Next Article in Journal
UV Induced Photocatalytic Degradation of Caffeine Using TiO2–H-Beta Zeolite Composite
Next Article in Special Issue
The Role of Te, As, Bi, and Sb in the Noble Metals (Pt, Pd, Au, Ag) and Microphases during Crystallization of a Cu-Fe-S Melt
Previous Article in Journal
Editorial for Special Issue “Composition, Geochronology and Geodynamic Implications of Igneous Rock”
Previous Article in Special Issue
Au-Ag-Se-Te-S Mineralization in the Maletoyvayam High-Sulfidation Epithermal Deposit, Kamchatka Peninsula
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Minerals
Volume 13
Issue 4
10.3390/min13040464
minerals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Comparative Experiments on the Role of CO2 in the Gold Distribution between Pyrite and a High-Salinity Fluid

by
Yuri Laptev
1,*,
Anna Doroshkevich
1,2 and
Ilya Prokopyev
1,3
1
Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Akademika Koptyuga Avenue 3, 630090 Novosibirsk, Russia
2
Dobretsov Geological Institute, Siberian Branch of the Russian Academy of Sciences, Sakhyanovoy 6a, 670047 Ulan-Ude, Russia
3
Department of Geology and Geophysics, Novosibirsk State University, Pirogova Street 1, 630090 Novosibirsk, Russia
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(4), 464; https://doi.org/10.3390/min13040464
Submission received: 6 February 2023 / Revised: 23 March 2023 / Accepted: 23 March 2023 / Published: 25 March 2023
(This article belongs to the Special Issue Precious Metals vs. Base Metals: Nature and Experiment)
Browse Figures
Versions Notes

Abstract

:
Experimental studies were conducted to identify the physical and chemical features of gold’s behaviour in hydrothermal processes linked to ore formation and involving CO2 in oxidized deposits. With the aid of the autoclave method, in a temperature range of between 200 and 400 °C, the isochoric dependences of the PVT parameters of concentrated sulphate chloride fluids were plotted, both in the presence and absence of CO2. Our experiments established that concentrated sulphate–chloride fluids (22 wt % Na2SO4 + 2.2 wt % NaCl) that lack CO2 are characterized by a wide supercritical temperature range, with homogenization temperatures of between 250 and 325 °C. In the presence of CO2, the same type of fluids showed heterogenization at a molar fraction of XCO2 = 0.18 (t = 192 °C, P = 176 bar). The process of homogenization for these low-density and high-salinity fluids was impossible at temperatures between 375 and 400 °C and at pressures between 600 and 700 bar. The behaviour of gold was studied during its interaction with a basic composition fluid of sulphate–chloride. We applied the autoclave method under the conditions of a simultaneous synthesis of pyrite and gold dissolution (metallic Au), at a temperature of 340 °C and at a pressure of 440 bar. High Au concentrations (up to 4410 ppm of Au in CO2-bearing fluids) were attained at high gold solubilities (up to 13.5 ppm in the presence of CO2), owing to the process of Au reprecipitation within the pyrite phase. We did not detect Au in the pyrite when we used the XRD or SEM methods, which suggested that it might be present as invisible gold. High values of the distribution coefficient (KD = CAu(solid)/CAu(solution)) in the fluids lacking (KD = 62) and bearing CO2 (KD = 327) empirically confirmed the possibility that gold concentrates in pyrite in structurally non-binding forms.

1. Introduction

Physicochemical studies into the genesis of magmatic fluids and the assessment of their composition and metal endowment are fundamental concerns in endogenetic, ore-forming processes. As a result of a study into melt and fluid inclusions in igneous rock-forming minerals and the associated metasomatic ore formations in commonly named oxidized fluids, distinctive compositions have been revealed [1,2]. These fluids are characterized by the following physicochemical features: (i) a heterophase state, coexisting with concentrated solutions and a gas phase; (ii) high concentrations of salt components in the liquid phase (more than 30–70 wt %), with a prevalence of a mix of chloride–sulphate, significant sulphate, fluoride–sulphate and carbonate–sulphate fluids; (iii) a gas phase predominantly of CO2, with smaller amounts of N2 and H2S; (iv) high Eh values in fluid corresponding to an increased sulphate–sulphide ratio, as evidenced by the presence of sulphides (pyrite, chalcopyrite and galena), sulphates (anhydrite, thenardite and gypsum, etc.) and oxides (magnetite and hematite) in the inclusions.
The critical role of CO2 in ore-bearing fluids of a hydrothermal origin has been recurrently considered in numerous publications, as “oxidized” fluids in the presence of CO2 have been found in a number of ore-magmatic systems: porphyry Cu-Mo (Butte and Kalmakyr) [2,3,4,5,6,7]; Cu-Ni-Pt (Bushveld, Stillwater and Chineyskoe) [8,9]; deposits of rare metals [10]; and carbonatites [11,12]. Analyses of fluid inclusions in these deposits have revealed that the CO2 concentration in slightly saline fluids is responsible for the formation of intrusive orogenic gold deposits in metamorphic belts at over 50 wt % [13,14,15]. Meanwhile, the CO2 content in high-salinity fluids from magmatic–hydrothermal porphyries is, on average, 10 wt % (5 mol %), sometimes reaching up to 20 wt % [16]. Carbon dioxide is also ubiquitous in liquids from epithermal and Carlin gold deposits, with its content varying between approximately 5–10 wt % [17,18,19].
On the other hand, there have been a limited number of physicochemical studies into the effect of CO2 on the behaviour of ore elements in different natural environments [20]. Some of these studies have conducted complex experimental and thermodynamic modelling of the interaction of sulphide and oxide minerals with water–salt-phase fluids in the presence of CO2, in a multicomponent system, and under hydrothermal–magmatic conditions, with temperatures of up to 500 °C. Thus, they have considered a multivariable mechanism to account for the effect of CO2 on the solubility and precipitation of metals such as gold. These data, however, should be contrasted with a more comprehensive experimental study of the role of CO2 in the fractionation of metals under the conditions of liquid–vapor equilibrium in water–salt–sulphur systems [21].
In this experimental study, we aimed to identify new features of the geochemical behaviour of gold in CO2-bearing brines, with the participation of pyrite as the most common sulphide in “oxidized” deposits [1,20,22]. In our experimental approach, we used two different setups. In the first arrangement, we used the autoclave method to plot the PVT correlations in a water–salt system of an H2O-Na2SO4-NaCl composition, within a temperature range of 200–400 °C and pressures of 200–700 bar, both in the presence and absence of CO2. Previous investigations have mainly focused on the solubility of thenardite (Na2SO4) in water at ordinary temperatures [23]. The phase diagram of the H2O-Na2SO4 system is well known for temperatures of up to 800 °C and pressures of up to 1200 bar [24]. However, to choose the appropriate conditions for conducting experiments, additional data on the specific volumes of fluids and their homogenization parameters are required. From a methodological point of view, in our study, the method for constructing isochoric dependencies was preferred when measuring the coordinate values of temperature and pressure as a function of P = f(T). To demonstrate the possibilities of this approach and the significance of the results of our experiments with concentrated fluids, the measurements were carried out in a water–salt solution, with a composition of 22 wt % Na2SO4–2.2 wt % NaCl, both in the presence of carbon dioxide and without it. This concentration of Na2SO4, with the considered limiting value, corresponded to thenardite solubility in water at a temperature of 25 °C [23,24,25]. NaCl (2.2 wt %) is an arbitrary value that is common to many fluid compositions in hydrothermal deposits. However, an increase in the temperature and pressure causes complex changes in the solubility of sulphate, but at the parameters chosen for our experiments (up to 340 °C and 440 bar), the solubility remained higher than in the initial state.
In our second experimental setup, the experiments were conducted under a simultaneous synthesis of pyrite and magnetite. This was achieved by dissolving gold in sulphate-bearing chloride-CO2 fluid (340 °C, 440 bar). Comparative experiments in pure chloride–sulphate solutions, with additional CO2, have offered the possibility of revealing the mechanism that regulates the geochemical behaviour of gold in hydrothermal processes, owing to the effect of CO2. It should be noted that the experimental nature of our study, which was based on methodologically complex individual experiments, allowed us to reveal a new phenomenon: the intense redeposition of Au into pyrite phases. However, further investigations into this effect should be considered in future studies to generate additional experimental data.

