Mg-Fe Carbonate Research Articles

Giant Rare Earth Element Accumulation Related to Voluminous, Highly Evolved Carbonatite: A Microanalytical Study of Carbonate Minerals From the Bayan Obo Deposit, China

Abstract The giant Bayan Obo deposit in China represents the largest rare earth element (REE) resource in the world, but the mechanisms for its highly anomalous REE enrichment have long been controversial. The central debate concerns the nature and origin of the ore-hosting dolomite. In this study, a texturally constrained microanalytical study of carbonate minerals from the Bayan Obo ore-hosting dolomite is presented to resolve this controversy. The dolomite rocks contain two major components: the early-stage coarse-grained dolomite (CD) and the late-stage fine-grained dolomite (FD). The CD dolomite grains have C-O isotope ratios (δ13CVienna-PeeDee Belemnite (V-PDB): –4.8 to –3.3‰; δ18OVienna-standard mean ocean water (V-SMOW): 7.1 to 11.9‰) plotting in or adjacent to the primary igneous carbonatite field, with a narrow range of low 87Sr/86Sr ratios (0.70262–0.70327). The mantle-like C-O and Sr isotopes indicate that the coarse-grained dolomite rocks are magmatic in origin. Dolomite grains from the FD have experienced extensive hydrothermal alteration related to both REE mineralization and post-ore metamorphism of the Bayan Obo deposit. The domains of primary unaltered dolomite have high SrO and MnO contents that clearly distinguish them from sedimentary carbonates. The 87Sr/86Sr ratios of unaltered dolomite domains range from 0.70271 to 0.70473, with the majority lower than 0.7035, contrasting with higher 87Sr/86Sr ratios of Mesoproterozoic sedimentary carbonates globally. Thus, the primary unaltered fine-grained dolomite, the precursor to the FD, is also proposed to be a carbonatite. Compared with dolomite grains from the CD, most of the unaltered dolomite domains within the FD are more enriched in FeO and MnO and have higher δ13CV-PDB (-4.9 to 0.3 ‰) and δ18OV-SMOW (9.4 to 17.1 ‰) values. In addition, the FD contains abundant REE- and volatile-rich hydrothermal minerals and Fe-Mg carbonates, which are rare in the CD. The geochemical and mineralogical data in conjunction indicate that the melts forming the late-stage FD were much more evolved than those forming the early-stage CD. It is noteworthy that the unaltered dolomite domains within the FD have a wider range of 87Sr/86Sr ratios than those within the CD, which implies that the CD and FD, at least a proportion of them, are unlikely to have crystallized from the same progenitor magmas. Some FD was possibly the product of fractionation of less-evolved carbonatitic magma that generated the CD, whereas other FD crystallized from new pulses of magmas that were highly fractionated at depth. When compared with other carbonatite complexes, the Bayan Obo carbonatite suite is notable for having a large surface area (~48 km2 in outcrop) and containing an anomalously large proportion of highly evolved components. The voluminous evolved carbonatite clearly provided a basis for the accumulation of significant ore metals. Therefore, the giant-sized REE deposit is proposed to be associated with large-volume, highly evolved carbonatite at Bayan Obo.

Monazite behaviour during metamorphic evolution of a diamond-bearing gneiss: a case study from the Seve Nappe Complex, Scandinavian Caledonides

Abstract Monazite is a common mineral in metapelitic rocks including those which underwent ultra-high pressure (UHP) metamorphism. During metamorphic evolution monazite adapts its composition to the changing mineral assemblage, especially in its heavy rare earth element contents. We studied this process in diamond-bearing gneiss containing monazite, from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides. Although the rock has been re-equilibrated under granulite facies and partial melting conditions, it still preserves minerals from the UHP stage: garnet, kyanite, rutile, and especially diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single-crystals or polyphase inclusions with Fe-Mg carbonates, rutile and CO2. Both monazite and diamond occur in the rims of garnet showing the highest pyrope content and a secondary peak of yttrium. Such a position indicates thermally activated diffusion under high temperature at the end of prograde metamorphism. Monazite compositions show negative Eu anomalies, which we interpret to be inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar which are unstable at UHP conditions. Our results suggest that the effect of whole-rock composition may be more important than that of coexisting phases. The UHP monazite was most likely formed from allanite during subduction and prograde metamorphism. The monazites included in garnet and kyanite are mostly unaltered, whereas those in the matrix show breakdown coronas consisting of apatite, REE-epidote/allanite and REE carbonate, likely formed due to pressure decrease and cooling. U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 ±3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltica continental margin in Early Ordovician time.

