exsolution is the substantial inter-mineral iron isotope fractionation. The magnetite separates are characterized by high δ⁵⁷Fe (+0.27 − +0.65‰), whereas the ilmenite separates have lower δ⁵⁷Fe (−0.65 to −0.28‰). Two types of pyrite are petrographically observed, each of which has a distinctive iron isotope fingerprint. Type I pyrite (pyrite_I) with higher δ⁵⁷Fe (δ⁵⁷Fe = +0.63 − +0.95‰) is consistent with magmatic origin, and type II pyrite (pyrite_II) with lower δ⁵⁷Fe (δ⁵⁷Fe = −0.90 to −0.11‰) was likely to have precipitated from fluids. Iron isotopic fingerprints of the pyrite_I probably indicate variations of oxygen fugacity, whereas those of the pyrite_II may result from fluid activities. The iron isotopic fractionation between the magnetite and ilmenite is the net result of sub-solidus processes (including ulvöspinel oxidation and inter-oxide re-equilibration) without needing varying oxygen fugacity albeit its presence. Although varying composition of magnetite-ilmenite pairs reflects variations of oxygen fugacity, inter-oxide iron isotopic fractionation does not.     

Oxide minerals in lithic fragments from Luna 20 fines

One hundred and seventy-six oxide mineral grains in the Luna 20 samples were analyzed by electron microprobe. Spinel is the most abundant oxide, occurring in troctolite fragments. Next most abundant is ilmenite, which occurs in all rock types except those containing spinel. Chromite also occurs in all rock types except those containing spinel. Minor amounts of ulvöspinel, armalcolite, zirkelite, baddeleyite and an unidentified TiO₂-rich phase were also found.Spinel grains are predominantly spinel-hercynite solid solutions, commonly with very minor chromite. The $\text{Fe}\text{(Fe + Mg)}$ ratio is generally lower than in spinel from Apollo 14 rocks. Chromites in non-mare rocks are similar to those from mare rocks. Ilmenite of mare origin is Mg-poor and Zr-rich compared to non-mare ilmenite; these elements may therefore be useful in determining the origin of ilmenite grains.Phase equilibria considerations suggest that spinel troctolite crystallized from a melt high in alumina; a likely candidate is the high-alumina basalt of Prinzet al. (1973a).Sub-micron wide rods of metallic Fe occur in plagioclase grains and may have formed by sub-solidus reduction processes.     

Applied Mineralogical Studies on Iranian Titanium Deposits

Qara-aghaj and Skandian as hard rock titanium deposit and Kahnooj one as a placer deposit were investigated from applied mineralogical point of view. The mineralogical studies were carried out using XRD, XRF, optical microscopy, scanning electron microscopy and microprobe analysis. These studies indicated that ilmenite and magnetite are main valuable minerals in the studied ores. Pyroxene, olivine and plagioclase are the main gangue minerals in Qara-aghaj ore while chlorite and plagioclase are the major gangue minerals in Skandian ore. Plagioclase, clinopyroxene, amphibole, feldspate and some quartz are the important gangue minerals in kahnooj deposit. In all three ores ilmenite is mainly in the form of ilmenite grains but some lamellae of ilmenite with thickness between 0.1 to 20 μm have been occurred as exsolution textures inside magnetite grains, where the magnetite here can be referred to as ilmenomagnetite. In the hard rock ores some fine ilmenites have been disseminated in silicate minerals. The liberation degree of granular ilmenite was determined 150, 140 and 200 μm for Qara-aghaj, Skandian and Kahnooj, respectively. So, only the granular form of ilmenite is recoverable by physical methods. Some sphene and rutile as titanium containing minerals were observed mainly inside ilmenite phase in kahnooj ore. Some fine rutile was also found inside Skandian ilmenite while there were not any other titanium minerals inside Qara-aghaj ilmenite. Apatite is another valuable mineral which was found only in Qara-aghaj ore. Using SEM and microprobe analysis it was found that there are different amounts of exsolved fine lamellae of hematite inside ilmenite in Qara-aghaj and Kahnooj ores while it was not observed in Sckandian one. The average contents of TiO2 in the lattice of Qara-aghaj, Skandian and Kahnooj ilmenite were determined 51.13, 50.9% and 52.02%, respectively. FeO content of ilmenite lattice for all three samples is clearly lower than the theoretical content. This is due to the substitution of Mg and Mn for some Fe2+ ions in the ilmenite lattice. V2O3 content of magnetite lattice is up to 1%. So, magnetite can be a suitable source for production of vanadium as a by-product in all three deposits.     

