Solubility Of Gold Research Articles

Solubility of Gold in FeO–SiO2–CaO–Al2O3–MgO Slag for Smelting of Gold-Containing Secondary Resources

Solubility of Gold in FeO–SiO2–CaO–Al2O3–MgO Slag for Smelting of Gold-Containing Secondary Resources

Gold in sulfide fluids revisited

Gold solubility was measured at temperatures of 350, 400, 450, and 490 °C and pressures of 500 and 1000 bar in an ’oxidized sulfide’ system, as a function of pHT (2 – 10) and sulfur concentration (m(Stotal) = 0.03 – 1.2 [mol·(kg H2O)-1]). In this system, sulfur primarily exists as H2S, H2SO3, H2SO4, their dissociation products, and radical species such as S2- and S3-. The complexes Au(HS)2-, Au2S22-, AuHS(aq), AuHS(H2S)3(aq), and AuOH(aq) were identified as the primary gold species in the experimental fluids, varying with pH and m(Stotal). The solubility constants for Au(HS)2-, a critical (hydro)sulfide complex, align excellently with literature for ’reduced sulfide’ fluids, where sulfur predominantly exists in the 2- oxidation state. New experimental data from the ’oxidized sulfide’ system were regressed along with reliable literature data from ’reduced sulfide’ systems to calculate standard thermodynamic properties and parameters of the Helgeson-Kirkham-Flowers (HKF) model. The solubility constants for charged complexes, Au(HS)2- and Au2S22-, increase sharply with temperature, whereas those for neutral species, AuHS(aq) and AuHS(H2S)3(aq), show a pronounced peak near 300 °C. These (hydro)sulfide complexes account for gold solubility ranging from a few tens of ppb to a few tens of ppm in natural sulfide fluids, depending on the fluid pH. Thermodynamic calculations also indicate that, in addition to (hydro)sulfide species and the hydroxide complex, AuCl2- significantly contributes to Au mobility in high-temperature acidic chloride fluids. Based on new experimental data and prior studies using solubility and X-ray absorption spectroscopy methods, other gold complexes including mixed Au-HS-Cl, Au-HS-S3- species, and complexes with alkali metal cations, are deemed redundant. Above 250 °C, the influence of chloride salts on gold solubility can be accurately modeled using a simple extended Debye-Hückel equation with the term bγ·I = 0. The Setchenov coefficient bn = 0 suffices for calculating the activity coefficients of neutral species. This streamlined thermodynamic model aligns closely with earlier experimental work by Terry Seward and his team, effectively describing the state of Au in all types of natural fluids where Au exists in the 1+ oxidation state, under any set of P-T-f(O2)-f(S2)-compositional parameters.

Occurrence of State of Gold in Crude Oil and Its Economic Significance

Gold and petroleum are also strategic resources of great importance to national security. With the increasing demand for energy, multi-energy cooperative exploration has become an inevitable trend of resource development and utilization. Petroleum and hydrothermal gold deposits may form together, with similar evolutionary trends in their formation, migration, and enrichment. Petroleum reservoirs and gold deposits are closely coupled under certain geological conditions. The solubility of gold in crude oil and its forms of occurrence are important in determining the mechanisms of interaction between gold and petroleum and in facilitating the recovery of gold from gold-bearing petroleum. In this study, the occurrence of gold in crude oil from the Linnan Depression in the Bohai Bay Basin, China, was studied using inductively coupled plasma–mass spectrometry and X-ray photoelectron spectroscopy. Concentrations of gold in crude oil from the Linpan and Shanghe oilfields averaged 44.5 ppb, which is well above the minimum concentration required for hydrothermal gold mineralization. Gold has an affinity with carbon, oxygen, and sulfur, and its concentration in crude oil is positively correlated with total acid and sulfur contents. We speculate that gold may exist in crude oil as complexes with organic acids or thiols, with crude oil thus being a transport medium for gold.

Formation of Intergrowths of Platinum-Group Minerals and Gold from Magmatogenic Ores in Relation to Phase Changes in Pt-Pd-Fe-Cu-Au System