Tracing olivine carbonation and serpentinization in CO2-rich fluids via magnesium exchange and isotopic fractionation

Chemical exchange between seawater and the oceanic crust is thought to play a significant role in the regulation of the global magnesium (Mg) cycle, yet relatively little is known about the rates and mechanisms of Mg exchange in these crustal environments. In this study we experimentally characterize the extent, and nature, of Mg isotope fractionation during the carbonation and serpentinization of olivine (one of the principal minerals found in ultramafic rocks) under hydrothermal conditions. Olivine alteration was found to be incongruent, with the reactant fluid composition varying according to the extent of olivine dissolution and the precipitation of secondary minerals. In mildly acid water (pH ∼ 6.5), olivine dissolved to form Mg-Fe carbonate solid solutions and minor chrysotile. Upon carbonation and a decrease of CO2 in the water, the pH increased to >8, with chrysotile and brucite becoming the dominant alteration minerals. The Mg-rich carbonates preferentially incorporated lighter Mg isotopes, resulting in a ∼0.5‰ increase of the δ26Mg composition of the fluid relative to olivine during the initial carbonation and serpentinization reactions. This was followed by a decrease in δ26Mg under higher pH conditions associated with the formation of brucite. Our experimental and modeling results therefore demonstrate that the δ26Mg composition of fluids involved in olivine alteration reflect the type and quantity of secondary Mg minerals formed, which in turn depend on the pH and CO2 concentration of the water. Comparison of these results with natural groundwaters and geothermal waters from basaltic terrains indicate that the δ26Mg composition of natural waters are likely to also be controlled by mafic rock dissolution and the preferential incorporation of isotopically light Mg into carbonates and isotopically heavy Mg into Mg-Si minerals. Together, these findings improve our understanding of Mg isotope systematics during water-rock interaction, and suggest that δ26Mg may be a useful tool for tracing reactions that are critical to geological CO2 sequestration.

Cathodo- and photoluminescence emission of a natural Mg-Cr carbonate layered double hydroxide

Abstract Spectral emission of a natural layered double hydroxide (LDH) studied by cathodo- and photoluminescence (PL), displays complex glow curves with wavebands that could be attributable to (i) the paramagnetic Non-bridging Oxygen Hole Center defects (peaked at 330 nm), (ii) F o centers (oxygen vacancies trapping two electrons) associated with O vacancy-hole centers and Me–O bonding defects and dehydroxylation process (in the range of 360–500 nm), (iii) presence of Mn 2+ (0.15%) substituting Mg 2+ ions (530–640 nm) and (iv) a red-IR band mainly associated with Cr and Fe point defects. The presence of Cr 3+ ion activators induces characteristic PL emission peaks in the red-infrared region at 681, 688 and 696 nm linked to 2 E → 4 A 2 transitions. The sample (Mg 6 Cr 2 CO 3 (OH) 16 ·4H 2 O) previously characterized by environmental scanning electron microscope, X-ray diffraction, and X-ray fluorescence indicates that this natural hydrotalcite-like compound is associated with hydroxyl-rich silicates (nimite and clinochlore) and Mg-Fe carbonate and Ni-Fe carbonate LDH (pyroaurite and reevesite).

Reaction path modelling of in-situ mineralisation of CO2 at the CarbFix site at Hellisheidi, SW-Iceland

Results from injection of 175 tonnes of CO2 into the basaltic subsurface rocks at the CarbFix site in SW-Iceland in 2012 show almost complete mineralisation of the injected carbon in less than two years (Matter et al., 2016; Snæbjörnsdóttir et al., 2017). Reaction path modelling was performed to illuminate the rate and extent of CO2-water-rock reactions during and after the injection. The modelling calculations were constrained by the compositions of fluids sampled prior to, during, and after the injection, as reported by Alfredsson et al. (2013) and Snæbjörnsdóttir et al. (2017). The pH of the injected fluid, prior to CO2 dissolution was ∼9.5, whereas the pH of the background waters in the first monitoring well prior to the injections was ∼9.4. The pH of the sampled fluids used in the modelling ranged from ∼3.7 at the injection well to as high as 8.2 in the first monitoring well. Modelling results suggest that CO2-rich water-basalt interaction is dominated by crystalline basalt dissolution along a faster, high permeability flow path, but by basaltic glass dissolution along a slower, pervasive flow path through which the bulk of the injected fluid flows. Dissolution of pre-existing calcite at the onset of the injection does not have a net effect on the carbonation, but does contribute to a rapid early pH rise during the injection, and influences which carbonate minerals precipitate. At low pH, Mg, and Fe are preferentially released from crystalline basalts due to the higher dissolution rates of olivine, and to lesser extent pyroxene, compared to plagioclase and glass (Gudbrandsson et al., 2011). This favours the formation of siderite and Fe-Mg carbonates over calcite during early mineralisation. The model suggests the formation of the following carbonate mineral sequences: siderite at pH < 5, Mg-Fe-carbonates and Ca-Mg-Fe-carbonates at pH > 5, and calcite at higher pH. Other minerals forming with the carbonates are Al- and Fe-hydroxides and chalcedony, and zeolites and smectites at elevated pH. The most efficient carbonate formation is when the pH is high enough for formation of carbonates, but not so high that zeolites and smectites start to form, which compete with carbonates over both cations and pore space. The results of reaction path modelling at the CarbFix site in SW-Iceland indicate that this “sweet spot” for mineralisation of CO2 is at pH from ∼5.2 to 6.5 in basalts at low temperature (20–50 °C).

