Potential for offsetting diamond mine carbon emissions through mineral carbonation of processed kimberlite: an assessment of De Beers mine sites in South Africa and Canada

De Beers kimberlite mine operations in South Africa (Venetia and Voorspoed) and Canada (Gahcho Kué, Victor, and Snap Lake) have the potential to sequester carbon dioxide (CO₂) through weathering of kimberlite mine tailings, which can store carbon in secondary carbonate minerals (mineral carbonation). Carbonation of ca. 4.7 to 24.0 wt% (average = 13.8 wt%) of annual processed kimberlite production could offset 100% of each mine site’s carbon dioxide equivalent (CO₂e) emissions. Minerals of particular interest for reactivity with atmospheric or waste CO₂ from energy production include serpentine minerals, olivine (forsterite), brucite, and smectite. The most abundant minerals, such as serpentine polymorphs, provide the bulk of the carbonation potential. However, the detection of minor amounts of highly reactive brucite in tailings from Victor, as well as the likely presence of brucite at Venetia, Gahcho Kué, and Snap Lake, is also important for the mineral carbonation potential of the mine sites.

     

Searching for parental kimberlite melt

Constraining the composition of primitive kimberlite magma is not trivial. This study reconstructs a kimberlite melt composition using vesicular, quenched kimberlite found at the contact of a thin hypabyssal dyke. We examined the 4 mm selvage of the dyke where the most elongate shapes of the smallest calcite laths suggest the strongest undercooling. The analyzed bulk compositions of several 0.09-1.1 mm² areas of the kimberlite free from macrocrysts were considered to be representative of the melt. The bulk analyses conducted with a new “chemical point-counting” technique were supplemented by modal estimates, studies of mineral compositions, and FTIR analysis of olivine phenocrysts. The melt was estimated to contain 26-29.5 wt% SiO₂, ∼7 wt% of FeO_T, 25.7-28.7 wt% MgO, 11.3-15 wt% CaO, 8.3-11.3 wt% CO₂, and 7.6-9.4 wt% H₂O. Like many other estimates of primitive kimberlite magma, the melt is too magnesian (Mg# = 0.87) to be in equilibrium with the mantle and thus cannot be primary. The observed dyke contact and the chemistry of the melt implies it is highly fluid (η = 10¹-10³ Pa s at 1100-1000 °C) and depolymerized (NBO/T = 2.3-3.2), but entrains with 40-50% of olivine crystals increasing its viscosity. The olivine phenocrysts contain 190-350 ppm of water suggesting crystallization from a low SiO₂ magma (a_SiO2 below the olivine-orthopyroxene equilibrium) at 30-50 kb. Crystallization continued until the final emplacement at depths of few hundred meters which led to progressively more Ca- and CO₂-rich residual liquids. The melt crystallised phlogopite (6-10%), monticellite (replaced by serpentine, ∼10%), calcite rich in Sr, Mg and Fe (19-27%), serpentine (29-31%) and minor amounts of apatite, ulvöspinel-magnetite, picroilmenite and perovskite. The observed content of H₂O can be fully dissolved in the primitive melt at pressures greater than 0.8-1.2 kbar, whereas the amount of primary CO₂ in the kimberlite exceeds CO₂ soluble in the primitive kimberlite melt. A mechanism for retaining CO₂ in the melt may require a separate fluid phase accompanying kimberlite ascent and later dissolution in residual carbonatitic melt. Deep fragmentation of the melt as a result of volatile supersaturation is not inevitable if kimberlite magma has an opportunity to evolve.     

