Tabular formations in titanomagnetite

A visually homogeneous intrusive titanium magnetite is shown microscopically to consist of two phases: a) solid solution of magnetite in ulvospinel and b) solid solution of ulvospinel in magnetite; the phase ratio is about 1 : 2. On heating in air two new phases are produced in titanium magnetite, as oriented plates forming the mineral's characteristic reticulate microstructure: ilmenite and hematite. Ilmenite disappears on further heating and the end-phase becomes a solid solution of ilmenite in hematite. It is believed that natural oxidation in depths also yields to ilmenite plates in titanium magnetite, if ulvospinel is present. Oriented platelets of hematite may be similarly produced in TiO₂ -free magnetite and in magnesium ferrite, although MgO retards their development in the latter. -- V. P. Sokoloff.     

Vanadium in magnetite gabbros and its behaviour during lateritic weathering,Windimurra Complex,Western Australia

Shephards Discordant Zone is a 500–600 m thick interlayered sequence of deformed, altered and metamorphosed magnetite metagabbro and about 50 layers or lenses of magnetitite (> 80–90% magnetite). The sequence shows progressive magmatic fractionation upwards: Ti and Ti/Fe increase, and V, V/Ti and Cr decrease upwards in magnetite and in whole‐rock compositions. The main magnetite‐rich sequence (about 400 m thick) is deeply weathered, with 40 m of saprolite showing vertical zonation of weathering minerals due to progressive weathering. Magnetitites (average 1% V₂O₃) are resistant to weathering and show little chemical change, but magnetite gabbros (average 0.27% V₂O₃) are extensively weathered and show progressive loss of Ca, Na, Mg and S. Plagioclase, magnetite (1.37% V₂O₃), chlorite (up to 0.35% V₂O₃), actinolite, epidote and minor sulfides in unweathered rocks weather to kaolinite, hematite, goethite and minor vermiculite, ilmenite remaining largely unaffected. Vanadium is essentially immobile during weathering and is unaffected during weathering of magnetitites (1% V₂O₃), but is slightly depleted during weathering of magnetite gabbros (0.23% V₂O₃).     

Phlogopite-olivine rocks of the Taukha terrane in southeastern Sikhote Alin

The Paleocene ultraferrous Mn-rich phlogopite-olivine rocks of the Taukha terrane belong to the alkaline ultrabasic rocks of the potassic series. The olivine is represented by hortonolite, while the phlogopite is enriched in Cl. Other minerals are represented by Ti-magnetite, Mn-rich ilmenite, Zn-rich pleonaste, apatite, and zircon. There are also epigenetic serpentine, talc, carbonates, magnetite, breithauptite, nickeline, hedleyite, cobaltite, tsumoite, auricupride, cuproauride, and other minerals. The phlogopite-olivine rocks possibly represent a part of a magmatic complex previously unknown in Sikhote Alin, the rocks of which are associated with fluidolites of a large diatreme. There are grounds to suggest that they were formed by the injection of fluid-rich (mainly, H₂O, Cl, F, and S) deep magmas into the upper lithosphere. Based on these specific features, as well as the sharp K predominance over Na and the enrichment in some incompatible elements (Sn, Ta, Nb, and Zr), the phlogopite-olivine rocks are the most close to lamproites but differ in the high contents of Fe, Mn, Au, Pt, and Pd and in the olivine’s composition. The manifestation of such magmatism in the Taukha terrane records the transition from subduction to transform continental margin settings.     

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     

Mineralogy, Chemical Characteristics and Upgrading of Beach Ilmenite of the Top Meter of Black Sand Deposits of the Kafr Al-Sheikh Governorate, Northern Egypt

