Geological Fluid Research Articles

Ore-forming fluids of the Fancha gold deposit, Lingbao, Henan Province

The Fancha gold deposit is a representative of the auriferous quartz vein type gold deposits in the Xiaoqingling gold district where the ore-forming fluid is one of the key controlling factors in the deposits formation, and also the main focus of ore deposit research. Here, the geological characteristics and fluid inclusions of the Fancha deposit are presented in detail, with a description of the potential origin and evolution of the fluid and the genesis of the deposit. Field investigation and petrographic observation show that ore-bodies in the Fancha deposit are preferentially hosted in the amphibolites of the Taihua Group and are controlled by brittle fracture. Hydrothermal alteration in the Fancha deposit includes silicification, potash feldspathization, sericitization, pyritization and carbonatization. The mineral assemblage, cross-cutting and replacement relationships indicate that the ore-forming process can be divided into quartz-K-feldspar (I), quartz-coarse pyrite (Ⅱ), quartz-fine pyrite (III), quartz-polymetallic sulfide (IV) and carbonate stages (V). Gold is mainly precipitated in stages III and IV. Three types of fluid inclusions have been identified in the Fancha deposit, based on their characteristics at room temperature: three-phase CO2-rich, three-phase CO2-bearing and two-phase aqueous inclusions. The fluid inclusions demonstrate the modes of homogenization temperatures of 363.3℃, 325.5℃, 273℃, 258.62℃ and salinities of 14.53, 9.59, 11.61, 8.03 wt% NaCl.equiv., for stages II, III, IV, and V respectively. The ore-forming fluid in the Fancha deposit belonged to the CO2-H2O-NaCl-(±CH4) fluid system with mid-low temperature and mid-low salinity. The δ18OH2O of the metallogenic fluids was between 1.36 and 6.28, and the δDH2O was between -87 and -53.1. Hydrogen and oxygen isotope data indicate that the ore-forming fluids were dominated by metamorphic fluids in the early stage, and that the metamorphic fluid gradually mixed with circulating meteoric water during the later stages. Petrographic observation and microthermometry suggest that fluid mixing and immiscibility obviously occurred during the main metallogenic stage. Fluid mixing and fluid immiscibility probably led to gold deposition in the Fancha deposit.

Gravity‐Induced Bubble Ripening in Porous Media and Its Impact on Capillary Trapping Stability

AbstractCapillary or residual trapping is considered one of the safest geologic CO2 storage mechanisms due to its hydrodynamic stability. We present a first study of the impact of gravity on Ostwald ripening in porous media and show it may render capillary trapping thermodynamically unstable. Gravity induces a vertical chemical potential gradient that leads to the upwards diffusion of CO2. Thus, bubbles at shallower depths grow at the expense of bubbles in deeper strata, leading to the formation of an overriding gas cap. We first develop a pore‐scale model for two bubbles trapped within adjacent pores and then upscale it to obtain a one‐dimensional continuum model. We use the latter to predict the macroscopic evolution of a trapped bubble population. Factors controlling the ripening process are isolated to assist in selecting CO2 storage sites. Gravity‐induced ripening may also play a role in geologic fluid emplacement and migration over millions of years.

Equation of state of the fluid system H2O-CO2-CaCl2 and properties of fluid phases at P-T parameters of the middle and lower crust

A numerical thermodynamic model is proposed for one of the most important geological fluid system – triple system H2O-CO2-CaCl2 at P-T conditions of the middle and lower crust, as well as for the crust-mantle boundary. The model is based on the equation for concentration dependence of the excess Gibbs free energy, proposed earlier, and for the first time obtained P-T dependencies of the coefficients of the equation of state (EOS) expressed via molar volumes of the components. The EOS allows predictions of the properties of the fluid, participating in the majority of the processes of depth petrogenesis: its phase state (homogeneous or multi-phase), densities of the fluid phases, concentrations of the components in the co-existing phases, and chemical activities of the components. The model precisely reproduces all available experimental data on the phase state of the ternary fluid system H2O-CO2-CaCl2 in the ranges of temperature 773.15–1073.15 K and pressures 0.1-0.9 GPa and allows, as well, correct application of the EOS beyond the experimentally studied domain of temperatures and pressures – namely up to P = 2 GPa and up to T = 1673.15 K. The possibility of the correct extrapolation of our EOS is ensured by using the parametrization of P-T dependencies via the molar volume of water. The latter remains in the experimental domain of values or very near to its boundaries, when increasing temperatures and pressures.