Mineralogical,geochemical, and microbial investigation of a sulfide-rich tailings deposit characterized by neutral drainage

Mineralogical, geochemical and microbial characterization of tailings solids from the Greens Creek Mine, Juneau, Alaska, was performed to evaluate mechanisms controlling aqueous geochemistry of near-neutral pH pore water and drainage. Core samples of the tailings were collected from five boreholes ranging from 7 to 26 m in depth. The majority of the 51 samples (77%) were collected from the vadose zone, which can extend >18 m below the tailings surface. Mineralogical investigation indicates that the occurrence of sulfide minerals follows the general order: pyrite [FeS₂] >> sphalerite [(Zn,Fe)S] > galena [PbS], tetrahedrite [(Fe,Zn,Cu,Ag)₁₂Sb₄S₁₃] > arsenopyrite [FeAsS] and chalcopyrite [CuFeS₂]. Pyrite constitutes <20 to >35 wt.% of the tailings mineral assemblage, whereas dolomite [CaMg(CO₃)₂] and calcite [CaCO₃] are present at ?30 and 3 wt.%, respectively. The solid-phase geochemistry generally reflects the mineral assemblage. The presence of additional trace elements, including Cd, Cr, Co, Mo, Ni, Se and Tl, is attributed to substitution into sulfide phases. Results of acid–base accounting (ABA) underestimated both acid-generating potential (AP) and neutralization potential (NP). Recalculation of AP and NP based on solid-phase geochemistry and quantitative mineralogy yielded more representative results. Neutrophilic S-oxidizing bacteria (nSOB) and SO₄-reducing bacteria (SRB) are present with populations up to 10⁷ and 10⁵ cells g⁻¹, respectively. Acidophilic S-oxidizing bacteria (aSOB) and iron-reducing bacteria (IRB) were generally less abundant. Primary influences on aqueous geochemistry are sulfide oxidation and carbonate dissolution at the tailings surface, gypsum precipitation–dissolution reactions, as well as Fe reduction below the zone of sulfide oxidation. Pore-water pH values generally ranged from 6.5 to 7.5 near the tailings surface, and from approximately 7–8 below the oxidation zone. Elevated concentrations of dissolved SO₄, S₂O₃, Fe, Zn, As, Sb and Tl persisted under these conditions.     

Post-emplacement serpentinization and related hydrothermal metamorphism in a kimberlite from Venetia, South Africa

The common serpentine–diopside matrix assemblage in volcaniclastic kimberlite (VK) at the Venetia Mine, South Africa is ascribed to a secondary origin, because of post‐emplacement serpentinization and associated hydrothermal metamorphism. Volcaniclastic deposits with 20–30% porosity infill kimberlite pipes in the waning stages of kimberlite eruptions. Olivine macrocrysts are typically rimmed by talc and are pseudomorphed by lizardite, with minor magnetite. The fine matrix consists of mixtures of lizardite, chlorite, smectite, brucite, calcite, titanite and andradite, an assemblage which either pseudomorphed microcrysts or in‐filled voids. Locally we recognize microcryst pseudomorphs rich in sub‐microscopic mixtures of lizardite with smectite, and other microcryst pseudomorphs and void‐filling matrix rich in chlorite and lizardite. Interstitial lizardite and associated phyllosilicates (brucite, smectite and chlorite) crystallized progressively from meteoric or hydrothermally derived pore waters, and Si⁴⁺ and Mg²⁺ released into the fluid phase during serpentinization of olivine macrocrysts. Radial‐fibrous fringes of diopside microlites around crystals display void‐filling textures because of unrestricted growth into pore spaces. Secondary diopside is attributed to Si⁴⁺, Mg²⁺ and Ca²⁺ cations released into the fluid phase by interaction with olivine, calcite and plagioclase in siliceous xenoliths. The paucity of primary, fine‐grained groundmass phases resistant to alteration, for example, perovskite and spinel, precludes an origin for the intergrain matrix as altered interstitial ash, glass or a late‐stage kimberlite melt. Isovolumetric replacement of olivine results in a volume increase of 60% so that pore spaces in the original deposit can be easily filled up with serpentine. The source of Al³⁺ to form chlorite and smectite is attributed to alteration of plagioclase in xenoliths which comprise 20–30 vol.% of the deposit. Titanite, hydro‐andradite and second‐generation diopside precipitate as hydrothermal minerals from calcium‐bearing serpentinizing fluids in replacement reactions and as void‐filling minerals. Consideration of mineral equilibria in the CaO‐MgO‐SiO₂‐H₂O‐CO₂ system constrains the common matrix assemblage of lizardite and diopside in X_CO2)–T space. At 300 bar, the assemblage is stable only at temperatures below 370 °C and X_CO2 < 0.01. This upper limit on temperature is well below the plausible solidus of ultrabasic magmas. Furthermore, the requirement of trace CO₂ in the fluid phase implies a post‐emplacement external source rather than ‘autometamorphism’ from kimberlite‐derived fluids, because of high P_CO2 commonly inferred for kimberlite magmas.     

