Environmental assessment of abandoned mine tailings in Adak,Västerbotten district (northern Sweden)

Sulfide-rich mine tailings in Adak that are exposed to weathering cause acid mine drainage characterized by low pH (2–4) and high SO₄ (up to 800 mg L⁻¹). Surface water, sediment and soil samples collected in this study contain higher concentrations of As, Cu, Fe and Zn, compared to the target and/or intervention limits set by international regulatory agencies. In particular, high As concentrations in water (up to 2900 μg L⁻¹) and sediment (up to 900 mg kg⁻¹) are of concern. There is large variability in trace element concentrations, implying that both physical (grain size) and chemical factors (pH, secondary phases as sulfides, Al-oxides or clay minerals) play an important role in their distribution. The low pH keeps the trace elements dissolved, and they are transported farther downstream. Trace element partition coefficients are low (log K_d = 0.3–4.3), and saturation indices calculated with PHREEQC are <0 for common oxide and sulfidic minerals. The sediment and soil samples indicate an enhanced pollution index (up to 17), and high enrichment factors for trace elements (As up to 38,300; Zn up to 800). Finally, leaves collected from different plant types indicate bioaccumulation of several elements (As, Al, Cu, Fe and Zn). However, some of the plants growing in this area (e.g., Salix, Equisétum) are generally resistant to metal toxicity, and hence, liming and phytoremediation could be considered as potential on-site remediation methods.     

Ra-partitioning between phlogopite and silicate melt and 226Ra/Ba–230Th/Ba isochrons

In this study we experimentally determine phlogopite/melt partition coefficients of Ra and other trace elements in a lamproitic system. This work was achieved using an analytical technique (LA-ICP-MS) with low detection limits (~ 0.01 fg) permitting the measurement of the very low Ra concentrations feasible in experiments (~ 1 ppb). D_Ra^{phlogopite/melt} was determined to 2.28 ± 0.44 and 2.84 ± 0.47 in two experiments, the ratio D_Ra/D_Ba is around 1.6. The compatibility of Ra in phlogopite results from an ionic radius being close to the apex of the lattice strain parabola for earth alkalis in the large XII-coordinated interlayer site of phlogopite. A re-evaluation of D_Ra and D_Ra/D_Ba for magmatic minerals containing appreciable Ra, yields D_Ra^mineral/melt ranging from ~ 2.6 for phlogopite down to 2–3 ? 10^? 5 for pyroxenes, and D_Ra/D_Ba^mineral/melt from ~ 4 for leucite to 2 ? 10^? 2 for orthopyroxene. The influence of melt composition on D_Ra/D_Ba is less than 10%. All investigated minerals have different D_Ra/D_Ba, strongly fractionating Ra from Ba. Thus, for magmatic systems, (²²⁶Ra)/Ba in the various minerals is not constant, these minerals do not form a straight line in the (²²⁶Ra)/Ba–(²³⁰Th)/Ba system at the time of crystallization and thus, there is no (²²⁶Ra)/Ba–(²³⁰Th)/Ba isochron at t₀. ²²⁶Ra–²³⁰Th–Ba mineral dating is thus applicable only to model ages calculated from mineral–glass pairs with known D_Ra.     

Source characteristics of the ∼2.5 Ga Wangjiazhuang Banded Iron Formation from the Wutai greenstone belt in the North China Craton: Evidence from neodymium isotopes

Here we first present samarium (Sm)–neodymium (Nd) isotopic data for the ∼2.5 Ga Wangjiazhuang BIF and associated lithologies from the Wutai greenstone belt (WGB) in the North China Craton. Previous geochemical data of the BIF indicate that there are three decoupled end members controlling REE compositions: high-T hydrothermal fluids, ambient seawater and terrigenous contaminants. Clastic meta-sediment samples were collected for major and trace elements studies in an attempt to well constrain the nature of detrital components of the BIF. Fractionated light rare earth elements patterns and mild negative Eu anomalies in the majority of these meta-sedimentary samples point toward felsic source rocks. Moreover, the relatively low Th/Sc ratios and positive ε_Nd(t) values are similar to those of the ∼2.5 Ga granitoids, TTG gneisses and felsic volcanics in the WGB, further indicating that they are derived from less differentiated terranes. Low Chemical Index of Weathering (CIW) values and features in the A-CN-K diagrams for these meta-sediments imply a low degree of source weathering. Sm–Nd isotopes of the chemically pure BIF samples are characterized by negative ε_Nd(t) values, whereas Al-rich BIF samples possess consistently positive ε_Nd(t) features. Significantly, the associated supracrustal rocks in the study area have positive ε_Nd(t) values. Taken together, these isotopic data also point to three REE sources controlling the back-arc basin depositional environment of the BIF, the first being seafloor-vented hydrothermal fluids (ε_Nd(t) < −2.5) derived from interaction with the underlying old continental crust, the second being ambient seawater which reached its composition by erosion of parts of the depleted landmass (likely the arc) (ε_Nd(t) > 0), the third being syndepositional detritus that received their features by weathering of a nearby depleted source (likely the arc) (ε_Nd(t) > 0).     