Equation of State of the H2O–CO2–CaCl2 Fluid System and Properties of Fluid Phases at Р-Т Parameters of the Middle and Lower Crust

A numerical thermodynamic model is proposed for one of the most important geological fluid system, ternary H2O–CO2–CaCl2 system, at P-T conditions of the middle and lower crust and crust–mantle boundary. The model is based on the previously proposed equation for concentration dependence of the excess Gibbs free energy and on the first obtained P-T dependencies of the coefficients of the equation of state (EOS) expressed via molar volumes of the components. The EOS allows to predict the properties of the fluid participating in the majority of deep petrogenetic processes: its phase state (homogeneous or multi-phase), densities of fluid phases, concentrations of components in the co-existing phases, and the chemical activities of the components. The model precisely reproduces all available experimental data on the phase state of the ternary H2O–CO2–CaCl2 fluid system in the ranges of temperatures 773.15–1073.15 K and pressures 0.1–0.9 GPa and also allows the correct application of the EOS beyond the experimentally studied range of temperatures and pressures up to P = 2 GPa and T = 1673.15 K. The possibility of the correct extrapolation of our EOS is ensured by using the parametrization of P-T dependencies via the molar volume of water. The latter remains in the experimental domain of values or falls slightly beyond its boundaries, when increasing temperatures and pressures.

Determination of CO2 Solubility in Water and NaCl Solutions under Geological Sequestration Conditions Using a Fused Silica Capillary Cell with in Situ Raman Spectroscopy

To investigate the solubility of carbon dioxide (CO2) in geological fluids at different temperatures and pressures, we developed a method for measuring solubility using an optically transparent fus...

Nanotopographic characterization of microfractures in rocks by Atomic Force Microscopy

The study of microfractures is one of the keys to understand a variety of geological issues as diverse as seismic rupturing mechanisms, process zone characterization and evolution toward large-scale fracturing, characterization of reservoirs of geological fluids, or identification of microhabitats outside Earth. In earlier works, microcrack initiation and propagation in rocks has been observed at microscale. We test Atomic Force Microscopy (AFM) as a tool to retrieve nanoscale digital elevation models (nDTM) of rock surfaces. Basalt samples from two different geological environments are studied. In a basalt from Siberia, the topography of the granular material infilling of the crack can be accurately characterised, and the pristine crack shape retrieved, opening access to an estimate of the volume of the infilling material. In a basalt from Ethiopia, mixed-mode fracturing (opening-shear) is identified from nanofracture topography, as well as a textbook example of along-strike variations of fracture displacement. These observations demonstrate that fractures already initiate and propagate at nanoscale, and AFM is a powerful tool to conduct quantitative structural geology analyses with a resolution of ∼1 nm.

Uranium-rich diagenetic fluids provide the key to unconformity-related uranium mineralization in the Athabasca Basin

The Proterozoic Athabasca Basin is well known for its unusually large-tonnage and high-grade ‘unconformity-related’ uranium (U) deposits, however, explanations for the basin-wide U endowment have not been clearly identified. Previous studies indicate that U-rich brines with up to ~600 ppm U and variable Na/Ca ratios (from Na-dominated to Ca-dominated) were present at the sites of U mineralization, but it is unknown whether such fluids were developed solely in the vicinity of the U deposits or at a basinal scale. Our microthermometric and LA-ICP-MS analyses of fluid inclusions in quartz overgrowths from the barren part of the basin indicate that U-rich brines (0.6 to 26.8 ppm U), including Na-dominated and Ca-dominated varieties, were widely developed in the basin. These U concentrations, although not as high as the highest found in the U deposits, are more than two orders of magnitude higher than most naturally occurring geologic fluids. The basin-scale development of U-rich diagenetic fluids is interpreted to be related to several geologic factors, including availability of basinal brines and U-rich lithologies, and a hydrogeologic framework that facilitated fluid circulation and U leaching. The combination of these favorable conditions is responsible for the U fertility of the Athabasca Basin.