Egyptian beach ilmenite occurs in a relatively high content in the naturally highly concentrated superficial black sand deposits at specific beach zones in the northern parts of the Nile Delta at Rosetta. Microscopic study shows that the ilmenite occurs as fresh homogeneous black or heterogeneous multicoloured altered grains and exhibits three types (homogeneous, exsolved and altered) of ilmenite varieties. XRD data of ilmenite indicates their association with minor hematite and quartz, whereas leucoxene shows its association with Nb‐rutile, pseudorutile and hematite. Grain size distribution suggests a very fine sand size of >89% and 80% and a fine sand size of 10.5% and 18.3% for fresh and altered ilmenites, respectively. The density of fresh, altered ilmenite and leucoxene concentrates varies from 2.70, 2.50 to 2.40 ton/m³, suggesting a gradual decrease from high grade fresh to leucoxene and consistent with variation in magnetic susceptibility as a consequence of the leaching of iron. Mass magnetic susceptibility reveals 97.6% of ilmenite and 92% of the altered form are obtained at 0.20 and 0.48 ampere. Fresh ilmenite exhibits variable TiO₂ (47.18%) and Fe₂O₃^T (46.10%) with minor MnO, MgO and Cr₂O₃ (1.22, 1.10 and 0.51%). The altered ilmenite is higher in TiO₂ (76.16%) and SiO₂ (4.68%) and lower in Fe₂O₃^T (14.45%), MnO, MgO and Cr₂O₃ (0.39, 0.52 and 0.11%) compared with the fresh form. Three concentrates of ilmenites (G1, G2 and G3) were prepared from crude ore using a Reading cross belt magnetic separator under different conditions, revealing a gradual increase of TiO₂, SiO₂, Al₂O₃ and CaO accompanied by a decrease of Fe₂O₃^T, MgO and Cr₂O₃ with repetition of the separation processes. Several ore dressing techniques were carried out to upgrade the ilmenite concentrate.     

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.     

Natural Fe-oxide and -oxyhydroxide nanoparticles: an EPR and SQUID investigation

Summary Magnetic hematite (α-Fe₂O₃) and goethite (α-FeOOH) nanoparticles, formed in the abandoned Libiola Mine (Liguria, Italy) as a result of Acid Mine Drainage (AMD) processes that involved sulfide-bearing rocks and sulfide-rich waste material, were investigated through the study of their magnetic properties in combination with electron paramagnetic resonance (EPR) spectroscopy. Both techniques revealed that the behaviour of the nanominerals involved was predominantly superparamagnetic. In contrast, the bulk antiferromagnetic behaviour of both hematite and goethite was substantially reduced. The observed magnetic properties reflected a great variability in the particle dimensions and confirmed that both nanominerals were the transformation products of a metastable amorphous species. Moreover, two peculiar phenomena were hypothesised: the transformation to hematite may occur before the coarsening of the amorphous transient and hematite and goethite can interconvert without intermediate crystallisation of metastable precursors. Goethite, in fact, appears to be formed as an alteration product of hematite, after a drastic change of the physico-chemical boundary conditions.     

