Solubility Of Gold Research Articles

Precious metals in high-temperature geothermal systems in New Zealand

High-temperature geothermal systems are the modern analogue of epithermal precious metal ore deposits. In these systems, gold and silver are transported primarily as bisulfide complexes, with the precious metals being deposited in response to boiling and mixing of the deep geothermal fluid. In order for the geothermal system to produce an economic precious metal deposit within the typical lifetime of a geothermal system, the deposition process must be efficient, and the gold and silver concentrations in the geothermal fluid must be sufficient. Thermodynamic data have been measured for gold and silver bisulfide complexes, but the question remains as to whether the deep geothermal fluid is saturated with respect to these complexes. Measurement of gold and silver concentrations in geothermal fluid sampled at the surface shows very low concentrations, as these metals precipitate down the well. Indirect measurements of the deep reservoir gold and silver concentrations have been calculated by estimating precipitate concentrations on scale deposits recovered from high-pressure apparatus at the surface. These estimates produce precious metal concentrations that are below saturation. We therefore designed and built a downhole sampling device specifically to measure precious and other trace metal concentrations in the deep reservoir fluids. The device is manufactured from titanium to be chemically inert, and therefore capable of scavenging the trace metals through acid rinsing. Analyses for precious and related metals on deep waters obtained from Kawerau and Ngawha geothermal systems, New Zealand, have been measured. Solutions were analysed by ICP-MS. Laboratory testing using blank solutions shows minimal sources of contamination from materials used in the construction and sampling procedure (i.e. titanium, MilliQ water and aqua regia). At Kawerau, samples were obtained from deep wells at ⩾1000 m depth at temperatures of 260 to 295 °C, while at Ngawha samples were taken from > 800 m depth at temperatures of 225–>235 °C. Gold, silver and thallium are at ppb levels, while arsenic, antimony and copper range from hundreds to thousands of ppb. Comparing these results with calculated solubilities of gold, silver and acanthite suggests deep waters are undersaturated in gold but close to saturation in silver.

The hydrolysis of gold(I) in aqueous solutions to 600°c and 1500 bar

The solubility of gold has been measured in aqueous solutions at temperatures between 300 and 600°C and pressures from 500 to 1500 bar to determine the stability and stoichiometry of the hydroxy complexes of gold(I) in hydrothermal solutions. The experiments were carried out using a flow-through autoclave system. The solubilities, measured as total dissolved gold, were in the range 1.2 × 10 −8 to 2.0 × 10 −6 mol kg −1 (0.002 to 0.40 mg kg −1), in solutions of total dissolved sodium between 0.0 and 0.5 mol kg −1, and total dissolved hydrogen between 4.0 × 10 −6 and 4.0 × 10 −4 mol kg −1. At constant hydrogen molality, the solubility of gold increases with increasing temperature and decreases with increasing pressure. The solubilities were found to be independent of pH but increased with decreasing hydrogen molality at constant temperature and pressure. Consequently, gold dissolves in aqueous solutions of acidic to alkaline pH according to the reactionAu(s)+H 2O(l)=AuOH(aq)+0.5H 2(g) K s,1The solubility constant, log K s,1, increases with increasing temperature from a minimum of −8.76 (±0.18) at 300°C and 500 bar to a maximum of −7.50 (±0.11) at 500°C and 1500 bar and decreases to −7.61 (±0.08) at 600°C and 1500 bar. From the equilibrium solubility constant and the redox potential of gold, the formation constant to form AuOH(aq) was calculated. At 25°C the complex formation is characterised by an exothermic enthalpy and a positive entropy. With increasing temperature and decreasing pressure, the formation reaction becomes endothermic and is accompanied by a large positive entropy, indicating a greater electrostatic interaction between Au + and OH −.