Hydrochemistry as a tool for interpreting brine origin and chemical equilibrium in oilfields: Zubair reservoir southern Iraq case study

The oil reservoir brines are very common geological fluids coexisting with hydrocarbons. The chemistry of brines is a powerful tool for determining the brine evolution and its origin, constraining fluid flow at the basin and predicting reservoir scales. This study targeted the Zubair brines in seven oilfields and investigated the geochemical evolution of brine compositions. The composition of the Zubair brine is characterized by the high average of TDS (219,408 mg/l). The contribution of cations as epm% are Na (73.7), Ca (17.32), Mg (7.45), and K (1.27), while anions contribute as Cl (99.6), SO4 (0.23), HCO3 (0.08), and CO3 (0.005). The Zubair brines are of Na–Ca–chloride type, whereas sodium content is 6.3 times greater than seawater; Ca and Mg contents are thirty-four and three times greater, respectively, while Cl is seven times greater. The SO4 ion is depleted to 0.16, due to a biodegradation. The salinity of the Zubair brine is six times more than that of seawater. The Zubair brines are characterized by an acidic pH (5.24–5.68) with a specific gravity of 1.1436, hydrocarbon saturation in pore spaces of 39%, and water saturation of 61%. The mineral saturation model indicates that Zubair brines are unsaturated. The predicted hypothetical salts are NaCl (76%), CaCl2 (15%), and MgCl2 (7.5%). The salts Ca(HCO3)2, CaSO4, and KCl together form only 1.6%. The Zubair brines are characterized by Cl/Br (305) in average, greater than that of seawater (289) confirming the fluids (brines and hydrocarbons) migrated upwards from Sargelu Formation to Zubair reservoir through fractures and cracks in Gotnia Formation.

Tracing the palaeoredox conditions at Forsmark, Sweden, using uranium mineral geochronology

U-Pb isotope systems have been used to constrain the timing of formation, alteration, and oxidation of U minerals from the meta-granitic bedrock at Forsmark, eastern Sweden. Secondary ion mass spectrometry (SIMS) has been used to collect U-Pb data from uraninite. Discordant data suggest a ~1.8 Ga emplacement of uraninite-bearing pegmatites and an event of uraninite alteration at ~1.6 Ga. The latter age is contemporaneous with the Gothian orogeny in Scandinavia, which was associated with hydrothermal fluid circulation in the Fennoscandian Shield. Ca-uranyl silicates haiweeite and uranophane predominately formed 1.3–1.2 Ga, contemporaneous with the emplacement of the Satakunta complex of the Central Scandinavian Dolerite Group. A Palaeozoic group of Ca-U(VI)-silicates is also present, which indicates that the geochemical composition of geologic fluids was heterogeneous throughout the fracture network during this time. Low Pb concentrations in the U(VI) silicates of several samples are compatible with a recent (<100 Ma) alteration or precipitation of these minerals in connection to reaction with carbonate-rich fluids. The results support a geologically early oxidation of U(IV) to U(VI) and provide insight into the palaeoredox conditions that may impart an on-going influence on the mobility of natural U in the Forsmark fracture network.

Direct analyses of fluorine in aqueous fluids extracted from 1-GPa experiments

Fluorine has been proposed as a useful geochemical tracer for tracking the transport of volatile elements in subduction zones as it is the last volatile element to be exsolved during magma ascent and crystallization. Accurate determination of the concentration of fluorine in geological fluids under high P-T conditions would greatly further the understanding of its geochemical behavior during aqueous transport and magma genesis in arcs. Mass balance is the most widely used method for calculating fluid compositions in hydrothermal experiments. However, the method is inaccurate when phase compositions and/or proportions have large uncertainties. In the current study, we present an alternative method based on the direct measurement of the concentration of F in fluids by extracting the enclosed liquid within experimental samples, and by performing high-pressure liquid chromatography (HPLC) analysis. We then compared the results of the direct analysis with those obtained by mass-balance calculations. Our experimental data demonstrated that direct analysis yielded more accurate concentration values than mass-balance calculation. The direct method is a useful alternative, when i) the phase assemblage is complex, ii) the total F content or the fluid proportion is low, or iii) when there is significant presence of hydrous melts. Caution should be exercised when applying direct analysis to samples that contained insoluble quench phases and/or were low in fluid. Direct HPLC analysis, combined with an improved extraction method that ensures the separation of quench phase, minerals and melts, could be of great utility for studying water-rock reactions.