2. Experimental and Analytical Methods

Both experimental setups were carried out using a high-pressure apparatus (UVD—2000) for hydrothermal research, developed at the Institute of Experimental Mineralogy of the Academy of Sciences of the Russian Federation. The apparatus setup diagram is shown in Figure 1 and includes the following: (i) a 50 cm3 hydraulic press to pressurize liquid CO2 into the autoclave; (ii) a system of capillaries and valves connecting both the piezoelectric pressure sensor and the autoclave into the circuit; and (iii) an electric furnace, with an autoclave of 21 cm3 placed inside it. All essential components were built at TopIndustriR, from a titanium-based alloy VT-8 (V, Ti, Cr, Zr and Al). Standard capillaries, with an outside diameter of 3 mm, were used in the modified alloy (stainless steel (316 SS)), with mushroom-like seals. Two pressure sensors, SDV–I–100 MPa and SDV–I–160 MPa, with a maximum capacity of 100 and 160 MPa, respectively, were supplied by NKP VIP company (Yekaterinburg, Russia). Temperature was controlled using an EClerk-M-11 electronic unit (RELSIB Research and Production), via a thermocouple built into the autoclave frame, with an accuracy of ±2 °C.
To make isochoric correlations from independent measurements of temperature and pressure, a specific procedure for the experiments was followed: In both the first and second setups, the autoclave was filled with a solution of known composition, volume and specific gravity. In the CO2 system, a pre-calculated, limited amount of oxide was transferred from the hydraulic press, in the form of liquid CO2, into the autoclave through a system of valves. Next, the loaded quantities of CO2 were calibrated by heating the autoclave to a temperature of 50 °C, which ensured the transition of the liquid phase of carbon dioxide to the supercritical state, in the form of a low-density gas.
The amount of CO2 was calculated, at a fixed volume, temperature and pressure, using an equation of state, according to the handbook [26], with virial coefficients of the form PV = RT(1 + aP + bP2). An approximate estimate of the amount of dissolved CO2 was made using an analogy with the data of the H2O-NaCl-CO2 system [27]. This showed that its proportion in the solution at the indicated temperature, in the total balance of carbon dioxide, did not exceed 1%–2%. Hence, this did not significantly affect the variations in the mole fractions of CO2. At the end of each experiment, the autoclave was quenched after it was removed from the electric furnace and cooled in cold water in the capillary assembly.
A solution of Na2SO4 (22 wt %) and NaCl (2.2 wt %) was prepared by weighing and then dissolving “chemically pure” chemical reagents in water. Density was determined by measuring the weight and volume of the solution at room temperature. Liquid CO2 (CO2: 99.99% purity) was supplied to the hydraulic press (Figure 1) through a capillary located at the bottom of a separate cylinder.
The synthesis of pyrite (FeS2) was attained by reacting elemental sulphur (high-purity grade) and metallic iron (sufficiently pure reagent). The obtained mass ratio of sulphur over iron was 1.30, which exceeded the stoichiometric S/Fe = 1.14 ratio. The following amounts were used: sulphur weight—262 mg and iron—201 mg. An excess amount of elemental sulphur allowed the formation of sulphide (S), through the well-known mechanism of disproportionation [28]. Pyrite was synthesized at t = 340 °C and pressure at P = 440 bar. An Au metal foil (Au: 99.99%, Heraeus) was used as a source of metallic gold. The partial amount of gold consumed in the total dissolved gold was determined by the loss weight method, using a Mettler Instrument AG microbalance, with an accuracy of ±0.00005 g.
Powder samples of pyrite and gold foil were investigated using an energy dispersive spectrometer, X-Max (by Oxford Instruments), combined with back-scattered electron imaging (BSE) from a TESCAN MIRA 3 LMU JSM-6510LV scanning electron microscope.
X-ray studies (XRD) were carried out in a DRON-4 automated powder diffractometer (CuKα radiation, graphite monochromator). The diffraction patterns were sampled in the 2θ scale, for the interval 2–65θ, with a step of 0.05θ. The scanning time per point was 4 s.
After the experiments, the sulphide content in aliquots from solutions was determined by iodometry. In microcells, the pH value was measured using an Ecotest-2000 instrument (SPE ECONIKS). The solid phase products derived from the experiments, as well as selected samples of solutions after their oxidation with aqua regia, were used to estimate gold content by means of atomic absorption spectroscopy (AAS), in a Perkin Elmer Analyst 400 instrument.
All analytical work was performed at the Analytical Centre for Multi-Elemental and Isotope Research, at the V.S. Sobolev Institute of Geology and Mineralogy (Novosibirsk, Russia).