Cl-bearing lizardite in metamorphosed kimberlites from the Udachnaya Vostochnaya Pipe,Yakutia

Anhydrite-bearing dolomites and kimberlites from the contact zone of the Udachnaya Vostochnaya pipe, northern part of the Eastern Siberian Platform, were affected by low-grade metamorphism to the zeolite facies. The kimberlites are serpentinized and carbonatized and contain metasomes of anhydrite and saponite pockets. Twenty lizardite pseudomorphs after olivine (Fo _91–82) in the kimberlites were examined on an electron microprobe. The lizardite is rimmed by saponite and contains dolomite, calcite, magnetite, and anhydrite inclusions. Lizardite in the central parts of the pseudomorphs contains 1.5–1.9 wt % Cl and 6–9 wt % Fe₂O₃, and this mineral in the outer portions of the pseudomorphs bears 0.7–1.0 wt % Cl and 2–6 wt % Fe₂O₃ (the paper presents nine microprobe analyses and images showing the distribution of Cl, Mg, Al, Si, S, Ca, Ti, and Fe obtained in characteristic X-ray radiation). The amounts of Fe³⁺ in octahedrally and tetrahedrally coordinated sites of the Cl-bearing lizardite are roughly equal. Cl was borrowed in the course of serpentinization from the host Early Paleozoic evaporites and brines contained in them. The Cl concentration in our lizardite from the metamorphosed kimberlites from the Eastern Siberian Platform (continental lizardite) is much higher than the Cl concentration in oceanic lizardite from serpentine replacing peridotites (0.03–0.2 wt % Cl). This is likely explained by differences in the Cl concentrations in the metamorphic fluids, their salinity (3% for oceanic water and 65% for brines in the platform cover).     

Metal extractability in acidic and neutral mine tailings from the Cartagena-La Unión Mining District (SE Spain)

Mine tailings are ubiquitous in the landscapes of mined areas. Metal solubilities were compared in two chemically distinct mine tailings from the old Mining District of Cartagena-La Unión (SE Spain). One of the tailings was acidic (pH 3.0) with 5400 mg/kg Zn, 1900 mg/kg As and 7000 mg/kg Pb. The other was neutral (pH 7.4) with 9100 mg/kg Zn, 5200 mg/kg Pb and 350 mg/kg As. In samples from the acidic tailings, more than 15% of the Zn and 55% of the Cd were extractable with 0.1 M NaNO₃, and distilled water. In the neutral tailings, using the same reagents, less than 1% of the metals were extractable. A sequential extraction procedure revealed that the sum of the residual and the Fe oxide fractions of Cu, Zn and Pb comprised 80–95% in the acidic tailings and 70–90% in the neutral tailings. The acidic mine tailings had a higher metal solubility, resulting in more metal leaching in the short-term, but also a higher fraction of inert metal. In contrast, in the neutral tailings, the metals were evenly distributed between, oxides and the residual fraction. This implies lower metal mobility in the short-term, but that metal mobility may increase in the long-term. When applied to mine tailings, sequential extractions may provide misleading results because the strong cation exchange capacity of some extractants may induce pH changes and thereby significantly change metal solubility.     