The article discusses the features of the chemical composition and the formation of intergrowths of platinum-group minerals, gold, gold-bearing phases, and other ore minerals present in placers collected from the Anabar River in the northeast part of the Siberian platform. Based on an analysis of changes in the phase compositions of these intergrowths of noble metals with other ore minerals on (Pt, Pd)-Fe-Au and Pd-Cu-Au phase equilibrium diagrams, potential trends in the crystallization of natural polymineral alloys from multicomponent low-sulfide metallic liquids are discussed. The similarity of the microstructures of natural and metallurgical alloys indicates that the formation of natural multiphase Au-PGE intergrowths occurred in a similar manner to the crystallization of multicomponent synthetic alloys. The authors suggest that magmatic Au-PGE mineralization occurs during the crystallization of a noble-metal-containing, low-sulfide, Cr-rich oxide melt separated from silicate mafic–ultramafic magma. Magmatic gold–platinum deposits are commonly associated with sulfide or oxide disseminated-schlieren ores in layered mafic–ultramafic intrusions. However, due to the high solubility of gold and platinoids in sulfide minerals, PGMs in sulfide ores occur as isomorphic impurities or as microphases and dispersed inclusions that cannot form placers. Therefore, the authors suggest that magmatic Au-PGE mineralization occurs during the crystallization of an immiscible low-sulfide, high-Cr oxide liquid separated from silicate mafic–ultramafic magma. In the northeast part of the Siberian platform, potential sources for these placers are likely alkaline, high-Ti mafic–ultramafic intrusions, as confirmed by the presence of silicate inclusions in ferroan platinum similar in composition to melteigite.

3D Plotting of Gold Solubility and Gold Fineness: Quantitative Analysis of Ore-Forming Conditions in Hydrothermal Gold Deposits

Abstract The 3D plotting of gold solubility and gold fineness aims to illustrate how to quantify their correlations with ore-forming conditions in hydrothermal gold deposits. The thermodynamic calculation of the Au-Ag solid solutions in Mathematica and the 3D plotting in MATLAB are used to build isopleths of gold solubility and gold fineness at different temperatures (200℃, 400℃), pressures (0.1, 5 kbar), salinities (1, 40 wt% NaCl eq.), and sulfur concentrations (0.01, 0.5 mol/kg). The plot indicates that the ore-forming conditions have different correlations with gold solubility and gold fineness. Average rates of change for the correlations are quantified, showing distinct values in the four pH-logfO2 fields of (I) HSO4−, (II) SO42−, (III) H2S, and (IV) HS−, where dominant gold and silver complexes have different dependencies on the conditions. The quantification of the plots illustrates that a decrease in gold solubility by one order of magnitude is possibly caused by a decrease in temperature of ≥40℃, the salinity of ≥9.6 wt% NaCl eq. or sulfur concentration of ≥0.14 mol/kg, or an increase in pressure of ≥3 kbar, while a decrease in gold fineness by 100 units is possibly caused by a decrease in temperature of ≥14 ℃, pressure of ≥1.4 kbar, or salinity of ≥4 wt% NaCl eq., or an increase in sulfur concentration of ≥0.07 mol/kg. Quantification results suggest that a sharp decrease in temperature may result in large-scale gold mineralization and a great variation in gold fineness. In addition, the quantification reveals that the correlation between gold solubility and gold fineness can be expressed by a function, providing a rapid method for 3D plotting.

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

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.

Analyses under the curve, identifying how invisible gold is held in pyrite

AbstractWhen laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of pyrite plot below the gold solubility line on a gold vs. arsenic plot and have relatively flat counts on laser ablation time-resolved output graphs, it is often interpreted that the gold is held within the pyrite structure. The study by Ehrig et al. (2023, this issue) shows, using a combination of LA-ICP-MS spot analyses of gold in pyrite, transmission electron microscopy, and electron backscatter diffraction that this is not necessarily the case. Furthermore, they use these same techniques to identify how trace elements, including gold, are remobilized in pyrite during deformation and metamorphism.

The solubility and speciation of gold in ammonia-bearing aqueous solutions at elevated temperature: An experimental study

The discovery, using short wavelength infrared radiation (SWIR), that some low-sulfidation epithermal gold deposits are associated with hydrothermal alteration involving ammonium minerals, particularly buddingtonite ((NH4)AlSi3O8), has led to the suggestion that the gold may have been transported as a complex involving ammonia (NH3). The speciation of gold in ammonia-rich solutions, however, has received little attention from experimentalists and, consequently, it is not known whether gold forms stable complexes with NH3 and whether such species could contribute significantly to the transport of gold in epithermal ore-forming systems. To resolve this issue, we have conducted experiments to investigate the speciation and solubility of gold in ammonia-bearing solutions at temperatures of 225 and 250 °C under vapour-saturated water pressure with oxygen fugacity buffered by the assemblage magnetite-hematite. Based on the results of these experiments, gold does form stable species with ammonia at elevated temperature. The gold is interpreted to have been dissolved predominantly as Au(NH3)OH0 in solutions with a NH30 concentration of 0.005 ∼ 0.49 m and a pH(T) of 3.29 ∼ 7.47. This species formed via the reaction, Au (s) + NH3 (aq) + H2O = Au(NH3)OH(aq) + 1/2 H2 (g) The logarithms of the equilibrium constant for this reaction are −8.95 ± 0.84 and −7.71 ± 0.85 for 225 and 250 °C, respectively. In order to determine whether this species is important for the transport of gold in epithermal systems, we modeled the speciation of gold in a fluid at conditions typical of those epithermal gold depositing fluids. This modeling shows that the solubility of gold as AuHS0 and Au(HS)2- is much higher than that as Au(NH3)OH0 except in alkaline ammonia-bearing fluids. The association of gold mineralization with hydrothermal alteration involving the formation of ammonium-bearing minerals in the deposits is therefore probably unrelated to the transport of gold as a Au-NH3 complex but simply reflects the high concentration of NH4+ in the ore-forming fluid.

