Dynamics of physical properties of soils and humus reserves in salted soils under desertification

The dynamics of the physical properties and reserves of humus in saline soils of an arid semi-desert zone is presented. The basis for the research was observational data for 2010-2022. in a typical hilly landscape of the Astrakhan region. A grid of 100 × 100 m was laid. Soil studies were carried out at the grid nodes along the sampling layers. We studied the change in physical properties (soil moisture; salt content according to the amount of dense residue; soil density; filtration coefficient; soil porosity; moisture reserve), humus reserves and salt state of saline soils under desertification. Comparative analysis showed that negative changes have taken place in the study area and it can be stated that the processes of soil degradation and desertification are intensively developing. It has been established that in the soil layer of 0–40 cm there was a significant reduction in soil moisture reserves against the background of an increase in maximum air and soil surface temperatures, as well as changes in the hydrological regime because of the embankment of the territory. Dehumification processes are observed. The most clearly negative trend was revealed for meadow solonchaks. The change in the hydrological regime of the territory because of embankment contributes to an increase in the areas of automorphic soils in the landscape and a reduction in the areas of meadow soils. An increase in the degree of salinity and compaction was recorded for automorphic zonal soils. The physical destruction of the Baer's hills exacerbates the processes of degradation and the development of desertification in such conditions

Gold remobilisation and formation of high grade ore shoots driven by dissolution-reprecipitation replacement and Ni substitution into auriferous arsenopyrite

Both gold-rich sulphides and ultra-high grade native gold oreshoots are common but poorly understood phenomenon in orogenic-type mineral systems, partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations. The world-class Obuasi gold deposit, Ghana, has gold-rich arsenopyrite spatially associated with quartz veins, which have extremely high, localised concentrations of native gold, contained in microcrack networks within the quartz veins where they are folded. Here, we examine selected samples from Obuasi using a novel combination of quantitative electron backscatter diffraction analysis, ion microprobe imaging, synchrotron XFM mapping and geochemical modelling to investigate the origin of the unusually high gold concentrations. The auriferous arsenopyrites are shown to have undergone partial replacement (~15%) by Au-poor, nickeliferous arsenopyrite, during localised crystal-plastic deformation, intragranular microfracture and metamorphism (340-460 °C, 2 kbars). Our results show the dominant replacement mechanism was pseudomorphic dissolution-reprecipitation, driven by small volumes of an infiltrating fluid that had relatively low fS2 and carried aqueous NiCl2. We find that arsenopyrite replacement produced strong chemical gradients at crystal-fluid interfaces due to an increase in fS2 during reaction, which enabled efficient removal of gold to the fluid phase and development of anomalously gold-rich fluid (potentially 10 ppm or more depending on sulphur concentration). This process was facilitated by precipitation of ankerite, which removed CO2 from the fluid, increasing the relative proportion of sulphur for gold complexation and inhibited additional quartz precipitation. Gold re-precipitation occurred over distances of 10 µm to several tens of metres and was likely a result of sulphur activity reduction through precipitation of pyrite and other sulphides. We suggest this late remobilisation process may be relatively common in orogenic belts containing abundant mafic/ultramafic rocks, which act as a source of Ni and Co scavenged by chloride-bearing fluids. Both the preference of the arsenopyrite crystal structure for Ni and Co, rather than gold, and the release of sulphur during reaction, can drive gold remobilisation in many deposits across broad regions

A numerical criterion for existence of the equilibrium state in an open chemical system. Un critère numérique de l'existence d'un état d'équilibre pour un système chimique ouvert

A numerical criterion for verifying attainability of equilibrium in an open chemical system is proposed. The criterion may also be used for testing compatibility of minerals (constant composition phases) in given phase assemblages.On propose un critère numérique pour vérifier que l'équilibre d'un système chimique ouvert peut être atteint. Il est possible aussi d'utiliser ce critère pour tester la possibilité de la coexistence des minéraux (phases à composition constante) dans une association donnée.Shvarov Yuri. A numerical criterion for existence of the equilibrium state in an open chemical system. Un critère numérique de l'existence d'un état d'équilibre pour un système chimique ouvert. In: Sciences Géologiques. Bulletin, tome 42, n°4, 1989. Thermodynamique des processus naturels. Thermodynamics of natural processes pp. 365-369