Variation of mineralogy and organic material during the early stages of aqueous activity recorded in Antarctic micrometeorites

Micrometeorites (MMs) recovered from surface snow near the Dome Fuji Station, Antarctica are almost free from terrestrial weathering and contain very primitive materials, and are suitable for investigation of the evolution and interaction of inorganic and organic materials in the early solar system. We carried out a comprehensive study on seven porous and fluffy MMs [four Chondritic porous (CP) MMs and three fluffy fine-grained (Fluffy Fg) MMs] and one fine-grained type 1 (Fg C1) MM for comparison with scanning electron microscope, transmission electron microscope, X-ray absorption near-edge structure analysis, and secondary ion mass spectrometer.They show a variety of early aqueous activities. Four out of the seven CP MMs contain glass with embedded metal and sulfide (GEMS) and enstatite whiskers/platelets and do not have hydrated minerals. Despite the same mineralogy, organic chemistry of the CP MMs shows diversity. Two of them contain considerable amounts of organic materials with high carboxyl functionality, and one of them contains nitrile (CN) and/or nitrogen heterocyclic groups with D and 15N enrichments, suggesting formation in the molecular cloud or a very low temperature region of the outer solar system. Another two CP MMs are poorer in organic materials than the above-mentioned MMs. Organic material in one of them is richer in aromatic C than the CP MMs mentioned above, being indistinguishable from those of hydrated carbonaceous chondrites. In addition, bulk chemical compositions of GEMS in the latter organic poor CP MMs are more homogeneous and have higher Fe/(Si+Mg+Fe) ratios than those of GEMS in the former organic-rich CP MMs. Functional group of the organic materials and amorphous silicate in GEMS in the organic-poor CP MMs may have transformed in the earliest stage of aqueous alteration, which did not form hydrated minerals.Three Fluffy Fg MMs contain abundant phyllosilicates, showing a clear evidence of aqueous alteration. Phyllosilicates in thee MMs are richer in Fe than those in hydrated IDPs, typical fine-grained hydrated (Fg C1) MMs, and hydrated carbonaceous chondrites. One of the Fluffy Fg MMs contains amorphous silicate, which is richer in Fe than GEMS and contains little or no nanophase Fe metal but contains Fe sulfide. Because the chemical compositions of the amorphous silicate are within the compositional field of GEMS in CP IDPs, the amorphous silicate may be alteration products of GEMS. The entire compositional field of GEMS in the CP MMs and the amorphous silicate in the Fluffy Fg MM matches that of the previously reported total compositional range of GEMS in IDPs.One Fluffy Fg MM contains Mg-rich phyllosilicate along with Fe-rich phyllosilicate and Mg-Fe carbonate. Mg-rich phyllosilicate and Mg-Fe carbonate may have been formed through the reaction of Fe-rich phyllosilicate, Mg-rich olivine and pyroxene, and water with C-bearing chemical species.These data indicate that CP MMs and Fluffy Fg MMs recovered from Antarctic surface snow contain materials that throw a light on the earliest stages of aqueous alteration on very primitive solar system bodies. Because mineralogy and isotopic and structural features of organic materials in D10IB009 are comparable with isotopically primitive IDPs, its parent body could be comets or icy asteroids showing mass ejection (active asteroids). By contrast, organic-poor CP MMs may have experienced the earliest stage of aqueous alteration and Fluffy Fg MMs experienced weak aqueous alteration. The precursor materials of the parent bodies of Fluffy Fg MMs probably contained abundant GEMS or GEMS-like materials like CP IDPs, which is common to fine-grained matrices of very primitive carbonaceous chondrites such as CR3s. However, highly porous nature of organic-poor CP MMs and Fluffy Fg MMs suggests that parent bodies of these MMs must have been much more porous than the parent bodies of primitive carbonaceous chondrites. Given no phyllosilicate among the returned samples of 81P/Wild 2 comet, the MMs may have been derived from porous icy asteroids such as active asteroids as well as P- and D-type asteroids rather than comets.