BIF-hosted iron mineral system: A review

The BIF-hosted iron ore system represents the world's largest and highest grade iron ore districts and deposits. BIF, the precursor to low- and high-grade BIF hosted iron ore, consists of Archean and Paleoproterozoic Algoma-type BIF (e.g., Serra Norte iron ore district in the Carajás Mineral Province), Proterozoic Lake Superior-type BIF (e.g., deposits in the Hamersley Province and craton), and Neoproterozoic Rapitan-type BIF (e.g., the Urucum iron ore district).The BIF-hosted iron ore system is structurally controlled, mostly via km-scale normal and strike-slips fault systems, which allow large volumes of ascending and descending hydrothermal fluids to circulate during Archean or Proterozoic deformation or early extensional events. Structures are also (passively) accessed via downward flowing supergene fluids during Cenozoic times.At the depositional site the transformation of BIF to low- and high-grade iron ore is controlled by: (1) structural permeability, (2) hypogene alteration caused by ascending deep fluids (largely magmatic or basinal brines), and descending ancient meteoric water, and (3) supergene enrichment via weathering processes. Hematite- and magnetite-based iron ores include a combination of microplaty hematite–martite, microplaty hematite with little or no goethite, martite–goethite, granoblastic hematite, specular hematite and magnetite, magnetite–martite, magnetite-specular hematite and magnetite–amphibole, respectively. Goethite ores with variable amounts of hematite and magnetite are mainly encountered in the weathering zone.In most large deposits, three major hypogene and one supergene ore stages are observed: (1) silica leaching and formation of magnetite and locally carbonate, (2) oxidation of magnetite to hematite (martitisation), further dissolution of quartz and formation of carbonate, (3) further martitisation, replacement of Fe silicates by hematite, new microplaty hematite and specular hematite formation and dissolution of carbonates, and (4) replacement of magnetite and any remaining carbonate by goethite and magnetite and formation of fibrous quartz and clay minerals.Hypogene alteration of BIF and surrounding country rocks is characterised by: (1) changes in the oxide mineralogy and textures, (2) development of distinct vertical and lateral distal, intermediate and proximal alteration zones defined by distinct oxide–silicate–carbonate assemblages, and (3) mass negative reactions such as de-silicification and de-carbonatisation, which significantly increase the porosity of high-grade iron ore, or lead to volume reduction by textural collapse or layer-compaction. Supergene alteration, up to depths of 200 m, is characterised by leaching of hypogene silica and carbonates, and dissolution precipitation of the iron oxyhydroxides.Carbonates in ore stages 2 and 3 are sourced from external fluids with respect to BIF. In the case of basin-related deposits, carbon is interpreted to be derived from deposits underlying carbonate sequences, whereas in the case of greenstone belt deposits carbonate is interpreted to be of magmatic origin. There is only limited mass balance analyses conducted, but those provide evidence for variable mobilization of Fe and depletion of SiO₂. In the high-grade ore zone a volume reduction of up to 25% is observed.Mass balance calculations for proximal alteration zones in mafic wall rocks relative to least altered examples at Beebyn display enrichment in LOI, F, MgO, Ni, Fe₂O_3total, C, Zn, Cr and P₂O₅ and depletions of CaO, S, K₂O, Rb, Ba, Sr and Na₂O. The Y/Ho and Sm/Yb ratios of mineralised BIF at Windarling and Koolyanobbing reflect distinct carbonate generations derived from substantial fluid–rock reactions between hydrothermal fluids and igneous country rocks, and a chemical carbonate-inheritance preserved in supergene goethite.Hypogene and supergene fluids are paramount for the formation of high-grade BIF-hosted iron ore because of the enormous amount of: (1) warm (100–200 °C) silica-undersaturated alkaline fluids necessary to dissolve quartz in BIF, (2) oxidized fluids that cause the oxidation of magnetite to hematite, (3) weakly acid (with moderate CO₂ content) to alkaline fluids that are necessary to form widespread metasomatic carbonate, (4) carbonate-undersaturated fluids that dissolve the diagenetic and metasomatic carbonates, and (5) oxidized fluids to form hematite species in the hypogene- and supergene-enriched zone and hydroxides in the supergene zone.Four discrete end-member models for Archean and Proterozoic hypogene and supergene-only BIF hosted iron ore are proposed: (1) granite–greenstone belt hosted, strike-slip fault zone controlled Carajás-type model, sourced by early magmatic (± metamorphic) fluids and ancient “warm” meteoric water; (2) sedimentary basin, normal fault zone controlled Hamersley-type model, sourced by early basinal (± evaporitic) brines and ancient “warm” meteoric water. A variation of the latter is the metamorphosed basin model, where BIF (ore) is significantly metamorphosed and deformed during distinct orogenic events (e.g., deposits in the Quadrilátero Ferrífero and Simandou Range). It is during the orogenic event that the upgrade of BIF to medium- and high-grade hypogene iron took place; (3) sedimentary basin hosted, early graben structure controlled Urucum-type model, where glaciomarine BIF and subsequent diagenesis to very low-grade metamorphism is responsible for variable gangue leaching and hematite mineralisation. All of these hypogene iron ore models do not preclude a stage of supergene modification, including iron hydroxide mineralisation, phosphorous, and additional gangue leaching during substantial weathering in ancient or Recent times; and (4) supergene enriched BIF Capanema-type model, which comprises goethitic iron ore deposits with no evidence for deep hypogene roots. A variation of this model is ancient supergene iron ores of the Sishen-type, where blocks of BIF slumped into underlying karstic carbonate units and subsequently experienced Fe upgrade during deep lateritic weathering.     