Mixing of metamorphic and surficial fluids during the uplift of the Hercynian upper crust: consequences for gold deposition

A detailed geochemical study of fluids from representative quartz-sealed faults hosting late Hercynian gold concentrations shows that fluids percolating the mineralised faults had two main distinct reservoirs: one was a quite shallow and the other rather deep-seated. Both fluids have lost a great part of their original geochemical signature through interactions with host metamorphic formations. Early fluids, present during the primary sealing of the faults by quartz, are considered to have effectively equilibrated with the metamorphic pile and then predominantly flowed upwards along the faults. They are characterised by CH 4/CO 2/H 2O ratios rather typical of fluids equilibrated with graphite, and moderate to medium chlorinities with a high Br/Cl ratio. The striking feature of the gold-bearing quartz is that gold is not synchronous within any quartz deposition, and appears located in late microfractures and associated with Pb–Bi–Sb sulphosalts and sulphides. These late stages are characterised by fluids whose salinities decrease to very low values indicating their progressive dilution by waters of more surficial origin in the fault system. The long-lived activity of the fault favoured the connection between two distinct fluid reservoirs at a critical time during the basement uplift. The fluids evolved through two main driving mechanisms which were responsible for the Au deposition: (i) decrease in temperature accompanying decompression from supra-lithostatic to hydrostatic conditions which yielded, in some instances, volatile unmixing in the faulted systems, (ii) mixing of the resulting fluids with waters entering the hydrological systems from shallower reservoirs. In addition to dilution and fluid mixing which are favourable factors for decreasing the gold solubility, the presence of microfractured sulphides could have enhanced gold precipitation through electrochemical processes.

The formation of gold metal fibers during the production of E-glass fibers

We first examined the relation between the solubility of gold and the reduction-oxidation (redox) condition of E-glass, but it was difficult to find a universal relation between the gold solubility and the redox of the glasses. Next, we examined in relation between the solubility of gold and the theoretical optical basicity of the glass and found that the theoretical optical basicity of the glasses was closely related to the gold solubility. We considered ways of increasing the solubility of the gold, probably through the formation of some complex with oxide ions. In the industrial process, no metal fiber was formed when glass composition of theoretical optical basicity larger than about 0.601 was spun.

Thermodynamics and Kinetics of the Dissolution of Gold Under Pressure Oxidation Conditions in the Presence of Chloride

It has been established that gold can be dissolved under conditions typical of those encountered during the pressure oxidation of refractory gold ores provided that the solutions contain chloride ions and that a high ratio of iron(III)/iron(II) is maintained. This study has been conducted in order to understand the behaviour of gold under such conditions and has comprised both thermodynamic and kinetic measurements over a wide range of temperature and chloride concentration.Preliminary electrochemical studies have demonstrated that the effective oxidant for gold under pressure leach conditions is iron(III) and not dissolved oxygen. Measurements of the equilibrium potentials of the gold(III)/gold and iron(III)/iron(II) couples over a temperature range from 25 °C to 200 °C in acidic sulphate solutions containing various concentrations of chloride ions have confirmed thermodynamic predictions that the equilibrium solubility of gold increases with increasing temperature, chloride concentration and iron(III)/iron(II) ratio.Detailed electrochemical studies of the anodic behaviour of gold and the cathodic reduction of oxygen and iron(III) have been made under the above conditions in an autoclave and have been combined with mixed potential measurements to develop an electrochemical model for the dissolution of gold. The model has been verified by measurements of the actual rate of dissolution of gold as a function of the chloride ion concentration and the temperature.The implications of these results as they relate to the deportment of gold during pressure oxidation and also to the possibility of an alternative process to cyanidation are discussed.

Gold solubility, speciation, and partitioning as a function of HCl in the brine-silicate melt-metallic gold system at 800°C and 100 MPa