3. Results

3.1. Isochoric Correlations in a Water–Salt Fluid, without CO2

An aqueous sulphate–chloride solution (composition: H2O-Na2SO4 22 wt %–NaCl 2.2 wt %) was prepared at room temperature. This composition represented the maximum amount of sodium sulphate at which the precipitation of its solid phase Na2SO4 (thenardite) did not occur, at all selected temperatures and pressures [23,24,25]. The isochores that were obtained constrained the existence of a supercritical fluid in the P-T range of 250–400 °C and at 43–600 bar. For a given specific volume, i.e., ρ = 1.04 cm3/g, homogenization occurred at a temperature of 325 °C. With a decrease in the specific volume, the homogenization temperatures decreased. Dependencies (P = f(T)) above the homogenization point corresponded to a linear function with the same slope.
Our choice of experimental conditions—340 °C and 440 bar, with an average specific volume of ρ = 0.99–0.97 cm3/g (Table 1)—were well justified as they were the optimal combination within the isochore region (Figure 2).

3.2. Isochoric Correlations in a Water–Salt Fluid, with CO2

In this experiment, the isochores were obtained using the water–salt solution described in Section 3.1. The molar fraction of CO2 corresponded to the total values in the conventional system (H2O-CO2), which were calculated using the known mass of its components. When the autoclave was filled with a water–salt mixture in the range of 81%–26.7%, the volume of the liquid CO2 changed accordingly and reached a maximum value of 45.2%, while there was still 27.6% CO2 in gas form. In terms of the molar fraction of carbon dioxide (XCO2), it corresponded with the values up to 0.38. The visualizations of the liquid water–salt phase, the liquid CO2 and the gas phase ratios in the isochoric experiments were demonstrated from the perspective of their similarity to the compositions of the gas–liquid natural inclusions at room temperature (Table 2).
The isochores allowed us to compare the thermobaric properties of a pure water–salt fluid (see Section 3.1) and its mixture with carbon dioxide, in the range of 200–400 °C and pressures of up to 700 bar (Figure 3). When mixed fluid loads of different CO2 molar fractions were run, the isochores were estimated for a wide temperature range (190–425 °C), with a successive increase in pressure (from 160 to 700 bar). The smooth shape of the curves, in the absence of a classical break at the onset of homogenization, consistently indicated the conditions of heterophase liquid–gas equilibrium over the entire temperature range that we investigated. Supercritical conditions of concentrated sulphate–chloride fluids with carbonic acid may only be attainable at higher temperatures and pressures. The comparative data for the non-bearing CO2 isochore (in the same figure) demonstrated the pressure-dependent temperature “path” for a purely aqueous–salt fluid (with a specific volume of ρ = 1.02 cm3/g) from the homogenization point, at 275 °C and from P = 50 bar to P = 700 bar. Contrary to the isochores of the CO2-bearing fluids, in this case, a sharp increase in the pressure occurred within a narrow temperature range: approximately 275–350 °C.
In general, heterophase fluids with CO2, when compared to purely aqueous–salt fluids, correspond to significantly lower densities, e.g., ρ = 1.02–1.49 and 1.04–0.95 cm3/g, respectively. It is worth noting that the autoclave method does not allow us to determine the volume of the liquid and gas phases at heterogeneous equilibrium under experimental conditions. This is the gross (total) value of the specific volume, which is calculated through the ratio of the autoclave volume to the mass of the substance.

3.3. Gold Concentration in Pyrite That Was Synthetized in CO2-Bearing and Non-Bearing Fluids

To attain the ideal conditions for medium-temperature hydrothermal ore formation, the experiments in this study were carried out at a temperature and pressure range of 340–342 °C and of 440–442 bar, respectively. The results of these comparative experiments are given in Table 3. The molar fraction of CO2, in terms of the specific volume in the run, including carbon dioxide, was 0.22. This value corresponded to the molar concentration of CO2 (molCO2/1 kgH2O) and was equal to 16.2 mol, when in a limiting homogeneous fluid state. The percentage of CO2 in the H2O-CO2 mixture was 41.7 wt %. The total duration of these experiments was 48 h.
The solid-phase by-products of the experiments were represented by a black powder with ferromagnetic properties. According to the XRD spectra, the dominant phase was FeS2 (pyrite), with additional small quantities of magnetite and marcasite (Figure 4). The crystal sizes varied from 5–8 µm (Figure 5). The same method also revealed etching on the surface of the gold foil (Figure 6), which indicated that there was an intense interaction between the gold and the fluids under experimental conditions. No fundamental differences were found in the composition of the by-products from the experiments with and without CO2.
In both cases, the gold’s solubility was significantly high, at between 10.1–13.5 ppm (Table 3); however, larger values were measured in the system with CO2. In addition to the high concentrations of Au in the liquid phase of fluids, a very high gold enrichment was observed in the newly formed pyrite. In the CO2-free experiment, the concentration of gold was approximately 630 ppm (0.63 mg/g). In the CO2-bearing test, the gold content in the pyrite reached up to 4400 ppm (4.4 mg/g). When we assessed the overall mass balance of the gold leaf, the concentration of the gold in the precipitate allowed us to accurately estimate the gold consumption via the weight loss method, with values of up to 2.0 mg per 460 mg of solid sample in the CO2-bearing experiments. The recalculation of dissolved Au against the total loss in the gold mass balance did not exceed 5%. Such a significant concentration of Au in the newly formed sulphide–oxide phase brought about a distribution coefficient (KD = CAu(solid)/CAu(solution)) equal to 62 in the system lacking CO2, which increased to 327 in the system with CO2 (Table 3).

