Secondary Mineral Phases Research Articles

Oxidation of black shale and its deterioration mechanism in the slip zone of the Xujiaping landslide in Sichuan Province, Southwestern China

This work investigated the chemical water–rock interaction of black shale interbedded with limestone along a bedding slip zone and how its deterioration affects the surrounding rock mass in the Xujiaping (XJP) landslide. Many dissolved pits were found on the limestone, and geochemical phenomena were investigated. Rock and water samples from the site were analysed for mineralogy, chemical composition and hydrochemistry. In slip zones, water–rock chemical processes occur in the bedding fractures of interface between black shale and limestone. Thus, a bedding water–rock cyclic reaction experiment was designed with limestone, black shale and black shale interbedded with limestone. Many major elements and heavy elements (Fe, Mn, Si, Zn, Ni, Al, S, Mg, Ca, Na, K, Co and Sr) dissolved out, demonstrating that strong dissolution occurred because of the acidic water during the black shale water–rock interaction. The limestone neutralized the acidic water through black shale oxidation in the interfacial fractures between black shale and limestone, causing deterioration of the slip zone. The acidic water from the fissure network inside the black shale strata migrated along the bedding to the exposed surfaces of cliffs and rock fractures, then evaporated to form secondary mineral phases, including melanterite, rozenite, szomolnokite, and gypsum. The water–rock chemical interaction in the XJP landslide included dissolution, oxidation, dehydration, and neutralization reactions. The accumulation trends and hydrochemical properties at different reaction stages of ions dissolved from the surrounding rock mass in the bedding direction were revealed. The deterioration mechanism was expanded: (i) rock-forming and carbonate minerals were especially prone to dissolution by sulfuric acid from black shale oxidation in the slip zone, and (ii) volume expansion due to the crystallization force of precipitated minerals caused further fracture expansion and deformation. Therefore, geochemical analyses can effectively elucidate the long-term development and nature of slip zones in landslide investigations.

Competitive TcO4-, IO3-, and CrO42- Incorporation into Ettringite.

Ettringite is a naturally occurring mineral found in cementitious matrices that is known for its ability to incorporate environmentally mobile oxyanion contaminants. To better assess this immobilization mechanism for contaminants within cementitious waste forms intended for nuclear waste storage, this work explores how mixed oxyanion contaminants compete for ettringite incorporation and influence the evolving mineralogy. Ettringite was precipitated in the presence of TcO4-, IO3-, and/or CrO42-, known contaminants of concern to nuclear waste treatment, over pre-determined precipitation periods. Solution analyses quantified contaminant removal, and the collected solid was characterized using bulk and microprobe X-ray diffraction coupled with pair distribution function and microprobe X-ray fluorescence analyses. Results suggest that ≥96% IO3- is removed from solution, regardless of ettringite precipitation time or the presence of TcO4- or CrO42-. However, TcO4- removal remained <20%, was not significantly improved with longer ettringite precipitation times, and decreased to zero in the presence of IO3-. When IO3- is co-mingled with CrO42-, calcite and gypsum are formed as secondary mineral phases, which allows for oxyanion partitioning, e.g., IO3- incorporation into ettringite, and CrO42- incorporation into calcite. Results from this work exemplify the importance of competitive immobilization when assessing waste form performance and environmental risk of contaminant release.

Geochemical and mineralogical characterization of stream sediments impacted by mine wastes containing arsenic, cadmium and lead in North-Central Mexico

This study aimed to evaluate the chemical distribution and natural attenuation of relevant potentially toxic elements (As, Cd, and Pb) contained in stream sediments that were partly generated by the weathering of waste dumps from mining operations. Sequential extraction procedure was used to evaluate the heavy metals and metalloid fractionation related to the primary and secondary mineral phases, showing that there is still a significant amount of As (12–75%), Cd (8–29%) and Pb (1–32%) present in the form of sulfides, representing a potential release of As, Cd and Pb associated with the weathering of the mine wastes. Furthermore, significant amounts of As, Cd and Pb are associated with the most available fractions of mine wastes and stream sediments. Based on these results, the main mechanisms that control the release of As, Cd and Pb are the progressive oxidation of sulfides, the acidic dissolution of carbonates, the reductive dissolution of Fe oxides and arsenate mineral phases, and the processes of ionic exchange. Although background values for As, Cd, and Pb have been documented in stream sediments in this mineralized zone, the high concentration and spatial distribution of these elements suggests that mine wastes are their main source in the studied area. The results show that the hydric dispersion of fine particles from both waste dumps and stream sediments represent environmental risks since these particles could undergo different chemical release mechanisms for As, Pb and Cd.