A vapor-undersaturated synthetic brine was equilibrated with metallic gold and a haplogranitic melt at 800°C and 100 MPa to examine the solubility, speciation and partitioning of gold in the silicate melt-brine-metallic gold system. The starting composition of the NaCl-KCl-HCl-H2O brine was 70 wt.% NaCl (equivalent) with starting KCl/NaCl ranging from 0.5 to 1. KCl/HCl was varied from 3.2 to 104 to evaluate the solubility and partitioning of gold as a function of the concentration of HCl in the brine. Inclusions of brine were trapped in a silicate glass during quench. Inclusion-poor and inclusion-rich portions of glass were analyzed for gold and chloride by using neutron activation analysis. The inclusion-poor glass yielded an estimate of the solubility of gold and chloride in the silicate melt. The solubility of gold in the melt, at gold metal saturation, was estimated as ≈1 ppm. The solubility of gold in the brine was estimated by mass balance, given the concentration of gold and chloride in the inclusion-poor and inclusion-rich glasses. The solubility of gold metal at low-HCl concentrations in the brine, CHClb, (3 × 103 to 1.1 × 104 ppm) is ≈40 ppm (by weight) and is independent of the HCl concentration under those conditions. For CHClb of 1.1 × 104 to 4.0 × 104 ppm, the solubility of gold increased from 40 to 840 ppm, and the solubility is given by: log CAub = [2.2 · log CHClb] − 7.2(1) These data suggest that a significant amount of gold is not chloride complexed in brines at low-HCl concentrations (< 1.1 × 104 ppm), but that gold-chloride complexes, possibly AuCl2H, are important at elevated concentrations of HCl (> 1.1 × 104 ppm). The calculated Nernst partition coefficient (DAub/m) for gold between a brine and melt varied from 40 to 830 over a range of brine HCl concentrations of 3 × 103 to 1.1 × 104 ppm. Our results indicate a significant amount of gold can be transported by a brine in the magmatic-hydrothermal environment independent of the fugacity of sulfur in the system. Thus brines provide an effective mechanism for the scavenging of gold from a crystallizing melt and transport into an associated magmatic-hydrothermal system, regardless of their sulfur contents.

The stability of Au-chloride complexes in water vapor at elevated temperatures and pressures

The solubility of gold in liquid-undersaturated HCl-bearing water vapor was investigated experimentally at temperatures of 300 to 360°C and pressures up to 144 bars. Results of these experiments show that the solubility of gold in the vapor phase is significant and increases with increasing fHCl and fH2O. This behavior of gold is attributed to formation of hydrated gold-chloride gas species, interpreted to have a gold-chlorine ratio of 1:1 and a hydration number varying from 5 at 300°C to 3 at 360°C. These complexes are proposed to have formed through the following reaction: (A1)Ausolid+m·HClgas+n·H2Ogas=AuClm·(H2O)ngas+m2·H2gas which was determined to have log K values of −17.28 ± 0.36 at 300°C, −18.73 ± 0.66 at 340°C, and −18.74 ± 0.43 at 360°C. Gold solubility in the vapor was retrograde, i.e., it decreased with increasing temperature, possibly as a result of the inferred decrease in hydration number.Calculations based on our data indicate that at 300°C and fO2-pH conditions, encountered in high sulfidation epithermal systems, the vapor phase can transport up to 6.6 ppb gold, which would be sufficient to form an economic deposit (e.g., Nansatsu, Japan; 36 tonnes) in ∼ 30,000 yr.

Tectonically driven fluid flow and gold mineralisation in active collisional orogenic belts: comparison between New Zealand and western Himalaya

Hydrothermal activity and mesothermal-styled gold mineralisation occurs near the main topographic divide of most active or young collisional mountain belts. The Southern Alps of New Zealand is used in this study as a model for the mineralising processes. The collisional tectonics results in a two-sided wedge-shaped orogen into which rock is transported horizontally. Upper crustal rocks pass through the orogen and leave the orogen by erosion, whereas lower crustal rocks are deformed into the mountain roots. High relief drives meteoric water flow to near the brittle–ductile transition. Lower to upper greenschist facies metamorphic reactions, driven by deformation at the crustal decollement and in the root, release water-rich fluids that rise through the orogen. Intimate chemical interaction between fluid and rock results in dissolution and later precipitation of gold, arsenic and sulphur. Fluid flow and mineralisation in the topographic divide region is facilitated by a network of steeply dipping faults and associated rock damage zones where oblique strike-slip faults intersect the thrust faults that strike subparallel to the main mountain range. The Nanga Parbat massif of the western Himalaya is an example of an active collisional zone which hosts hydrothermal activity but no gold mineralisation. The lack of gold mineralisation is due to the following factors: CO 2-dominated rising metamorphic fluid in dehydrated amphibolite-granulite facies metamorphic rocks does not dissolve gold and arsenic; hot (up to 400 °C) meteoric water confined to fractures in the gneiss limits dissolution of gold and arsenic; low density of hot water/dry steam, and low reduced sulphur content of fluid, restrict solubility of gold and arsenic; absence of fracture networks in the core of the massif and the small volumes of circulating fluid limit metal concentration; and lack of reactive rock compositions limits chemically mediated metal deposition.