In situ carbon isotopic exploration of an active submarine volcano

The geologic and biogeochemical cycling of carbon in deep ocean environments has important implications for our understanding of the functioning of Earth systems across a wide range of spatial and temporal scales. To improve our ability to study the cycling and fluxes of carbon in the deep ocean, new technologies are emerging for making in situ measurements of carbon compounds over a range of environmental contexts. Within many of these deep-sea environments, fluxes of carbon compounds often occur as either venting fluids or rising gas bubbles. Key compounds of interest include methane (CH4), dissolved inorganic carbon (DIC), and carbon dioxide (CO2) – a component of DIC. In particular, measurement of the carbon isotopic composition (δ13C) of these pools can offer a better understanding of the nature of sources, fluxes, and cycling processes involving these compounds. Here we present the advancement of an in situ laser spectrometer (initially developed for measurement of δ13CCH4 only) into a sensor that can measure δ13C of both CH4 and CO2 in both deep-sea bubble plumes as well as geologic fluids. We present results of a 2014 telepresence-enabled investigation of a back arc submarine volcano (Kick’em Jenny) in the Caribbean Sea. In situ isotopic analysis of both bubbles and fluids suggest a primarily thermogenic origin for CH4 and a magmatic origin for CO2, yet highlight the occurrence of some heterogeneities indicating locally elevated contribution of organic matter to DIC fluxes.

Structure, acidity, and metal complexing properties of oxythioarsenites in hydrothermal solutions

In the pursuit of understanding the aqueous chemistry of oxythioarsenites (i.e., H3AsO2S and H3AsOS2) on a microscopic scale, first principles molecular dynamics (FPMD) simulations were performed to investigate the hydration structures, acidity constants, and metal-complexing properties of these species in aqueous solutions at temperatures up to 573K. The simulations showed that the oxythioarsenite species are stable and have trigonal pyramidal structures in both low- and high-temperature fluids. Their solvated structures were characterized in detail and it was found that the S or H atoms of the SH ligands form very weak H-bonds with the solvating water molecules and the OH ligands form H-bonds with water as both donors and acceptors. Furthermore, the dangling S or O sites act as strong acceptors to form H-bonds. Their acidity constants were calculated by using the FPMD-based vertical energy gap method. Together with our previous work (Liu et al., 2015, Chemical Geology, 411, 192–199), we found that in the arsenite-thioarsenite series (i.e., H3AsO3−xSx for x varying from 0 to 3), the pKa values of the oxythioarsenite species (H3AsO2S and H3AsOS2) were always located between those of the end members, H3AsO3 and H3AsS3, and decreased with the increasing temperature up to 573K. Based on the calculated acidity constants, the speciation versus pH distributions were obtained at temperatures ranging from ambient temperature to 573K. It was found that both H2AsO2S− and H2AsOS2− are the predominant species at near neutral pH for H3AsO2S and H3AsOS2 species, respectively. In addition, the structures and the dissociation free energy of the oxythioarsenite-metal complexes at 573K indicated that these As-S moieties are effective ligands for complexing with ore-forming metals in geological fluids.

