[en] Pyrochlore-group minerals are the main concentrators of niobium in carbonatites of the Belaya Zima alkaline pluton. Fluorcalciopyrochlore, kenopyrochlore and hydropyrochlore were identified in chemical composition. Their main characteristics are given: compositional variation, morphology, and zoning. During evolution from early calcite to late ankerite carbonatites, the UO₂, TiO₂, REE, and Y contents gradually increased. All carbonatite types are suggested to contain initial fluorcalciopyrochlore. However, in calcite–dolomite and ankerite carbonatites, it is partially or completely hydrated due to hydrothermal processes at the late stage of the pluton. This hydration resulted in the appearance of kenopyrochlore and hydropyrochlore due to removal of Ca, Na and F, and input of Ba, H₂O, K, Si, Fe, and probably U and REE. At the last stage of the pluton, this hydrated pyrochlore was replaced by Fe-bearing columbite.

[en] Ankeritic alteration zones are associated with gold mineralisation in the Nugget vein system, Shotover valley, northwest Otago. These zones consist of sheared and brecciated schist in Miocene normal faults. The zones are also extensively silicified. Mineralisation resulted from passage of low-salinity, low-CO₂ water at 200-250 degrees C within a few kilometres of the surface. Alteration has involved replacement and veining of host pelitic schist in the mineralised zone. Most elements have been diluted by quartz and carbonate, with insignificant gains or losses. Ba has been leached as it became more soluble with decreasing fluid temperature. Considerable addition of Sr has occurred, as the fluid had higher Sr/Ca than most Otago mineralising fluids. Enrichment in Sr may be a distinctive feature of the Shotover style of mineralisation. Metallic mineralisation is due to addition of S, As, and Au during alteration. In contrast to many Otago gold vein systems, there has been no scheelite mineralisation. Carbonates have δ¹⁸O = +15.4 to +18.6, and δ¹³C = -8.2 to +0.5 per thousand. The oxygen isotopic signature is similar to other Otago gold veins. Carbon isotopes in aqueous CO₂ which deposited Nugget ankerites are generally heavier (by 2-4 per thousand) than those in other Otago gold vein systems. This is caused by changes in dissolved carbonate species at the lower temperature, and fluid/rock interaction during ankerite mineralisation. (author). 29 refs., 10 figs., 2 tabs

[en] Previous water-CO₂ interaction studies have studied few basalt types (rich in Ca, Mg, and Fe cations) to determine pyroxene, olivine and plagioclase reaction rates; however, limited research has examined the Deccan basalt. Therefore, in this study, basalt-CO₂-water-saturated interaction experiments and numerical simulations were performed under hydrothermal-like conditions. X-ray Diffraction (XRD) data revealed the appearance of calcite, aragonite, ankerite, huntite and siderite; additionally, smectite, chlorite, smectite/chlorite mixed layers and chabazite were also formed. SEM images showed that tiny calcite crystals developed over larger calcite crystals, incipient-disordered calcite formed with imperfections on its crystal faces and cubic chabazite crystals were bounded by smectite grains. Furthermore, these observations were confirmed by EDS analyses and mineral formulae calculations. Major shifts in the carbonate peaks observed at 155 and 282, 178, and 849 cm⁻¹ on Raman spectra confirmed the formation of calcite, dolomite and aragonite, respectively. Mixing trends between basalt and Ca-Mg-Fe carbonates and chlorite/smectite were observed in the case of phyllosilicates. However, ankerite, calcite and siderite recorded the enrichment of (i) Ca, (ii) Fe-Mg and (iii) Fe. High degrees of carbonation and progressive mineral growth were observed with the increasing pH of the solution. The formation of secondary carbonates predominated over that of silicates in short-term experiments; however, with increasing reaction time, the carbonates no longer persisted in the system, as they were dissolved and replaced by silicates. However, the degree of carbonation increased with the increasing pCO₂ and pH of the solution. Transition-state-theory (TST)-based numerical simulation models do not agree well with the results of experiments, as carbonate growth was greater in the former case. These models work well to predict the dissolution rates of most minerals but overpredict the growth of non-hydrous Mg carbonates at low temperatures. - Graphical abstract: Basaltic glass experiments and modelling of primary minerals with CO₂ and water revealed appearance of calcite, aragonite, siderite, smectite, chlorite, and chabazite. These results largely correspond to reaction modelling, explained by reaction progress, enthalpy and entropy. Display Omitted - Highlights: • Basalt-CO₂-water experiments and simulations revealed formation of calcite, aragonite, siderite and secondary silicates. • Simulation models do not fit well with the experiments. • Carbonates and secondary silicates formation is influenced by time, but, pCO₂, pH and temperature play subordinate role.