Features of the Formation of Geochemical Types of Natural Waters in Certain Regions of Central Eurasia

The report presents extensive factual data on the chemical composition of natural waters in different regions of Central Eurasia. The features of the chemical composition of natural waters are revealed for several regions with a humid and arid climate, as well as for tectonomagmatic activation conditions. The abundance of over 50 chemical elements in natural waters is characterized. The geochemical types of waters corresponding to rigorously defined evolution stages of the water–rock system are established for natural waters of all considered regions based on saturation of waters in secondary minerals. The influence of the geochemical conditions on the dissolution of primary minerals and the formation of secondary mineral phases is revealed. The composition of groundwaters is systematized by the features of the origin of their chemical compositions. The role of climatic, geological, and structural factors in the formation of geochemical types of waters in several Central Eurasian regions is shown.

Quantitative microscale Fe redox imaging by multiple energy X-ray fluorescence mapping at the FeKpre-edge peak

AbstractFe oxidation/reduction reactions play a fundamental role in a wide variety of geological processes. In natural materials, Fe redox state commonly varies across small spatial scales at reaction interfaces, yet the approaches available for quantitatively mapping the Fe redox state at the microscale are limited. We have designed an optimized synchrotron-based X-ray spectroscopic approach that allows microscale quantitative mapping of Fe valence state by extending the Fe XANES pre-edge technique. An area of interest is mapped at nine energies between 7109–7118 eV and at 7200 eV, allowing reconstruction, baseline subtraction, and integration of the pre-edge feature to determine Fe(III)/ΣFe with 2 μm spatial resolution. By combining the Fe redox mapping approach with hyperspectral Raman mineralogy mapping, the Fe oxidation state distributions of the major mineral phases are revealed. In this work, the method is applied to a partially serpentinized peridotite with various Fe-bearing secondary mineral phases to trace the Fe transformations and redox changes that occurred during its alteration. Analysis with the Fe redox mapping technique revealed that the peridotite contained relict olivine with abundant Fe(II), while serpentine, pyroaurite, and another hydroxide phase are secondary mineral reservoirs of Fe(III). Although serpentine is not Fe-rich, it contained approximately 74% ± 14% Fe(III)/ΣFe. These analytical results are integral to interpreting the sequence of alteration reactions; serpentinization of primary olivine formed Fe(II)-rich brucite and oxidized serpentine, which could have contributed to H2 production during serpentinization. Subsequent weathering by oxidizing, CO2-bearing fluids led to the partial carbonation and oxidation of brucite, forming pyroaurite and a hydroxide phase containing dominantly Fe(III). This Fe redox imaging approach is applicable to standard petrographic thin sections or grain mounts and can be applied to various geologic and biogeochemical systems.

Lithium isotopes in the Loire River Basin (France): Hydrogeochemical characterizations at two complementary scales

We illustrate two different, but complementary, applications of lithium (Li) isotope tracers for river-basin characterization at two different scales within the Loire River basin (LRB) in France. The first example deals with the behaviour of Li and the fractionation of its isotopes during river weathering at the basin scale of the LRB (117,800 km2). The wide δ7Li range (+5.0 to +13.3‰) in Loire basin streams, spatialized between the headwaters and the lowlands, is consistent with distinct weathering conditions and the distribution of Li from bedrock in the basin between the rivers and secondary mineral phases during water/rock interaction. Additionally, suspended sediments in the LRB streams are significantly 6Li enriched (δ7Li from −8.7 to −7.6‰) compared to average river waters that range from +5.0 to +13.3‰. The second example focuses on the smaller scale Egoutier watershed (13 km2), part of the LRB, which shows that Li isotopes can be useful for distinguishing between natural input and anthropogenic pollution, like effluents from a water treatment plant connected to a hospital. Overall, we confirm that Li isotopes cannot be used as lithological tracers for river waters. However, we did find that Li isotopes can be good tracers of weathering conditions and of anthropogenic sources in an urbanized watershed.