On the statics and dynamics of fully confined bubbles

We investigate theoretically the statics and dynamics of bubbles in fully confined liquids, i.e. in liquids surrounded by solid walls in all directions of space. This situation is found in various natural and technological contexts (geological fluid inclusions, plant cells and vessels, soil tensiometers, etc.), where such bubbles can pre-exist in the trapped liquid or appear by nucleation (cavitation). We focus on volumetric deformations and first establish the potential energy of fully confined bubbles as a function of their radius, including contributions from gas compressibility, surface tension, liquid compressibility and elastic deformation of the surrounding solid. We evaluate how the Blake threshold of unstable bubble growth is modified by confinement and we also obtain an original bubble stability phase diagram with a regime of liquid superstability (spontaneous bubble collapse) for strong confinements. We then calculate the liquid velocity field associated with radial deformations of the bubble and strain in the solid, and we predict large deviations in the kinematics compared to bubbles in extended liquids. Finally, we derive the equations governing the natural oscillation dynamics of fully confined bubbles, extending Minnaert’s formula and the Rayleigh–Plesset equation, and we show that the compressibility of the liquid as well as the elasticity of the walls can result in ultra-fast bubble radial oscillations and unusually quick damping. We find excellent agreement between the predictions of our model and recent experimental results.

Polyaxial stress-dependent permeability of a three-dimensional fractured rock layer

A study about the influence of polyaxial (true-triaxial) stresses on the permeability of a three-dimensional (3D) fractured rock layer is presented. The 3D fracture system is constructed by extruding a two-dimensional (2D) outcrop pattern of a limestone bed that exhibits a ladder structure consisting of a “through-going” joint set abutted by later-stage short fractures. Geomechanical behaviour of the 3D fractured rock in response to in-situ stresses is modelled by the finite-discrete element method, which can capture the deformation of matrix blocks, variation of stress fields, reactivation of pre-existing rough fractures and propagation of new cracks. A series of numerical simulations is designed to load the fractured rock using various polyaxial in-situ stresses and the stress-dependent flow properties are further calculated. The fractured layer tends to exhibit stronger flow localisation and higher equivalent permeability as the far-field stress ratio is increased and the stress field is rotated such that fractures are preferentially oriented for shearing. The shear dilation of pre-existing fractures has dominant effects on flow localisation in the system, while the propagation of new fractures has minor impacts. The role of the overburden stress suggests that the conventional 2D analysis that neglects the effect of the out-of-plane stress (perpendicular to the bedding interface) may provide indicative approximations but not fully capture the polyaxial stress-dependent fracture network behaviour. The results of this study have important implications for understanding the heterogeneous flow of geological fluids (e.g. groundwater, petroleum) in subsurface and upscaling permeability for large-scale assessments.

Origins and differences in condensate gas reservoirs between east and west of Tazhong uplift in the Ordovician Tarim Basin, NW China

The Ordovician of the Tazhong area in the Tarim Basin has suffered multi-cyclic hydrocarbon charging, making Tazhong a typical condensate gas district. In this paper, production and test data were gathered and a detailed comparison was conducted on the geology and the fluid distribution and characteristics between the eastern and western Tazhong area. Eastern and western regions exhibit significant differences in tectonic structure, fluid distribution, and physical-chemical properties of oil and gas. Compared with the eastern region, the western part has a greater development of discordogenic gas associated with strike-slip faults which, combined with the Tazhong No. 1 fault zone, control the fluid distribution. The eastern region is mainly controlled by the Tazhong No. 1 fault zone. Fluid have markedly homogeneous properties in the east, but are heterogeneous in the west. The origins of oil and gas are different between the east and the west. In the east, hydrocarbons are mainly from Ordovician source rocks and natural gas is mostly derived from kerogen pyrolysis. In the west, the hydrocarbons mainly originated from Cambrian source rocks, and the gas was mostly generated by crude oil cracking. In sum, the east region is dominated by primary condensate gas reservoirs, and the western region is dominated by secondary condensate gas reservoirs. Because of the different geological settings and fluid physical properties, differences in the condensate gas reservoirs in the eastern and the western Tazhong area have been analyzed, and appropriate formation mechanisms for condensate gas origins are established.