[en] Thermal decomposition of Bakal siderite ore (that consists of magnesium siderite and ankerite traces) was investigated by thermomagnetic analysis. Thermomagnetic analysis was carried-out using laboratory-built facility that allows automatic registration of sample magnetization with the temperature (heating/cooling rate was 65°/min, maximum temperature 650 °C) at low- and high-oxygen content. Curie temperature gradually decreases with each next cycles of heating/cooling at low-oxygen content. Curie temperature decrease after 2nd cycle of heating/cooling at high-oxygen content and do not change with next cycles. Final Curie temperature for both modes was ~320 °C. Saturation magnetization of obtained samples increases up to 20 Am"2/kg. The final product of phase transformation at both modes was magnesioferrite. It was shown that intermediate phase of thermal decomposition of Bakal siderite ore was magnesiowustite. - Highlights: • Mg-siderite decomposition was investigated by thermomagnetic analysis. • Magnetization and Curie temperature change with each next cycle of heating/cooling. • Magnesioferrite is the final phase of Mg-siderite thermal decomposition. • Transformation exclude the hematite formation.

[en] Carbon dioxide disposal into deep aquifers is a potential means whereby atmospheric emissions of greenhouse gases may be reduced. However, our knowledge of the geohydrology, geochemistry, geophysics, and geomechanics of CO₂ disposal must be refined if this technology is to be implemented safely, efficiently, and predictably. As a prelude to a fully coupled treatment of physical and chemical effects of CO₂ injection, the authors have analyzed the impact of CO₂ immobilization through carbonate mineral precipitation. Batch reaction modeling of the geochemical evolution of 3 different aquifer mineral compositions in the presence of CO₂ at high pressure were performed. The modeling considered the following important factors affecting CO₂ sequestration: (1) the kinetics of chemical interactions between the host rock minerals and the aqueous phase, (2) CO₂ solubility dependence on pressure, temperature and salinity of the system, and (3) redox processes that could be important in deep subsurface environments. The geochemical evolution under CO₂ injection conditions was evaluated. In addition, changes in porosity were monitored during the simulations. Results indicate that CO₂ sequestration by matrix minerals varies considerably with rock type. Under favorable conditions the amount of CO₂ that may be sequestered by precipitation of secondary carbonates is comparable with and can be larger than the effect of CO₂ dissolution in pore waters. The precipitation of ankerite and siderite is sensitive to the rate of reduction of Fe(III) mineral precursors such as goethite or glauconite. The accumulation of carbonates in the rock matrix leads to a considerable decrease in porosity. This in turn adversely affects permeability and fluid flow in the aquifer. The numerical experiments described here provide useful insight into sequestration mechanisms, and their controlling geochemical conditions and parameters

[en] The Shuangwang gold deposit, located in the Fengxian-Taibai fore-arc basin in the western Qinling Orogen of Central China, has yielded over 70 tons of gold. It is an orogenic gold deposit occurring in an NW-trending breccia belt. Most of the ores are hydrothermal breccia type containing fragments of adjacent strata cemented by ankerite and pyrite. Pyrite is the most abundant metallic mineral and the major gold-bearing mineral in the ores. A total of 58 pyrite samples from main ore bodies of the Shuangwang gold deposit have been analysed for 44 trace elements by HR-ICP-MS. Sb, Ba, Cu, Pb, Zn, Bi, Mo, Co are selected as indicator elements to investigate the potential usefulness of trace elements in pyrite as an indicator in gold exploration. The results show that the supra-ore halo elements Sb and Ba, which may have been more active than other near-ore halo elements and sub-ore halo elements, are best to characterize the shape of ore bodies. Five target areas are pointed out for deep ore exploration based on a comprehensive study of supra-ore, near-ore and sub-ore halos. This study provides evidence that trace elements in pyrite can be used to depict the deep extension of ore bodies and to vector towards undiscovered ore bodies.