4. Discussion

4.1. Temperature, Pressure and Specific Volume of Fluids

A systematic study of the thermobaric properties of fluids is usually limited by the comparison of experimental studies in complex systems with similar properties to the natural ones. The study of the PVT parameters of a sulphate–chloride fluid, both in the presence of CO2 and without it, is a fundamental part of the initial dataset required for subsequent experiments using gold.
The isochore area covers a temperature and pressure range of 200–450 °C and 50–700 bar, respectively. A comparison of PVT data from a pure water–salt fluid and one containing CO2 indicates the following features (in terms of their critical parameters): (i) in almost the entire temperature range (starting from t ≥ 250 °C), saline fluids occur as a single phase (Table 1 and Figure 2); (ii) when CO2 is added to the system, already at its minimum value (XCO2 = 0.18), fluid heterogenization is observed at a minimum temperature and pressure of 192 °C and 176 bar, respectively (Table 2 and Figure 3); and (iii) in the presence of CO2, fluids have a higher overall specific volume than they do in experiments without CO2, due to heterogenization. Particularly, the volumetric parameters of comparative gold-bearing experiments show values of 1.49 and 1.03 cm3/g, respectively (see Table 3).
This method of constructing isochores is promising, especially in combination with the microthermobaric observations of gas–liquid inclusions in the study. The representative physicochemical data of microinclusions at the Bogunai deposit, Yenisei Ridge, Russia [29], illustrates this well. In this metamorphogene and hydrothermal deposit, the volumetric proportions of liquid and gas phases (Vliq/Vgas) for pure water–salt inclusions in quartz veins (type I) vary from 9.1 to 4.3; however, for water-carbon dioxide inclusions (type II), this ratio ranges from 0.25 to 1.5. Independently of the type of inclusions, their homogenization temperature occurs in the range of 225–310 °C. In line with our data in Figure 2, a wide range of homogenization temperatures for CO2-free inclusions indicates a variable liquid/gas ratio, which corresponds to different specific volumes of the fluid (see Table 1). Moreover, the process of the homogenization of natural water–salt inclusions in the presence of CO2 begins at 280 °C. It is likely that, in agreement with the well-known concepts of phase equilibria in the H2O-NaCl-CO2 system [30], the homogenization temperatures in natural materials [29] decrease due to a rise in the solubility of CO2 in low-salinity fluids (4–6 wt % NaCl-equiv.).

4.2. Gold Solubility in Hydrothermal Environments

As noted in the introduction, the first study on the solubility of ore minerals, which included gold in mixed aqueous–salt fluids, with the participation of CO2, was conducted relatively recently [20]. The experiments conducted in the investigation differed significantly from ours, in terms of the assumed physicochemical conditions. First, they referred to a more reducing environment, which was recreated in an aqueous chloride fluid, due to the presence of a pyrrhotite–magnetite–pyrite buffer association. Furthermore, their range of CO2 concentrations, from 0 to 50% in the H2O-CO2 system, allowed for a very low solubility of gold at t = 450 °C and P = 680 bar. Increasing the CO2 content caused a drop in the concentration from 6 × 10−6 to 1 × 10−7 mol/kg of fluid (1.8–0.2 ppm Au). This offers a probable explanation for their hypothesis about gold deposition during CO2-rich magma intrusions into the hydrothermal system, at critical ore stages in Cu-Mo porphyry deposits.
According to our experimental modelling of the behaviour of gold in sulphate–chloride fluids (under oxidizing conditions), the redox properties—including a high sulphate–sulphide ratio—correspond to pyrite–magnetite equilibrium (see Section 3.3). As we can see from the well-known data on the formation of gold hydrosulphide complexes in the presence of sulphide sulphur [31,32]—Au(met) + H2S(aq) = Au(HS)o + 0.5H2(aq), and Au(met) + 2H2S(aq) = Au(HS)2− + 0.5H2(aq) + H+—Au solubility should increase as the hydrogen concentration, H2(aq), decreases. The complete absence of pyrrhotite in our experiments clearly indicated lower values of H2(aq) compared to the pyrite–magnetite–pyrrhotite buffer dataset. In this regard, significantly higher gold concentrations—e.g., 10.1 ppm in pure water–salt fluids and 13.5 ppm in the presence of CO2—established in our experiments did not contradict the data of the aforementioned paper [20].