Speciation and colloid transport of arsenic from mine tailings

In addition to affecting biogeochemical transformations, the speciation of As also influences its transport from tailings at inoperative mines. The speciation of As in tailings from the Sulfur Bank Mercury Mine site in Clear Lake, California (USA) (a hot-spring Hg deposit) and particles mobilized from these tailings have been examined during laboratory-column experiments. Solutions containing two common, plant-derived organic acids (oxalic and citric acid) were pumped at 13 pore volumes d⁻¹ through 25 by 500 mm columns of calcined Hg ore, analogous to the pedogenesis of tailings. Chemical analysis of column effluent indicated that all of the As mobilized was particulate (1.5 mg, or 6% of the total As in the column through 255 pore volumes of leaching). Arsenic speciation was evaluated using X-ray absorption spectroscopy (XAS), indicating the dominance of arsenate [As(V)] sorbed to poorly crystalline Fe(III)-(hydr)oxides and coprecipitated with jarosite [KFe₃(SO₄, AsO₄)₂(OH)₆] with no detectable primary or secondary minerals in the tailings and mobilized particles. Sequential chemical extractions (SCE) of <45 μm mine tailings fractions also suggest that As occurs adsorbed to Fe (hydr)oxides (35%) and coprecipitated within poorly crystalline phases (45%). In addition, SCEs suggest that As is associated with 1 N acid-soluble phases such as carbonate minerals (20%) and within crystalline Fe-(hydr)oxides (10%). The finding that As is transported from these mine tailings dominantly as As(V) adsorbed to Fe (hydr)oxides or coprecipitated within hydroxysulfates such as jarosite suggests that As release from soils and sediments contaminated with tailings will be controlled by either organic acid-promoted dissolution or reductive dissolution of host phases.     

Arsenic attenuation in tailings at a former Cu–W–As mine,SW Finland

Nearly half a century after mine closure, release of As from the Ylöjärvi Cu–W–As mine tailings in groundwater and surface water run-off was observed. Investigations by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), synchrotron-based micro-X-ray diffraction (μ-XRD), micro-X-ray absorption near edge structure (μ-XANES) and micro-extended X-ray absorption fine structure (μ-EXAFS) spectroscopy, and a sequential extraction procedure were performed to assess As attenuation mechanisms in the vadose zone of this tailings deposit. Results of SEM, EMPA, and sequential extractions indicated that the precipitation of As bearing Fe(III) (oxy)hydroxides (up to 18.4 wt.% As₂O₅) and Fe(III) arsenates were important secondary controls on As mobility. The μ-XRD, μ-XANES and μ-EXAFS analyses suggested that these phases correspond to poorly crystalline and disordered As-bearing precipitates, including arsenical ferrihydrite, scorodite, kaňkite, and hydrous ferric arsenate (HFA). The pH within 200 cm of the tailings surface averaged 5.7, conditions which favor the precipitation of ferrihydrite. Poorly crystalline Fe(III) arsenates are potentially unstable over time, and their transformation to ferrihydrite, which contributes to As uptake, has potential to increase the As adsorption capacity of the tailings. Arsenic mobility in tailings pore water at the Ylöjärvi mine will depend on continued arsenopyrite oxidation, dissolution or transformation of secondary Fe(III) arsenates, and the As adsorption capacity of Fe(III) (oxy)hydroxides within this tailings deposit.     

Jurassic kimberlites from Picton and Varty Lake,Ontario: Geochemical and stable isotopic characteristics

Micaceous ultramafic dikes of Jurassic age from Picton and Varty Lake, Ontario, consist mineralogically of olivine — phlogopite — serpentine — calcite-spinel. The rocks are characterized by abundant Ba-rich phlogopite (up to 6.5 wt.% BaO) and spinels with a diagnostic kimberlite trend-1. Compositionally the dikes are characterized by extreme silica-undersaturation (21–30 wt.% SiO₂), primitive Mg/(Mg + Fe^T) ratios (0.75–0.83), large enrichments of volatile components (CO₂ and H₂O), and relatively high abundances of both incompatible and compatible trace elements. The dikes exhibit pronounced enrichments of light rare earth elements (LREE) (La_N=320–1330) combined with strongly fractionated patterns (La_N/Yb_N=45–108). Calcite in the dikes is a primary magmatic phase, from textural relations and C-isotopic compositions ( ¹³C= –4.0 to –8.3). A calcite-rich aphanitic phase of the Picton dike is interpreted to be a late stage magmatic differentiate, which possibly involved olivine fractionation. Although the dikes lack most of the macrocrysts generally considered to be important diagnostic minerals of kimberlite magmatism, the geochemical, mineralogical and C- and O-isotopic characteristics collectively indicate that the dikes are evolved varieties of hypabyssal facies kimerlite.     