Macrocrystic corundum and Fe–Ti oxide minerals entrained in alkali basalts from the Eger (Ohře) Rift: Mg−Fe3+-rich ilmenite as tracer of an oxidized upper mantle

Macrocrysts of corundum, ilmenite, and spinel-group minerals from alluvial deposits of the Eger Rift were studied for composition, texture, and mineral inclusions. All macrocrysts show usually magmatic corrosion textures indicating disequilibrium with the transporting alkali-basalt magma. Corundum grains, exclusively sapphires, were classified by trace-element signatures as magmatic and metamorphic types. Some sapphire grains show erratic compositions that may have resulted from a metasomatic overprint. The inclusion inventory of magmatic corundum suggests crystallization from a differentiated alkaline silicate melt. Corundum itself was never observed as an inclusion mineral. Magnesium- and Fe³⁺-rich ilmenite, described as typical mantle-derived species, is the dominant heavy mineral in almost all alluvial deposits of the Eger Rift. Most discrete macrocrysts are similar in appearance and composition to kimberlite- and basanite-related ilmenite. Ilmenite included in alluvial corundum and zircon grains differ from the bulk of discrete ilmenite grains by larger concentrations of Nb and Mn. The mantle origin of the Mg–Fe³⁺-rich ilmenite is confirmed by compositional and thermo-barometric comparison with ilmenite from clinoproxenitic and hornblenditic xenoliths, which probably originated in the Moho region. The Fe–Ti two-oxide geothermometry and oxygen-barometry of coexisting ilmenite–magnetite pairs yield equilibrium temperatures between 900 and 1,080 °C and oxygen fugacities log₁₀fO₂ between ?0.1 and 1.1 (relative to the NNO buffer), which indicate that the upper mantle as well as the mantle/crust transition zone below the rift is at least partially oxidized. The ilmenite macrocrysts were transported from the source region to the surface by explosive alkali-basalt magmas, as implied by the presence of basaltic-pipe breccias in close vicinity to some placer deposits.     

In-situ LA-ICP-MS trace elemental analyses of magnetite: Fe–Ti–(V) oxide-bearing mafic–ultramafic layered intrusions of the Emeishan Large Igneous Province,SW China

The Taihe, Baima, Hongge, Panzhihua and Anyi intrusions of the Emeishan Large Igneous Province (ELIP), SW China, contain large magmatic Fe–Ti–(V) oxide ore deposits. Magnetites from these intrusions have extensive trellis or sandwich exsolution lamellae of ilmenite and spinel. Regular electron microprobe analyses are insufficient to obtain the primary compositions of such magnetites. Instead, laser ablation ICP-MS uses large spot sizes (~ 40 μm) and can produce reliable data for magnetites with exsolution lamellae. Although magnetites from these deposits have variable trace element contents, they have similar multi-element variation patterns. Primary controls of trace element variations of magnetite in these deposits include crystallography in terms of the affinity of the ionic radius and the overall charge balance, oxygen fugacity, magma composition and coexisting minerals. Early deposition of chromite or Cr-magnetite can greatly deplete magmas in Cr and thus Cr-poor magnetite crystallized from such magmas. Co-crystallizing minerals, olivine, pyroxenes, plagioclase and apatite, have little influence on trace element contents of magnetite because elements compatible in magnetite are incompatible in these silicate and phosphate minerals. Low contents and bi-modal distribution of the highly compatible trace elements such as V and Cr in magnetite from Fe–Ti oxide ores of the ELIP suggest that magnetite may not form from fractional crystallization, but from relatively homogeneous Fe-rich melts. QUILF equilibrium modeling further indicates that the parental magmas of the Panzhihua and Baima intrusions had high oxygen fugacities and thus crystallized massive and/or net-textured Fe–Ti oxide ores at the bottom of the intrusive bodies. Magnetite of the Taihe, Hongge and Anyi intrusions, on the other hand, crystallized under relatively low oxygen fugacities and, therefore, formed net-textured and/or disseminated Fe–Ti oxides after a lengthy period of silicate fractionation. Plots of Ge vs. Ga + Co can be used as a discrimination diagram to differentiate magnetite of Fe–Ti–(V) oxide-bearing layered intrusions in the ELIP from that of massif anorthosites and magmatic Cu–Ni sulfide deposits. Variable amounts of trace elements of magmatic magnetites from Fe–Ti–(P) oxide ores of the Damiao anorthosite massif (North China) and from Cu–Ni sulfide deposits of Sudbury (Canada) and Huangshandong (northwest China) demonstrate the primary control of magma compositions on major and trace element contents of magnetite.