Dehydration and melting during continental collision: Constraints from element and isotope geochemistry of low-T/UHP granitic gneiss in the Dabie orogen

A combined study of petrography, whole-rock major and trace elements as well as Rb?Sr and Sm?Nd isotopes, and mineral oxygen isotopes was carried out for two groups of low-T/UHP granitic gneiss in the Dabie orogen. The results demonstrate that metamorphic dehydration and partial melting occurred during exhumation of deeply subducted continent. Zircon δ¹⁸O values of ? 2.8 to + 4.7‰ for the gneiss are all lower than normal mantle values of 5.3 ± 0.3‰, consistent with ¹⁸O depletion of protolith due to high-T meteoric-hydrothermal alteration at mid-Neoproterozoic. Most samples have extremely low ⁸⁷Sr/⁸⁶Sr ratios at t₁ = 780 Ma, but very high ⁸⁷Sr/⁸⁶Sr ratios at t₂ = 230 Ma. This suggests intensive fluid disturbance due to the hydrothermal alteration of protoliths during Neoproterozoic magma emplacement and the metamorphic dehydration during Triassic continental collision. Rb–Sr isotopes, Th/Ta vs. La/Ta and Th/Hf vs. La/Nb relationships suggest that Group I gneiss experienced lower degrees of hydrothermal alteration, but higher degrees of dehydration, than Group II gneiss. The two groups of gneiss have similar patterns of REE and trace element partition. Group I gneiss displays good correlations between Nb and LREEs but no correlations between Nb and LILEs (Rb, Ba, Pb, Th and U), indicating differential mobilities of LILEs during the dehydration. Thus the correlation between Nb and LREEs is inherited from protolith rather than caused by metamorphic modification. Relative to Group I gneiss, Group II gneiss has stronger negative Eu anomaly, lower contents of Sr and Ba but higher contents of Rb, Th and U. In particular, Nb correlates with LILEs (e.g., Rb, Sr, Ba, Th and U), but not with LREEs (La and Ce). This may indicate decoupling between the dehydration and LILEs transport during continental collision. Furthermore, dehydration melting may have occurred due to breakdown of muscovite during “hot” exhumation. Group II gneiss has extremely low contents of FeO + MgO + TiO₂ (1.04 to 2.08 wt.%), high SiO₂ contents of 75.33 to 78.23 wt%, and high total alkali (Na₂O + K₂O) contents (7.52 to 8.92 wt.%), comparable with compositions predicted from partial melting of felsic rocks by experimental studies. Almost no UHP metamorphic minerals survived; felsic veins of fine-grain minerals occurs locally between coarse-grain minerals, resulting in a kind of metatexite migmatites due to dehydration melting without considerable escape of felsic melts from the host gneiss. In contrast, Group I gneiss only shows metamorphic dehydration. Therefore, the two groups of gneiss show contrasting behaviors of fluid–rock interaction during the continental collision.     

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle: 2. Trace element compositions of melts and minerals

The trace element compositions of melts and minerals from high-pressure experiments on hydrous pyroxenites containing K-richterite are presented. The experiments used mixtures of a third each of the natural minerals clinopyroxene, phlogopite and K-richterite, some with the addition of 5% of an accessory phase ilmenite, rutile or apatite. Although the major element compositions of melts resemble natural lamproites, the trace element contents of most trace elements from the three-mineral mixture are much lower than in lamproites. Apatite is required in the source to provide high abundances of the rare earth elements, and either rutile and/or ilmenite is required to provide the high field strength elements Ti, Nb, Ta, Zr and Hf. Phlogopite controls the high levels of Rb, Cs and Ba.Since abundances of trace elements in the various starting mixtures vary strongly because of the use of natural minerals, we calculated mineral/melt partition coefficients (D_Min/melt) using mineral modes and melting reactions and present trace element patterns for different degrees of partial melting of hydrous pyroxenites. Rb, Cs and Ba are compatible in phlogopite and the partition coefficient ratio phlogopite/K-richterite is high for Ba (1 3 6) and Rb (12). All melts have low contents of most of the first row transition elements, particularly Ni and Cu ((0.1–0.01) × primitive mantle). Nickel has high D_Min/melt for all the major minerals (12 for K-richterite, 9.2 for phlogopite and 5.6 for Cpx) and so behaves at least as compatibly as in melting of peridotites. Fluorine/chlorine ratios in melts are high and D_Min/melt for fluorine decreases in the order apatite (2.2) > phlogopite (1.5) > K-richterite (0.87). The requirement for apatite and at least one Ti-oxide in the source of natural lamproites holds for mica pyroxenites that lack K-richterite. The results are used to model isotopic ageing in hydrous pyroxenite source rocks: phlogopite controls Sr isotopes, so that lamproites with relatively low ⁸⁷Sr/⁸⁶Sr must come from phlogopite-poor source rocks, probably dominated by Cpx and K-richterite. At high pressures (>4 GPa), peritectic Cpx holds back Na, explaining the high K₂O/Na₂O of lamproites.     