Mineralogy,petrography and micro-chemical characteristics of enclaves of mylonitic BIFs within Sukinda ultramafic complex,Odisha

Elongated NE-SW trending bodies of iron-rich rock are exposed adjacent to pyroxenite dyke within Sukinda ultramafic complex, Odisha. Field study followed by optical and electron microscopy, XRD and EPMA investigation reveal the rocks to be fine grained, weathered, limonitised; containing quartz, magnetite, hematite/martite and goethite. The rock has suffered from deformation during intrusion of chromiferous magma. It rarely shows banding/lamination, but largely exhibits mylonitic fabric, resulting from magmatic intrusion. The stronger deformation is evident from sub-grain formation, deformed mineral grains; often with orientation, stretching (boudinage) and shortening (folding); presence of porphyroclasts, pull-apart structure, undulose extinction, dynamic recrystallisation etc. From the microstructure and mineral abundance, the rock is designated as “Mylonitic Magentite Quartzite” (MMQ).Enrichment of some elements like Ni, Mg, Cr in the magnetite phase of MMQ is attributed to solid state diffusion of these elements from chromiferous mafic magma during thermal metamorphism. This is determined from electron probe microanalysis of iron-rich phase in MMQ, which is found to contain 88-90 wt% of FeO^(t) with ~1%, NiO, ~1%, MgO and 0.1% Cr₂O₃ having around 3 mole% of trevorite; 4-6% of magnesioferrite; 0.15-0.3% of chromite; 86-87% of magnetite and 3-4% of wustite. Considering presence of wustite as temperature indicator, the temperature of magma envisaged to be around 950-1100°C.In a later period, the MMQ has undergone oxidation and lateritisation owing to its prolonged exposure. During this process, new minerals like hematite and goethite substituted magnetite, resulting leaching of iron (FeO: 62-68%) and magnesium (MgO: 0.1-0.35) and enrichment of chromium (Cr₂O₃:4-7%) and nickel (NiO: 1.6-2.3%). The silica (SiO₂: 4-5%), alumina (Al₂O₃:~1%) are contributed by kaolinite, formed during lateritisation.The field and laboratory studies confirm these iron-rich exposures to be enclaves of BIFs, banded magnetite quartzite (BMQ) in particular, within the Sukinda chromiferous ultramafic complex. Micro-structural features and microchemical composition of iron minerals in these exposures are interpreted as the influence of forceful ultramafic intrusion into the existing BMQ and effect of thermal metamorphism followed by oxidation, weathering/lateritisation.     

Stable isotope and Rare Earth Element evidence for recent ironstone pods within the Archean Barberton greenstone belt, South Africa

There is considerable debate about the mode and age of formation of large (up to ∼200 m long) hematite and goethite ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite δ¹⁸O values range from −0.7 to +1.0‰ and δD from −125 to −146‰, which is consistent with formation from modern meteoric waters at 20 to 25 °C. Hematite δ¹⁸O values range from −0.7 to −2.0‰, which is consistent with formation at low to moderate temperatures (40-55 °C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREE_Avg/LREE_Avg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs.     

Composition and assemblage characteristics of magnetic minerals in layer S<Subscript>5-1</Subscript> of Xifeng and Duanjiapo sections