Alteration and mineralization patterns in orogenic gold deposits: Constraints from deposit observation and thermodynamic modeling

Hydrothermal alteration and mineralization types of orogenic gold deposits display many variations depending on formation depths, types of host rock, compositions of ore fluids, spatial positions, fluid/rock ratios, and repetitive modifications during multiple fluid infiltrations in the complicated processes of fluid-rock interaction. Therefore, establishing holistic reference patterns of hydrothermal alteration with associated mineralization for the orogenic gold system is necessary. Based on the observation of typical orogenic gold deposits and thermodynamic simulation of fluid-rock reaction, the general features of proposed patterns include:(1)Orogenic ore fluids are capable of transporting sufficient gold from deep (5 kbar and 600°C) to shallow (1 kbar and 200°C) crustal levels, resulting in alteration and mineralization occurred in a much-varied depth. Only considering the effects of fluid-rock reaction and sulfidation, the formation of pyrrhotite and pyrite at relatively high and low P-T conditions, respectively, is the most important mechanism driving gold precipitation. In addition, the changes of fluid fO2 and pH can also trigger a decrease in the gold solubility in fluids.(2)Multiple waves of fluid infiltration are capable of dissolving the early-formedulfides and gold, and reprecipitate them in the distal alteration zones, which can explain the irregular gold concentration distribution patterns in wide alteration zones. The fluids under higher P-T conditions (5 kbar and 600°C and 3 kbar and 400°C) are more capable of dissolving early-formed sulfides and gold. Additionally, the later infiltrated fluids can transform early-formed pyrrhotite into pyrite.(3)The model of interactions between ore fluids and aluminosilicate rocks at varied P-T conditions indicates that the alteration mineral assemblages in hypozonal deposits (>12 km) mainly include biotite, amphibole, anorthite, K-feldspar, quartz, and pyrrhotite assemblages; that those in mesozonal deposits (6 to 12 km) are characterized by chlorite, calcite, quartz, muscovite, pyrite or pyrrhotite. The epizonal deposits (<6 km) commonly develop more abundant Fe-Mg bearing carbonates (e.g., dolomite, ankerite, and siderite), quartz, muscovite, and pyrite. The compositional variations of ore fluids and host rocks have important controls on the development of iron-oxides and carbonaceous material.(4) The alteration halo is relatively restricted under hypozonal to lower mesozonal conditions. However, broad carbonation develops under upper mesozonal to epizonal conditions, with Fe-Mg bearing carbonates and calcite distributed in proximal and distal zones, respectively.

The solubility of Cu, Ag and Au in magmatic sulfur-bearing fluids as a function of oxygen fugacity