ФОРМЫ НАХОЖДЕНИЯ ГЕРМАНИЯ И ВОЛЬФРАМА В ГЕРМАНИЙ-УГОЛЬНОМ МЕСТОРОЖДЕНИИ СПЕЦУГЛИ (ДАЛЬНИЙ ВОСТОК)

Актуальность исследования обусловлена необходимостью оценки форм нахождения германия и других попутных элементов примесей в германий-угольных месторождениях. Цель: изучить формы нахождения германия и вольфрама в металлоносных углях месторождения Спецугли (Дальний Восток). Объекты: угли и углистые алевролиты германий-угольного месторождения. Методы: корреляционный анализ, изучение группового состава бурого угля, масс-спектрометрия с индуктивно связанной плазмой, инструментальный нейтронно-активационный анализ, сканирующая электронная микроскопия. Результаты. Изучены основные формы нахождения германия и вольфрама в углях месторождения Спецугли. В высокогерманиеносных углях месторождения Спецугли германий характеризуется разнообразными формами нахождения. Наряду с преобладающей германий-органической формой, значительную роль играют минеральные фазы. Установлено, что значение минеральных форм нахождения в концентрировании германия в высокогерманиеносных углях значительно выше, чем предполагалось ранее. В угольных пластах, особенно отчетливо вблизи коры выветривания по грейзенизированным гранитам, широко распространены различные германийсодержащие минеральные фазы микронной и нанометровой размерности. В углях выявлены алюмосиликатная (силикатная) форма германия, разнообразные железистые (гидрогётит, гётит, ярозит) и полиминеральные фазы. Алюмосиликатная (силикатная) фаза представлена пленкоподобными агрегатами, содержащими 0,15–0,4 % германия. В железистых минеральных агрегатах содержится от 0,1 до 6,2 % германия. Аномальные содержания германия совместно с вольфрамом и мышьяком установлены в железомарганцевых корках – продуктах выветривания грейзенизированных гранитов фундамента, сформировавшихся в условиях палеоболота. Наличие вторичных минеральных фаз германия в углях, развитых по диагенетическим минералам, позволяет предполагать, что формирование германиевой минерализации в месторождении может быть полихронным и не ограничивается торфяной стадией. Для вольфрама выявлена преимущественно органическая форма нахождения. Роль минеральных форм вольфрама в общем его балансе в углях месторождения Спецугли незначительна. В изученных 9 пробах угля выявлено всего 1 зерно шеелита. Вблизи коры выветривания вольфрам в углях также встречается в качестве примеси в железомарганцевых «корках». Эти данные согласуются с результатами анализа распределения вольфрама по фракциям группового состава. После извлечения гумусовых кислот в щелочную вытяжку переходит в среднем 99 % вольфрама. При этом в щелочной вытяжке около половины его связано с гуминовыми кислотами, а остальная часть – с низкомолекулярными органическими кислотами.

A Flow-Through Reaction Cell for Studying Minerals Leaching by In-Situ Synchrotron Powder X-ray Diffraction

A flow-through reaction cell has been developed for studying minerals leaching by in-situ time-resolved powder X-ray diffraction, allowing for a better understanding of the leaching mechanisms and kinetics. The cell has the capability of independent control of temperature (up to 95 °C) and flow rate (&gt;0.5 mL min−1) for atmospheric pressure leaching. It was successfully tested at the powder diffraction beamline at the Australian Synchrotron. Galena powder was leached in a citrate solution under flow-through condition at a flow rate of 0.5 mL min−1, while diffraction patterns were collected during the entire leaching process, showing rapid galena dissolution without the formation of secondary mineral phases. The flow-through cell can be used to study leaching processes of other ore minerals.