4.3. Gold Concentration in Oxidizing Fluids

At the highest gold concentrations in the newly formed pyrite, X-ray (XRD) and SEM data could not reveal any individual form of gold, including metallic nanoparticles of Au. This allowed us to suggest that trapped gold in synthesized pyrite may occur in an “invisible” form; however its phase state (nanoparticles or solid solution) remains unknown. Furthermore, the influence of CO2 is still inscrutable.
Recent data [33,34,35] from natural samples have shown that invisible gold is a common variety of gold. A candidate for the existence of invisible gold may be found in the gold-bearing pyrite at the Bakoshi-Kundila deposit, in the northern part of the West Nigerian Subshield [33]. There, two generations of pyrite were identified in quartz veins, which contained structurally bound Au (Au+), Au nanoparticles (Au0) and native Au grains that had been absorbed onto the surface of pyrite type II (Py1b).
The need to conduct extensive research into the chemical and physical properties in the classification of the internal relationships of the several forms of invisible gold in sulphides is considered an independent problem. It has been shown that the criteria for the form of gold has to do with the perfection of the matrix structure in the host mineral [36] and the uniformity of its distribution [37]. Any defect in the pyrite crystals, which occurs simultaneously, is associated with a decrease in size from 1.5 to 0.3 mm. This leads to an increase in the proportion of non-structural gold from 22 to 77%, against total Au [37]. This common concentration of gold in non-structural forms suggests that there is a more complex mechanism involved in its conservation than a direct co-crystallization, which forms solid solutions [38,39,40].
However, there is strong evidence to support gold extraction from the surface of pyrite grains, using adsorption mechanisms [33,41]. Based on this approach, we previously conducted an experimental study [42] to analyse the combined solubility of gold—Au(met)—and pyrite in a slightly acidic chloride–sulphide solution, at higher temperatures (200 °C, P = 150–200 bar). In this study, the phenomenon of gold concentration (redeposition) into the originally used pure pyrite was revealed. The experimental calculations of the distribution coefficients (KD = CAu(solid)/CAu(solution)) were equal to 22–26; however, in our newest results they were significantly higher: 62 in pure aqueous–sulphate fluid and 327 in the same fluid, but in the presence of CO2 (see Table 3). Higher KD values can certainly be linked to the small size of the newly formed pyrite (5 µm, Figure 5), compared to the large size of the previously ground pyrite crystals (approximately 2 mm) up to 50 µm, which were used in our earlier experiments.
Unfortunately, we have no analytical techniques to distinguish the physical form of gold in pyrite (see above). Hopefully, in the future it will be possible to identify different forms of gold via other analytical methods and to study the mechanism involved in increasing its solubility in the presence of CO2. Overall, and owing to the conclusions of other extensive studies on the limits of incorporating gold into the pyrite structure (units in terms of CAu, ppm) [36], it should be assumed that the distribution coefficients (KD = CAu(solid)/CAu(solution)) obtained in our experiments were of an “apparent” high value. This also confirms the possibility that gold concentrates in sulphides as invisible forms (i.e., in nanoparticles and, possibly, as a solid solution).

5. Conclusions

(1)
Experimentally obtained isochoric dependencies of the PVT parameters in concentrated sulphate–chloride fluids (with or without CO2) function as the necessary thermobaric constraints in the reconstruction of the physicochemical conditions—in the temperature range of 200–400 °C and pressures up to 700 bars—of hydrothermal, ore-forming processes, which involve oxidized deposits.
(2)
In the absence of CO2, concentrated sulphate–chloride fluids with specific volumes of 0.95–1.04 cm3/g are characterized by a wide temperature range within a supercritical state and by homogenization temperatures of approximately 250–325 °C. In the presence of CO2, with molar fractions of XCO2 = 0.18–0.38, high-salinity sulphate–chloride fluids occur in a heterogeneous (heterophase) state across a wide range of temperatures and pressures: t = 192–400 °C and P = 180–700 bar.
(3)
The experimental modelling of the behaviour of gold in sulphate–chloride fluids, under conditions similar to oxidizing fluids and as recognized by the manifestation of the pyrite–magnetite association, indicates that Au has high mobility in these types of ore-forming processes. Dissolved Au may reach concentrations of greater than 10 ppm.
(4)
The interaction of Au with concentrated sulphate–chloride fluids, both with and without CO2, during the simultaneous synthesis of pyrite, and at a temperature of 340 °C and a pressure of 440 bar, enables the redeposition of Au into pyrite grains. In this way, it reaches very high concentrations (up to 4410 ppm Au in experiments with CO2) as an invisible form of gold. The high distribution coefficient (KD = CAu(solid)/CAu(solution)) in fluids without CO2 (KD = 62) and in its presence (KD = 327) experimentally confirm the possibility that gold concentrates in non-structural forms in sulphide minerals.

Author Contributions

Conceptualization, Y.L. and A.D.; data curation, Y.L. and A.D.; investigation, Y.L.; methodology, Y.L., I.P. and A.D.; project administration, A.D.; writing—original draft, Y.L., A.D. and I.P.; writing—review and editing, Y.L., A.D. and I.P. All authors have read and agreed to the published version of the manuscript.

Funding

The experimental investigation was supported and performed on state assignment of IGM SB RAS (FWZN2022-0024) and GIN SB RAS (AAAA-A21-121011390002-2).

Data Availability Statement

The original data used during the current study are available from the corresponding author on reasonable request.