Magma-derived fluids containing chlorine and sulfur are critical for the transport of ore metals to porphyry ore-forming environments. However, experimental data on the speciation and the solubility of ore metals in these fluids at conditions relevant to the arc magmatism are scarce. In particular, the effect of redox conditions on ore metal speciation and solubilities in sulfur-bearing fluids has not yet been experimentally constrained. We performed experiments to determine the effect of oxygen fugacity (fO2) on the solubility of Cu, Ag and Au in high-temperature, low- density, low-salinity fluids and hypersaline brine. The experiments were conducted at T = 900 °C, P = 2000 bar varying fO2 in 7 steps between 0.5 log units below the Ni-NiO buffer (NNO − 0.5) to NNO + 2.5. A prototype rapid-quench Molybdenum-Hafnium Carbide (MHC) externally heated pressure vessel assembly was used, which was fitted with a Shaw membrane for precise control of fO2. The fluid phase was sampled as synthetic fluid inclusions (SFI) by in situ fracturing of quartz chips during the experiments. As capsule material, Au97Ag2Cu1 alloy was used, which imposed activities of 0.962, 0.0082 and 0.0097 for Au, Ag and Cu, respectively. The apparent solubility of Cu and Ag at the imposed metal activities increases by a factor of 7 in the H2O-NaCl-KCl-S low-salinity fluid with fO2 increasing from NNO − 0.5 to NNO + 2.5. The addition of 0.198 m HCl increases the overall solubility of Cu by a factor of 1.2–2.4. The apparent solubility of Au decreases by a factor of 9 as fO2 changes from NNO − 0.5 to NNO + 2.5. The relationship between the logarithms of the apparent Cu and Ag solubilities and fO2 is linear and the slope of the fitted line corresponds to 1+ oxidation state of these metals indicating that they are dominantly complexed by ligands that are S-free (e.g., chloride). Thermodynamic model calculations indicate that the dominant species for Cu and Ag are NaCuCl2 and NaAgCl2, respectively. For Au, the dominant species is predicted to be NaAu(HS)2 at low and intermediate fO2 conditions, and AuCl and NaAuCl2 at oxidizing conditions. The measured gold solubilities at intermediate fO2 do not indicate significant Au complexation with S species containing S in intermediate oxidation states. Considering previous studies on silicate melts and our experimental data for volatiles, we conclude that Cu and Ag likely have constant fluid/melt partition coefficients in the typical fO2 range of arc magmatism because they dissolve in the same oxidation state in the fluid and the melt (1+) and are dominantly chloride complexed in the fluid.

Leaching of complex gold ore using a cyanide-glycine solution

In this research, the leaching of complex gold ore was studied using cyanide and glycine. In the absence of glycine, the gold dissolution increases from 6.47 to 62.21% with increasing the cyanide concentration from 250 to 4500 g/t. The ore contains sulfide minerals and base metals, such as copper, zinc, and silver, which are high cyanide consumers. At 2000 g/t cyanide concentration, the gold dissolution significantly increases from 30 to 92% with adding 0.5 mol/L of glycine. A further increase in the glycine concentration has no significant effect on the dissolution of gold. At 2000 g/t cyanide and 0.5 mol/L glycine concentrations in the absence of H2O2, the dissolution of gold is 78%, which increases to 88% and 93% with increasing the H2O2 concentration to 1 and 1.5%, respectively. The solid content is very effective in the range of 20–25%. Further increases in the solid content to 30, 35, and 40% do not have a significant effect on the gold dissolution. When only glycine is in the system, the gold dissolution is very low, and when only cyanide is in the system, the gold and copper dissolutions are almost the same. The solubility of copper in the presence of cyanide and glycine is the same, but the solubility of gold in cyanide is much higher than glycine.

High solubility of gold in H2S-H2O ± NaCl fluids at 100–200 MPa and 600–800 °C: A synthetic fluid inclusion study

Numerous previous experimental and theoretical studies of gold deposits in the Earth’s crust have shown that chloride and hydrosulfide are the most abundant ligands in the relevant hydrothermal fluids. However, most of these studies were performed with solutions under relatively oxidizing conditions in the presence of SO42-, HSO4-, and S3-. Reduced sulfur with a −2 valence (e.g., H2S, HS-, etc.) may play a key role in controlling redox conditions and promoting the migration of gold during the formation of epithermaland porphyry gold-copper deposits. To examine this possibility, we performed experiments in reduced H2S-bearing fluids to constrain the effects of temperature, pressure and NaCl concentration on Au solubility through the synthetic fluid inclusion method. Measured amounts of H2S and aqueous solutions (H2O ± NaCl) were loaded in an Au capsule containing a fractured quartz column, and fluid inclusions were formed through healing of these fractures at specified pressures (100 or 200 MPa) and temperatures (600, 700 or 800 °C) for 20 days. Thin sections with both sides polished were prepared from quenched quartz columns, and conventional microthermometric measurements were performed. Raman spectra were collected from all phases in the fluid inclusions from room temperature to 300 °C to identify sulfur species and potential Au-bearing complexes. In all fluid inclusions, S and H2Sn (n ≥ 1) species were identified, but SO2 and SO42-, HSO4- and S3- ions were not observed. Au concentrations in these fluid inclusions, which were analyzed using LA-ICP-MS with NaCl or RbCl reference standards, ranged from 91 to 3599 ppm; they were greatly affected by pressure and NaCl concentration but not obviously by temperature. H2Sn (n > 1) analogs have chemical properties similar to those of H2S, and their deprotonation products (HSn- and Sn2-) exhibit high affinities for Au in aqueous solution. Therefore, HSn-Au (or AuSn-) and Au-Cl could be the dominant species in H2S-bearing hydrothermal fluids and may play important roles during the dissolution and transport stages of Au in ore formation processes.

Textures of gold-bearing sulfides and gold precipitation mechanism, Wangu gold deposit, Jiangnan Orogen

万古金矿床位于江南造山带中部，赋存于新元古界冷家溪群浅变质岩系中，受NNE-NE向长沙-平江断裂带和近EW向九岭-清水韧性剪切带联合控制，金资源量约85t。其主要矿石类型为毒砂-黄铁绢英岩型和石英-硫化物脉型，其次为构造角砾岩型。毒砂和黄铁矿为该矿床主要的载金矿物，分布广泛。金成矿作用可分为四个阶段，I为乳白色石英-绢云母-白钨矿阶段；Ⅱ为烟灰色石英-绢云母-毒砂-黄铁矿-金阶段；Ⅲ为烟灰色石英-绢云母-黄铁矿-毒砂-多金属硫化物-金阶段；Ⅳ为乳白色石英-方解石阶段。其中，Ⅱ、Ⅲ为成矿主阶段。根据成矿主阶段毒砂电子探针分析结果，Ⅱ阶段毒砂中As的含量在42.19%~44.84%之间，均值为43.42%（n=56），Ⅲ阶段毒砂中As的含量在40.08%~43.36%之间，均值为42.08%（n=19）。通过毒砂温度计相图估算出Ⅱ、Ⅲ阶段的形成温度和硫逸度分别为364±21℃、319±22℃和10^-9.7~10^-7、10^-11.5~10^-8.6。电子探针数据揭示的载金毒砂和黄铁矿中不可见金含量分别为0.01%~0.66%和0.01%~0.11%。黄铁矿Au-As元素投点分布于金溶解度曲线两侧，说明其内金主要以纳米级颗粒和固溶体金或晶格金的形式赋存；其中Ⅱ阶段黄铁矿纳米级金颗粒占比为73.33%，多于Ⅲ阶段黄铁矿（67.80%）。以上数据说明在水岩反应过程中，围岩中的含铁矿物与成矿流体中的H₂S发生反应，生成毒砂和黄铁矿。伴随着强烈的水岩反应，成矿温度和硫逸度降低，成矿Ⅱ阶段至Ⅲ阶段主要载金硫化物由毒砂转变为黄铁矿，强烈的硫化作用导致金-硫络合物失稳并释放金，金以置换的方式进入硫化物晶格或以显微-超显微金颗粒的形式沉淀，形成含金硫化物；即硫化作用是导致万古矿床不可见金沉淀的主导机制。;The Wangu gold deposit, with a proven gold resource of~85t, is located in the middle section of the Jiangnan Orogen and occurs in metamorphic rock series of the Neoproterozoic Lengjiaxi Group. It formed under the control of the NNE-NE-trending Changsha-Pingjiang fault zone and the EW-trending Jiuling-Qingshui ductile shear zone. The main ore types of the deposit include arsenopyrite-, pyrite-, sericite-, and quartz-altered slate and quartz-sulfide veins, followed by slate breccia within hydrothermal quartz. Arsenopyrite and pyrite are the main gold bearing minerals in the deposit, which are widely distributed in the deposit. Gold mineralization can be divided into four stages:I, the milky quartz-muscovite-scheelite; Ⅱ, the smoky gray quartz-muscovite-arsenopyrite-pyrite-gold; Ⅲ, the smoky gray quartz-muscovite-pyrite-arsenopyrite-polymetallic sulfide-gold; and Ⅳ, the milky quartz-calcite. Among them, the Ⅱ and Ⅲ are the main mineralization stages. Based on the results of Electron Probe Micro Analysis (EPMA) of arsenopyrite from the main mineralization stages, the contents of As in arsenopyrite at stage Ⅱ range from 42.19% to 44.84%, with an average of 43.42% (n=56). The contents of As in arsenopyrite at stage Ⅲ range from 40.08% to 43.36%, with an average of 42.08% (n=19). According to the phase diagram of arsenopyrite thermometer, the formation temperature of Apy-1 in the stage Ⅱ is estimated to be 364±21℃ with a sulfur fugacity varying within 10^-9.7~10^-7. The formation temperature of Apy-2 in the stage Ⅲ is 319±22℃, and its sulfur fugacity is 10^-11.5~10^-8.6. The contents of invisible gold in gold-bearing arsenopyrite and pyrite were 0.01%~0.66% and 0.01%~0.11%, respectively, revealed by EPMA data. The Au-As data of pyrite are plotted on both sides of the gold solubility curve, indicating that gold in pyrite mainly exists in the form of nano-scale particles and solid solution or lattice gold. The proportion of nanometer gold particles in Py-1 pyrite is 73.33%, more than that in Py-2 (67.80%). The above data indicate that in the process of water-rock reaction, the iron-bearing minerals in surrounding rocks react with H₂S in ore-forming fluid to form arsenopyrite and pyrite. Accompanied by a strong water-rock reaction, the main gold-bearing sulfides changed from arsenopyrite to pyrite during mineralization from stage Ⅱ to Ⅲ with the decrease of mineralization temperature and sulfur fugacity, i.e., strong sulfurization results in instability of the gold-sulfur complex and the release of gold. As a result, gold is deposited into the sulfide lattice by substitution or in the form of microscopic-ultra microscopic gold particles to form gold-bearing sulfide. Therefore, sulfidation is the main mechanism of the precipitation of invisible gold in sulfides at the Wangu gold deposit.

The solubility of gold and palladium in magmatic brines: Implications for PGE enrichment in mafic-ultramafic and porphyry environments

We performed experiments to determine the solubility of Au and Pd in magmatic aqueous fluids as a function of oxygen fugacity (ƒO2), temperature (T), pH and total chloride concentration (Cltotal). Experiments were conducted at 800–1000 °C and 200 MPa in an externally-heated rapid-quench Molybdenum-Hafnium Carbide (MHC) cold-seal pressure vessel assembly. We employed a synthetic fluid inclusion (SFI) technique to entrap equilibrated, hydrothermal fluids in response to in situ fracturing of quartz cylinders at experimental run conditions. The solubility of Au and Pd both have positive relationships with ƒO2, temperature, acidity and chlorinity. Concentrated aqueous brines containing 63 wt.% NaCl can dissolve wt.% levels of Au (∼1.2 wt.%) and Pd (∼1.7 wt.%) at metal saturation in relatively oxidized conditions, 1.44 log units above the Ni-NiO oxygen buffer (NNO+1.44), and mildly acidic pH at 900 °C and 200 MPa. Thermodynamic modeling of experimental results suggests that Au is mainly transported as AuCl(aq) at high pH and low Cltotal conditions, whereas HAuCl2(aq) and potentially AuCl2(aq)− predominates at low pH and high Cltotal conditions. Results from thermodynamic modeling also suggest Pd is mobilized in significant contributions by both PdCl2(aq) and PdCl3(aq)− with the latter gaining predominance in response to increasing Cltotal. Calculated fluid/melt partition coefficients for Au and Pd in low-density, magmatic vapors at 1000 °C and 200 MPa suggest that Pd may experience fractionation from Au in porphyry Au-Cu (±Pd, Pt) systems due to the restricted compatibility of Pd in the fluid phase (requiring strongly acidic and substantially high ƒO2 conditions). Moreover, high-density, concentrated aqueous brines facilitate the compatibility of Pd in the fluid phase which may be important with respect to the formation of platinum-group element (PGE)-enriched horizons in layered mafic intrusions (e.g., J-M Reef, Stillwater Complex, U.S.A.). The potential for magmatic, near-neutral pH, high-salinity brines to dissolve significant amounts of Pd as Pd(II)-chloride complexes (∼400 to ∼900 µg/g) well below the NNO buffer suggests that such fluids may be responsible for late-stage hydrothermal remobilization of Pd within mafic-ultramafic igneous environments (e.g., Cu-Ni-PGE footwall deposits and low-sulfide PGE deposits in the Sudbury Igneous Complex, Canada).

Quantitative In Situ Visualization of Thermal Effects on the Formation of Gold Nanocrystals in Solution.

Understanding temperature effects in nanochemistry requires real-time in situ measurements because this key parameter of wet-chemical synthesis simultaneously influences the kinetics of chemical reactions and the thermodynamic equilibrium of nanomaterials in solution. Here, temperature-controlled liquid cell transmission electron microscopy is exploited to directly image the radiolysis-driven formation of gold nanoparticles between 25 °C and 85 °C and provide a deeper understanding of the atomic-scale processes determining the size and shape of gold colloids. By quantitatively comparing the nucleation and growth rates of colloidal assemblies with classical models for nanocrystal formation, it is shown that the increase of the molecular diffusion and the solubility of gold governs the drastic changes in the formation dynamics of nanostructures in solution with temperature. In contraction with the common view of coarsening processes in solution, it is also demonstrated that the dissolution of nanoparticles and thus the Ostwald ripening is not only driven by size effects. Furthermore, visualizing thermal effects on faceting processes at the single nanoparticle level reveals how the competition between the growth speed and the surface diffusion dictates the final shape of nanocrystals.

The extensive hydrocarbon-mediated fixation of hydrothermal gold in the Witwatersrand Basin, South Africa

There is a close spatial relation between high-grade gold mineralization in the Witwatersrand basin and carbonaceous nodules, veins and seams. Hydrocarbons thus may well have been essential in ore genesis. We have sampled four major gold-, uranium- and hydrocarbon-bearing ore horizons, namely the Carbon Leader, Vaal, B and Black reefs, to determine the role of hydrocarbons in the accumulation and hydrothermal fixation of gold. Our multipronged approach included high-resolution scanning and transmission electron microscopy (SEM, TEM), nanotomography with video clips, and geochemical modeling. Post-depositional hydrothermal activity at the peak of regional metamorphism produced an assemblage of quartz, phyllosilicates, brannerite, crandallite, florencite, monazite and gold in all four reefs. The gold, hydrocarbons and associated mineral assemblages are closely related on the micro to nano scale. Gold deposition occurred in interstices, as fracture fillings in detrital minerals, and on the surface of migrated solid hydrocarbon residues. The spherical to elliptical inclusions in the gold consist of an outer pyrobitumen phase and a central void space, partially associated with nanometric gold, uraninite, coffinite and silica. The hydrocarbon-bearing inclusion likely formed by the entrapment of a fossil liquid oil precursor during gold precipitation. The oil was subsequently thermally altered and converted into the final pyrobitumen and gaseous residues. Geochemical calculations to simulate the interaction of an invading hot hydrothermal fluid with the hydrocarbons in the reefs reveal that a very small amount of hydrocarbons will drastically decrease the aqueous solubility of gold and hence cause its instant precipitation.We extend our genetic model for the epigenetic formation of gold in the Witwatersrand. Regional metamorphism promoted the extensive and likely basin-wide circulation of hydrothermal fluids; these were capable of mobilizing substantial amounts of gold. The liquid, gaseous and solid hydrocarbons in the reefs acted as efficient chemical traps for the concentration of gold. Being strong chemical reductants, they caused the rapid precipitation and accumulation of gold on the surface of the fossil oil droplets and already solidified hydrocarbons. The release of the gases from accessible hydrocarbons into the sediments away from their source buffered the redox state of the hydrothermal solutions even at a considerable distance from the pyrobitumen seams and veins, likely resulting in the deposition of gold in the absence of visible hydrocarbons. Although our findings do not explain the ultimate origin and exceptional endowment of gold in the Witwatersrand, we do provide intriguing evidence for the large-scale hydrothermal mobilization, accumulation and fixation of gold mediated by hydrocarbons during post-depositional metamorphism.

Gold speciation in hydrothermal fluids revealed by in situ high energy resolution X-ray absorption spectroscopy

Abstract Gold mobilization, transfer, and concentration in the Earth’s crust are controlled by hydrothermal sulfur- and chloride-bearing fluids. Yet the exact chemical identity, structure, and stability of Au-bearing species and, in particular, the respective contributions of the sulfide (HS−) and trisulfur ion (S3⋅−) ligands to Au transport lack direct in situ evidence. Here we employed high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) on aqueous sulfate/sulfide/S3⋅−-bearing solutions at typical hydrothermal temperatures and pressures (T = 350 °C, P = 600 bar) to reveal differences in dissolved Au spectral signatures indicative of contrasting fluid-phase Au speciation as a function of acidity and redox conditions. Combined with in situ Au solubility measurements and quantum-chemical and thermodynamic modeling, our spectroscopic data provide direct evidence for the Au(HS)S3− and Au(HS)2− complexes predominant at acidic-to-neutral and alkaline conditions, respectively. Our findings thus directly confirm a recent speciation scheme for Au in aqueous S-bearing fluids established using less direct methods, and highlight an important role of the trisulfur ion in gold mobilization and concentration in hydrothermal-magmatic deposits associated with subduction zones. More generally, our results show that HERFD-XAS enables the identification of structural and coordination features in metal complexes virtually unresolvable using classical XAS techniques. By avoiding limitations of less direct techniques, our integrated high-resolution spectroscopic approach opens perspectives for studies of the speciation and solubility of gold and other metals in high T-P fluids, and potentially silicate melts, inaccessible to direct observation in nature.

Gold solubility in silicate melts and fluids: Advances from high-pressure and high-temperature experiments

Gold solubility in silicate melts and fluids: Advances from high-pressure and high-temperature experiments

Are Vapor-Like Fluids Viable Ore Fluids for Cu-Au-Mo Porphyry Ore Formation?

Abstract Ore formation in porphyry Cu-Au-(Mo) systems involves the exsolution of metal-bearing fluids from magmas and the transport of the metals in magmatic-hydrothermal plumes that are subject to pressure fluctuations. Deposition of ore minerals occurs as a result of cooling and decompression of the hydrothermal fluids in partly overlapping ore shells. In this study, we address the role of vapor-like fluids in porphyry ore formation through numerical simulations of metal transport using the Gibbs energy minimization software, GEM-Selektor. The thermodynamic properties of the hydrated gaseous metallic species necessary for modeling metal solubility in fluids of moderate density (100–300 kg/m3) were derived from the results of experiments that investigated the solubility of metals in aqueous HCl- and H2S-bearing vapors. Metal transport and precipitation were simulated numerically as a function of temperature, pressure, and fluid composition (S, Cl, and redox). The simulated metal concentrations and ratios are compared to those observed in vapor-like and intermediate-density fluid inclusions from porphyry ore deposits, as well as gas condensates from active volcanoes. The thermodynamically predicted solubility of Cu, Au, Ag, and Mo decreases during isothermal decompression. At elevated pressure, the simulated metal solubility is similar to the metal content measured in vapor-like and intermediate-density fluid inclusions from porphyry deposits (at ~200–1,800 bar). At ambient pressure, the metal solubility approaches the metal content measured in gas condensates from active volcanoes (at ~1 bar), which is several orders of magnitude lower than that in the high-pressure environment. During isochoric cooling, the simulated solubility of Cu, Ag, and Mo decreases, whereas that of Au reaches a maximum between 35 ppb and 2.6 ppm depending on fluid density and composition. Similar observations are made from a compilation of vapor-like and intermediate-density fluid inclusion data showing that Cu, Ag, and Mo contents decrease with decreasing pressure and temperature. Increasing the Cl concentration of the simulated fluid promotes the solubility of Cu, Ag, and Au chloride species. Molybdenum solubility is highest under oxidizing conditions and low S content, and gold solubility is elevated at intermediate redox conditions and elevated S content. The S content of the vapor-like fluid strongly affects metal ratios. Thus, there is a decrease in the Cu/Au ratio as the S content increases from 0.1 to 1 wt %, whereas the opposite is the case for the Mo/Ag ratio; at S contents of &amp;gt;1 wt %, the Mo/Ag ratio also decreases. In summary, thermodynamic calculations based on experiments involving gaseous metallic species predict that vapor-like fluids may transport and efficiently precipitate metals in concentrations sufficient to form porphyry ore deposits. Finally, the fluid composition and pressure-temperature evolution paths of vapor-like and intermediate-density fluids have a strong effect on metal solubility in porphyry systems and potentially exert an important control on their metal ratios and zoning.

Gold dispersion in tropical weathering profiles at the Belikombone gold anomaly (Bétaré Oya gold district), east Cameroon

The distribution of gold in the weathering blanket at the Belikombone gold prospect in east Cameroon provides insights into gold mobility in the secondary environment and in tropical terrains worldwide. Both gridline-controlled sampling of topsoil (surface samples) and sampling of various layers in pits are used and the gold assay for each sample determined by NiS fire assay with ICP-AES finish. One hundred and thirty-two (132) surface samples and a total of 206 samples from 19 exploration pits were analyzed. The results from the topsoil samples show an anomaly with the highest Au concentration at 5.9 mgkg−1. The mineralization corridor follows a NE-SW trend. The horizons within the pits range from sap rock at the base, through saprolite, rubble layer rich in relict quartz material to a ferruginous loose layer at the top although some horizons are missing in some pits. All the layers contain gold and the highest concentration in the sap rock horizon is 3.4 mgkg−1 while the rubble layer has a gold high of 6.1 mgkg−1. The individual soil horizons show no systematic gold trends and given the presence of gold in all layers, the patterns point towards supergene dissolution and redistribution of gold. Gold enrichment within the upper horizons in the weathering blanket is attributed to sequestration by Fe oxides of chemically remobilized gold. However, the high gold content within the sap rock and saprolite layers suggests that migration of gold in the particulate form supersedes chemical gold redistribution. Particulate gold obtained by panning samples from the pits varies in shape from euhedral, elongated to irregular. Electron microprobe analysis on the grains record high contents of gold in the rim zones (90.0 to 99.8 wt%). The cores are relatively rich in Ag (12.6 to 14.2 wt%) while the rims are poor in Ag. The low Ag content in the rims is attributed to the preferential leaching of Ag. The soil pH value in this area varies between 3.6 and 7.3. Under such acidic to near neutral conditions, bisulfide and thiosulfate ions can dissolve and transport Au and Ag to be precipitated under surficial conditions creating authigenic Au haloes especially in the saprolite and sap rock layers. Such pH values together with oxidizing Eh conditions explain the solubility of gold in the area. These results are important for geochemical exploration of gold in tropical terrains, and confirm previous studies.