Release,transport and attenuation of metals from an old tailings impoundment

The oxidation of sulfide minerals from mine wastes results in the release of oxidation products to groundwater and surface water. The abandoned high-sulfide Camp tailings impoundment at Sherridon, Manitoba, wherein the tailings have undergone oxidation for more than 70 a, was investigated by hydrogeological, geochemical, and mineralogical techniques. Mineralogical analysis indicates that the unoxidized tailings contain nearly equal proportions of pyrite and pyrrhotite, which make up to 60 wt% of the total tailings, and which are accompanied by minor amounts of chalcopyrite and sphalerite, and minute amounts of galena and arsenopyrite. Extensive oxidation in the upper 50 cm of the tailings has resulted in extremely high concentrations of dissolved SO₄ and metals and As in the tailings pore water (pH < 1, 129,000 mg L⁻¹ Fe, 280,000 mg L⁻¹ SO₄, 55,000 mg L⁻¹ Zn, 7200 mg L⁻¹ Al, 1600 mg L⁻¹ Cu, 260 mg L⁻¹ Mn, 110 mg L⁻¹ Co, 97 mg L⁻¹ Cd, 40 mg L⁻¹ As, 15 mg L⁻¹ Ni, 8 mg L⁻¹ Pb, and 3 mg L⁻¹ Cr). The acid released from sulfide oxidation has been extensive enough to deplete carbonate minerals to 6 m depth and to partly deplete Al-silicate minerals to a 1 m depth. Below 1 m, sulfide oxidation has resulted in the formation of a continuous hardpan layer that is >1 m thick. Geochemical modeling and mineralogical analysis indicate that the hardpan layer consists of secondary melanterite, rozenite, gypsum, jarosite, and goethite. The minerals indicated mainly control the dissolved concentrations of SO₄, Fe, Ca and K. The highest concentrations of dissolved metals are observed directly above and within the massive hardpan layer. Near the water table at a depth of 4 m, most metals and SO₄ sharply decline in concentration. Although dissolved concentrations of metals and SO₄ decrease below the water table, these concentrations remain elevated throughout the tailings, with up to 60,600 mg L⁻¹ Fe and 91,600 mg L⁻¹ SO₄ observed in the deeper groundwater. During precipitation events, surface seeps develop along the flanks of the impoundment and discharge pore water with a geochemical composition that is similar to the composition of water directly above the hardpan. These results suggest that shallow lateral flow of water from a transient perched water table is resulting in higher contaminant loadings than would be predicted if it were assumed that discharge is derived solely from the deeper primary water table. The abundance of residual sulfide minerals, the depletion of aluminosilicate minerals in the upper meter of the tailings and the presence of a significant mass of residual sulfide minerals in this zone after 70 a of oxidation suggest that sulfide oxidation will continue to release acid, metals, and SO₄ to the environment for decades to centuries.     

Insights into the extreme PGE enrichment of the W Horizon,Marathon Cu-Pd deposit,Coldwell Alkaline Complex,Canada: Platinum-group mineralogy,compositions and genetic implications

The W Horizon, Marathon Cu-Pd deposit in the Mesoproterozoic Midcontinent rift is one of the highest grade PGE repositories in magmatic ore deposits world-wide. The textural relationships and compositions of diverse platinum-group mineral (PGM) and sulfide assemblages in the extremely enriched ores (>100 ppm Pd-Pt-Au over 2 m) of the W Horizon have been investigated in mineral concentrates with ∼10,000 PGM grains and in situ using scanning electron microprobe and microprobe analyses.Here we show, from ore samples with concentrations up to 23.1 Pd ppm, 8.9 Pt ppm, 1.4 Au ppm and 0.73 Rh ppm, the diversity of minerals (n = 52) including several significant unknown minerals and three new mineral species marathonite (Pd₂₅Ge₉; McDonald et al., 2016), palladogermanide (Pd₂Ge; IMA 2016-086, McDonald et al., 2017), kravtsovite (PdAg₂S, IMA No 2016-092, Vymazalová et al., 2017). The PGM are distributed as PG-, sulfides (52 vol%), -arsenides (34 vol%), -intermetallics of Au-Ag-Pd-Cu and Pd-Ge(10 vol%) and -bismuthides and tellurides (4 vol%). The discovery of abundant (>330 grains) large unknown sulfide minerals with Rh allows us to present analyses three significant potentially new minerals (WUK-1, WUK-2, WUK-3) that are all clearly enriched in Rh (averaging 4.2, 8.5 and 28.21 wt% Rh respectively). Several examples of paragenetic sequences and mineral chemical changes for enrichment of Cu, Pd and Rh with time are revealed in the PGM and base-metal sulfides. We suggest this enhanced metal enrichment formed in response to increasing fO₂ causing the oxidation of Fe²⁺ to Fe³⁺ and to a lesser extent, S.Phase relations in the Ag-Pd-S, Rh-Ni-Fe-S, Pd-Ge, Au-Pd-Cu-Ag, Pd-Ag-Te systems help constrain the crystallization temperatures of the majority of ore minerals in the W Horizon at ∼500 °C or moderate to high subsolidus temperatures (400–600 °C). Local transport by aqueous fluids becomes evident as minerals recrystallize down to <300 °C. The PGE-enriched W Horizon ores exhibit a complex post-magmatic history dominated by the effects of oxidation during cooling of a Cu-PGE enriched magma source from a deep reservoir.     