The role of fluid chemistry on permeability evolution in granite: Applications to natural and anthropogenic systems

Efforts to maintain and enhance reservoir permeability in geothermal systems can contribute to sourcing more sustainable energy, and hence to lowering CO2 emissions. The evolution of permeability in geothermal reservoirs is strongly affected by interactions between the host rock and the fluids flowing through the rock's permeable pathways. Precipitation of secondary mineral phases, the products of fluid-rock interactions, within the fracture network can significantly reduce the permeability of the overall system, whereas mineral dissolution can enhance reservoir permeability. The coupling between these two competing processes dictates the long-term productivity and lifetime of geothermal reservoirs. In this study, we simulate the conditions within a geothermal system from induced fracturing to the final precipitation stage. We performed batch and flow-through experiments on cores of the Carnmenellis granite, a target unit for geothermal energy recovery in Cornwall (UK), to understand the role of mineral dissolution and precipitation in controlling the permeability evolution of the system. The physico-chemical properties of the cores were monitored after each reaction-phase using ICP-OES, SEM, hydrostatic permeability measurements, and X-ray Computed Tomography. Results show that permeability evolution is strongly dependent on fluid chemistry. Undersaturated alkaline fluids dissolve the most abundant mineral phases in granite (quartz and feldspars), creating cavities along the main fractures and generating pressure-independent permeability in the core. Conversely, supersaturated alkaline fluids, resulting from extended periods of fluid-rock interactions, promote the precipitation of clay minerals, and decrease the permeability of the system. These results suggest that chemical dissolution during geothermal operations could generate permeable pathways that are less sensitive to effective stress and will remain open at higher pressures. Similarly, maintaining the circulation of undersaturated fluids through these granitic reservoirs can prevent the precipitation of pore-clogging mineral phases and preserve reservoir permeability in granite-hosted geothermal systems.

Crystal chemistry and microfeatures of gadolinite imprinted by pegmatite formation and alteration evolution

AbstractGadolinite [REE2Fe2+Be2Si2O10] is a common mineral in certain types of rare element and rare earth element (REL-REE) pegmatites. Changes in pegmatite environment during and after gadolinite formation may be devised by studying its crystal-chemical properties and a thorough observation of microfeatures in the mineral matrix. Post-crystallization processes in pegmatite might trigger alteration mechanisms in gadolinite like in other REE-rich pegmatite minerals, whereby various late-magmatic or metasomatic events may affect mineral chemistry. Three gadolinite samples originating from various pegmatite occurrences in southern Norway offer an excellent opportunity in studying post-crystallization evolution of the pegmatites; by determining their crystallographic, chemical, and micro-textural features, imprints of the related processes in the pegmatites have been characterized in this study. Relevant mineral information was collected in recrystallization experiments of fully or slightly metamictized gadolinite samples and subsequent XRD analyses. Micro-Raman spectroscopy, electron microprobe analysis (EMPA), and scanning electron microscope–backscattered electron–energy-dispersive X-ray spectroscopy (SEM-BSE-EDS) analyses were employed to retrieve micro-chemical properties and related micro-textural features of the mineral matrix. With a reference to the gadolinite supergroup, a general alteration path can be envisaged outlining the pegmatite evolution and suggesting the occurrence of the secondary REE mineral phases: altered gadolinite domains prove Ca enrichment with a tendency toward the hingganite composition, while a slight fluorine increase and sporadic secondary fluorite occurrence imply a significant role of fluorine as a complexing agent in the dissolution-reprecipitation mechanism of metasomatic alteration in the mineral. Micro-Raman spectra show improved vibration statistics for the altered gadolinite domains, which could be linked to the substitution of rare earth elements (REE) by Ca and a possible increase of structural ordering within the gadolinite structure, being at the same time an indication of structural healing of metamictized domains by metasomatic processes. A study of microfeatures in the complex silicates like gadolinite proves to be an excellent tool to trace post-crystallization processes in a pegmatitic environment. With a slight redistribution of radionuclides during an alteration in gadolinite, a moderate precaution has to be taken when selecting gadolinite for U-Th-Pb dating.

Native metals and intermetallic compounds in subduction-related ultramafic rocks from the Stanovoy mobile belt (Russian Far East): Implications for redox heterogeneity in subduction zones