Acknowledgments

The authors express their sincere appreciation to the editor and reviewers for their contribution to improving the manuscript. Analytical equipment for this study was provided by the Analytical Centre for multi-elemental and isotope research of the Sobolev Institute of Geology and Mineralogy, SB RAS.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Borisenko, A.S.; Borovikov, A.A.; Vasyukova, E.A.; Pavlova, G.G.; Ragozin, A.L.; Prokopév, I.R.; Vladykin, N.V. Oxidized magmatogene fluids: Metal-bearing capacity and role in ore formation Russ. Geol. Geophys. 2011, 52, 144–164. [Google Scholar] [CrossRef]
  2. Borisenko, A.S.; Borovikov, A.A.; Zhitova, L.M.; Pavlova, G.G. Composition of magmatogene fluids and factors of their geochemical specialization and metal-bearing capacity. Russ. Geol. Geophys. 2006, 47, 1308–1326. [Google Scholar]
  3. Naumov, V.B.; Kamenetsky, V.S.; Thomas, R.; Kononkova, N.N.; Ryzhenko, B.N. Inclusions of silicate and sulfate Melts in chrome diopside from the Inagli deposit. Yakutia Geochem. Intern. 2008, 6, 554. [Google Scholar] [CrossRef] [Green Version]
  4. Heinrich, C.A.; Gunther, D.; Audetat, A.; Ulrich, T.; Frischknecht, R. Metal fractionation between magmatic brine and vapor, determined by microanalysis of fluid inclusions. Geology 1999, 27, 755–758. [Google Scholar] [CrossRef]
  5. Ulrich, T.; Gunther, D.; Heinrich, C.A. Gold concentrations of magmatic brines and the metal budget of porphyry copper deposits. Nature 1999, 399, 676–679. [Google Scholar] [CrossRef]
  6. Rusk, B.; Reed, M.; Dilles, J.H.; Klemm, L.; Heinrich, C. Compositions of magmatic hydrothermal fluids determined by LA-ICP-MS of fluid inclusions from the porphyry copper-molybdenum deposit at Butte. Montana Chem. Geol. 2004, 210, 173–199. [Google Scholar] [CrossRef]
  7. Williams-Jones, A.E.; Heinrich, C.A. Vapor transport of metals and the formation of magmatic hydrothermal ore deposits. Econ. Geol. 2005, 100, 1287–1312. [Google Scholar] [CrossRef]
  8. Hanley, J.J.; Mungall, J.E.; Spooner, E.T.C. Fluid and melt inclusion evidence for platinum-group element transport by high salinity fluids and halide melts below the J-M reef, Stillwater Complex, Montana, USA. In Extended Abstracts of the X International Platinum Symposium; Geological Survey of Finland: Oulu, Finland, 2005; pp. 94–97. [Google Scholar]
  9. Zhitova, L.M.; Kinnaird, J.A.; Gora, M.P.; Shevko, E.P. Magmatogene fluids of metal-bearing reefs in the Bushveld Complex, South Africa: Based on research data on fluid inclusions in quartz. Geol. Ore Depos. 2016, 58, 58–81. [Google Scholar] [CrossRef]
  10. Reyf, F.G. Immiscible phases of magmatic fluid and their relation to Be and Mo mineralization at the Yermakovka F-Be deposit, Transbaikalia, Russia. Chem. Geol. 2004, 21, 49–71. [Google Scholar] [CrossRef]
  11. Panina, L.I. Multiphase carbonate-salt immiscibility in carbonatite melts: Data on melt inclusions from the Krestovskiy massif minerals (Polar Siberia). Contr. Miner. Petrol. 2005, 150, 19–36. [Google Scholar] [CrossRef]
  12. Panina, L.I.; Motorina, I.V. Liquid immiscibility in deep-seated magmas and the generation of carbonatite melts. Geochem. Intern. 2008, 46, 448. [Google Scholar]
  13. Boiron, M.-C.; Cathelineau, M.; Banks, D.; Fourcade, S.; Vallance, J. Mixing of metamorphic and surficial fluids during the uplift of the Hercynian upper crust: Consequences for gold deposition. Chem. Geol. 2003, 194, 119–141. [Google Scholar] [CrossRef]
  14. Phillips, G.N.; Evans, K.A. Role of CO2 in the formation of gold deposits. Nature 2004, 429, 860–863. [Google Scholar] [CrossRef]
  15. Garofalo, P.S.; Ficker, M.B.; Gunther, D.; Bersani, D.; Lottici, P.P. Physical-chemical properties and metal budget of Au transporting hydrothermal fluids in orogenic deposits. Geol. Soc. London Spec. Publ. 2014, 402, 71–102. [Google Scholar] [CrossRef]
  16. Rusk, B.G.; Reed, M.H.; Dilles, J.H. Fluid inclusion evidence for magmatic-hydrothermal fluid evolution in the porphyry copper–molybdenum deposit at Butte, Montana. Econ. Geol. 2008, 103, 307–334. [Google Scholar] [CrossRef]
  17. Bodnar, R.J.; Reynolds, T.J.; Kuehn, C.A. Fluid inclusion systematics in epithermal systems. Rev. Econ. Geol. 1985, 2, 73–97. [Google Scholar]
  18. Koděra, P.; Lexa, J.; Rankin, A.H.; Fallick, A.E. Epithermal gold veins in a caldera setting: Banská Hodruša Slovakia. Mineral. Depos. 2005, 39, 921–943. [Google Scholar] [CrossRef]
  19. Catchpole, H.; Kouzmanov, K.; Fontboté, L.; Guillong, M.; Heinrich, C.A. Fluid evolution in zoned Cordilleran polymetallic veins—Insights from microthermometry and LAICP-MS of fluid inclusions. Chem. Geol. 2011, 281, 293–304. [Google Scholar] [CrossRef] [Green Version]
  20. Kokh, M.A.; Akinfiev, N.N.; Pokrovski, G.S.; Salvi, S.; Guillaume, D. The role of carbon dioxide in the transport and fractionation of metals by geological fluids. Geochim. Cosmochim. Acta 2017, 197, 433–466. [Google Scholar] [CrossRef]
  21. Kokh, M.A.; Lopez, M.; Gisquet, P.; Lanzanova, A.; Candaudap, F.; Besson, P.; Pokrovski, G.S. Combined effect of carbon dioxide and sulfur on vapor-liquid partitioning of metals in hydrothermal systems. Geochim. Cosmochim. Acta 2016, 187, 311–333. [Google Scholar] [CrossRef]
  22. Prokopyev, I.R.; Doroshkevich, A.G.; Ponomarchuk, A.V.; Sergeev, S.A. Mineralogy, age and genesis of apatite-dolomite ores at the Seligdar apatite deposit (Central Aldan, Russia). Ore Geol. Rev. 2017, 81, 296–308. [Google Scholar] [CrossRef]