Metasomatic column with the apobasite listvenites end-member from the Slovinky-Gelnica ore field (Gemeric Superunit, Western Carpathians) and comparison to their type localities from Ural

A metasomatic column (or metasomatic zone pattern) is the complete sequence of metasomatic zones characterising an individual metas omatic facies (Zharikov, 2007). The genesis of this column is the result of a series of processes, in the case of basic rock protolith leading to the formation of apobasite listvenites (in the sense of Sazonov, 1975, 1978). It includes three partial processes: 1) carbonatization, 2) silicification and 3) formation of mica (e.g. Plissart et al., 2009) involving the infiltration of a CO 2 -rich fluids and shows similarity to processes of listvenitisation (e.g. Clayton, 1993). Field research in Slovinky-Gelnica ore field has revealed the presence of such a metasomatic column (cf. Ivan, 1987) . It comprises metabasites as the protolith, carbonatised metabasites, listvenite-type metasomatites and listvenites s.s. The metabasites have features of subvolcanic rock types ( Ivan, 2009) , what makes them comparable with similar rocks from Ural. The petrographic study of selected samples has showed that amphibole, chlorite and albite became progressively unstable in this sequence of alterations, replaced by carbonate, silica and lastly by the muscovite minerals, sericite or fuchsite (Ivan, 1987). Carbonates change their quantity as well as mineralogical composition with the development of the column in two trends of changes: towards Mg-Fe carbonates or Mg-Ca carbonates. Regarding the samples from Ural, Fe-dolomites and ankerites predominate, the metasomatic columns derived from gabbros and dolerites are essentially the same. The geochemical study of rock samples from the study area (22 whole rock analyses) as well as the comparison with similarly metasomatised rock series from Ural (56 analyses) allowed as to give more detailed characterization of the metasomatic column. The most obvious change is a clear increase of K with decreasing Na as a sign of the albite break-up and K-rich minerals (such as muscovite) formation. The increase of LOI and CO 2 indicate the process of carbonatisation. Listvenitisation is often associated with the hydrothermal ore veins formation and the rare metal mineralization with concentration of elements such as Au, Co, Sb, Cu, Ni (e.g. Dinel, 2008; Buckman & Ashley 2010 ). This could be significant as regards the genesis of stratiform ore deposits within the Slovinky-Gelnica ore field.

Hydrothermal phosphate vein-type ores from the southern Central Iberian Zone, Spain: Evidence for their relationship to granites and Neoproterozoic metasedimentary rocks

Hydrothermal quartz–apatite veins, called “Iberian-type”, occur exclusively in the southern Central Iberian Zone. This study presents a multidisciplinary approach leading to the mineralogical, fluid inclusions, and geochemical characterization of these veins from two representative areas, Logrosán and Belvís–Navalmoral, in order to establish their relationship with nearby phosphorous-rich granites and apatite in Neoproterozoic metasediments. The mineral assemblage of these veins comprises quartz, apatite, minor sulfides, dolomite, Fe–Mg-carbonates and Fe–Mn-oxides. Three texturally different types of apatite have been recognized in the veins: apatite I showing prismatic habit associated with minor sulfides, apatite II occurring as white fibrous radial crystal aggregates called “dahllite”, and apatite III as grayish to greenish hexagonal crystals. Hydrothermal apatite from veins (H-apatite) is enriched in Sr and depleted in Mn–Y–REE–Th–U–Pb compared to magmatic apatite (M-apatite) from the granitic plutons. However, trace element compositions of apatite from metasedimentary phosphorous-rich levels or nodules (S-apatite) of the Schist–Greywacke Complex show similar characteristics to the H-apatite, although the H-apatite stands out for its relatively high Sr-contents. This relative Sr enrichment in H-apatite is interpreted as inherited from both phosphate in Neoproterozoic metasediments (S-apatite) and carbonate levels. REE, fluid inclusion and stable isotope data are consistent with a long episode of hydrothermal activity implying cooling and dilution processes while interacting with phosphate-rich shale and carbonate beds in the SGC. Fluid inclusion study undertaken on hydrothermal apatite and quartz reveals the presence of aqueous low salinity fluids (0.2–6.7wt.% NaCl equiv.) at moderate to low Th (125–350°C). All available data point at a recycling event of the southern CIZ metasediments (the SGC) during post-Variscan hydrothermal fluid circulation as the more plausible origin of the phosphate vein-type mineralizations.