Plutonic associations in orogenic belts are potent indicators of multi-stage fractionation of primitive arc magmas and growth of island-arc crust under evolving redox conditions in ancient subduction zones. Ildeus-Lucha ultramafic–mafic complex (ILC) was emplaced at 232–233 Ma within the Mesozoic Stanovoy convergent margin and subsequently underwent multi-stage hydrothermal alteration and greenschist to amphibolite facies metamorphism (~140 Ma), followed by adakite and K-lamprophyre magmatism (114–117 Ma). Dunite, wehrlite, pyroxenite and gabbro in the ILC are composed of olivine, clinopyroxene, orthopyroxene, plagioclase and late-magmatic amphibole. Mineral compositions in the ILC are typical of island-arc plutonic complexes characterized by early crystallization of orthopyroxene, plagioclase intercumulus and presence of magmatic amphibole. Ultramafic-mafic rocks display high-field strength (HFS) element depletions coupled with large-ion lithophile (LIL) element enrichments, indicating formation via crystal fractionation of mafic primary magma derived from a subduction-related mantle source. Some ultramafic rocks from the ILC contain native metals (W, Pt, Au, Ag, Zn, Bi) and intermetallic compounds (Cu-Au-Ag, Pt-Rh-Pd, Cu-Sn-Zn, etc.) included in silicate minerals, or observed as discrete phases in the finer-grained silicate-oxide-sulfide matrix. Textural evidence and association with either primary (magmatic) or secondary (metasomatic or metamorphic) mineral phases, suggest either magmatic (high-temperature) or metasomatic (low-temperature) origin of metals and metallic compounds. The following models of their formation can be proposed on the basis of the presented data: a) crystallization (W and, possibly, Pt) from metal-rich arc magmas under unusually reduced conditions in subduction-related lithosphere; b) formation from magmatic sulfides during pervasive serpentinization and release of abiogenic hydrocarbons and c) precipitation from chlorine-rich, saline aqueous fluids associated with collision-related metasomatism of the Stanovoy island arc crust.

Conventional analysis methods underestimate the plant-available pools of calcium, magnesium and potassium in forest soils

The plant-available pools of calcium, magnesium and potassium are assumed to be stored in the soil as exchangeable cations adsorbed on the cation exchange complex. In numerous forest ecosystems, despite very low plant-available pools, elevated forest productivities are sustained. We hypothesize that trees access nutrient sources in the soil that are currently unaccounted by conventional soil analysis methods. We carried out an isotopic dilution assay to quantify the plant-available pools of calcium, magnesium and potassium and trace the soil phases that support these pools in 143 individual soil samples covering 3 climatic zones and 5 different soil types. For 81%, 87% and 90% of the soil samples (respectively for Ca, Mg and K), the plant-available pools measured by isotopic dilution were greater than the conventional exchangeable pool. This additional pool is most likely supported by secondary non-crystalline mineral phases in interaction with soil organic matter and represents in many cases (respectively 43%, 27% and 47% of the soil samples) a substantial amount of plant-available nutrient cations (50% greater than the conventional exchangeable pools) that is likely to play an essential role in the biogeochemical functioning of forest ecosystems, in particular when the resources of Ca, Mg and K are low.

Long-term mobility of uranium in the granitic KURT site using isotopic analysis and sequential chemical extraction

In this study, the long-term mobility of natural uranium in the KAERI Underground Research Tunnel (KURT) site was investigated using sequential chemical extraction and isotopic analyses of natural uranium in granite. It was found that uranium(VI) would be leached out due to preferential leaching of 234U relative 238U and concentrated by an uraniferous enrichment process through long-term interactions with the oxic KURT groundwater. Our results also suggest that the migration of the preferentially released uranium(VI) through fractured granite can be significantly retarded by secondary mineral phases formed by the weathering processes of granite.

Process network modelling of the geochemical reactions responsible for acid mine drainage emanating from the Witwatersrand tailings facilities