Relatively strongly magnetic fine components (< 30μm, XS-4J and DS-4J) which are most environmentally sensitive were separated from layer S_5-1 in the Xifeng and Duanjiapo loess sections and analyzed by MPV-3 for their morphometric characteristics and reflectance, SEM-ESD for their element contents and XRD for their mineral phases, respectively. The results showed that minerals in both samples are dominated by detrial Fe-Ti oxides of aeolian origin. In sample XS-4J the reflectance and iron contents of magnetic minerals are usually high. In addition to magnetite (Fe₃O₄), maghemite (γFe₂O₃) and hematite (Fe₂O₃), some Fe-high oxide (72.25 wt%–86.67 wt%), ilmenite (FeTiO₃), and magnetite-ulvöspinel [Fe(FeCr)O₄, Fe (FeNi)O₄] were also detected. In sample DS-4J obvious negative linear correlations were found between Ti and Fe, and the contents of Mn, Si, Al and Ca are usually high and the minerals are dominated by magnetite (maghemite), goethite (FeOOH) and limonite (containing Si and OH). In addition, the signs of corrosion of magnetic minerals and newly crystallized magnetite (maghemite) were recognized. Differences in the composition and assemblage characteristics of magnetite minerals between XS and DS reflect significant differences in source rocks and preserving conditions.     

FeO*-Al2O3-TiO2-Rich Rocks of the Tertiary Bana Igneous Complex, West Cameroon

FeO*‐Al₂O₃‐TiO₂‐rich rocks are found associated with transitional tholeiitic lava flows in the Tertiary Bana plutono‐volcanic complex in the continental sector of the Cameroon Line. These peculiar rocks consist principally of iron‐titanium oxides, aluminosilicates and phosphates, and occur as layers 1–3 m thick occupying the upper part of lava flows on the southwest (site 1) and northwest (site 2) sites of the complex. Mineral constituents of the rocks include magnetite, ilmenite, hematite, rutile, corundum, andalusite, sillimanite, cordierite, quartz, plagioclase, alkali feldspar, apatite, Fe‐Mn phosphate, Al phosphate, micas and fine mixtures of sericite and silica. Texturally and compositionally, the rocks can be subdivided into globular type, banded type, and Al‐rich fine‐gained massive type. The first two types consist of dark globule or band enriched in Fe‐Ti oxides and apatite and lighter colored groundmass or bands enriched in aluminosilicates and quartz, respectively. The occurrence of andalusite and sillimanite and the compositional relations of magnetite and ilmenite in the FeO*‐Al₂O₃‐TiO₂‐rich rocks suggest temperatures of crystallization in a range of 690–830°C at low pressures. The Bana FeO*‐Al₂O₃‐TiO₂‐rich rocks are characterized by low concentrations of SiO₂ (25–54.2 wt%), Na₂O + K₂O (0–1%), CaO (0–2%) and MgO (0–0.5%), and high concentrations of FeO* (total iron as FeO, 20–42%), Al₂O₃ (20–42%), TiO₂ (3–9.2%), and P₂O₅ (0.26–1.30%). TiO₂ is positively correlated with Al₂O₃ and inversely correlated with FeO*. The bulk rock compositions cannot be derived from the associated basaltic magma by crystal fractionation or by partial melting of the mantle or lower crustal materials. In ternary diagrams of (Al₂O₃)?(CaO + Na₂O + K₂O)?(FeO*+ MnO + MgO) and (SiO₂)?(FeO*)?(Al₂O₃), the compositional field of the rocks is close to that of laterite and is distinct from the common volcanic rocks, suggesting that the rocks are derived from lateritic materials by recrystallization when the materials are heated by the basaltic magmas. A hydrothermal origin is discounted because the rocks contain high‐temperature mineral assemblages and lack sulfide minerals. It is proposed that the FeO*‐Al₂O₃‐TiO₂‐rich rocks of the Bana complex were formed by pyrometamorphism of laterite by the heat of basaltic magmas.     

Microlaboratory study on magnetic,gravity and high-tension separation of hercynite and pleonaste from low-grade ilmenite concentrates

Some ilmenite concentrates obtained from oleate flotation of ilmenite ores from NE Poland contain less than the required 45% TiO₂ due to the presence of green spinels (hercynite and pleonaste). Such concentrates were further upgraded by different separation techniques. It was established at microlaboratory scale that magnetic, gravity and high-tension separations can provide qualified ilmenite concentrates with TiO₂ recovery in the order of 50 to 80%.     