First evidence of the trisulfur radical ion S3− and other sulfur polymers in natural fluid inclusions

Sulfur plays a key role in numerous processes occurring in the Earth's crust. However, its speciation in deep and hot geological fluids remains poorly constrained. Here, we used quantitative in-situ Raman spectroscopy on natural fluid inclusions from deep sedimentary environments where thermochemical sulfate reduction (TSR) occurred to determine the sulfur speciation at temperatures representative of their entrapment conditions (100–300°C). Results unambiguously demonstrate the presence of the trisulfur ion S3− and other polymeric S species (Sn2−±Sn0) at temperature (T)>100°C, whereas only sulfide and sulfate were detected at 25°C. From 200 to 300°C, sulfate and sulfide, the two dominant S species, contribute to 41±9% and 59±9% of the mean total dissolved S concentration ([Stot]=0.25mol/kgH2O=0.8wt%), respectively. The S3− concentration accounts for 0.2 to 3% of Stot in this T range, with a maximum recorded concentration of 2.9×10−2mol/kgH2O (2780ppm) at 300°C. This observation implies that the TSR process occurs under physico-chemical conditions that enhanced the stability of S3− and other polymeric S species. This conclusion has important consequences for the genesis of base metal sulfide deposits and sour gas fields where reduced sulfur originates from TSR.

Geological fluid flow in sedimentary basins

For over 50 years, sedimentary basins have been considered as the lithosphere's surface film, belonging to the subsurface domain and containing the vast majority of accessible mineral and energy resources. Beyond their human use, sedimentary basins are more importantly the ultimate exchange interface between the earth's main reservoirs. Firstly, between the upper lithosphere and the atmosphere-hydrosphere reservoirs, exchanges are mainly vertical. Next, between the onshore reservoirs, i.e., on the continental part, and the offshore reservoirs, i.e., in the submerged part of the margins, exchanges are lateral and may take place over great distances. Unexpected low accumulations and/or dry...

Hydrothermal mineral replacement reactions for an apatite-monazite assemblage in alkali-rich fluids at 300–600 °C and 100 MPa

Mineral replacement reactions are common in the various environments where rocks have undergone re-equilibration with geologic fluids. Replacement reactions commonly take the form of fluid-aided, coupled dissolution-precipitation and often result in pseudomorph formation. One class of environment that frequently shows significant examples of mineral replacements is hydrothermal ore deposit systems. The goal of this study was to test the simultaneous reactivity of fluorapatite and monazite in Na- and Si-rich hydrothermal fluids, which partially mimic the mineralogy and fluid chemistry of the Llallagua tin deposit in Bolivia. A series of experiments were performed at 300 to 600 °C and 100 MPa, utilizing various combinations of monazite, fluorapatite, and H 2 O + Na 2 Si 2 O 5 . Reaction products were evaluated using scanning electron microscopy, electron microprobe analysis, and single-crystal X-ray diffraction. The results of this experimental study show that fluorapatite and monazite are differentially reactive under the conditions studied. The reaction products, pathways, and kinetics have a large temperature dependence. The 300 and 400 °C experiments show variable amounts of monazite replacement and only minor, if any, dissolution or reactivity of fluorapatite. The high-temperature 500 and 600 °C experiments are characterized by massive replacement of monazite by vitusite and britholite. Exclusively at 600 °C, monazite alteration takes the form of symplectite development at the reaction front as vermicular intergrowths of vitusite and britholite. The higher-temperature experiments also show substantially more reactivity by fluorapatite, which is partially pseudomorphically altered into britholite. This is an example of regenerative mineral replacement where both fluorapatite and britholite share the same atomic structure and are crystallographically coherent after the partial replacement. The britholite replacement is characterized by the presence of oriented nanochannels, which facilitate fluid-based mass transfer between the bulk solution and the reaction front. The fluorapatite replacement is enhanced by monazite alteration through a self-perpetuating, positive feedback mechanism between these two reactions, which enhance the REE mobility in alkali-bearing fluids and further drives bulk re-equilibration. These results have potential geochronologic implications and may be significant in the evaluation of monazite and fluorapatite as potential solid nuclear waste forms. They also give us deeper insights into the mechanism of mineral replacement reactions and porosity development.