Evidence for a Noachian-aged ephemeral lake in Gusev crater, Mars

Gusev crater has long been considered the site of a lake early in Martian history, but the Mars Exploration Rover Spirit found no apparent evidence of lake deposits along its 7 km traverse from 2004 to 2010. Although outcrops rich in Mg-Fe carbonate, dubbed Comanche, were discovered in the Noachian-aged Columbia Hills, they were inferred to result from volcanic hydrothermal activity. We now find evidence that the alteration of the Comanche outcrops is consistent with evaporative precipitation of low-temperature, near-surface solutions derived from limited water-rock interaction with rocks equivalent to nearby outcrops called Algonquin. Additional observations show that the Algonquin outcrops are remnants of volcanic tephra that covered the Columbia Hills and adjacent plains well before emplacement of basalt flows onto the floor of Gusev crater. Water-limited leaching of formerly widespread Algonquin-like tephra deposits by ephemeral waters, followed by transport and evaporative precipitation of the fluids into the Comanche outcrops, can explain their chemical, mineralogical, and textural characteristics.

On the potential for CO2mineral storage in continental flood basalts – PHREEQC batch- and 1D diffusion–reaction simulations

Continental flood basalts (CFB) are considered as potential CO2 storage sites because of their high reactivity and abundant divalent metal ions that can potentially trap carbon for geological timescales. Moreover, laterally extensive CFB are found in many place in the world within reasonable distances from major CO2 point emission sources.Based on the mineral and glass composition of the Columbia River Basalt (CRB) we estimated the potential of CFB to store CO2 in secondary carbonates. We simulated the system using kinetic dependent dissolution of primary basalt-minerals (pyroxene, feldspar and glass) and the local equilibrium assumption for secondary phases (weathering products). The simulations were divided into closed-system batch simulations at a constant CO2 pressure of 100 bar with sensitivity studies of temperature and reactive surface area, an evaluation of the reactivity of H2O in scCO2, and finally 1D reactive diffusion simulations giving reactivity at CO2 pressures varying from 0 to 100 bar.Although the uncertainty in reactive surface area and corresponding reaction rates are large, we have estimated the potential for CO2 mineral storage and identified factors that control the maximum extent of carbonation. The simulations showed that formation of carbonates from basalt at 40 C may be limited to the formation of siderite and possibly FeMg carbonates. Calcium was largely consumed by zeolite and oxide instead of forming carbonates. At higher temperatures (60 – 100 C), magnesite is suggested to form together with siderite and ankerite. The maximum potential of CO2 stored as solid carbonates, if CO2 is supplied to the reactions unlimited, is shown to depend on the availability of pore space as the hydration and carbonation reactions increase the solid volume and clog the pore space. For systems such as in the scCO2 phase with limited amount of water, the total carbonation potential is limited by the amount of water present for hydration of basalt.

Geochemical Evolution of the Banded Iron Formation-Hosted High-Grade Iron Ore System in the Koolyanobbing Greenstone Belt, Western Australia