Geochemistry and mineralogy of laterites in the Sula Mountains greenstone belt, Lake Sonfon gold district, Sierra Leone

Geochemical and mineralogical investigations have been carried out on laterite profiles developed in the Lake Sonfon Au district of northern Sierra Leone. The area is underlain by Archean metavolcanics and constitutes part of the Sula Mountains greenstone belt, which is mineralized in Au. Extensive lateritization has affected the rocks of this region, resulting in a profile which from bottom to top consists typically of a decomposed bedrock zone, a pisolitic laterite layer and a duricrust layer. Both the pisolitic and duricrust layers of the laterite are sometimes punctuated by lenses of ironstones containing high amounts of Cu, Zn, Ni, Co and Ce. Gold occurs as small grains within the heavy mineral fraction recovered from the decomposed rock zones and pisolitic layers of the profiles and also in gravels of streams draining the area. The mineralogy of the duricrust and pisolitic layers is dominated by goethite, gibbsite and quartz, with minor amounts (<5% by volume) of ilmenite, magnetite, haematite, rutile and kaolinite. The kaolinite content increases towards the decomposed rock zone, where talc, vermiculite and other layer lattice silicates become abundant. The heavy-mineral fraction of stream sediments is composed essentially of ilmenite, magnetite, haematite, and traces of rutile, zircon, tourmaline and Au. The Au grains are often characterized by a 10–200-μm-wide rim having a much lower content of Ag (0.3 wt.% or lower) than the grain interior (about 5 wt.% on average). Dissolution effects are also observed on the grain surfaces. It is considered that Au derived from the amphibolite parent rock is dissolved, transported, and redeposited during laterization.The duricrust cover of the laterite profiles is characterized by high contents of Fe₂O₃ (ca. 60 wt.%) and Al₂O₃ (ca. 32wt.%) and low content of SiO₂ (ca. 9 wt.%). In comparison, the pisolitic layer is higher in SiO₂ (ca. 18 wt.%) as well as a slightly higher in Al₂O₃ (ca. 34 wt.%). Lateritic weathering has resulted in the removal of CaO, Na₂O, MgO and SiO₂, with relative enrichment of Fe₂O₃ and Al₂O₃. The geochemical distribution of the trace elements in the laterite profiles can be related to the occurrence of the auriferous mineralization. The significance of these observations is discussed in relation to the origin of the lateritic Au and the role of the associated trace elements as indicators of the mineralization.     

The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

Mineral equilibria calculations in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO₂ and Fe₂O₃ on greenschist and amphibolite facies mineral equilibria in metapelites. The end‐member data and activity–composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity–composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO₂ and Fe₂O₃ on the stability of these minerals. Thermodynamic models for ilmenite–hematite and magnetite–ulvospinel solid solutions accounting for order–disorder in these phases allow the distribution of TiO₂ and Fe₂O₃ between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO₂ and Fe₂O₃ in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO₂ to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe₂O₃ develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe₂O₃‐rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids.     

Using oxygen isotope chemistry to track hydrothermal processes and fluid sources in itabirite-hosted iron ore deposits in the Quadrilátero Ferrífero,Minas Gerais,Brazil

The Quadrilátero Ferrífero, Brazil, is presently the largest accumulation of single itabirite-hosted iron ore bodies worldwide. Detailed petrography of selected hypogene high-grade iron ore bodies at, e.g. the Águas Claras, Conceição, Pau Branco and Pico deposits revealed different iron oxide generations, from oldest to youngest: magnetite → martite (hematite pseudomorph after magnetite) → granoblastic (recrystallised) → microplaty (fine-grained, <100 μm) → specular (coarse-grained, >100 μm) hematite. Laser-fluorination oxygen isotope analyses of selected iron ore species showed that the δ¹⁸O composition of ore-hosted martite ranges between ?4.4 and 0.9?‰ and is up to 11?‰ depleted in ¹⁸O relative to hematite of the host itabirite. During the modification of iron ore and the formation of new iron oxide generations (e.g. microplaty and specular hematite), an increase of up to 8?‰ in δ¹⁸O values is recorded. Calculated δ¹⁸O values of hydrothermal fluids in equilibrium with the iron oxide species indicate: (1) the involvement of isotopically light fluids (e.g. meteoric water or brines) during the upgrade from itabirite-hosted hematite to high-grade iron ore-hosted martite and (2) a minor positive shift in δ¹⁸O_fluid values from martite to specular hematite as result of modified meteoric water or brines with slightly elevated δ¹⁸O values and/or the infiltration of small volumes of isotopically heavy (metamorphic and/or magmatic) fluids into the iron ore system. The circulation of large fluid volumes that cause the systematic decrease of ¹⁸O/¹⁶O ratios from itabirite to high-grade iron ore requires the presence of, e.g. extensive faults and/or large-scale folds.     