Brucite [Mg(OH2)] carbonation in wet supercritical CO2: An in situ high pressure X-ray diffraction study

Understanding mechanisms and kinetics of mineral carbonation reactions relevant to sequestering carbon dioxide as a supercritical fluid (scCO₂) in geologic formations is crucial to accurately predicting long-term storage risks. Most attention so far has been focused on reactions occurring between silicate minerals and rocks in the aqueous dominated CO₂-bearing fluid. However, water-bearing scCO₂ also comprises a reactive fluid, and in this situation mineral carbonation mechanisms are poorly understood. Using in situ high-pressure X-ray diffraction, the carbonation of brucite [Mg(OH)₂] in wet scCO₂ was examined at pressure (82 bar) as a function of water concentration and temperature (50 and 75 °C). Exposing brucite to anhydrous scCO₂ at either temperature resulted in little or no detectable reaction over three days. However, addition of trace amounts of water resulted in partial carbonation of brucite into nesquehonite [MgCO₃·3H₂O] within a few hours at 50 °C. By increasing water content to well above the saturation level of the scCO₂, complete conversion of brucite into nesquehonite was observed. Tests conducted at 75 °C resulted in the conversion of brucite into magnesite [MgCO₃] instead, apparently through an intermediate nesquehonite step. Raman spectroscopy applied to brucite reacted with ¹⁸O-labeled water in scCO₂ show it was incorporated into carbonate at a relatively high concentration. This supports a carbonation mechanism with at least one step involving a direct reaction between the mineral and water molecules without mediation by a condensed aqueous layer.     

Arsenic mobilization from mine tailings in the presence of a biosurfactant

Batch experiments were conducted to investigate As mobilization from mine tailings in the presence of a biosurfactant (JBR425, mixed rhamnolipids) and to evaluate the feasibility of using biosurfactant in remediating As contaminated mine tailings/soils. Introduction of the biosurfactant increased As mobilization greatly. When the mass ratio was 10 mg biosurfactant/g mine tailings at pH 11, As mobilization by the biosurfactant was greatest after 24 h, with a corresponding concentration ratio (the ratio of As mobilization by the biosurfactant to that by distilled water at same adjusted pH, wt/wt) of 21.6. Selective sequential extraction indicated that As was easily mobilized from the weakly bound and relatively more mobile fractions by washing with the biosurfactant. A mobilization isotherm was developed to predict As mobilization from the mine tailings in the presence of biosurfactant. It was shown that biosurfactant sorption to the mine tailings is essential for As mobilization. Arsenic mobilization was found to be positively correlated with the mobilization of Fe and other metals (i.e., Cu, Pb and Zn), which might further enhance As mobilization by helping incorporate it into soluble complexes or micelles. Capillary electrophoresis analyses indicated that As redox or methylation reactions had insignificant effect on As mobilization. Biosurfactants might be used potentially to remove bulk As from mine tailings or contaminated soils under alkaline conditions.     

Major, trace element and oxygen isotope study of glass cosmic spherules of chondritic composition: The record of their source material and atmospheric entry heating