Effect of Surface Condition on the Solid Solubility of Substitutional Gold in the Out-Diffusion of Supersaturated Gold in Silicon

Effect of Surface Condition on the Solid Solubility of Substitutional Gold in the Out-Diffusion of Supersaturated Gold in Silicon

Fabrication of micromechanical structures on substrates selectively etched using a micropatterned ion-implantation method

An advanced micromachining technique using ion implantation to modify materials was studied. Gold ion implantation into silicon decreased the etching rate when the silicon was etched in potassium hydroxide solution after the ion implantation; the implanted region remained, thus forming the microstructure. Observation of the cross-section of the resulting etched structure by transmission electron microscopy showed that the structure was made only from the ion-implanted region, and that gold was precipitated on the surface. To clarify the mechanism involved in the decrease in the etching rate, we varied the etching conditions. Our results show that precipitation of implanted gold on the surface decreased the etching rate, because solubility of gold is lower.

Solubility of gold in sulfide-containing aqueous solutions at T=300°C

Solubility of gold in sulfide-containing aqueous solutions at T=300°C

The Bronzewing lode-gold deposit, Western Australia: P– T– X evidence for fluid immiscibility caused by cyclic decompression in gold-bearing quartz-veins

The Bronzewing lode-gold deposit is located in the Yandal greenstone belt in the Yilgarn Craton of Western Australia. Gold mineralization is hosted in tholeiitic basalt that is metamorphosed to greenschist facies. Individual ore bodies are controlled by a complex, gold-bearing quartz vein system that comprises shear and extension veins formed during a progressive D 2 deformation event. Gold is localized in quartz along fractures and deformed grain boundaries, and is interpreted to have formed late in the formation of the veins. Detailed petrography and microthermometry on primary, pseudosecondary and secondary fluid inclusions trapped in gold-bearing shear and extension veins revealed five types of fluid inclusions: (1) CO 2±CH 4–H 2O–NaCl inclusions of variable salinity (0.1 to 17.5 eq. wt.% NaCl) containing between 10 and 99 mol% CO 2 and molar volumes that range from 22 to 76 cm 3; (2) H 2O–NaCl inclusions of variable salinity (0.4 to 22.1 eq. wt.% NaCl); (3) CO 2±CH 4 inclusions with up to 58 mol% CH 4 and molar volumes between 54 and 73 cm 3; (4) CH 4–H 2O inclusions with CH 4 ranging from 80 to 90 mol%; and (5) CH 4 inclusions with low molar volumes of 19 to 23 cm 3. Types 1, 2 and 3 constitute a fluid inclusion assemblage that occurs consistently in primary, pseudosecondary and secondary trails and clusters within the gold-bearing quartz vein system. These co-existing fluids are interpreted to have formed by fluid immiscibility of a low-salinity, homogeneous parent fluid at about 300°C and 1500 bars. Locally, Type 2 fluid inclusions exhibit total homogenization temperatures that are on average 100°C less than the co-genetically trapped Type 1 aqueous-carbonic inclusions. This discrepancy is interpreted to have involved CO 2 effervescence in response to fluid pressure fluctuations. Types 4 and 5 fluid inclusions are rare, and are only present locally in secondary trails and clusters in extension veins. Significant pressure fluctuations, but relatively constant homogenization temperatures for Types 1, 2 and 3 fluid inclusions, suggest that cyclic decompression of the hydrothermal fluids, due to seismic activity along the shear zones that host the gold-bearing veins, triggered fluid immiscibility. The process of fluid immiscibility and subsequent lowering of gold solubility is interpreted to be the most efficient precipitation mechanism for gold in the D 2 shear zone hosted vein system at Bronzewing.