Geology and geochemistry of bauxite deposits in Lushoto District, Usambara Mountains, Tanzania

Bauxite deposits in the Usambara Mountains of north eastern Tanzania occur as remnants of residual deposits on two geomorphologically related plateaus of Mabughai-Mlomboza and Kidundai at Magamba in Lushoto, Usambara Mountains. The parent rocks for the deposits are mainly granulites and feldspathic gneisses of Neoproterozoic Mozambique belt. The plateaus represent a preserved Late Cretaceous–Lower Tertiary old land surface (African surface). Other parts of the Usambara Mountains and the neighbouring Pare Mountains are covered mostly by red–brown lateritic soils and impure reddish-brown kaolinitic clays. The bauxite deposits contain mainly Al₂O₃ (40–69 wt.%), Fe₂O₃ (3–10 wt.%), SiO₂ (0.16–7 wt.%) and other elements occur in quantities not substantial to affect the quality or processing of the bauxite, and are attributed to the presence of relic minerals. Gibbsite makes up to 98 vol.% of the bauxite ore in special cases. Gibbsite is accompanied by goethite in the ore. Boehmite occurs in small amounts and is usually accompanied by hematite. Impurities include goethite, hematite, kaolinite, and minor relic quartz and microcline. Kaolinite is the sole clay mineral encountered in the bauxite ore, suggesting mature soil profiles and a development of the bauxite deposits on a well-drained peneplanation. Ore reserve estimates from the drilling data and surface geological mapping of the deposits yielded bauxite reserves of about 37 million tonnes.     

Structural constraints of ferric (hydr)oxides on dissimilatory iron reduction and the fate of Fe(II)

Due to the strong reducing capacity of ferrous Fe, the fate of Fe(II) following dissimilatory iron reduction will have a profound bearing on biogeochemical cycles. We have previously observed the rapid and near complete conversion of 2-line ferrihydrite to goethite (minor phase) and magnetite (major phase) under advective flow in an organic carbon-rich artificial groundwater medium. Yet, in many mineralogically mature environments, well-ordered iron (hydr)oxide phases dominate and may therefore control the extent and rate of Fe(III) reduction. Accordingly, here we compare the reducing capacity and Fe(II) sequestration mechanisms of goethite and hematite to 2-line ferrihydrite under advective flow within a medium mimicking that of natural groundwater supplemented with organic carbon. Introduction of dissolved organic carbon upon flow initiation results in the onset of dissimilatory iron reduction of all three Fe phases (2-line ferrihydrite, goethite, and hematite). While the initial surface area normalized rates are similar (∼10⁻¹¹ mol Fe(II) m⁻² g⁻¹), the total amount of Fe(III) reduced over time along with the mechanisms and extent of Fe(II) sequestration differ among the three iron (hydr)oxide substrates. Following 16 d of reaction, the amount of Fe(III) reduced within the ferrihydrite, goethite, and hematite columns is 25, 5, and 1%, respectively. While 83% of the Fe(II) produced in the ferrihydrite system is retained within the solid-phase, merely 17% is retained within both the goethite and hematite columns. Magnetite precipitation is responsible for the majority of Fe(II) sequestration within ferrihydrite, yet magnetite was not detected in either the goethite or hematite systems. Instead, Fe(II) may be sequestered as localized spinel-like (magnetite) domains within surface hydrated layers (ca. 1 nm thick) on goethite and hematite or by electron delocalization within the bulk phase. The decreased solubility of goethite and hematite relative to ferrihydrite, resulting in lower Fe(III)_aq and bacterially-generated Fe(II)_aq concentrations, may hinder magnetite precipitation beyond mere surface reorganization into nanometer-sized, spinel-like domains. Nevertheless, following an initial, more rapid reduction period, the three Fe (hydr)oxides support similar aqueous ferrous iron concentrations, bacterial populations, and microbial Fe(III) reduction rates. A decline in microbial reduction rates and further Fe(II) retention in the solid-phase correlates with the initial degree of phase disorder (high energy sites). As such, sustained microbial reduction of 2-line ferrihydrite, goethite, and hematite appears to be controlled, in large part, by changes in surface reactivity (energy), which is influenced by microbial reduction and secondary Fe(II) sequestration processes regardless of structural order (crystallinity) and surface area.     