ABSTRACTAcid mine drainage (AMD) and associated metal(loid) and SO42- pollution of soil, surface water and groundwater is ubiquitously associated with tailings material generated by Au mining in the Witwatersrand Basin in South Africa. The individual geochemical processes responsible for the AMD generation in this tailings material are relatively well understood. What is less clear are how these different processes interact as a network within the tailings system. Process network modelling (PNM) is a tool that can be used to study such interactive and complex networks of geochemical processes, especially when stochastic methods, e.g. Monte Carlo simulation, are included in the model development. Secondary mineral phase supersaturation requirements from classical nucleation theory are also built into the model.A PNM was developed for a tailings facility in the Witwatersrand gold basin focussing on pH, Fe(total) and SO42- concentrations in the tailings pore water and the relationship of these parameters to the dissolution of pyrite, O2 diffusion into the tailings, oxidation of Fe2+ and the precipitation of secondary minerals, specifically goethite and jarosite. The model indicated that AMD conditions develop fairly rapidly after the sulphidic material is exposed to the Earth’s oxygenated atmosphere. The concentration of H+, and hence the pH, in the tailings pore water is controlled by a number of feedbacks. The positive feedback, implying addition of H+, is the dissolution of pyrite and the precipitation of the secondary Fe3+-bearing minerals goethite and jarosite. Jarosite precipitation was shown to increase the median H+ addition rate by ~2%. The negative feedback, i.e. decrease in H+, is the oxidation of Fe2+ to Fe3+. This feedback loop produces a net excess of H+. Together with the buffer effect of goethite and jarosite precipitation, system steady-state conditions are eventually achieved with respect to pH.The pore water SO42- concentration is controlled by the positive and negative feedback of pyrite dissolution and jarosite precipitation. This feedback loop produces a large excess of SO42- and steady state conditions can only be achieved if SO42- is physically removed from the tailings system, e.g. seepage to groundwater.Oxidation of the Fe2+ produced by pyrite dissolution to Fe3+ is the only positive feedback for tailings pore water Fe3+ concentrations. The negative feedbacks are precipitation of goethite and jarosite and the oxidation of pyrite by Fe3+. The former effect is delayed as these phases first have to achieve a certain level of supersaturation in the tailings pore water solution before they can form. The precipitation of jarosite and goethite, by removing Fe3+ from solution, decreases the effect of Fe3+ pyrite oxidation causing O2 to remain the dominant oxidation mechanism. This feedback loop produces a small excess of Fe3+ over time, however, the model is very sensitive to other factors, e.g. O2 diffusion deeper into the tailings facility.

Pore network analysis of Brae Formation sandstone, North Sea

In this work, we apply digital rock physics (DRP) to characterize the pore networks of the Brae Formation sandstones from two different wells in the Miller field area (North Sea, UK). Using X-ray micro-CT scans, we calculate the porosity and permeability and generate pore network models to assess pore shape characteristics. The porous samples are marked by macroporosities ranging from 4.9% to 15.2% with the effective porosities varying from 0 to 14.8%. The samples also contained some microporosity hosted in secondary and accessory mineral phases, varying between 2.6% and 10.7%. Pore network model results for total porosity indicate that the samples have median pore and throat radii ranging from 5.5 μm to 16.8 μm and 6.4 μm–12.9 μm, respectively. The throat length of all samples has a median value ranging between 36.3 μm and 82.4 μm. The ratio between effective porosity and total porosity (φ*) varies with total porosity (φ) following the exponential relation φ*=0.98−e−(φ−0.032)/0.028. Pore network connectivity is established at a porosity of 3% and full communication is achieved at porosities exceeding 10%. Permeability was found to vary with total porosity with an exponent of 3.67. Based on these observations and the results from our models, the connectivity of the pore network has important implications for predicting reservoir performance during large scale subsurface projects such as hydrocarbon production and CO2 storage.

Acid mine drainage formation and arsenic mobility under strongly acidic conditions: Importance of soluble phases, iron oxyhydroxides/oxides and nature of oxidation layer on pyrite

Acid mine drainage (AMD) formation and toxic arsenic (As) pollution are serious environmental problems encountered worldwide. In this study, we investigated the crucial roles played by common secondary mineral phases formed during the natural weathering of pyrite-bearing wastes—soluble salts (melanterite, FeSO4·7H2O) and metal oxides (hematite, Fe2O3)—on AMD formation and As mobility under acidic conditions (pH 1.5–4) prevalent in historic tailings storage facilities, pyrite-bearing rock dumps and AMD-contaminated soils and sediments. Our results using a pyrite-rich natural geological material containing arsenopyrite (FeAsS) showed that melanterite and hematite both directly—by supplying H+ and/or oxidants (Fe3+)—and indirectly—via changes in the nature of oxidation layer formed on pyrite—influenced pyrite oxidation dynamics. Based on SEM-EDS, DRIFT spectroscopy and XPS results, the oxidation layer on pyrite was mainly composed of ferric arsenate and K-Jarosite when melanterite was abundant with/without hematite but changed to Fe-oxyhydroxide/oxide and scorodite when melanterite was low and hematite was present. This study also observed the formation of a mechanically ‘strong’ coating on pyrite that suppressed the mineral’s oxidation. Finally, As mobility under acidic conditions was limited by its precipitation as ferric arsenate, scorodite, or a Fe/Al arsenate phase, including its strong adsorption to Fe-oxyhydroxides/oxides.