The banded iron formation (BIF)-hosted iron ore deposits in the lower greenstone succession of the Koolyanobbing greenstone belt, 50 km north of Southern Cross in Western Australia, are a ∼200 Mt high-grade Fe (>58%) pre-mining resource and represents one of the most important iron ore districts in the Yilgarn craton. Four hypogene alteration (ore-forming) stages and one supergene upgrading event took place: (1) During ore stage 1, LREE-depleted, transition metal-enriched, Mg-Fe (±Ca) carbonates replaced quartz in BIFs. The deposit-scale alteration was most likely induced by devolatilization of sea-floor-altered, Ca-Si-depleted mafic rocks in the vicinity of the BIF during early regional (syn-D 1), very low to low-grade metamorphism and was most strongly developed on reactivated BIF-basalt contacts. (2) Ore stage 2 involved the formation of patchy magnetite ore by a syn-D 2 to-D 4 dissolution of early carbonate. Enrichment of Fe 2O 3total in magnetite iron ore was by a factor of 2 to 2.4, and compatible trace elements in magnetite, such as Ga, V, and Al, were immobile. A subdeposit-scale ferroan talc-footprint proximal to magnetite iron ore in the largest deposit (K deposit) was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the BIF under greenschist facies conditions and potentially high fluid/rock ratio. (3) Magnetite growth, during ore stage 3, forming granular magnetite-martite ore is related to a subsequent hydrothermal event, occurring locally throughout the belt, especially in D 2b faults. (4) Ore stage 4 was associated with Fe-Ca-P-(L)REE-Y-enriched hydrothermal fluids, possibly from a magmatic source such as the postmetamorphic Lake Seabrook granite that crops out about 10 km west of the Koolyanobbing deposits and at the southern margin of the greenstone belt. These Ca-enriched fluids interacted with distal metamorphosed mafic rock and influenced the BIF-ore system in a small number of deposits. They were channelled through regional D 4 faults and caused specularitedolomite- quartz alteration, resulting in Fe grades of up to 68%. (5) Supergene upgrade (ore stage 5) by (further) gangue leaching in the weathering zone was most effective in carbonate-altered BIFs and magnetite ore. This process, together with supergene martitization and goethite replacement of magnetite, led to the formation of high-grade, locally (at K deposit) high P goethite-martite ore. At Koolyanobbing, the two geochemically distinct stages of Archean carbonate alteration clearly controlled the formation of hypogene magnetite-specularite- martite-rich ore and recent supergene modification, including the further upgrade of Fe ore. © 2012 by ECONOMIC GEOLOGY.

Sound velocities of ferromagnesian carbonates and the seismic detection of carbonates in eclogites and the mantle

[1] Ferromagnesian carbonates are the stable high pressure carbonates in eclogite and peridotite and their physical properties are thus important for understanding the dynamics of reintroduction of carbon into the Earth's mantle. Using Brillouin scattering spectroscopy, we have determined the sound velocities and elasticity of carbonates along the (Mg1−xFex)CO3 join (with x = 0, 0.65, 0.95 and 0.99) at room conditions to evaluate the effect of carbonates on the seismic velocity structure of the subducting lithosphere. Within experimental uncertainties, Fe substitution has a negligible effect on the adiabatic bulk modulus Ksof Mg-Fe carbonates whereas the shear modulusμ decreases by 34% from MgCO3 to FeCO3. We find that the seismic velocity contrast between carbonate-bearing eclogite and peridotite with 5 wt% CO2and their carbon-free counterparts is less than 1%, indicating that moderately carbonated regions in the subducting mafic crust and mantle may be difficult to detect seismically. The threshold of seismic detection with present methods (∼2%) would require significantly high carbonate contents of 24.5 vol% (Mg0.79Fe0.21)CO3 (15 wt% CO2) in eclogite and more than 30 vol% (Mg0.93Fe0.07)CO3 (>20 wt% CO2) in peridotite.

The Darasun gold deposit, Eastern Transbaikal region: Chemical composition, REE patterns, and stable carbon and oxygen isotopes of carbonates from ore veins

The chemistry, REE patterns, and carbon and oxygen isotopic compositions of carbonates from ore veins of the Darasun deposit are discussed. In addition to the earlier described siderite, calcite, and carbonates of the dolomite-ankerite series, kutnahorite is identified. The total REE content in Fe-Mg carbonates of the dolomite-ankerite series (2.8–73 ppm) is much lower than in later calcite (18–390 ppm). δ13C of Fe-Mg carbonates and calcite varies from +1.1 to −6.7‰ and from −0.9 to −4.9‰, respectively. δ18O of Fe-Mg carbonates and calcite varies from +17.6 to 3.6‰ and from +15.7 to −0.5‰, respectively. The REE sum and carbon and oxygen isotopic compositions reveal zonal distribution relative to the central granodiorite porphyry stock. The correlation between the carbon and oxygen isotopic compositions and REE sum reflects variations in the physicochemical formation conditions and composition of ore-forming fluid. The isotopic composition of fluid is calculated, and possible sources of its components are considered. Earlier established evidence for a magmatic source of ore-forming fluid and participation of meteoric water in ore formation is confirmed. Geochemical evidence for interaction of ore-forming fluid with host rocks is furnished. The relationships between the REE sum, on the one hand, and carbon and oxygen isotopic compositions of hydrothermal ore-forming fluid, on the other, are established.