New geochemical data on cosmic spherules (187 major element, 76 trace element, and 10 oxygen isotope compositions) and 273 analyses from the literature were used to assess the chemical diversity observed among glass cosmic spherules with chondritic composition. Three chemical groups of glass spherules are identified: normal chondritic spherules, CAT-like spherules (where CAT refers to Ca-Al-Ti-rich spherules), and high Ca-Al spherules. The transition from normal to high Ca-Al spherules occurs through a progressive enrichment in refractory major elements (on average from 2.3 wt.% to 7.0 wt.% for CaO, 2.8 wt.% to 7.2 wt.% for Al₂O₃, and 0.14 wt.% to 0.31 wt.% for TiO₂) and refractory trace elements (from 6.2 μg/g to 19.3 μg/g for Zr and 1.6CI-4.3CI for Rare Earth Elements-REEs) relative to moderately refractory elements (Mg, Si) and volatile elements (Rb, Na, Zn, Pb). Based on a comparison with experimental works from the literature, these chemical groups are thought to record progressive heating and evaporation during atmospheric entry. The evaporative mass losses evaluated for the high Ca-Al group (80-90%) supersede those of the CAT spherules which up to now have been considered as the most heated class of stony cosmic spherules. However, glass cosmic spherules still retain isotopic and elemental evidence of their source and precursor mineralogy. Four out of the 10 normal and high Ca-Al spherules analysed for oxygen isotopes are related to ordinary chondrites (δ¹⁸O = 13.2-17.3‰ and δ¹⁷O = 7.6-9.2‰). They are systematically enriched in Ni and Co (Ni = 24-500 μg/g) with respect to spherules related to carbonaceous chondrites (Ni < 1.2 μg/g, δ¹⁸O = 13.1-28.0‰ and δ¹⁷O = 5.1-14.0‰). REE abundances in cosmic spherules, which are not fractionated according to parent body or atmospheric entry heating, can then be used to unravel the precursor mineralogy. Spherules with flat REE pattern close to unity when normalized to CI are the most abundant in our dataset (54%) and likely derive from homogeneous, fine-grained chondritic precursors. Other REE patterns fall into no more than five categories, a surprising reproducibility in view of the mineralogical heterogeneity of chondritic lithologies at the micrometeorite scale.     

Primitive Magma From the Jericho Pipe, N.W.T., Canada: Constraints on Primary Kimberlite Melt Chemistry

We report the first estimates of primary kimberlite melt compositionfrom the Slave craton, based on samples of aphanitic kimberlitefrom the Jericho kimberlite pipe, N.W.T., Canada. Three samplesderive from the margins of dykes where kimberlite chilled againstwall rock (JD51, JD69 and JD82) and are shown to be texturallyconsistent with crystallization from a melt. Samples JD69 andJD82 have geochemical characteristics of primitive melts: theyhave high MgO (20–25 wt %), high mg-numbers (86–88),and high Cr (1300–1900 ppm) and Ni (800–1400 ppm)contents. They also have high contents of CO₂ (10–17 wt%). Relative to bulk macrocrystal kimberlite, they have lowermg-numbers and lower MgO but are enriched in incompatible elements(e.g. Zr, Nb and Y), because the bulk kimberlite compositionsare strongly controlled by accumulation of mantle olivine andother macrocrysts. The compositions of aphanitic kimberlitefrom Jericho are similar to melts produced experimentally bypartial melting of a carbonate-bearing garnet lherzolite. Onthe basis of these experimental data, we show that the primarymagmas from the Jericho kimberlite could represent 0·7–0·9%melting of a carbonated lherzolitic mantle source at pressuresand temperatures found in the uppermost asthenosphere to theSlave craton. The measured CO₂ contents for samples JD69 andJD82 are only slightly lower than the CO₂ contents of the correspondingexperimental melts; this suggests that the earliest hypabyssalphase of the Jericho kimberlite retained most of its originalvolatile content. As such these samples provide a minimum CO₂content for the primary kimberlite magmas from the Slave craton. KEY WORDS: kimberlite; melt; primitive; primary magma; Slave craton     

Coalingite from kimberlite breccia of the Manchary pipe,Central Yakutia

Coalingite, Mg₁₀Fe₂(CO₃)(OH)₂₄ · 2H₂O, rare Mg–Fe hydrous carbonate, has been found in the course of the mineralogical study of a disintegrated kimberlite breccia from the Manchary pipe of the Khompu–May field located in the Tamma Basin, Central Yakutia, 100 km south of Yakutsk. Coalingite occurs as small reddish brown platelets, up to 0.2 mm in size. It is associated with lizardite, chrysotile and brucite, which are typical kimberlitic assemblage. Coalingite is a supergene mineral, but in this case, it is produced by the interaction of brucite-bearing kimberlite and underground water circulating through a vertical or oblique fault zone.     