Role of CO2-water-rock interactions and implications for CO2 sequestration in Eocene deeply buried sandstones in the Bonan Sag, eastern Bohai Bay Basin, China

This study explores CO2-water-rock interactions in Eocene hydrocarbon sandstone reservoirs in eastern Bohai Bay Basin, China, to document mineral alterations and their implications for CO2 geological sequestration. Petrographic, geochemical and reactive transport modeling evidences reveal that various inorganic mineral alterations occur during organic CO2 intrusion from adjacent source rocks into sandstone reservoirs including: (1) pervasive dissolution of calcite and chlorite minerals that are re-precipitated as secondary, more stable dolomite and ankerite cements; (2) precipitation of low-temperature calcite cements (27–49 °C) and relatively high-temperature dolomite (88–111 °C) and ankerite (103–136 °C) cements based on their δ18OPDB values (−11.91‰ to −7.86‰ for calcite; −15.16‰ to −12.48‰ for dolomite and −17.71‰ to −14.32‰ for ankerite); (3) Carbon sources for dolomite and ankerite cements (δ13CPDB from −7.59‰ to −3.58‰) are a mixture of carbon derived from early-formed calcite precursors (δ13CPDB from–0.26‰ to +3.75‰) with significant proportions of organic carbon (δ13CPDB from −25‰ to −10‰); (4) plagioclase dissolution spatially associated with precipitation of quartz and kaolinite. Reactive transport modeling results illustrate that mineral dissolution is coupled temporally and spatially with precipitation of secondary mineral phases. It is inferred that advective and diffusive mass transport of dissolved species in the extremely low pore-fluids is greatly restricted and mineral kinetics exerts more influences on CO2–water–rock interactions in the deep burial environments. The overall mineral alterations in the CO2-mediated geochemical system are the consequences of complicated interconnected reactions in the simulated model. Kinetically slow dissolution reactions of aluminosilicate minerals (e.g. chlorite) are regarded as the rate-limiting step for precipitation of more stable carbonate phases and CO2 sequestration. Therefore, the occurrences, abundances and distribution patterns of the aluminosilicate minerals coupled with their kinetic reaction rates are likely to affect the long-term fate of the safe CO2 storage in the deeply buried siliciclastic reservoirs.

Gold Remobilization: Insights from Gold Deposits in the Archean Swayze Greenstone Belt, Abitibi Subprovince, Canada

Abstract Recognizing if and how Au is remobilized, in solid, melt, or fluid state, is critical for understanding the origin of high-grade ore zones in Au deposits. When evidence for Au remobilization can be demonstrated, then primary versus secondary processes can be distinguished, resulting in a more complete understanding of Au deposit formation. To address this, samples from two Au deposits, Jerome and Kenty, in the Archean Swayze greenstone belt of northern Ontario, Canada, together with archived samples from 39 high-grade Au deposits from the Abitibi greenstone belt across Ontario and Quebec, were geochemically characterized using integrated scanning electron microscopy-energy dispersive spectroscopy and electron microprobe imaging and analyses in addition to laser ablation-inductively coupled plasma-mass spectrometry elemental mapping. These data provided the basis to develop a model for Au remobilization and upgrading of Au that is widely applicable to orogenic gold settings. Data for the Jerome deposit indicate that Au uptake into early pyrite was not due to pulsing of different fluids, but instead was predominantly controlled by S availability, whereby the oscillatory/sector zoning in pyrite resulted from the substitution of As into S sites during rapid growth due to local chemical disequilibrium. In addition, Au-bearing pyrite from both the Jerome and Kenty deposits records textures, such as porosity development coincident with the presence of native gold and accessory sulfide phases, that are strongly suggestive of coupled dissolution-reprecipitation (CDR) reactions that liberated Au and associated elements from earlier auriferous (100–5,000 ppm Au) pyrite. During the remobilization process, Au and Ag were decoupled, which resulted in (1) a change in Au/Ag ratios of 0.5 to 5 in early pyrite to ≈9 in the new native gold (900 Au fineness) and (2) incorporation of Ag into cogenetic secondary mineral phases (e.g., chalcopyrite, tetrahedrite, and galena). Evidence for an association of low-melting point chalcophile elements (LMCE; Hg, Te, Sb, and Bi) with Au at the Jerome, Kenty, and many (&amp;gt;50%) of the 39 historic deposits sampled, along with native gold filling structurally favorable sites in vein quartz in all samples, indicates a fluid might not have been the only factor contributing to remobilization. This systematic Au-LMCE association strongly supports a model whereby Au is released by CDR reactions and is then remobilized by fluid-mediated, LMCE-rich melts that began to form at 335°C and/or by local, nanoparticle (nanomelt?) transport during deformation and metamorphism. Conclusions drawn from this study have implications for Au deposits globally and can account for the common presence of coarse-grained, commonly crystalline, native gold filling fractures in quartz and the paragenetically late-stage origin of gold in veins. They can also better explain the inability of Au in solution remobilization models to account for locally high gold grades, given the relatively low solubility of Au in hydrothermal fluids.