Numerical modelling of CO2-water-basalt interaction

AbstractThe effects of CO2on water-basaltic glass interaction have been simulated at 25ºC. The calculations indicate that addition of CO2(2—30 bar) to water significantly changes the reaction path. Initially, the pH is buffered between 4 and 6 by CO2ionization, with dissolution of basaltic glass and the formation of secondary minerals with SiO2, Mg-Fe carbonates and dolomite predominating. Upon the dissolution of additional basaltic glass and mineral fixation of CO2, the pH increases to &gt;8 and (Ca)-Fe-Mg smectites, SiO2, Ca-Na zeolites and calcite become the dominant secondary minerals forming. The overall reaction path depends on the initial water composition, reactive surface area, and the composition of the phyllosilicates and carbonates forming. The key factors are the mobility of Mg2+, Fe2+and Ca2+and the competing reactions for these solutes among secondary minerals.

Mineral-melt-fluid composition of carbonate-bearing cumulate xenoliths in Tertiary alkali basalts of southern Slovakia

AbstractTwo types of carbonatic cumulate xenoliths occur in alkali basalts of the northern part of the Carpatho-Pannonian region, Central Europe. One is dominated by Ca-Fe-Mg carbonates with randomly distributed bisulphide globules (Fe1+xS2, x = 0–0.1), Mg-Al spinel, augite, rhönite, Ni-Co-rich chalcopyrite, and a Fe(Ni,Fe)2S4 phase. The second, carbonatic pyroxenite xenolith type, is composed of diopside, subordinate fluorapatite, interstitial Fe-Mg carbonates, and accessory K-pargasite, F-Al-rich ferroan phlogopite, Mg-Al spinel, albite and K-feldspar. All accessory minerals occur in ultrapotassic dacite-trachydacite glass in primary silicate melt inclusions in diopside, together with calcio-carbonatite and CO2-N2-CO inclusions. Textural evidence is provided for multiphase fluid-melt immiscibility in both xenolith types. The carbonatic pyroxenite type is inferred to have accumulated from differentiated, volatile-rich, ultrapotassic magma derived by a very low-degree partial melting of strongly metasomatized mantle. Mineral indicators point to a genetic link between the carbonatite xenolith with olivine-fractionated, silica-undersaturated alkalic basalt ponded at the mantle-crust boundary.

Genesis of Mg-Fe Carbonates from the Sierra Menera Magnesite-Siderite Deposits, Northeast Spain: Evidence from Fluid Inclusions, Trace Elements, Rare Earth Elements, and Stable Isotope Data

The Mg-Fe carbonate deposits of Sierra Menera, underneath exploited goethite gossan, are located in the Iberian Range of northeasten Spain within the Late Ordovician dolomitic formation of the Sierra Menera unit. MgO and FeO contents of the carbonates range between 13.74 and 44.02 and 5.52 and 41.73 wt percent, respectively, corresponding to the breunnerite and pistomesite members. The morphology, petrographic, mineralogical, and geochemical characteristics of the Mg-Fe carbonates are consistent with a metasomatic hydrothermal origin by replacement of a dolomitic protolith as the result of the influx of hot Fe-Mg–rich fluids under acid and reducing conditions. These deposits have similarities to Alpine-type magnesite deposits (Austria, Slovakia, western Carpathian Mountains, Pyrenees) and Mississippi Valley-type Fe deposits (Basque-Cantabrian basin, northern Spain). The Mg-Fe carbonates have normalized rare earth element (REE) patterns characterized by low light REE/heavy REE ratios and positive Eu anomalies. Dolomite rocks that host Mg-Fe occurrences have been recrystallized to saddle dolomite crystals and have less fractionated normalized REE patterns but are also characterized by positive Eu anomalies. These patterns are distinct from the pristine host dolomite, which has REE distribution typical of marine carbonates. Based on REE, fluid inclusion, and stable isotope data, the Mg-Fe carbonates are interpreted to have been formed by interaction between hot (100°–150°C), 18O-enriched, and saline (20 wt % NaCl equiv) iron-rich hydrothermal fluids and marine carbonates. Recrystallization of the host dolomite resulted from moderate to low fluid/rock ratios ( >10). The Mg-Fe carbonates were partially replaced by late dolomite (redolomitization) due to the action of more dilute solutions. Although this event resulted in 18O depletion, REE patterns and 13C values of the precursors were preserved.

Genesis of Mg-Fe Carbonates from the Sierra Menera Magnesite-Siderite Deposits, Northeast Spain: Evidence from Fluid Inclusions, Trace Elements, Rare Earth Elements, and Stable Isotope Data

Genesis of Mg-Fe Carbonates from the Sierra Menera Magnesite-Siderite Deposits, Northeast Spain: Evidence from Fluid Inclusions, Trace Elements, Rare Earth Elements, and Stable Isotope Data

Local replacement of calcite by Fe-Mg carbonates in the albite-biotite zone in Sanbagawa basic schists.