Ultrahigh-temperature metamorphism and anticlockwise P–T–t path of Paleozoic granulites from north Qinling-Tongbai orogen,Central China

Mafic and semi-pelitic granulites from the Qinling-Tongbai orogen in central China preserve petrological evidence and mineral paragenesis suggesting four distinct stages of metamorphic evolution. The prograde history (M₁) is recorded by the occurrence of cordierite, orthopyroxene and biotite inclusions in garnet porphyroblasts of the peak-metamorphic (M₂) assemblage. Peak-metamorphism was followed by cooling with minor decompression (M₃), which formed symplectites and coronitic textures. The greenschist facies retrograde metamorphic assemblage (M₄) is represented by hydrous minerals replacing minerals of the M₂ and M₃ assemblages. We present LA-ICPMS zircon U-Pb data which show ages of 432 ± 4 Ma for the peak metamorphism and 403 to 426 Ma for the retrograde stage. Microstructural analysis, P–T pseudosections, and mineral isopleths in conjunction with the zircon U-Pb ages define an anticlockwise P–T–t path. The P–T estimates for peak metamorphic conditions of 880–920 °C and 8.0–10 kbar suggest that these rocks witnessed extreme crustal metamorphism under ultrahigh-temperature conditions. The anticlockwise trajectory reported in this study is comparable with similar P–T paths recorded from subduction–collision settings, and correlate the Tongbai granulites to hot orogens developed within a Paleozoic collisional suture. We propose a ridge subduction and slab window setting to explain the formation of the Tongbai orogen, in a convergent plate setting associated with the northward subduction of the Paleo-Tethyan Qinling Ocean.     

Evolution of rare-earth mineralization in the Bear Lodge carbonatite,Wyoming: Mineralogical and isotopic evidence

The Bear Lodge alkaline complex in northeastern Wyoming (USA) is host to potentially economic rare-earth mineralization in carbonatite and carbonatite-related veins and dikes that intrude heterolithic diatreme breccias in the Bull Hill area of the Bear Lodge Mountains. The deposit is zoned and consists of pervasively oxidized material at and near the surface, which passes through a thin transitional zone at a depth of ~ 120–183 m, and grades into unaltered carbonatites at depths greater than ~ 183–190 m. Carbonatites in the unoxidized zone consist of coarse and fine-grained calcite that is Sr-, Mn- and inclusion-rich and are characterized by the presence of primary burbankite, early-stage parisite and synchysite with minor bastnäsite that have high (La/Nd)cn and (La/Ce)_cn values. The early minerals are replaced with polycrystalline pseudomorphs consisting of secondary rare-earth fluorocarbonates and ancylite with minor monazite. Different secondary parageneses can be distinguished on the basis of the relative abundances and composition of individual minerals. Variations in key element ratios, such as (La/Nd)_cn, and chondrite-normalized profiles of the rare-earth minerals and calcite record multiple stages of hydrothermal deposition involving fluids of different chemistry. A single sample of primary calcite shows mantle-like δ¹⁸O_V-SMOW and δ¹³C_V-PDB values, whereas most other samples are somewhat depleted in ¹³C (δ¹³C_V-PDB ≈ − 8 to − 10‰) and show a small positive shift in δ¹⁸O_V-SMOW due to degassing and wall-rock interaction. Isotopic re-equilibration is more pronounced in the transitional and oxidized zones; large shifts in δ¹⁸O_V-SMOW (to ~ 18‰) reflect the input of meteoric water during pervasive hydrothermal reworking and supergene oxidation. The textural relations, mineral chemistry and C and O stable-isotopic variations record a polygenetic sequence of rare-earth mineralization in the deposit. With the exception of one Pb-poor sample showing an appreciable positive shift in ²⁰⁸Pb/²⁰⁴Pb value (~ 39.2), the Bear Lodge carbonatites are remarkably uniform in their Nd, Sr and Pb isotopic composition: ¹⁴³Nd/¹⁴⁴Nd_t = 0.512591–0.512608; εNd_t = 0.2–0.6; ⁸⁷Sr/⁸⁶Sr_t = 0.704555–0.704639; εSr_t = − 1.5–2.7; ²⁰⁶Pb/²⁰⁴Pb_t = 18.071–18.320; ²⁰⁷Pb/²⁰⁴Pb_t = 15.543–15.593; and ²⁰⁸Pb/²⁰⁴Pb_t = 38.045–39.165. These isotopic characteristics indicate that the source of the carbonatitic magma was in the subcontinental lithospheric mantle, and modified by subduction-related metasomatism. Carbonatites are interpreted to be generated from small degrees of partial melt that may have been produced via interaction of upwelling asthenosphere giving a small depleted MORB component, with an EM1 component likely derived from subducted Farallon crust.     