Arsenic and antimony contamination of waters,stream sediments and soils in the vicinity of abandoned antimony mines in the Western Carpathians,Slovakia

Environmental contamination with As and Sb caused by past mining activities at Sb mines is a significant problem in Slovakia. This study is focused on the environmental effects of the 5 abandoned Sb mines on water, stream sediment and soil since the mines are situated in the close vicinity of residential areas. Samples of mine wastes, various types of waters, stream sediments, soils, and leachates of the mine wastes, stream sediments and selected soils were analyzed for As and Sb to evaluate their geochemical dispersion from the mines. Mine wastes collected at the mine sites contained up to 5166 mg/kg As and 9861 mg/kg Sb. Arsenic in mine wastes was associated mostly with Fe oxides, whereas Sb was present frequently in the form of individual Sb, Sb(Fe) and Fe(Sb) oxides. Waters of different types such as groundwater, surface waters and mine waters, all contained elevated concentrations of As and Sb, reaching up to 2150 μg/L As and 9300 μg/L Sb, and had circum-neutral pH values because of the buffering capacity of abundant Ca- and Mg-carbonates. The concentrations of Sb in several household wells are a cause for concern, exceeding the Sb drinking water limit of 5 μg/L by as much as 25 times. Some attenuation of the As and Sb concentrations in mine and impoundment waters was expected because of the deposition of metalloids onto hydrous ferric oxides built up below adit entrances and impoundment discharges. These HFOs contained >20 wt.% As and 1.5 wt.% Sb. Stream sediments and soils have also been contaminated by As and Sb with the peak concentrations generally found near open adits and mine wastes. In addition to the discharged waters from open adits, the significant source of As and Sb contamination are waste-rock dumps and tailings impoundments. Leachates from mine wastes contained as much as 8400 μg/L As and 4060 μg/L Sb, suggesting that the mine wastes would have a great potential to contaminate the downstream environment. Moreover, the results of water leaching tests showed that Sb was released from the solids more efficiently than As under oxidizing conditions. This might partly explain the predominance of Sb over As in most water samples.     

Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada

Peridotites that sample Archean mantle roots are frequentlyincompatible trace element enriched despite their refractorymajor element compositions. To constrain the trace element budgetof the lithosphere beneath the Canadian craton, trace elementand rare earth element (REE) abundances were determined fora suite of garnet peridotites and garnet pyroxenites from theNikos kimberlite pipe on Somerset Island, Canadian Arctic, theirconstituent garnet and clinopyroxene, and the host kimberlite.These refractory mantle xenoliths are depleted in fusible majorelements, but enriched in incompatible trace elements, suchas large ion lithophile elements (LILE), Th, U and light rareearth elements (LREE). Mass balance calculations based on modalabundances of clinopyroxene and garnet and their respectiveREE contents yield discrepancies between calculated and analyzedREE contents for the Nikos bulk rocks that amount to LREE deficienciesof 70–99%, suggesting the presence of small amounts ofinterstitial kimberlite liquid (0·4–2 wt %) toaccount for the excess LREE abundances. These results indicatethat the peridotites had in fact depleted or flat LREE patternsbefore contamination by their host kimberlite. LREE and Sr enrichmentin clinopyroxene and low Zr and Sr abundances in garnet in low-temperatureperidotites (800–1100°C) compared with high-temperatureperidotites (1200–1400°C) suggest that the shallowlithosphere is geochemically distinct from the deep lithospherebeneath the northern margin of the Canadian craton. The Somersetmantle root appears to be characterized by a depth zonationthat may date from the time of its stabilization in the Archean. KEY WORDS: Canada; mantle; metasomatism; peridotite; trace elements     

Continuous Gradations among Primary Carbonatitic, Kimberlitic, Melilititic, Basaltic, Picritic, and Komatiitic Melts in Equilibrium with Garnet Lherzolite at 3-8 GPa