Gilt by Association? Origins of Pyritic Gold Ores in the Victory Mesothermal Gold Deposit, Western Australia

Sulfur isotope data and calculated fluid-rock reaction paths constrain the origin of pyritic gold ores in the Victory mesothermal gold deposit of Western Australia. Pyrite δ 34S values range from –4.4 to +5.1 and are negatively correlated with gold tenor in mafic host rocks, indicating a significant increase in fluid oxidation state accompanied mineralization. Geologic evidence argues strongly that mineralization occurred with wall-rock sulfidation, but this process alone cannot produce the degree of fluid oxidation necessary to explain the sulfur isotope range. In contrast, either carbonation of ferric iron-bearing minerals or separation of immiscible fluid phases, both of which are consistent with the available geologic data, can drive fluid oxidation to the point of aqueous sulfate dominance. Coupled decreases in gold solubility and sulfide sulfur isotope values that are predicted by such a scenario provide an explanation for the trend between δ 34S and gold tenor observed at Victory. Because both wall-rock carbonation and fluid phase separation involve loss of aqueous sulfide but do not require precipitation of iron sulfides, these processes may explain the poor correlation between gold tenor and pyrite abundance in many mesothermal deposits and the occurrence of some gold ores in iron-poor rocks.

Experimental Study of Gold and Platinum Solubility in a Complex Fluid under Hydrothermal Conditions

Abstract: The solubility of gold was studied in water and aqueous NaCl (1– 5 m) solutions under oxygen and sulfur buffered conditions between 300–500C at a constant pressure 1 kb. Two buffer assemblages HMP and PPM were used. Analysis of the scatter in measured values in log mAu–mNaCl–T frame fixed linear dependence between log mAu and T at any studied iso-pleth (mNaCl) in the form of log mAu = a. T(C) + b. Coefficients of the equation were calculated for water and NaCl (1, 3, 5 m) solutions. The maximum solubility characterizes the NaCl-free system in the presence of HMP. In the case, Au solubility increases from (log mAu) –6. 72 to –5. 04 at 300 and 500C, respectively. In the presence of PPM, maximum of Au solubility was obtained for the 5 mNaCl solution. In a similar manner solubility rises from –6. 54 to –5. 77 at 300 and 500C, accordingly. In studied fO2/fS2 area the behavior of Au solubility testified that: (i) – a composite interaction between chloride and hydrosulfide speciation of gold affects its total solubility; (ii) – in addition of NaCl up to about 1. 5 m the solubility decreases, more pronounced in the presence of HMP; (iii) – the contribution of chloride in total Au solubility is more for PPM despite of lower fO2value, than for HMP. The solubility of platinum was studied in the Pt–Cl–S–H2O system between 300 and 500C, 1 kb. PPM solid buffer controlled oxidation state, pH and sulfur activity of solutions (H2O, 1 mNaCl and 0. 1 mHCl). Under the conditions, PtS precipitated from the solutions with increasing temperature and acidity. The PtS solubility in the 0. 1 mHCl solutions lowers slightly in the range of 300–500C from –5. 30 to –5. 60 (in log mPt) that is typical to the hydrosulfide species. It was deduced that reducing media, regulated by the PPM assemblage, suppress activity of chloride species of Pt. More oxidizing conditions were modeled in runs using mixtures of Mn(II), Mn(III) and Mn(IV) oxides to buffer the aqueous-chloride solutions between 300 and 500C, 1 kb. It was found that MnO tends to oxidize at T below 400C forming intermediate Mn-hydroxides (β–MnOOH, Mn (OH)2 and Mn2(OH)3Cl). These phases are metastable and transfer to Mn3O4 with increasing duration. Generation of the Mn-hydroxides leads to a change of physical-chemical parameters of the solutions, such as water activity, pH and Eh. The last results in abrupt increase in the noble metals dissolution. At stable existence of only Mn3O4, the solubility of both Pt and Au lowers to equilibrium values. Essential catalysis effect of Pt on intensity and rate of Mn(II) oxidation was found. The dominant role of chloride of Pt and Au was defined under most oxidized conditions, specified by Mn2O3–MnO2 buffer. So at 400C, dissolved Au (log mAu) increases from –4. 40 in water to –1. 00 in 0. 1 mHCl, and ones of Pt (log mPt) from –4. 80 to –2. 90 accordingly. Thus, mixing of hydrosulfide and chloride solutions, as well as transformation of the systems to the stable state act upon total solubility of the noble metals.