K2GWB: utility for generating thermodynamic data files for the geochemist's Workbench® at 0-1000 °C and 1-5000 °C bar from UT2K and the UNITHERM database

The Geochemists Workbench® (GWB) is a versatile geochemical modelling software package capable of reaction path modelling, equilibrium fluid speciation and activity diagramming, although it is restricted by the availability of thermodynamic (log K) data files with a temperature range of 0-300 °C and pressures on the critical curve for water. The K2GWB (log K to GWB) utility is used to produce a usable Geochemists Workbench data file from a generic log K and system file generated by the UT2K utility complementing the UNITHERM thermodynamic database system. The GWB data files can be generated for pressure temperature conditions within the bounds of the modified Helgeson Kirkham Flowers model (0-1000 °C, 1-5000 bar). Data can be generated with any combination of pressure temperature log K grids required, allowing the Geochemists Workbench to be used to model a wide range of geological scenarios under polythermal and polybaric conditions, as well as the generation of activity diagrams over a wide range of pressures and temperatures. A comparative HCh and Geochemists Workbench model using the same thermodynamic dataset was run to benchmark test the validity of the new data files. Modelling the cooling of 2 molal NaCl-H2O fluid in equilibrium with a ganodiorite-type rock from 600 to 300 °C gave close to identical results between the two codes, validating the thermodynamic data conversion process described here. An investigation of the speciation of Au across a temperature pressure gradient from 550 °C and 1500 bar to 150 °C and saturated vapor curve pressures has been used to illustrate the usefulness of coupling thermodynamic data file generation at wide temperatures and pressures from a central database and the functionality of The Geochemists Workbench

Role of water/rock interaction in the formation of ore-bearing solutions and deposition of hydrothermal ore, Sadon Mining District, North Caucasus Mountains, Russia

REE distribution patterns of the ores and host rocks of the Dzhimidon vein lead-zinc deposit (North Caucasus, Ossetia, Sadon mining district, Russia) have been analyzed to elucidate the source(s) of hydrothermal ore deposits. Two types of prevailing rocks are involved in ore formation - Paleozoic granites (the main ore-hosting rocks at the majority of deposits) and Precambrian schists (specific only the for host rocks of the Dzhimidon deposit). The source of ore components tends to be complex and includes host rocks in variable proportions that could be characterized by REE distribution in ores. Interaction of water with combined sources was thermodynamically modeled. Critical differences were found in the ore-forming models, with variable sequence and rock proportions during interaction with barren fluid

Experimental determination of calcite solubility and the stability of aqueous Ca– and Na–carbonate and –bicarbonate complexes at 100–160 °C and 1–50 bar pCO2 using in situ pH measurements

International audienceThe solubility of calcite was measured at 100, 120, 140 and 160°C at 1-50 bar pCO2 in10-3-0.1 mol·kg-1 NaCl solutions using a new experimental setup involving in situ pH measurements with high-temperature solid-contact H-selective glass and two types of reference electrodes: i) Ag/AgCl in 3.5 M KCl, saturated AgCl placed in a Teflon extensible container with liquid junction, and ii) solid-contact high-T Na-selective glass electrode in the cell without liquid junction. The stability constants of NaHCO3° and NaCO3-aqueous complexes formation were determined in NaCl-Na2CO3/NaHCO3 solutions in CO2-free media and under 10 bar pCO2 from 100 to 160 °C. These values allowed calculation of the pH of the calibration solution in the system NaCl-CO2-H2O used in the cell without liquid junction with Na+-selective electrode as a reference. This highly stable, low-cost electrode system can be recommended for routine pH measurements at 4 < pH < 10 in sodium-bearing solutions up to 160°C and the critical point of CO2. The values of the stability constants of CaCO3° and CaHCO3+ aqueous complexes and calcite solubility product were generated at 100, 120, 140 and 160 °C allowing a description of the solubility of calcite in a wide range of pH and pCO2