Arsenic associations in sediments from the loess aquifer of La Pampa,Argentina

Groundwater from the Quaternary loess aquifer of La Pampa, central Argentina, has significant problems with high concentrations of As (up to 5300 μg L⁻¹) as well as other potentially toxic trace elements such as F, B, Mo, U, Se and V. Total As concentrations in 45 loess samples collected from the aquifer have a range of 3–18 mg kg⁻¹ with a mean of 8 mg kg⁻¹. These values are comparable to world-average sediment As concentrations. Five samples of rhyolitic ash from the area have As concentrations of 7–12 mg kg⁻¹. Chemical analysis included loess sediments and extracted porewaters from two specially cored boreholes. Results reveal a large range of porewater As concentrations, being generally higher in the horizons with highest sediment As concentrations. The displaced porewaters have As concentrations ranging up to 7500 μg L⁻¹ as well as exceptionally high concentrations of some other oxyanion species, including V up to 12 mg L⁻¹. The highest concentrations are found in a borehole located in a topographic depression, which is a zone of likely groundwater discharge and enhanced residence time. Comparison of sediment and porewater data does not reveal unequivocally the sources of the As, but selective extract data (acid-ammonium oxalate and hydroxylamine hydrochloride) suggest that much of the As (and V) is associated with Fe oxides. Primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments, secondary oxide minerals are probably more important. Extract compositions also suggest that Mn oxide may be an As source. The groundwaters of the region are oxidising, with dissolved O₂, NO₃ and SO₄ normally present and As(V) usually the dominant dissolved As species. Under such conditions, the solubility of Fe and Mn oxides is low and As mobilisation is strongly controlled by sorption–desorption reactions. Desorption may be facilitated by the relatively high-pH conditions of the groundwaters in the region (7.0–8.8) and high concentrations of potential competitors (e.g. V, P, HCO₃). PHREEQC modelling suggests that the presence of V at the concentrations observed in the Pampean porewaters can suppress the sorption of As to hydrous Fe(III) oxide (HFO) by up to an order of magnitude. Bicarbonate had a comparatively small competitive effect. Oxalate extract concentrations have been used to provide an upper estimate of the amount of labile As in the sediments. A near-linear correlation between oxalate-extractable and porewater As in one of the cored boreholes investigated has been used to estimate an approximate K_d value for the sediments of 0.94 L kg⁻¹. This low value indicates that the sediments have an unusually low affinity for As.     

Mineralogical evidence for crystallization conditions and petrogenesis of ilmenite-series I-type granitoids at the Baogutu reduced porphyry Cu deposit (Western Junggar,NW China): Mössbauer spectroscopy,EPM and LA-(MC)-ICPMS analyses

Primary ore-forming minerals retain geochemical signatures of magmatic crystallization information and can reveal the petrochemical conditions prevalent at the time of their formation. The Baogutu deposit is a typical reduced porphyry Cu deposit. Amphibole and biotite Fe³⁺/ΣFe ratios, minerals (feldspar, biotite, amphibole, zircon and apatite), in situ elemental and apatite Nd isotopic compositions were determined by Mössbauer spectroscopy, electron probe microanalysis, and laser ablation multiple-collection inductively coupled plasma mass spectrometry, respectively, to investigate the magma oxidation state, petrogenesis, source features, and to constrain the carbon species at magmatic stages for the intrusive phases. The results show that the primary plagioclase and amphibole in the mineralized diorite to granodiorite porphyry and post ore hornblende diorite porphyry are distinct (An_26-55 versus An_60-69; Mg-hornblende versus tschermakite). In particular, the amphibole shows distinct major and trace element compositions with light rare earth element enrichments and negative Eu anomalies in Mg-hornblende and light rare earth element depletions and no Eu anomalies in tschermakite. All the analyzed biotites are primary igneous phases with a biotite phenocryst profile showing significant variations of Zn, Cr, Sc and Sr from core to rim. These results may indicate the occurrence of mixing between two distinct magmas during mineral formation. Titanium in zircon and Si¹ in amphibole thermometries indicate that magma crystallized at >900 °C and continued to ∼650 °C. In situ apatite Nd isotope (εNd(t) = 5.6–7.6, T_DM2 = 620–460 Ma), indicate absence of significant reduced sedimentary contamination and the source of juvenile lower crust. Slightly decreasing Fe³⁺/ΣFe ratios from biotite and amphibole to whole rock indicate decreasing oxygen fugacity during magma crystallization. Recalculated biotite compositions according to Fe³⁺/ΣFe ratios indicate fO₂ values of less than Ni-NiO buffer (NNO) which show slightly lower values than that estimated according to zircon/melt distribution coefficients Ce anomalies (∼ΔNNO + 0.6). These values are consistent with the features of reduced porphyry Cu deposits. Crystallization of other mineral phases significantly affects the reliability of oxybarometer of zircon/melt distribution coefficients Eu anomalies and Mn contents in apatite. This oxidation state suggests that only CO₂ was present at the magmatic stage, and implies that CH₄ formed during CO₂ reduction occurring later hydrothermal alteration. The alteration of primary amphibole to actinolite released Ti, Al, Fe, Mn, Na and K to the fluid with later precipitation of titanite, albite and minor ilmenite and magnetite during actinolite alteration.     

In situ U–Pb age determination and Nd isotopic analysis of perovskites from kimberlites in southern Africa and Somerset Island,Canada

Determination of the emplacement ages and initial isotopic composition of kimberlite by conventional isotopic methods using bulk rock samples is unreliable as these rocks usually contain diverse clasts of crustal- and mantle-derived materials and can be subject to post-intrusion sub-aerial alteration. In this study, 8 samples from 5 kimberlites in southern Africa and twelve samples from 7 kimberlites from Somerset Island, Canada have been selected for in situ perovskite U–Pb isotopic age determination and Nd isotopic analysis by laser ablation using thin sections and mineral separates. These fresh perovskites occur as primary groundmass minerals with grain-sizes of 10–100 μm. They were formed during the early stage of magmatic crystallization, and record data for the least contaminated or contamination-free kimberlitic magma. U–Pb isotopic data indicate that the majority of the southern Africa kimberlites investigated were emplaced during the Cretaceous with ages of 88 ± 3 to 97 ± 6 Ma, although one sample yielded an Early Paleozoic age of 515 ± 6 Ma. Twelve samples from Somerset Island yielded ages ranging from 93 ± 4 Ma to 108 ± 5 Ma and are contemporaneous with other Cretaceous kimberlite magmatism in central Canada (103–94 Ma). Although whole-rock compositions of the kimberlites from southern Africa have a large range of ε_Nd(t) values (? 0.5 to + 5.1), the analysed perovskites show a more limited range of + 1.2 to + 3.1. Perovskites from Somerset Island have ε_Nd(t) values of ? 0.2 to + 1.4. These values are lower than that of depleted asthenospheric mantle, suggesting that kimberlites might be derived from the lower mantle. This study shows that in situ U–Pb and Nd isotopic analysis of perovskite by laser ablation is both rapid and economic, and serves as a powerful tool for the determination of the emplacement age and potential source of kimberlite magmas.     