The role of carbon dioxide in the transport and fractionation of metals by geological fluids

Although carbon dioxide is one of the major components of crustal fluids responsible for ore deposit formation, its effect on transport and precipitation of metals remains unknown, due to a lack of direct experimental data and physical–chemical models for CO2-rich fluids. To fill this gap, we combined laboratory experiments and thermodynamic modeling to systematically quantify the role played by CO2 for the solubility of economically important metals such as Fe, Cu, Zn, Au, Mo, Pt, Sn under hydrothermal conditions. Solubility measurements of common ore minerals of these metals (FeS2, CuFeS2, ZnS, Au, MoS2, PtS, SnO2) were performed, using a flexible-cell reactor equipped with a rapid sampling device, in a single-phase fluid (CO2–H2O–KCl) at 350–450°C and 600–750bar, buffered with iron sulfide and oxide and alkali-aluminosilicate mineral assemblages. In addition, another type of experiments was conducted to measure gold solubility in more sulfur-rich supercritical CO2–H2O–S–NaOH fluids at 450°C and 700bar using a batch reactor that allows fluid quenching. Our results show that the solubilities of Si, Au, Mo, Pt and Cu either decrease (within <1log unit) or remain constant upon CO2 increase, whereas those of Fe, Zn and Sn increase significantly (>1log unit) with CO2 contents in the fluid increasing from 0 to 50wt%. These data were interpreted using a simple model that does not require any new adjustable parameters, and is based on the dielectric constant of the H2O–CO2 solvent and on the Born solvation parameter for the dominant metal-bearing species in an aqueous fluid. Our predictions using this model suggest that in a supercritical CO2–H2O–S-salt fluid typical of metamorphic Au deposits, in equilibrium with pyrite and chalcopyrite, the Cu/Fe ratio decreases by up to 2 orders of magnitude with an increase of CO2 content from 0 to 70wt%. This effect is due to the decrease of the fluid dielectric constant in the presence of CO2, which favors the stability of neutral species (FeCl20) compared to charged ones (CuCl2−). Our results explain the Fe enrichment and Cu depletion in metamorphic gold deposits formed by CO2-rich fluids. The transport of gold is unfavorable in the presence of CO2 only in S-rich (>0.5wt%S) fluids in which Au forms the negatively charged Au(HS)2− and Au(HS)S3− complexes. By contrast, it is only weakly affected in S-poor (<0.1wt%S) acidic-to-neutral fluids in which the uncharged Au(HS)0 complex predominates. Thus, even at very high CO2 contents (>50wt% CO2), the capacity of such fluids to transport gold (up to 100s ppb Au) remains comparable to that of aqueous fluids. These findings are in agreement with analyses of natural fluid inclusions in metamorphic deposits. In more saline oxidizing and S-rich fluids such as those in magmatic porphyry Cu–Au deposits, the Fe, Cu, and Au solubilities in the presence of CO2 decrease by ∼1 order of magnitude with CO2 increasing to 20–30wt%, following the decrease in the stability of their dominant charged species (FeCl42−, CuCl2−, Au(HS)2− and Au(HS)S3−), but stay almost constant at higher CO2 contents (30–70wt%) as controlled by the neutral species (FeCl20, Cu(HS)0 and Au(HS)0). Such solubility trends suggest a new potential trigger of ore precipitation in porphyry systems by CO2 pulses from the magmatic chamber, which may operate along with commonly admitted depositional mechanisms such as cooling, vapor-brine immiscibility, and water–rock interaction. The direct effect of CO2 on the mobility of Pt and Mo, metals that likely form hydrogen sulfide and oxy-hydroxide complexes, respectively, is expected to be weak in most settings. Among the studied elements, Sn is the only one whose solubility may be favored at high CO2 content (>20wt%) due to carbonate complexing. This study demonstrates, for the first time, that, contrary to common belief, the presence of CO2 in a supercritical fluid may lead to enhanced mobility or, on contrary, to massive precipitation of some metals, depending on salinity and sulfur content, and, more generally, to significant fractionations between different metals.

Spectral quasilinear numerical simulation of micropolar convective wall plumes in high permeability porous media

Laminar natural convection plume of a microstructural non-Newtonian fluid along a vertical surface about a line heat source in a saturated high permeability porous medium is studied. The transformed non-linear boundary value problem is solved numerically using a rigorously tested SQLM algorithm, which combines a spectral collocationmethod with the quasilinearization method (QLM). The effects of Grashof number, Prandtl number, Darcy number and Eringen micropolar rheological material parameters are examined. Excellent stability and convergence of the spectral method is demonstrated. Validation of solutions with the Keller box finite difference is included. Applications of the study arise in geological (petroleum) fluid dynamics.