  23. Song, P.; Yan, Y. Thermodynamics and phase diagram of the salt lake brine system at 298.15 K Computer Coupling of Phase. Diagr. Thermochem. 2003, 27, 343–352. [Google Scholar]
  24. Valyashko, V.M. Chapter 15—Phase equilibria of water-salt systems at high temperatures and pressures. In Aqueous Systems at Elevated Temperatures and Pressures, Physical Chemistry in Water, Steam and Hydrothermal Solutions; Donald, A.P., Roberto Fernández-Prini, A.H.H., Eds.; Academic Press: Cambridge, MA, USA, 2004; pp. 597–641. [Google Scholar] [CrossRef]
  25. Krumgalz, B.S. Temperature dependence of mineral solubility in water. part 3. Alkaline and alkaline earth sulfates. J. Phys. Chem. Ref. Data 2018, 47, 023101. [Google Scholar] [CrossRef]
  26. Naymov, G.B.; Ryzhenko, B.N.; Khodakovsky, I.L. Handbook of Thermodynamic Constants for Geologists; Atomizdat Moscow: Moscow, Russia, 1971; p. 240. (In Russian) [Google Scholar]
  27. Bowers, T.S.; Helgeson, H.C. Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H2O–CO2–NaCl on phase relations in geologic systems: Equation of state for H2O–CO2–NaCI fluids at high pressures and temperatures. Geochim. Cosmochim. Acta 1983, 47, 1247–1275. [Google Scholar] [CrossRef]
  28. Ellis, A.J.; Giggenbach, W. Hydrogen ionization and sulphur hydrolysis in high temperature solution. Geochim. Cosmochim. Acta 1971, 35, 247–260. [Google Scholar] [CrossRef]
  29. Ryabukha, M.A.; Gibsher, N.A.; Tomilenko, A.A.; Bul’bak, T.A.; Khomenko, M.O.; Safonov, A.M. P–T–X parameters of metamorphogene and hydrothermal fluids, isotopy and age of the Bogunai gold deposit, southern Yenisei Ridge (Russia). Russ. Geol. Geophys. 2015, 56, 903–918. [Google Scholar]
  30. Mao, S.; Zhang, D.; Li, Y.; Liu, N. An improved model for calculating CO2 solubility in aqueous NaCl solutions and the application to CO2–H2O–NaCl fluid inclusions Chemical. Geology 2013, 347, 43–58. [Google Scholar] [CrossRef]
  31. Akinfiev, N.N.; Zotov, A.V. Thermodynamic description of aqueous species in the system Cu–Ag–Au–S–O–H at temperatures of 0–600 C and pressures of 1–3000 bar. Geochem. Intern. 2010, 48, 714–720. [Google Scholar] [CrossRef]
  32. Benning, L.G.; Seward, T.M. Hydrosulphide complexing of Au(I) in hydrothermal solutions from 150–400°C and 500–1500 bar. Geochim. Cosmochim. Acta 1996, 60, 1849–1871. [Google Scholar]
  33. Elatikpo, S.M.; Li, H.; Liu, B.; Zhang, W.D. Metallogenesis of the bakoshi-kundila gold deposit in northern west Nigerian subshield: Insights from pyrite chemical and sulfur isotopic compositions and zircon U-Pb geochronology. Precambrian Res. 2022, 383, 106890. [Google Scholar] [CrossRef]
  34. Ma, Y.; Shao-Yong, J.; Frimmel, H.E.; Zhu, L.-Y. In situ chemical and isotopic analyses and element mapping of multiple-generation pyrite: Evidence of episodic gold mobilization and deposition for the Qiucun epithermal gold deposit in southeast China. Am. Mineral. 2022, 107, 1133–1148. [Google Scholar] [CrossRef]
  35. Large, R.R.; Maslennikov, V.V. Invisible gold paragenesis and geochemistry in pyrite from orogenic and sediment-hosted gold deposits. Minerals 2020, 10, 339. [Google Scholar] [CrossRef] [Green Version]
  36. Tauson, V.L. Gold solubility in the common gold-bearing minerals: Experimental evaluation and application to pyrite. Eur. J. Mineral. 1999, 11, 937–947. [Google Scholar] [CrossRef]
  37. Tauson, V.L.; Bessarabova, O.I.; Kravtsova, R.G.; Pastushkova, R.M.; Smagunov, V. Separation of forms of gold occurrence in pyrites by studying statistic samples of analytical data. Russ. Geol. Geophys. 2002, 43, 57–67. [Google Scholar]
  38. Tauson, V.L.; Lipko, S.V.; Shchegolkov, Y.V. Surface nanoscale relief of mineral crystals and its relation to nonautonomous phase formation. Crystallogr. Rep. 2009, 54, 1219–1227. [Google Scholar] [CrossRef]
  39. Mikhlin, Y.L.; Romanchenko, A.S. Gold deposition on pyrite and the common sulfide minerals: An STM/STS and SR-XPS study of surface reactions and Au nanoparticles. Geochim. Cosmochim. Acta 2007, 71, 5985–6001. [Google Scholar] [CrossRef]
  40. Mikhlin, Y.; Romanchenko, A.; Likhatski, M.; Karacharov, A.; Erenburg, S.; Trubina, S. Understanding the initial stages of precious metals precipitation: Nanoscale metallic and sulfidic species of gold and silver on pyrite surfaces. Ore Geol. Rev. 2011, 42, 47–54. [Google Scholar] [CrossRef]
  41. Widler, A.M.; Seward, T.M. The adsorption of gold(I) hydrosulphide complexes by iron sulphide surfaces. Geochim. Cosmochim. Acta 2002, 66, 383–402. [Google Scholar] [CrossRef]
  42. Laptev, Y.V.; Rozov, K.B. Interaction of gold with sulfide surface as a factor of its concentration in hydrothermal ore formation. Dokl. Earth Sci. 2006, 411, 1229–1232. [Google Scholar] [CrossRef]
Minerals 13 00464 g001 550
Figure 1. Schematic setup of the autoclave for measuring pressure and temperature in a water–salt fluid. CO2 was supplied through a hydraulic press.
Figure 1. Schematic setup of the autoclave for measuring pressure and temperature in a water–salt fluid. CO2 was supplied through a hydraulic press.
Minerals 13 00464 g001
Minerals 13 00464 g002 550
Figure 2. Isochoric dependencies, according to P-T measurements for a fluid mixture (composition: H2O + 22 wt % Na2SO4 + 2.2 wt % NaCl, with no CO2 additions). The numbers inscribed above the lines show the filling factor of the autoclave.
Figure 2. Isochoric dependencies, according to P-T measurements for a fluid mixture (composition: H2O + 22 wt % Na2SO4 + 2.2 wt % NaCl, with no CO2 additions). The numbers inscribed above the lines show the filling factor of the autoclave.