Re–Os isotopic constraints on the source of platinum-group minerals (PGMs) from the Vestřev pyrope-rich garnet placer deposit,Bohemian Massif

Mineralogical studies of the heavy fraction from a Holocene pyrope-rich garnet placer deposit at Vestřev (Krkonoše Piedmont Basin, Bohemian Massif) have identified the presence of very rare grains of platinum group minerals (PGM). Pt–Fe alloy grains are accompanied by Os–Ir–Ru minerals (native osmium, iridium, and ruthenium) with inclusions of Pt–Fe alloy and hongshiite (PtCu). This mineral assemblage is typical for several mantle settings including ophiolites. The chemistry of the Os–Ir–Ru minerals shows an enrichment of the PGM in Ru, which is typical of ophiolites. The grain morphology of PGM and pyrope-rich garnet (mostly rounded with numerous euhedral/subhedral grains) does not exclude a common source. In-situ laser-ablation MC-ICP-MS was used to measure the Re–Os isotopic compositions of single Os-rich grains, which show heterogeneous subchondritic Os isotopic compositions (¹⁸⁷Os/¹⁸⁸Os = 0.12082–0.12505 ± 0.00003). This precludes their low-temperature origin and indicates derivation of platinum-group elements (PGEs) essentially from mantle-derived rocks without a significant contribution of crustal Os. The mantle model age (T_MA) and Re-depletion model age (T_RD) model ages range from ~ 0.4 to ~ 1.0 Ga and most likely reflect a long history of melt depletion that affected the mantle sources of PGM.     

Long-lived magmatic systems and implications on the recognition of granite–pegmatite genetic relations: Characterization of the Pavia granitic pegmatites (Ossa-Morena Zone,Portugal)

It is generally accepted that pegmatites are derived from large masses of granite but, even in areas where complete mineralogical, chemical and isotopic datasets are available, the relation between pegmatites and host granitic rocks or nearby plutons is usually not simple to address. The Pavia pluton, located in the Ossa-Morena Zone (Iberian Massif), is a multiphase intrusive body constructed over ∼11 m.y. by the amalgamation of several batches of magma. At the first glance, pegmatites seem to constitute a very homogeneous pegmatite field. They are mainly “intragranitic” thin tabular dikes, unzoned, layered, or with simple internal structure and are composed by the ordinary minerals that constitute the different classes of igneous rocks. They also present identical whole rock major and trace elements geochemistry and isotopic signature [(⁸⁷Sr/⁸⁶Sr)_i = 0.70434–0.70581, ɛNd_t = −1.3 to −3.7 and δ¹⁸O = 8.2–9.6‰] but, based on previously published geochronological data, three generations of pegmatites were identified. Two of these are coeval with the emplacement of the host granites (s.l.) at 328 Ma and ca. 324 Ma. The other is related to a later magmatic event at 319–317 Ma. A similar and rather juvenile source is suggested for host granites (s.l.) and pegmatites but a simple and continuous process of intra-chamber magmatic differentiation is not supported by our data. It is suggested that pegmatites derived from slightly evolved batches of magma that interacted with fresh, newly emplaced, batches (from the same or from a similar source) with limited interaction with the crust. Therefore, the Pavia pegmatites do not represent the final products of magmatism at this level of the crust but slightly differentiated products of different batches of magma. This study demonstrates how long-lived magmatic systems can potentially affect the recognition of granite–pegmatite genetic relationships.     

Multi-stage metamorphic evolution of retrograde eclogite with a granulite-facies overprint in the Zhaigen area of the North Qinling Belt,China