Surface simulation studies of the hydration of white rust Fe(OH)2, goethite α-FeO(OH) and hematite α-Fe2O3

Computer modelling techniques were used to elucidate the hydration behaviour of three iron (hydr)oxide minerals at the atomic level: white rust, goethite and hematite. A potential model was first adapted and tested against the bulk structures and properties of eight different iron oxides, oxyhydroxides and hydroxides, followed by surface simulations of Fe(OH)₂, α-FeO(OH) and α-Fe₂O₃. The major interaction between the adsorbing water molecules and the surface is through interaction of their oxygen ions with surface iron ions, followed by hydrogen-bonding to surface oxygen ions. The energies released upon the associative adsorption of water range from 1 to 17 kJ mol⁻¹ for Fe(OH)₂, 26 to 80 kJ mol⁻¹ for goethite and 40 to 85 kJ mol⁻¹ for hematite, reflecting the increasing oxidation of the iron mineral. Dissociative adsorption at goethite and hematite surfaces releases larger hydration energies, ranging from 120 to 208 kJ mol⁻¹ for goethite and 76 to 190 kJ mol⁻¹ for hematite.The thermodynamic morphologies of the minerals, based on the calculated surface energies, agree well with experimental morphologies, where these are available. When the partial pressures required for adsorption of water from the gas phase are plotted against temperature for the goethite and hematite surfaces, taking into account experimental entropies for water, it appears that these minerals may well be instrumental in the retention of water during the cyclic variations in the atmosphere of Mars.     

Granulite facies metamorphism and metasomatism in the gabbro-anorthosites of the Kolvitsa massif,Kola Peninsula

The investigation of the Kolvitsa gabbro-anorthosite massif showed that its melanocratic layers conformable with metamorphic banding are mafic differentiates transformed into eclogite-like rocks during prograde metamorphism. During the peak and retrograde stage of the Svecofennian metamorphism in the White Sea region at t = 910–750°C and P = 14-7 kbar, the infiltration of Fe-, CO₂-, Si-, and Na-bearing fluids with X_{H2 O} < 0.4X_{H_2 O} < 0.4 resulted in metasomatic alterations of the melanocratic gabbro-anorthosite interlayers, dissolution of a number of elements, and their reprecipitation with the formation of cross-cutting zoned metasomatic veins with abundant magnetite and ilmenite. The high content of hematite in the ilmenite suggests that the veins were formed at an increase in oxygen fugacity from logf_O2 = - 14.5\log f_{O_2 } = - 14.5 to logf_O2 = - 11\log f_{O_2 } = - 11, which caused the Fe²⁺ → Fe³⁺ transition and iron precipitation. The increase in at the conditions corresponding to the metamorphic peak was probably related to the neutralization of solutions during their infiltration through the gabbro-anorthosites. The reprecipitation of components and the formation of cross-cutting veins occurred owing to interaction between the melanocratic layers in the gabbro-anorthosites and a fluid phase and, contrary to previous models, did not involve the fluid transport of components from the zones of charnockite formation and granitization located far away from the sites of reprecipitation. This is demonstrated by the similarity of mineral compositions and major component contents in the melanocratic gabbro-anorthosite layers and cross-cutting metasomatic veins and regular distribution of trace elements.