Multianvil melting experiments in the system CaO–MgO–Al₂O₃–SiO₂–CO₂(CMAS–CO₂) at 3–8 GPa, 1340–1800°C, involvingthe garnet lherzolite phase assemblage in equilibrium with CO₂-bearingmelts, yield continuous gradations in melt composition betweencarbonatite, kimberlite, melilitite, komatiite, picrite, andbasalt melts. The phase relations encompass a divariant surfacein P–T space. Comparison of the carbonatitic melts producedat the low-temperature side of this surface with naturally occurringcarbonatites indicates that natural magnesiocarbonatites couldbe generated over a wide range of pressures >2·5 GPa.Melts analogous to kimberlites form at higher temperatures alongthe divariant surface, which suggests that kimberlite genesisrequires more elevated geotherms. However, the amount of waterfound in some kimberlites has the potential to lower temperaturesfor the generation of kimberlitic melts by up to 150°C,provided no hydrous phases are present. Compositions resemblinggroup IB and IA kimberlites are produced at pressures around5–6 GPa and 10 GPa, respectively, whereas the compositionsof some other kimberlites suggest generation at higher pressuresstill. At pressures <4 GPa, an elevated geotherm producesmelilitite-like melt in the CMAS–CO₂ system rather thankimberlite. Even when a relatively CO₂-rich mantle compositioncontaining 0·15 wt % CO₂ is assumed, kimberlites andmelilitites are produced by <1% melting and carbonatitesare generated by even smaller degrees of melting of <0·5%. KEY WORDS: carbonatite; CO₂; kimberlite; melilitite; melt generation     

Geochemistry and origin of the Cretaceous sedimentary kaolin deposits,Red Sea,Egypt

This work reports, for the first time, the mineralogical and geochemical characteristics of the Cretaceous sedimentary kaolin deposits in the Red Sea area, Egypt and sheds the light on their source. Mineralogical and geochemical analyses of both bulk deposits and the sand and clay fractions of these deposits indicated that they are composed of kaolinite (average of 75 wt.%) and quartz (average of 22 wt.%). Traces of anatase (average of 1 wt.%) were identified in all kaolin samples, while traces of halite (average of 2 wt.%) and hematite (average of 1 wt.%) were reported in the majority of the analyzed samples. The clay fractions show relatively high contents of TiO₂ (average of 2.1%), Ni (average of 103 ppm), Nb (average of 98 ppm), Y (average of 67 ppm), and Zr (average of 630 ppm). Sum of the rare earth elements (ΣREE) in the clay fractions varies between 193 and 352 ppm. Chondrite-normalized REE patterns show enrichment of the light REE relative to the heavy REE ((La/Yb)_N = 9) and negative Eu anomaly (Eu*/Eu = 0.67).     

Production of amorphous hydrated impure magnesium carbonate through ex situ carbonation

The evaluation of the feasibility of ex situ carbonation in landfills utilizing raw natural substances (namely serpentinites as Mg-source and the CO₂-rich fraction of biogas as C-source) was tested through a laboratory procedure comprising three steps. The first step is the acid attack of a serpentinite at 70 °C, by means of HCl 2 M, to get MgCl₂-rich solutions. Attacks of different durations were performed to evaluate the time needed. The second step is the neutralization of the MgCl₂-rich solution by addition of concentrated ammonia. The third (carbonation) step is mixing of the neutralized MgCl₂-rich solution with a solution of ammonium carbonate. This was produced in a landfill by absorption of CO₂ contained in biogas in a solution of ammonia. The neutralization of acid MgCl₂-rich solutions caused the precipitation of ferrihydrite with secondary ammonium carnallite and salammoniac, whereas abundant precipitation of Amorphous Hydrated Impure Magnesium Carbonate (AHIMC), sometimes with minor nesquehonite, occurred in the third step. This solid carbonate acts as a stable CO₂ sink up to 380 °C. The geochemical behavior of some minor elements was also investigated during the experimental processes revealing that Al, Cr and Ni were removed during neutralization (second step), in contrast to Ca which remained in the circumneutral MgCl₂-rich solution and entered into the structure of AHIMC. During the carbonation step, precipitation of artinite, hydromagnesite, lansfordite, magnesite and nesquehonite was thermodynamically impossible as the aqueous phase was undersaturated with respect to these solid phases upon separation of AHIMC.