Retrograde eclogite from the central part of the Qinling Complex, Zhaigen area of the North Qinling Belt, was studied using integrated petrology, mineral chemistry, pseudosection modeling, and geochronology. Microstructures and mineral relationships reveal five metamorphic stages and associated mineral assemblages as follows: (1) pre-peak stage M₁, which is recorded by the inner cores of garnets together with mineral inclusions of clinopyroxene (Cpx₁) + amphibole (Am₁) + plagioclase (Pl₁) ± quartz ± rutile, occurred under conditions of 760–770 °C and 11.4–14.0 kbar; (2) eclogite-facies stage M₂, recorded by garnet cores + relic omphacite (with a high jadeite content up to 31%) + rutile + quartz under conditions of > 16.7 kbar and 679–765 °C; (3) high-pressure granulite-facies stage M₃, characterized by clinopyroxene (Cpx₂) + plagioclase (Pl₂) symplectites under conditions of 14.5–15.6 kbar and 800–850 °C; (4) medium-pressure granulite-facies stage M₄, characterized by the growth of plagioclase + orthopyroxene coronas around garnet under conditions of 8.3–10 kbar and 795–855 °C; and (5) retrogressive amphibolite-facies stage M₅, which is represented by amphibole (Am₃) + plagioclase (Pl₃) kelyphitic rims around garnet at conditions of < 4 kbar and < 620 °C. Based on Laser Raman analysis of mineral inclusions, cathodoluminescence images, in situ trace element concentrations from different domains within zircon grains, and LA-ICP-MS and SHRIMP U–Pb dating, the protolith age of the Zhaigen retrograde eclogite is suggested at 786 ± 10 Ma and the eclogite-facies metamorphic age recorded by metamorphic zircon cores is limited within 501–497 Ma. The retrograde zircon rims display ages of 476–447 Ma and 425 Ma that probably reflect the timing of two stages of retrograde metamorphism, respectively. The mineral assemblages, P–T conditions, and zircon U–Pb data define a clockwise P–T–t path for the retrograde eclogite, suggesting that the Neoproterozoic protolith of the retrograde eclogite might evolved into continental subduction and eclogite-facies metamorphism during 501–497 Ma before undergoing retrograde metamorphism during an initial stage of exhumation to middle–upper crust level at 474–447 Ma and subsequent exhumation to shallow upper crust at ~ 420 Ma.     

Reprint of “Depositional environment and tectonic implications of the Paleoproterozoic BIF in Changyi area,eastern North China Craton: Evidence from geochronology and geochemistry of the metamorphic wallrocks”

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. Three types of metamorphic wallrocks interbedded with the BIF bands are identified, including plagioclase gneisses and leptynites, garnet-bearing gneisses and amphibolites. Protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials, the garnet-bearing gneisses are Fe-rich pelites contaminated by clastics, and the amphibolites are tholeiitic rocks. Trace elements of La, Th, Sc and Zr of the plagioclase gneisses and leptynites and the garnet-bearing gneisses support that these meta-sedimentary rocks were probably derived from recycling of Archean rocks with felsic and mafic materials differentiated into different rock types. ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons from the meta-sedimentary rocks concentrate at 2.7–3.0 Ga, confirming their derivation from the Archean rocks. The presence of several Paleoproterozoic detrital zircons (2240 to 2246 Ma), however, also suggests minor involvement of Paleoproterozoic materials. The Archean detrital zircons have ε_Hf(t) values varying from − 0.7 to 7.6, which mainly fall between the 3.0 Ga and 3.3 Ga average crustal evolution lines on the age vs. ε_Hf(t) diagram, further illustrating that the rocks providing materials for the meta-sedimentary rocks mainly originated from partial melting of a Mesoarchean crust. This is strongly supported by their crust-like trace element distribution patterns (such as Nb, Ta, P and Ti depletion) and ancient Nd depleted mantle model ages (T_DM = 2.9–3.4 Ga). In addition, the remarkably high ε_Hf(t) values (7.5 to 9.3) of the Paleoproterozoic detrital zircons constrain the Paleoproterozoic materials to originate from a depleted mantle. The amphibolites show low SiO₂ (46.5 to 52.8 wt.%) and high MgO (5.68 to 10.9 wt.%) contents, crust-like trace element features and low ε_Nd(t) values (− 4.5 to − 0.3), suggesting that these ortho-metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by Archean crustal materials. Since an intra-continental environment was required for the formation of the above metamorphic rocks, these rocks not only confine the depositional environment of the Changyi BIF to be an intra-continental rift, but also support the rifting processes of the eastern NCC during Paleoproterozoic.     

Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella,Dronning Maud Land,East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Mineralogical and geochemical evidence for multi-stage origin of mineral veins hosted by teschenites at Tichá, Outer Western Carpathians,Czech Republic

Numerous mineral veins are hosted in a body of teschenite which is situated within the Lower Cretaceous flysch siliciclastics of the Silesian Unit at Tichá. Mineralogy, fluid inclusions, stable isotopes and trace elements have been studied in order to assess the origin of this mineralization. Three stages of vein cementation have been recognized, each of them being characterized by distinct mineral composition and genetic conditions. The first stage is composed of titanite, aegirine-augite to aegirine, annite, analcime and strontian apatite. These minerals originated from NaCl-rich, CaCl₂-poor magmatic brine (total fluid salinities range between 47 and 57 wt%), leaving after crystallization of host teschenite in low-pressure (<1 kbar) environment. Crystallization temperatures reached ～390–510 °C for early phases, titanite and aegirine-augite. The second stage is formed by calcite, chlorite, dolomite, siderite, strontianite, quartz, pyrite and sphalerite. The parent fluids were low-salinity (0.5–4.5 wt% NaCl eq.) aqueous solutions with low content of strong REE-complexing ligands, that were progressively cooled during mineral precipitation (up to ～190 °C at the beginning, ～90 °C at the end of crystallization). These fluids are interpreted to be predominantly of external origin, derived from surrounding sedimentary sequences during diagenetic dewatering of clay minerals. The highly positive δ¹⁸O and near-zero δ¹³C values indicate an interaction of fluids with sedimentary carbonates. The third stage is formed by a dense net of calcite veinlets, which probably originated during tectonic deformations connected with orogenetic movements during the Tertiary. The source of strontium for first stage mineralization was probably related to the special conditions of magmatic evolution of the host teschenite, whereas strontium for second stage minerals could have been remobilized during hydrothermal alteration from earlier teschenite-hosted mineral phases and/or limestone.     