[en] Thin section feature and electronic probe analysis of rocks show that middle-low temperature hydrothermal alteration has been developed in Baixintu uranium deposit, the alteration includes kaolinization, carbonatation, pyritization, hydromicazation, sericitization, hematization et.al. The uranium mineral of ore is maily coffinite, a little pitchblende and titanous uranium mineral and the ore is generally rich in phosphorus and titanium. Because uranium mineral coexists with colloidal, clumpy and fragiform pyrite and ankerite and hematite of alteration, it is suggested that the profitable uranium mineralization is closely related to hydrothermal process. (authors)

[en] Predictive uncertainty for reactive transport modeling of CO₂ geological storage arises due to high uncertainty in dissolution/precipitation rates. Here, the reactive transport modeling of the Frio sandstone formation is used as a case study. The major CO₂ trapping mineral is ankerite, while the main dissolution minerals are oligoclase and chlorite. In this context, unlike the commonly used local sensitivity analysis, the sensitivity analysis is global so that the potential co-operative effects among input parameters can be investigated. Nine key factors for kinetic rates and reactive mineral surface areas with respect to precipitating and dissolving minerals (only oligoclase) are considered. Sensitivity results from the Morris method show that the dissolution rate of oligoclase k^nu_O, and its reactive surface area A_O, are the most sensitive parameters, with the largest effects on CO₂ mineral capture. The variation of the total amount of CO₂ captured by minerals is pronounced with multiple model runs from Morris samples, which suggests that reactive surface areas and kinetic rates have significant impacts on CO₂ mineral sequestration. (authors)

[en] Orogenic hydrothermal systems in the South Island of New Zealand were active during Mesozoic and late Cenozoic collisional deformation and metamorphism of greywacke/schist terranes. Observations on the currently active mountain-building environment yield insights on processes occurring in the upper 5-15 km of the crust, and observations on an adjacent lithologically identical exhumed ancient mountain belt provide information on processes at 10-20 km in the crust. Hydrothermal fluids were mainly derived from metamorphic dehydration reactions and/or circulating topographically driven meteoric water in these mountain belts. Three geochemically and mineralogically different types of hydrothermal alteration and vein mineralisation occurred in these orogenic belts, and gold enrichment (locally economic) occurred in some examples of each of these three types. The first type of alteration involved fluids that were in or near chemical equilibrium with their greenschist facies host rocks. Fluid flow was controlled by discontinuous fractures, and by microshears and grain boundaries in host rocks, in zones from metres to hundreds of metres thick. Vein and alteration mineralogy was similar to that of the host rocks, and included calcite and chlorite. The second type of alteration occurred where the fluids were in distinct disequilibrium with the host rocks. Fracture permeability was important for fluid flow, but abundant host rock alteration occurred as well. The alteration zones were characterised by decomposition of chlorite and replacement by ankeritic carbonate in zones up to tens of metres thick. The mineralising fluid was deep-sourced and initially rock-equilibrated, with some meteoric input. The third type of mineralisation was controlled almost exclusively by fracture permeability, and host rock alteration was minor (centimetre scale). This mineralisation type commonly involved calcite and chlorite as vein and alteration minerals, and mineralisation fluids had a major meteoric water component. The three mineralisation types can be traced spatially and/or temporally from one to another with some overlap. The first type is characteristic of the deeper parts of an orogenic hydrothermal system, and this type gave way to the second type formed at shallower crustal levels, locally near to the surface. The third type of alteration is typically a late-stage, shallow-level phenomenon. (author). 58 refs., 9 figs., 3 tabs

[en] Igneous intrusions significantly affect the mineralogical composition of coal. The Jingxi Coalfield, North China was subjected to the intrusion of a Mesozoic aplite; however, the resulting coal mineralogy has not been well investigated. This paper reports on a study of the mineralogical composition of five coal bench samples collected from the No. M3 coal in the Jingxi Coalfield, which was intruded by an aplite sill along the roof. The minerals present in the highest proportions in the No. M3 coal are margarite–paragonite group mineral (21.4%) and ammonian illite (72.7%), followed by ankerite (4.2%), anatase (0.7%), and rutile (0.9%). The formation of margarite, paragonite and ammonian illite is attributed to the intrusion of the aplite sill. The margarite and paragonite were formed prior to the formation of the ammonian illite. The proportion of the margarite–paragonite group mineral decreases, and the proportion of ammonian illite increases from the sill to the roof. The Ca²⁺ and K⁺ are more easily incorporated into the aluminosilicate mineral lattices than Na⁺ and NH₄⁺, respectively. Thus, the Ca²⁺/Na⁺ ratio in the margarite–paragonite group mineral decreases, and the NH⁴⁺/K⁺ ratio in the ammonian illite increases from the sill to the roof. The formation of ankerite is also attributed to the igneous intrusion, but this mineral was formed later than the aluminosilicate minerals. The proportion of ankerite in the coal samples increases from the sill to the roof.