Au-Sb association and fractionation in micro-disseminated gold deposits, southwestern Guizhou—geochemistry and thermodynamics

The dual geochemical character of paragenesis and fractionation between gold, arsenic and antimony in micro-disseminated gold deposits in southwestern Guizhou is discussed in terms of spatial distribution of independent deposits, lateral and vertical enrichment, mineralization stage and factor and correlation analyses. Thermodynamic calculations of solubility and speciation of gold and antimony minerals indicate that gold is transported in hydrothermal solution as Au(HS) - 2 and antimony is mainly as Sb (OH 0 3 ) although HSb 2 S - 4 may be of increasing importance in acid environment at low temperatures during the late stage of mineralization. Changes in physicochemical conditions hold the key to the association and fractionation between gold and antimony. Gold and antimony response differently to physicochemical variations because they are distinct from each other in soluble speciation and mineral solubility, leading to precipitation at different times and localities during hydrothermal evolution.

Solubility of Native Gold in H—O—S Fluids at 100–400°C and High H2S Content

Reaction of MgS, water, and air in sealed gold capsules at 100 to 400°C and0.15 GPa is used to generate an aqueous fluid with very high (20.6 m) H2Scontent and to remobilize significant quantities of native gold as gold sulfides.A combination of X-ray photoelectron and Auger electron spectroscopy (XPS,AES), analytical scanning-electron microscopy (SEM—EDX),electron-micro-probe analysis (EPMA), and calculated solution properties shows that the goldsulfides precipitated during quenching and later perforation of the capsulesrepresent native gold dissolved as Au(I)-bisulfide under the experimental conditions.The equilibrium constant (logK) for the reaction:Au(s) H2S(aq) + HS− = Au(HS)2− + 1/2H2(g)ranges from −3.96 ± 0.40 at 115°C to −1.06 ± 0.32 at 400°C; it is in goodagreement with literature values for 25°C and 300−350°C, and varies inverselywith absolute temperature T[−ΔH01/(2.303R)= −2644 ± 33K; r = 1.00]. Themaximum solubility of native gold in this study (29.4 g/kg at 200°C) issignificantly greater than that from published studies on Au(I)-bisulfides and maystimulate interest in developing bisulfides as gold-complexing agents in goldextraction technology.

Gold solubility in the common gold-bearing minerals: Experimental evaluation and application to pyrite

A method is proposed for determining gold solubility in the common gold-bearing minerals (sulfides, etc.) using so-called (GAE). These elements increase gold solubility in the fluid phase, enabling minerals formed from hydrothermal fluids to be saturated with gold. Two experimental approaches are discussed: (1) identifying the solid-solution limit of gold in a mineral structure by determining the maximum content of uniformly distributed gold constituent which remains unchanged with increasing gold content in the coexisting fluid phase, and (2) determining the gold distribution between the mineral under study and a reference mineral with a sufficiently high and well-defined solid-solution limit of gold and utilizing the phase composition correlation principle. Statistical treatment of analytical data for single crystals permits inference of the structurally bound gold constituent. Greenockite (alpha -CdS) incorporates a maximum of 50 + or -7 ppm Au in solid solution at 500 degrees C and 1 kbar and this is used as a reference mineral to determine gold solubility in pyrite under the same conditions in the presence of As and Se as gold-assisting elements. The value obtained for gold solubility in pyrite (3+ or -1 ppm Au) is in reasonable agreement with the results of ion-probe microanalysis of natural pyrites. The data suggest that monovalent gold substitutes for divalent iron, giving rise to an acceptor center compensated by a donor defect, either a sulphur vacancy or a hydro-sulphide ion replacing S (super 2-) 2 .