Geology,mineralogy, and geochemistry of fault-controlled hydrothermal Cu–Au mineralization in the Shanmen Volcanic Basin,SE China

The Yukeng–Banling deposit is a typical fault-controlled hydrothermal Cu–Au deposit in the Shanmen Volcanic Basin (SVB), SE China. Ore bodies commonly occur as lodes, lenses and disconnected pods dipping SW with vertical zonation of ore minerals. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicification, sericitization, chloritization and argillation with a banded alteration zonation. The mineralization can be divided into three stages (stages I, II and III). Native gold is present as veinlets in fractures of fine-grained pyrite from stage II.Zircon U–Pb and Rb–Sr isochron ages indicate that the Cu–Au mineralization is coeval with the Caomen alkaline granite and Xiaokeng quartz-diorite, both emplaced at ca. 102 Ma. Microthermometric measurements of fluid inclusions in quartz and sphalerite from stage II veins indicate that the Yukeng–Banling deposit is an epithermal deposit. Six ore-related quartz grains have δD_H2O values of − 69 to − 43‰, and δ¹⁸O_H2O values calculated using total homogenization temperatures that range from − 2.0 to 0.7‰. All samples plot in an area between the magmatic field and the meteoric line, suggesting that the ore-forming fluids are derived from a mixed source of magmatic and meteoric waters. δ³⁴S values for eight pyrite separates range from − 2.1 to + 4.1‰ with an average of + 1.7‰, and δ³⁴S values for galena and sphalerite are 2.3‰ and 2.2‰, similar to magmatic sulfur. Four alkaline granite samples have Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb)_t = 18.175–18.411, (²⁰⁷Pb/²⁰⁴Pb)_t = 15.652–15.672 and (²⁰⁸Pb/²⁰⁴Pb)_t = 38.343–38.800. Three quartz-diorite samples have ratios (²⁰⁶Pb/²⁰⁴Pb)_t, (²⁰⁷Pb/²⁰⁴Pb)_t and (²⁰⁸Pb/²⁰⁴Pb)_t of 18.277–18.451, 15.654–15.693 and 38.673–38.846, respectively. These age-calculated lead isotopic data for alkaline granite are similar to those for the analyzed sulfides. Co/Ni ratios for stage II pyrites range from 1.42 to 5.10, indicating that the Yukeng–Banling deposit records the past involvement of magmatic hydrothermal fluids. The isotope data, together with geological, mineralogical and geochronological evidence, favor a primary magmatic source for sulfur and metals in the ore fluids. Mixing of the Cu- and Au-rich fluids with meteoric water led to precipitation of the Cu–Au veins along NW-trending faults.The Yukeng–Banling deposit, the contemporaneous Caomen alkaline granite and Xiaokeng quartz-diorite in the SVB formed under an extensional setting, due to high-angle subduction of the paleo-Pacific plate. The extensional setting facilitated the formation of Cu- and Au-rich magmas which was derived from enriched mantle and lower crust.     

Can the long-term potential for carbonatization and safe long-term CO2 storage in sedimentary formations be predicted?

A sedimentary formation perturbated by supercritical CO₂ reacts by dissolving primary minerals and forming new secondary phases. In this process CO₂ may be trapped in stable carbonate minerals and may thereby be immobilized for long time spans. The potential for mineral trapping can be estimated by solving kinetic expressions for the reservoir minerals and possible secondary phases. This is, however, not trivial as kinetic data are uncertain or even lacking for the minerals of interest. Here, the rate equations most commonly used for CO₂ storage simulations have been solved, and the rate parameters varied, to obtain sensitivity on the total amount of CO₂ stored as mineral carbonate. As various expressions are in use to estimate growth rates of secondary carbonates, three formulations were compared, including one taking into account mineral nucleation preceding growth. The sensitivity studies were done on two systems, the Utsira Sand being representative for a cold quartz-rich sand (37 °C, 100 bar CO₂), and the Gulf Coast Sediment, being representative for a medium temperature quartz–plagioclase-rich system (75 °C, 300 bar CO₂).The simulations showed that the total predicted CO₂ mineral storage is especially sensitive to the choice of growth rate model and the reactive surface area. The largest sensitivity was found on α, fraction of total surface area available for reactions, with a reduction of one order of magnitude for all reacting phases leading to 3–4 times lower predicted CO₂ mineral storage. Because the reactive surface area is highly uncertain for natural systems, the range in predicted results may be even larger. The short-term predictions (<100–1000 a), such as the onset of carbonate growth, were highly sensitive to nucleation and growth rates. Moreover, the type of carbonate minerals formed was shown to be model dependent, with the simplest model predicting an unlikely carbonate assemblage at low temperature (i.e., formation of dolomite at 37 °C). Therefore, to use kinetic models to upscale short-term (<months) laboratory experiments in time, to identify the past reactions and physical conditions of natural CO₂ storage analogues, and finally to predict the potential for CO₂ trapping in existing and future storage projects, more knowledge has to be collected, especially on the reactive surface area of CO₂ storage reservoirs, and on the rate of secondary carbonate nucleation and growth.