Influence of manganese abundances on iron and arsenic solubility in rice paddy soils

Arsenic (As) mobilization in rice paddy soils under fluctuating redox conditions is influenced by the biogeochemical cycling of redox sensitive elements such as iron (Fe) and manganese (Mn). Arsenic mobility in paddy soils is highly variable, and the influence of Mn abundances and Mn/Fe ratios on As mobility in these soils have received little attention. In this contribution, we developed a complementary set of field and laboratory experiments designed to evaluate the impact of Mn on interconnected Fe and As solubilization in rice paddy soils experiencing wetting-drying cycles through controlled irrigation. Porewater monitoring and synchrotron-based imaging and spectroscopy of thin sections prepared from an Arkansas paddy soil confirmed that As release was primarily governed by reductive dissolution of Fe (oxy)hydroxide phases. Experiments with laboratory soil microcosms amended with the synthetic nanocrystalline Mn oxide, δ-MnO2, showed that higher initial Mn/Fe inhibited Fe and As mobilization into porewater relative to unamended soil by up to 95% and 45%, respectively. Geochemical modeling suggests that pH increases driven by microbial MnO2 reduction, in conjunction with microbial Fe- and sulfate-reduction in carbonate-rich porewater, enhanced the precipitation of siderite (FeCO3(s)), mackinawite (FeS(s)), and potentially a Mn(II) arsenate phase. These secondary mineral phases likely played a greater role in controlling As solubilization than the role of Mn as a redox buffer regulating the redox conditions in the flooded soils. Field and laboratory experiments showed that alternate wetting and drying approaches with a single dry-down can be effective at reducing dissolved As concentrations in porewater through the oxidation of Fe. Differences in soil Mn/Fe ratios had no clear impact on the effectiveness of dry-downs as a strategy to reduce As mobilization.

A synthesis and meta-analysis of the Fe chemistry of serpentinites and serpentine minerals.

The iron chemistry of serpentinites and serpentine group minerals is often invoked as a record of the setting and conditions of serpentinization because Fe behaviour is influenced by reaction conditions. Iron can be partitioned into a variety of secondary mineral phases and undergo variable extents of oxidation and/or reduction during serpentinization. This behaviour influences geophysical, geochemical and biological aspects of serpentinizing systems and, more broadly, earth systems. Iron chemistry of serpentinites and serpentines is frequently analysed and reported for single systems. Interpretations of the controls on, and the implications of, Fe behaviour drawn from a single system are often widely extrapolated. There is a wealth of serpentinite/serpentine chemical composition data available in the literature. Consequently, compilation of a database including potential predictors of Fe behaviour and measures of Fe chemistry enables systematic investigation of trends in Fe behaviour across a variety of systems and conditions. The database presented here contains approximately 2000 individual data points including both bulk rock and serpentine mineral geochemical data which are paired whenever possible. Measures of total Fe and Fe oxidation state, which are more limited, are compiled with characteristics of the systems from which they were sampled. Observations of trends in Fe chemistry in serpentinites and serpentines across the variety of geologic systems and parameters will aid in verifying and strengthening interpretations made on the basis of Fe chemistry. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.