Gold solubility in supercritical hydrothermal brines measured in synthetic fluid inclusions

Dissolved gold contents in chloride brines that were experimentally saturated with gold, magnetite, iron sulfides, orthoclase, and muscovite at 550 degrees to 725 degrees C and 100 to 400 megapascals are reported here. Microsamples of the fluid were isolated at the experimental temperature and pressure as fluid inclusions in quartz. Individual fluid inclusions were opened by laser ablation and analyzed by inductively coupled plasma mass spectrometry. The results show that gold solubility as sulfide species in supercritical brines is far higher than previously supposed. Dissolved gases such as molecular hydrogen sulfide that are ineffective metal-complexing agents at low pressures evidently become highly effective at high pressures.

Gold mobilizing fluids in the Witwatersrand Basin: composition and possible sources

Crush-leach data were obtained, using High Performance Gradient Ion-Chromatography and Capillary Electrophoresis, on individual generations of aqueous fluid inclusions in hydrothermal quartz from three different auriferous conglomerate horizons (reefs) in the late Archaean Witwatersrand Basin, South Africa. These data, supplemented by oxygen isotope analyses of hydrothermal quartz and in combination with microthermometric analyses, help to constrain the chemical composition, pH, temperature of formation and the possible source of the mineralizing fluid which, in places, was capable of mobilizing some of the primarily detrital gold in the fluvial Witwatersrand sediments. The dominant cations in the aqueous fluid inclusions are Na+ and Ca2+, with Cl− or HCO 3 − being the dominant anion, whereas K+, Mg2+, and SO 4 2− are subordinate. Most fluid inclusions have elevated NH 4 + concentrations which are directly correlated with those of N03. In a number of samples small amounts of organic acids (formate, propionate, and acetate) were also detected. A largely meteoric source is inferred for the gold-mobilizing fluids in the Witwatersrand reefs because of a lack of Br in the fluid, a composition distinctly different from that of seawater, the presence of organic acids, and δ18Ofluid values around O%o. The fluids are ascribed to hydrothermal infiltration triggered by the 2020 Ma Vredefort impact which also created a secondary permeability in the form of a dense network of micro-fractures preferentially in the conglomerate beds of the already metamorphosed Witwatersrand rock sequence. This fluid differs from the regional metamorphic fluid in the basin by having a considerably higher pH (5.7–7.2). The difference in pH might explain why the older, fairly acidic metamorphic fluid was apparently less capable of mobilizing the gold as gold solubility reaches its peak at the pH calculated for the fluid ascribed to the impact.

Gold solubility and speciation in hydrothermal solutions: experimental study of the stability of hydrosulphide complex of gold (AuHS°) at 350 to 450°C and 500 bars

The solubility of gold was measured in aqueous KCl (0.5 m) solutions under oxygen, sulfur, and slightly acidic pH buffered conditions between 350 and 450°C at a constant pressure of 500 bars. Two buffer assemblages were used to constrain fO 2, fS 2, and consequently fH 2 and aH 2S: respectively, pyrite-pyrrhotite-magnetite (Py-Po-Mt) and pyrite-magnetite-hematite (Py-Mt-Hm). The measured solubility of gold at equilibrium with Py-Po-Mt and Qtz-KF-Mus is 52 ± 8 ppb at 350°C, 134 ± 17 ppb at 400°C and 496 ± 37 ppb at 450°C. With Py-Mt-Hm and Qtz-KF-Mus the solubility of gold is increased to 198 ± 9 ppb at 400°C and 692 ± 10 ppb at 450°C. These results are consistent with the aqueous complex AuHS° being the dominant gold-bearing species. The equilibrium constants (log K R10) for the reaction: Au (s)+H 2S (aq)=AuHS 0+ 1 2 H 2(g) R10 have been determined at 350, 400, and 450°C and are, respectively, −5.20 ± 0.25, −5.30 ± 0.15, and −5.40 ± 0.15. These values are similar to those suggested by Zotov (written pers. commun.) and those obtained by recalculating the experimental data of Hayashi and Ohmoto (1991). They are significantly higher than those derived by Benning and Seward (1996) and the possible causes of the discrepancies are discussed. The equilibrium constant for AuHS° shows that this species plays an important role in the deposition of gold in natural environments. Cooling, H 2S loss, pH change, and oxidation seem to be effective mechanisms for gold precipitation, depending on the local ore forming conditions.