Minerals 13 00464 g002
Minerals 13 00464 g003 550
Figure 3. Isochoric dependencies according to P-T measurements for the fluid mixture composition: H2O + 22 wt % Na2SO4 + 2.2 wt % NaCl, with CO2 additions. The mole fraction of CO2 is shown in the figure.
Figure 3. Isochoric dependencies according to P-T measurements for the fluid mixture composition: H2O + 22 wt % Na2SO4 + 2.2 wt % NaCl, with CO2 additions. The mole fraction of CO2 is shown in the figure.
Minerals 13 00464 g003
Minerals 13 00464 g004 550
Figure 4. XRD spectrum of synthesized by-products of the experiments. FeS2—pyrite; FeS2*—marcasite; and Fe3O4—magnetite.
Figure 4. XRD spectrum of synthesized by-products of the experiments. FeS2—pyrite; FeS2*—marcasite; and Fe3O4—magnetite.
Minerals 13 00464 g004
Minerals 13 00464 g005 550
Figure 5. SEM back-scattered electron (BSE) image of synthesized pyrite.
Figure 5. SEM back-scattered electron (BSE) image of synthesized pyrite.
Minerals 13 00464 g005
Minerals 13 00464 g006 550
Figure 6. SEM back-scattered electron (BSE) image showing dissolution patterns on gold leaf.
Figure 6. SEM back-scattered electron (BSE) image showing dissolution patterns on gold leaf.
Minerals 13 00464 g006
Table
Table 1. P-T values for an aqueous fluid composition (H2O-Na2SO4 22 wt %–NaCl 2.2 wt %) at temperatures and pressures above homogenization (Thom, Phom). This includes different values for the coefficient of filling, i.e., the free space in the autoclave occupied by liquid (K). Values for the specific volume of fluids are also given (ρ, cm3/g).
Table 1. P-T values for an aqueous fluid composition (H2O-Na2SO4 22 wt %–NaCl 2.2 wt %) at temperatures and pressures above homogenization (Thom, Phom). This includes different values for the coefficient of filling, i.e., the free space in the autoclave occupied by liquid (K). Values for the specific volume of fluids are also given (ρ, cm3/g).
Minerals 13 00464 i001Minerals 13 00464 i002Minerals 13 00464 i003Minerals 13 00464 i004Minerals 13 00464 i005
K = 0.80K = 0.82K = 0.84K = 0.86K = 0.88
ρ = 1.04 cm3/gρ = 1.02 cm3/gρ = 0.99 cm3/g ρ = 0.97 cm3/gρ = 0.95 cm3/g
Thom = 325 °CThom = 300 °CThom = 280 °CThom = 265 °CThom = 255 °C
Phom = 75 barPhom = 68 barPhom = 52 barPhom = 48 barPhom = 43 bar
T, °CP, barT, °CP, barT, °CP, barT, °CP, barT, °CP, bar
3257530068280522654825543
340204310157290143280188260140
350290320245300234290282270236
360376330334310325300375280332
370462340422320416310469290428
380548350511330507320562300524
--360599340598--310620
Table
Table 2. P-T values for an aqueous fluid composition (H2O-Na2SO4 22 wt %–NaCl 2.2 wt %), with and without CO2 additions, and including different values for the filling factor—i.e., the free space in the autoclave (K) using a liquid salt–water admixture and liquid carbon dioxide CO2 (liq). Values for the specific volume of fluids are also given (ρ, cm3/g).
Table 2. P-T values for an aqueous fluid composition (H2O-Na2SO4 22 wt %–NaCl 2.2 wt %), with and without CO2 additions, and including different values for the filling factor—i.e., the free space in the autoclave (K) using a liquid salt–water admixture and liquid carbon dioxide CO2 (liq). Values for the specific volume of fluids are also given (ρ, cm3/g).
Minerals 13 00464 i006Minerals 13 00464 i007Minerals 13 00464 i008Minerals 13 00464 i009
without CO2X CO2 = 0.18X CO2 = 0.22X CO2 = 0.38
K = 0.82K = 0.67K = 0.60K = 0.72
ρ = 1.02 cm3/gρ = 1.32 cm3/gρ = 1.49 cm3/g ρ = 1.43 cm3/g
T, °CP, barT, °CP, barT, °CP, barT, °CP, bar
25543192176190215190315
26548225209210245215350
28052230215215250235390
30068235224230277260454
310157270260255320310575
320245300310278360330640
330334308330325450355750
340422315345350525--
350511350418357562--
360599380500360576--
--390548370615--
--405600372625--
--408618373630--
----385700--
Table
Table 3. Main parameters obtained from the experiments, accounting for the effect of CO2 on the solubility and distribution of Au between the pyrite and the solution.
Table 3. Main parameters obtained from the experiments, accounting for the effect of CO2 on the solubility and distribution of Au between the pyrite and the solution.
System CompositionT, °CP, barρ, cm3/gSolutionSolidKD = CAu(solid)
/CAu(solution)
Au,
ppm 		CH2S, mpHAu, ppm
FeS2-Na2SO4–NaCl-H2O3404401.0310.10.033.963062
FeS2-Na2SO4–NaCl-H2O-CO23424421.4913.50.043.84410327
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Laptev, Y.; Doroshkevich, A.; Prokopyev, I. Comparative Experiments on the Role of CO2 in the Gold Distribution between Pyrite and a High-Salinity Fluid. Minerals 2023, 13, 464. https://doi.org/10.3390/min13040464

AMA Style

Laptev Y, Doroshkevich A, Prokopyev I. Comparative Experiments on the Role of CO2 in the Gold Distribution between Pyrite and a High-Salinity Fluid. Minerals. 2023; 13(4):464. https://doi.org/10.3390/min13040464

Chicago/Turabian Style

Laptev, Yuri, Anna Doroshkevich, and Ilya Prokopyev. 2023. "Comparative Experiments on the Role of CO2 in the Gold Distribution between Pyrite and a High-Salinity Fluid" Minerals 13, no. 4: 464. https://doi.org/10.3390/min13040464

APA Style

Laptev, Y., Doroshkevich, A., & Prokopyev, I. (2023). Comparative Experiments on the Role of CO2 in the Gold Distribution between Pyrite and a High-Salinity Fluid. Minerals, 13(4), 464. https://doi.org/10.3390/min13040464

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop