Bulk Au Research Articles

Fabrication of Uniform Nanoparticulate Gold through Potential-Modulated Electrochemical Deposition and Dissolution of Silver in Ionic Liquids

Nanoparticulate gold (NPG) has been successfully produced from polycrystalline Au by a novel method based on potential-modulated electrochemical deposition and dissolution of silver in an ionic liquid bath comprising of 1-ethyl-3-methylimidazolium chloride and silver nitrate at elevated temperatures. Cyclic voltammetric measurements exhibit that the potential cycles between −1.0 and 0.75 V versus a silver pseudo reference electrode can drive the electrochemical deposition and dissolution of silver onto and away from the Au electrode. Scanning electron microscope studies show that the NPG resulting from the potential cycles has very narrow particle size distribution with an average diameter of approximately 50 nm and is free of elemental Ag residue. As compared to a bulk Au electrode, the NPG electrode demonstrates substantially enhanced electrochemical responses. It exhibits a seven-time higher electro-catalytic activity toward the electrooxidation of nitrite in acidic media.

Electronic and optical correlation effects in bulk gold: Role of spin–orbit coupling

The modified Becke-Johnson exchange potential proposed by Tran and Blaha (TB-mBJ) is useful for accurate calculations of electronic band structures of solids. In this view, we used such potential together with spin–orbit coupling to calculate the electronic and optical correlation effects in bulk gold (Au) via first principle DFT. Inclusion of the relativistic effects in the simulation were found to play vital role on the optical properties of bulk Au. Furthermore, the imaginary part of the dielectric function of Au disclosed good agreement with the experimental results. Disclosure of good agreement between simulation and experimental results (up to Fermi level) was due to the improvement in the band structure of bulk Au achieved by incorporating spin–orbit coupling.

Gold partitioning between 1-dodecanethiol and brine at elevated temperatures: Implications of Au transport in hydrocarbons for oil-brine ore systems

Gold can be associated with hydrocarbons in hydrothermal gold deposits, but the near-absence of experimental data on gold-hydrocarbon interactions at hydrothermal conditions prevents a quantitative interpretation of this relationship. In this study, we investigate the ability of liquid hydrocarbons to act as gold-transporting ore fluids using experiments on the partitioning of gold between brine (10 wt% NaCl) and 1-dodecanethiol (DDT). Experiments were conducted at 105 °C and 150 °C and <5 bar in a hydrothermal flow-through vessel. Partition coefficients (DAuorg/aq) obtained from the 105 °C and 150 °C experiments range from 10 ± 3.0 to 91 ± 30 at 105 °C and from 4.9 ± 0.9 to 33 ± 6.9 at 150 °C, with averages of 37 ± 33 and 21 ± 12, respectively. Gold concentrations in both the brine (<1 ppm) and DDT (up to 10 ppm) are dependent on the initial bulk concentration of gold (up to 4 ppm) and on the Au loss to the vessel walls during the experiment, as the Au concentrations correlate linearly with Au loss, while DAuorg/aq appears independent of bulk composition and Au loss. This linear dependency results in a stronger Au loss from the oil that bears more Au (DDT), and enables the calculation of DAuorg/aq for zero Au loss, which is 19 ± 21. These findings indicate that DAuorg/aq is a constant at the P-T-X conditions investigated. While these experiments did not directly assess Au solubility in DDT, no indication of Au saturation was observed, suggesting that higher concentrations are possible. The pH of the brine decreases during the experiments, indicating that the DDT underwent deprotonation and possibly decomposition during the partitioning reaction, but the pH does not have an observable effect on the gold concentrations in the brine and DDT. Preferential partitioning of gold into DDT is attributed to bonding with the thiol group, and as thiols compose up to 7 wt% of the total organic sulfur content of crude oils, liquid hydrocarbons have the potential to transport significant amounts of gold in ore formation processes. This is of importance for ore systems forming at relatively low temperatures and ore deposits known to be associated with organic matter such as Carlin type Au deposits.

Effect of fs/ps laser pulsewidth on ablation of metals and silicon in air and liquids, and on their nanoparticle yields

The effect of laser pulsewidth, tuned in the range τ = 0.3–10 ps, on infrared (IR) laser ablation (LA) thresholds of bulk gold, silver and silicon in air and in liquid environments (deionized water, isopropyl alcohol (IPA)) was studied in this work. The influence of laser pulsewidth on the morphology and yield of the ejected nanoparticles (NPs), as well as its impact on the efficiency of NP generation, was studied by UV–vis spectral measurement of the extinction coefficient spectra of NP colloids, fabricated by multi-pass ablation of bulk targets in both solvents at different pulse duration values, by analytical disc centrifuge and by high-resolution scanning electron microscopy. The ablation thresholds are raising with the increase of laser pulsewidth, exhibiting sublinear dependences. The extinction coefficient dependence on τ exhibited a non-monotonous character in case of water medium for all materials, and was gradually descending in case of LA in IPA. Additional MD-TTM (molecular dynamics- two-temperature model) modeling was performed for single-shot laser exposition of bulk Au target in air with τ = 0.3 ps and 4 ps, which allowed distinguishing the main mechanisms of the material removal, the formation of NPs, and the observed crater topography. The performed simulations supported the trend on the ablation threshold increase with the increase of laser pulse duration.

Adsorption of Br2 onto Small Au Nanoclusters

Au nanoclusters grown on SiO2 by physical vapor deposition are exposed to Br2 and then measured with 1.5 keV Na+ low energy ion scattering. It is found that the clusters are able to dissociate the molecules, which then adsorb as individual Br atoms, but Br2 does not stick to the bare substrate nor to bulk Au. Adsorption is the first step in any surface chemical reaction, and this result shows how nanoclusters can induce adsorption of species that otherwise do not stick. Results from the literature indicate that catalysis involving nanoclusters occurs at the edges and that the edge atoms are positively charged. This information in conjunction with the ion scattering results lead to the conclusion that the Br adatoms are negatively charged and ionically bonded at the edges of the clusters. Br2 is also a known catalytic poison and this work shows how its adsorption blocks sites that would otherwise be involved in nanocatalysis.

Interference of coexisting copper and aluminum on the ammonium thiosulfate leaching of gold from printed circuit boards of waste mobile phones

Ammonium thiosulfate solution is an ideal lixiviant to extract gold (Au) from electronic wastes (E-wastes) because it is non-toxic, less corrosive, and more selective than conventional cyanide or halide solutions. It was reported recently, however, that Au leaching efficiency in ammonium thiosulfate medium dramatically decreased at high solid-to-liquid ratios (S/L), even though the amounts of reagents used were in excess. To understand how this occurred, leaching experiments were conducted using printed circuit boards (PCBs) from waste mobile phones, and Au distribution in the leaching residues was examined by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). Significant amounts of Au were found together with copper (Cu) and aluminum (Al), implying that extracted Au ions were likely re-deposited during leaching onto Cu and Al found in PCBs via cementation (i.e., reductive deposition). A more detailed elucidation of this phenomenon by cementation experiments using pure Cu and/or Al powders indicates that these metals could only recover Au ions alone via cementation at very high amounts, however, this process became more extensive when Cu and Al powders were suspended together in solution even though the amounts of the individual metals were much lower. Electrochemical experiments (chronoamperometry) in ammonium thiosulfate solutions containing Au ions using an Al working electrode also showed that Au ion cementation was dramatically enhanced when Cu powder was present in solution, and the bulk of Au was cemented on Cu powder rather than on the Al electrode. These results suggest that coexistence of Cu and Al interfered with the extraction of Au in ammonium thiosulfate medium at high S/L because of the enhanced re-deposition of extracted Au via galvanic interaction.

Size Stability Study of Catalytically Active Sub-2 nm Diameter Gold Nanoparticles Synthesized with Weak Stabilizers.

Here we report on the very low size stability of electrocatalytically active 1.5 to 2.0 nm diameter tetrakis(hydroxymethyl)phosphonium chloride-stabilized Au nanoparticles (THPC Au2nm NPs) chemically attached to glass/indium tin oxide electrodes. The potential for oxidative dissolution of THPC Au2nm NPs in the presence of bromide is about 250 mV negative of 4 nm diameter citrate-stabilized Au NPs (Cit Au4nm NPs) and 450 mV negative of bulk Au, which provides us with an easy method to assess the size stability using anodic stripping voltammetry. The THPC Au2nm NPs show a strong CO2 reduction wave at about -0.40 V (vs RHE), which is nonexistent for the Cit Au4nm NPs or bulk Au. The THPC Au2nm NPs are also comparatively more electroactive for the hydrogen evolution reaction. In acid electrolyte, however, the potential for surface Au2O3 formation on THPC Au2nm NPs is significantly negative relative to bulk Au, and a single cycle through the surface oxide and reduction waves leads to an increase in the NP size to about 4 nm. Similarly, the THPC Au2nm NPs undergo Ostwald ripening in the presence of bromide within 5 min at potentials well before oxidation, which increases their size to 4-10 nm in diameter by 35 min. Exposure to ozone for only 1-2 min also causes the THPC Au2nm NPs to increase in size to about 4 nm. In comparison, Cit Au4nm NPs are stable under all of these conditions, requiring much longer times to change in size. These differences in reactivity and size stability are due to the different Au NP size. Sub-2 nm diameter NPs with weak stabilizers are potentially very useful for electrocatalysis, but their low oxidation potential and poor size stability are major issues of concern.

SIESTA: A high current ion source for erosion and retention studies.

Second ion experiment for sputtering and TDS analysis is a high-current ion source for erosion and retention studies with focus on wall materials for fusion devices. The system is composed of a DuoPIGatron type ion source, three consecutive grids for ion extraction, acceleration and beam focusing, a differential pumping stage, a dipole magnet for mass filtering, a quadrupole doublet lens, a target chamber, a load-lock, and a chamber for thermal desorption spectrometry. The acceleration potential of the source can be varied between 500 V and 10 kV. The target chamber has a base pressure of 10-8 mbar and an operating pressure of 5 × 10-7 mbar. The target can be rotated to study angle-dependent effects and can be heated via electron-impact heating up to 1300 K for high temperature erosion and implantation studies. The target chamber is equipped with an in situ magnetic suspension balance. The operating parameters of the ion source were mapped to achieve the maximum ion current at the target for various gas species and accelerating potentials. The beam emittance for a D3 + ion beam was measured after deflection in the dipole magnet. This was used for ion beam simulations, which were instrumental for the design of the quadrupole lenses. If the quadrupole doublet is used, the ion flux to the target is increased by up to a factor of 4. Additionally, the relative population of neutral particles present in the beam at the target was quantified and is equal to 0.8% when averaged over the measurement positions. The typical beam footprint at the target under normal incidence has an area of 0.5 cm2. The ion current reaching the target increases with the accelerating potential. Due to this effect, the ion flux density at the target in the low-ion-impact-energy range can be increased by operating the source at a higher extraction potential and by applying a (decelerating) potential to the target. Ion impact energies as low as 200 eV/D are achieved this way with a D3 + current of 100 μA when focusing the beam with the quadrupole doublet lens, equating to a D particle flux density of 3.7 × 1019 m-2 s-1. At ion impact energies of 2 keV/D, the maximum achievable flux density with D3 + is 6 × 1019 D m-2 s-1. Experimental determination of sputter yields was performed via ex situ weight loss measurement for bulk Au samples, showing reasonably good agreement with simulations and experimental data from the literature.

Nanoindentation tests of heavy-ion-irradiated Au foams—molecular dynamics simulation

Irradiation by light ions may change the mechanical properties of nanofoams. Using molecular-dynamics simulation, we study the effect of irradiating a Au foam (porosity, 50%, and ligament diameter, 3 nm) with heavy ions: here, 10 keV Au ions up to a dose of 4 × 1016 m−2. We demonstrate that in consequence, the ligament morphology changes in the irradiated region, caused by local melting. The changes in mechanical properties are monitored by simulated nanoindentation tests. We find that the foam hardness is only around 1/3 of the hardness of a bulk Au crystal. Irradiation increases the hardness of the foam by around 10% in the central irradiated area. The plastic zone extends to only 1.5 ac, where ac denotes the contact radius; this value is unchanged under irradiation. The hardness increase after irradiation is attributed to two concurring effects. To begin with, irradiation induces melting and annealing of the ligaments, leading to their coarsening and alleviating surface stress, which in turn increases the dislocation nucleation threshold. In addition, irradiation introduces a stacking fault forest that acts as an obstacle to dislocation motion.

MD simulation-based study on the thermodynamic, structural and liquid properties of gold nanostructures

The thermodynamic, dynamical and structural properties of gold nanoparticles (AuNPs) of sizes 603–28897 atoms are studied by using molecular dynamics (MD) simulations based on the Embedded-Atom-Method (EAM) potential. The radial distribution function (RDF) and common neighbors analysis method (CNA) are used to characterize the local structure of the AuNP during the heating processes. The behavior of the Lindemann index of each region for the AuNP with respect to temperature is studied to comprehend the melting mechanism. The simulation results revealed that that the melting starts from the free-surface and proceeds towards the core for AuNP. Moreover, it is found that the complete melting temperature of AuNP at different heating rates decreases drastically with decreasing particle size. The global behavior of the size-dependent melting point evaluated in the present study is in good agreement with the previous experimental data. Furthermore, dynamical properties of AuNP in liquid state are carried out by using the velocity autocorrelation function (VAF) through which the temperature dependence of the self-diffusion coefficient of the AuNP is computed. Besides, it is found that the activation energy found through the Arrhenius equation is less than the experimental value for the Au bulk in the liquid state.

Optimising the visibility of graphene and graphene oxide on gold with multilayer heterostructures

Metals have been increasingly used as substrates in devices based on two-dimensional (2D) materials. However, the high reflectivity of bulk metals results in low optical contrast (<3%) and therefore poor visibility of transparent mono- and few-layer 2D materials on these surfaces. Here we demonstrate that by engineering the complex reflectivity of a purpose-designed multilayer heterostructure composed of thin Au films (2–8 nm) on SiO2/Si substrate, the optical contrast of graphene and graphene oxide (GO) can be significantly enhanced in comparison to bulk Au, up to about 3 and 5 times, respectively. In particular, we achieved ∼17% optical contrast for monolayer GO, which is even 2 times higher than that on bare SiO2/Si substrate. The experimental results are in good agreement with theoretical simulations. This concept is demonstrated for Au, but the methodology is applicable to other metals and can be adopted to design a variety of high-contrast metallic substrates. This will facilitate research and applications of 2D materials in areas such as plasmonics, photonics, catalysis and sensors.

HCHO oxidation over highly dispersed Au nanoparticles supported on mesoporous silica with superior activity and stability

We firstly reported highly active Au nanoparticles catalyst supported on mesoporous silica for low-temperature HCHO oxidation. Amine as the positively charged groups was post grafted onto the surface of mesoporous silica before gold loading (PG), and it showed 15% HCHO conversion to CO2 at room temperature and complete conversion was obtained at 55 °C. No activity decrease was observed within 8 h. As comparison, mesoporous silica supported Au catalysts was also prepared by straightforward chemical reduction (CR) and impregnation (IM), which behaved poor HCHO catalytic activity with complete conversions at 200 °C and 90 °C respectively. The activation energy (Ea) for HCHO oxidation over PG was 17.6 kJ/mol, which was much lower than those (22.2–50.2 kJ/mol) over IM and CR samples. It was found that amine-functionalized mesoporous silica contributed to the well-dispersed and uniform Au nanoparticles with the optimal size distribution about 2–4 nm, and large amounts of low-coordinated Au atoms and electronic structure like bulk Au were observed on these Au nanoparticles, which accelerated HCHO adsorption and transformation into intermediate species like DOM, formate and CO(a). Particularly, the interaction between amine and Au effectively inhibited the reduction of Auδ+ to Au0, which positively contributed to the best catalytic performance. Moreover, Au/SiO2 system prepared by amine post-grafting method showed poor activity of complete combustion at 210 °C. Comparatively, it was proposed that MCM-41 with well-ordered and open ends of pore structure showed better HCHO adsorption-diffusion capacity and the ability of controlling Au particles in high dispersion, which was great benefit to the high catalytic activity of HCHO oxidation.

Formic acid electrooxidation activity of Pt and Pt/Au catalysts: Effects of surface physical properties and irreversible adsorption of Bi

Revealing formic acid oxidation (FAO) behaviors and designing a catalyst are very important in direct formic acid fuel cell industry. Herein, we introduced various Pt substrates of Pt nanoparticles (Pt NPs), Pt deposits on Au nanoparticles (Pt/Au NPs), Pt disk, and Pt deposits on bulk Au disks (Pt/Au disk), and the catalysts modified with Bi by irreversible adsorption. For FAO in the absence of Bi-modification, the catalytic activity increased in the order of Pt disk < Pt NP < Pt/Au disk < Pt/Au NP. On the other hand, the presence of Bi on Pt surfaces enhanced the catalytic performance in the order of Pt/Au NP < Pt/Au disk ≈ Pt NP < Pt disk. The observed activities were found to be highly correlated with a synergistic effect by Bi modification and the surface physical properties of various modified Pt substrates. The single fuel cell performances of various Bi-modified Pt NPs and Pt/Au NPs were additionally examined for real application: the performances per substrate surface area, and Pt mass of Pt/Au NPs and Bi/Pt/Au NPs were better than those of Pt NP and Bi/Pt NPs. Furthermore, the presence of Bi corresponded to single cell potentials higher than those in the absence of Bi by more than 0.1 V at 100 mA cm−2. The full understanding of the effects of surface physical properties and the irreversible adsorption of Bi further deepens the catalytic pathways of formic acid oxidation.

Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations

The borohydride oxidation reaction (BOR) was characterized on the three most-studied noble metals (Au, Pt and Pd) in a range of NaBH4 concentration and temperature that enables to bridge model studies of the BOR (low concentration) to more practical ones, relevant to the direct borohydride fuel cell (DBFC) operation. BOR mechanistic insights were unveiled using the complementary techniques of rotating disk electrode cyclic voltamperometry, rotating ring-disk electrode measurements of the BH3OH− production and differential electrochemical mass spectrometry detection of H2 escape. When the concentration of sodium borohydride is brought to DBFC-like operating conditions, the H2 escape is more severe and the poisoning effect of the metal surfaces by the BOR intermediates (BHads or BH3,ads) is more significant, and stronger on Pt surfaces compared to Au and Pd ones. Even at high NaBH4 concentrations, Pd exhibits promising BOR kinetics, making of this material an interesting candidate for DBFC anode electrocatalysis. These data enabled to complement our previous kinetic model and to confirm the BH3 species oxidation pathways for NaBH4 concentrations of 5 mM and 50 mM. However, this model is incomplete for high borohydride concentrations; it does not take into account possible local pH variations and cannot explain the origin of the important reduction currents measured at high potential on the Au ring electrodes. Finally, it is shown that the BOR mechanism at Pd electrodes must take into account PdH formation and oxidation.

Electrochemical Observation of High Oxophilicity and its Effect on Oxygen Reduction Reaction Activity of Au Clusters Mass-Selectively Deposited on Glassy Carbon

The measurement procedure of oxophilicity and electrocatalytic activity for oxygen reduction reaction (ORR) on size-selected Au clusters was developed and applied. Mass-selected Au9 clusters were deposited on a glassy carbon substrate under ultrahigh vacuum (UHV) conditions, and the model electrode was transferred to an electrochemical cell with the rotating disk electrode (RDE) configuration. Cyclic voltammetry exhibited a significantly lower oxide-reduction peak potential of the Au clusters than that of bulk Au in the first cycle, thus confirming the exceptionally higher oxophilicities for the former. After performing cyclic voltammetry with an upper limit potential of 1.8 V versus a reversible hydrogen electrode, the ORR activity of the Au clusters was lower than that of bulk Au and bulk Pt. This result was partially ascribed to aggregations of Au clusters during the potential cycles.

Electrochemical reduction of CO2 at CuAu nanoparticles: size and alloy effects

Reduction of CO2 at Cu or Au electrodes typically yields methane or CO, respectively. Cu and Au nanoparticles and their alloys offer unique advantages over foil electrodes in terms of reduced overpotentials and product selectivities. In this work, we explore the electrochemical reduction of CO2 in aqueous electrolytes using alloys of Cu and Au including 2 and 6 nm nanoparticles along with polycrystalline foils. These results show the CuAu alloys primarily produce CO; however, yields are dramatically increased relative to Au. CuAu electrodes in the form of planar foils produce up to 3.4 times more CO yields relative to Au foil. Most remarkably, nanoparticle electrodes provide up to 12.5-fold CO yield increases relative to polycrystalline alloy foils and 175-fold CO yield increases relative to bulk Au foils. Voltammetry shows that onset potentials for CO2 reduction are shifted anodically with smaller nanoparticle sizes and with greater Au content. The dramatic increase in CO yields with nanoparticle alloys is attributed to the improved CO2 deoxygenation associated with Cu interfaces and the relatively facile desorption of CO from low-coordination Au sites.

Gold nanocatalysts supported on carbon for electrocatalytic oxidation of organic molecules including guanines in DNA.

Gold (Au) is chemically stable and resistant to oxidation. Although bulk Au is catalytically inert, nanostructured Au exhibits unique size-dependent catalytic activity. When Au nanocatalysts are supported on conductive carbon (denoted as Au@C), Au@C becomes promising for a wide range of electrochemical reactions such as electrooxidation of alcohols and electroreduction of carbon dioxide. In this mini-review, we summarize Au@C nanocatalysts with specific attention on the most recent achievements including the findings in our own laboratories, and show that Au nanoclusters (AuNCs, <2 nm) on nitrided carbon are excellent electrocatalysts for the oxidation of organic molecules including guanines in DNA. The state-of-the-art synthesis and characterization of these nanomaterials are also documented. Synergistic interactions among Au-containing multicomponents on carbon supports and their applications in electrocatalysis are discussed as well. Finally, challenges and future outlook for these emerging and promising nanomaterials are envisaged.

Study of oxygen evolution reaction on amorphous Au13@Ni120P50 nanocluster.

The pursuit of catalysts to promote effective water oxidization to produce oxygen has become a research subject of high priority for water splitting. Here, first-principles calculations are employed to study the water-splitting oxygen evolution reaction (OER) on ∼1.5 nm diameter Au13@Ni120P50 core-shell nanoclusters. Water splitting to produce oxygen proceeds in four intermediate reaction steps (OH*, O*, OOH* and O2). Adsorption configurations and adsorption energies for the species involved in OER on both Au13@Ni120P50 cluster and Ni12P5(001) supported by Au are presented. In addition, thermodynamic free energy diagrams and kinetic potential energy changes are systematically discussed. We show that the third intermediate reaction (O* reacting with H2O to produce OOH*) of the four elementary steps is the reaction-determining step, which accords with previous results. Also, the catalytic performance of OER for Au13@Ni120P50 is better than that for Ni12P5(001) supported by Au in terms of reactive overpotential (0.74 vs. 1.58 V) and kinetic energy barrier (2.18 vs. 3.17 eV). The optimal kinetic pathway for OER is further explored carefully for the Au13@Ni120P50 cluster. The low thermodynamic overpotential and kinetic energy barrier make Au13@Ni120P50 promising for industrial applications as a good OER electrocatalyst candidate.

Evaluation of Structural in Homogeneity by Small Angle Synchrotron Radiation X-ray Scattering and Elucidation of Plastic Deformation Induced Nanocrystallization Behavior in Zr-Cu-Ni-Al-(Pd, Pt, Ag or Au) Bulk Metallic Glasses

Evaluation of Structural in Homogeneity by Small Angle Synchrotron Radiation X-ray Scattering and Elucidation of Plastic Deformation Induced Nanocrystallization Behavior in Zr-Cu-Ni-Al-(Pd, Pt, Ag or Au) Bulk Metallic Glasses

Quantification and Prediction of Bulk Gold Fineness at Placer Gold Mines: A New Zealand Example

This study documents the bulk Au fineness (Au parts per thousand) of the bullion from a placer gold mine in southern New Zealand. The compositions of doré bars produced approximately every 10 days over nearly three years is compared to the range of compositions of gold particles which have been extracted. Silver is the principal impurity in the gold, and the doré bars contained 2–3 wt % Ag over the period examined. At the scale of a typical individual 0.5 mm gold particle, there are three different types of gold: an Ag-bearing core (2–9 wt % Ag), a 10–50 µm wide Ag-poor rim (typically &lt;1 wt % Ag), and micron scale overgrowth gold (0% Ag). The overgrowths are volumetrically negligible, and the average Ag content of a gold particle is controlled principally by the proportions of core and rim gold. The rims have been formed by recrystallisation of deformed core gold, with associated leaching of the Ag from the recrystallised gold. The volumetric proportion of cores has decreased with increasing flattening of gold particles, and highly flattened and folded flakes have little or no remnant cores. The bulk Au fineness of doré bars from the mine has decreased from ~980 to ~970 as the mine progressed upstream in a Pleistocene paleochannel because the upstream gold has been less flattened than the downstream gold.