During retrograde metamorphism of carbonate-rich basic schists from the Asemi-gawa area, a part of host calcite formed near the peak of metamorphism has been locally replaced by Fe-Mg containing two different types of carbonate inclusions. One type is Fe-dolomite (FeCO3=8.8 mol.%) accompanied by Cal2(FeCO3=1.5%, MgCO3=2.7%) in the inner part of the replacement. The other one, Mg-ankerite (FeCO3=16.6%) is accompanied by Cal3(FeCO3=3.2%, MgCO3=7%) and Cal4(FeCO3=3.2%, MgCO3=0.6%) in the outer part such of the replacement. An origin of carbonates can be explained by the reactions between a fluid phase containing Mg2+ and Fe2+ and a host calcite1 during retrograde metamorphism.

Lithological and geochemical evidence of Fe and Mn pathways during deposition of Lower Proterozoic banded iron formation in the Krivoy Rog Basin (Ukraine)

Abstract Data on stratigraphy, chemical and mineral composition of siliciclastic and ferrugionus rocks of greenschist to epidote-amphibolite metamorphic grade, multi-order cyclicity and banding, isotopic composition of sulphur and carbon and other features of the Krivoy Rog Supergroup metasedimentary rocks are summarized. They provide an insight into the primary sources and the nature of siliciclastic and chemogenic components of banded iron formation (BIF), as well as on pathways of Fe, Mn and Si in the Krivoy Rog palaeobasin at the time of BIF accumulation. Skelevat and Saxagan iron-ore Groups were deposited in a single cyclic transgressive-regressive sequence, probably in a trough-like landlocked basin with complex bottom topography defined by syn-sedimentary faults inherited from the earlier rift structures. Variations in sea level, humidity of climate, continental run-off, oceanic water inflows etc. many times provoked the rapid facies changes ranging from sedimentation of land-derived siliciclastics to deposition of nearly pure chemogenic sediments of the oxide facies BIF in the deeper parts of the basin. Up to seven pairs of schist ( s -) and BIF ( f -) horizons were deposited in the Saxagan Group. Lateral variations of their thicknesses, distribution of free carbon in the rocks and other data indicate that only parts of the basin at a given time were optimal for chemical sedimentation; major part of dissolved Fe, Si and Mn was supplied by constant or event inflow of suboxic ocean water. The Mn content in the Krivoy Rog Supergroup BIFs could serve as an indicator of the fraction of total Fe that precipitated chemically as carbonate. The main part of Fe precipitation in form of Fe(III) hydroxide particles, possibly coated with amorphous silica, is due to a number of oxidation mechanisms operating in the photic surface water layer. On the basis of physicochemical considerations, it is concluded that the Mn CO 3 content in siderite ( c. 2 mol %) contained in the BIF rocks reflects a uniform dissolved Mn concentration ( c. 1 ppm) in the ocean and basin waters. Sedimentation of the typical association of Mg-Fe carbonates and silicates of the carbonate-silicate facies BIF could occur in the near-bottom water only in the case of strong enrichment in dissolved Fe(II) (up to 100–200 ppm) relative to the ocean water (1–10 ppm), which required a stratified brackish-water basin with an estuarine-type circulation. No enrichment in dissolved Mn(II) was possible until p O 2 in the atmosphere raised high enough for precipitation of particulate Mn oxide in the photic zone and its dissolution in the suboxic near-bottom waters.

An interplanetary dust particle with links to CI chondrites

W7013F5 is a chondritic, hydrated interplanetary dust particle whose composition and mineralogy is nearly identical to that found in the CI chondrites. Transmission electron microscope observations show that the phyllosilicates in W7013F5 consist largely of a coherent intergrowth of Mg-Fe serpentine and Fe-bearing saponite on the unitcell scale. This distinctive intergrowth of phyllosilicates has only been observed previously in the CI chondrites. Other secondary minerals in W7013F5 include Mg-Fe carbonates, magnetite, and pentlandite. The mineral assemblage in W7013F5 is generally not as oxidized as that in the CI chondrites. The presence of kamacite in W7013F5 indicates that the particle is extraterrestrial, and a thin amorphous rim surrounding the particle provides evidence that it is not a piece of a meteorite that fragmented during transit through the atmosphere. The apparent lack of hydrated IDPs with CI mineralogy and chemistry may indicate that CI-type dust-producing asteroids are uncommon in the asteroid belt.