Au System Research Articles

Geology of the Souter Head subvolcanic complex, Aberdeenshire, Scotland: an Ordovician granite-related Mo–(Bi–As–Au) system

ABSTRACTAn Ordovician subvolcanic intrusive complex hosted by Neoproterozoic metasediments crops out at Souter Head about 6km S of Aberdeen, Scotland. The complex is composed mainly of two-mica red granite and breccia with minor dykes of pegmatite, quartz porphyry, felsite and dolerite, and widespread quartz veining, hydrothermal alteration and minor molybdenite mineralisation. Anomalous levels of bismuth (Bi), arsenic (As) and gold (Au) occur in quartz–pyrite veins. The complex has been mapped and the major- and minor-element geochemistry, including rare-earth elements of intrusives and mineralisation, has been determined. These data reveal a complex tectonic, intrusive and hydrothermal history. The intrusives are peraluminous and magnetite-, muscovite- and garnet-bearing. The youngest member, a quartz porphyry, is highly fractionated. There are two stages of hydrothermal activity: the first is linked to the explosive release of volatiles from a granite cupola and breccia formation; and the second, widespread quartz veining. Mo is associated with both stages, and Bi–As–Au anomalies are found in late quartz–pyrite veins. The mineralisation is classified as a granite-related vein-type Mo system. The unique preservation, in the Grampian terrane, of an Ordovician subvolcanic complex may be attributed to pre-Devonian movements on the nearby Dee fault and possibly also the collapse of the magma chamber following the explosive release of volatiles. The combination of large size, poor exposure and abundant multi-stage hydrothermal activity suggests that there is potential for further Mo and possibly Au mineralisation in this complex. Further mineralisation of this style may be present in the NE Grampian terrane.

Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

Abstract. An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (D1 and D2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early D1 generated a regionally distributed, penetrative S1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this D1 event. Based on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to D1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships. During D2 deformation, folding of S0–S1 structures generated F2 folds with steeply plunging fold axes in low-strain areas. NNW-trending D1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending D2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to D2 structures is a predominantly potassic–ferroan association comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in D2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales to understand the temporal–spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.

Fusion of the Borromean nucleus Be9 with a Au197 target at near-barrier energies

To probe the role of the intrinsic structure of the projectile on sub-barrier fusion, measurement of fusion cross sections has been carried out in $^{9}$Be + $^{197}$Au system in the energy range E$_{c.m.}$/V$_B$ $\approx$ 0.82 to 1.16 using off-beam gamma counting method. Measured fusion excitation function has been analyzed in the framework of the coupled-channel approach using CCFULL code. It is observed that the coupled-channel calculations, including couplings to the inelastic state of the target and the first two states of the rotational band built on the ground state of the projectile, provide a very good description of the sub-barrier fusion data. At above barrier energies, the fusion cross section is found to be suppressed by $\approx$ 39(2)\% as compared to the coupled-channel prediction. A comparison of reduced excitation function of $^{9}$Be + $^{197}$Au with other $x$ + $^{197}$Au shows a larger enhancement for $^9$Be in the sub-barrier region amongst Z=2-5 weakly and tightly bound projectiles, which indicates the prominent role of the projectile deformation in addition to the weak binding.

GENERAL CHARACTERISTICS OF INTERMEDIATE MASS FRAGMENT INCLUSIVE CROSS SECTIONS IN 20Ne INDUCED INTERACTIONS

Inclusive integrated cross sections of intermediate mass fragments have been measured in 20Ne induced reactions on silver and gold targets at 20, 30, 40, 50 and 60 MeV/nucleon incident energies. Whereas the production yields remain approximately constant for the Ne+Ag system, for incident energies greater than 30 MeV/nucleon, they increase by a factor of 3.5 for the Ne+Au system over the full energy range studied.

Morphology and reactivity of size-selected titanium oxide nanoclusters on Au(111).

The morphology and reactivity of mass-selected titania clusters, Ti3O6 and Ti3O5, deposited onto Au(111) were studied by scanning tunneling microscopy and temperature programmed desorption. Despite differing by only one oxygen atom, the stoichiometric Ti3O6 and the sub-stoichiometric ("reduced") Ti3O5 clusters exhibit very different structures and preferred binding sites. The Ti3O6 clusters bind at step edges and form small assemblies (2-4 clusters) on Au terraces, while the "reduced" Ti3O5 clusters form much larger fractal-like assemblies that can extend across step boundaries. Annealing the Ti3O5,6/Au(111) systems to higher temperatures causes changes in the size-distributions of cluster assemblies, but does not lead to the formation of TiOx nanoislands for temperatures ≤700 K. Reactivity studies show that the reduced Ti3O5 cluster has higher activity than Ti3O6 for 2-propanol dehydration, although both clusters exhibit substantial activity for dehydrogenation to acetone. Calculations using DFT+U suggest that the differences in aggregate morphology and reactivity are associated with the number of undercoordinated Ti3c sites in the supported clusters.

Correlation between Crystal-Geometry Parameters and Structural-Phase States in the Alloys Based on Ag–Me (Me = Co, Rh, Ir, Ni, Pd, Pt, Cu, Au)

The results of a search for correlations between the crystal-geometry parameters and structural-phase states in the alloys based on Ag–Me (Me = Co, Rh, Ir, Ni, Pd, Pt, Cu, Au) are presented. The limits of numerical values of the crystal-geometry and thermodynamic factors are identified, within which inorganic regions of solid solutions are formed in the state diagrams of binary systems or monotectic reactions take place in the systems due to a limited solubility of the system components in liquid states. It is shown that an approach relying on the analysis of state diagrams, including the crystal-geometry and thermodynamic factors of the alloy-forming elements into consideration, is promising in searching for common patterns of structural-phase states in the Ag-based alloys; it is found out that an Ag–Pt system is the most attractive binary system in this respect. A slight deviation of the experimental concentration dependences of the atomic volume in the solid solutions of the Ag–Pd and Ag–Au systems from the Zen relation is observed in intermetallic compounds in the alloys of the Ag–Pt system.

The mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface: A DFT study

Density functional theory (DFT) was employed to investigate the mechanism of CH3OH dehydrogenation on the PdAu(1 0 0) surface. The adsorption of intermediates involved in the methanol dehydrogenation process was identified and all the elementary reactions were calculated. The route 3 “CH3OH → CH2OH → CHOH → CHO → CO” is determined as the most preferable pathway, of which the rate-limiting step is the dehydrogenation of CH3OH to CH2OH. PdAu has an enhanced catalytic performance towards MOR comparing to the monometallic Pd and Au systems. The strengthened adsorption of CH3OH and the decreased energy barrier of the rate-limiting step account for the enhanced activity, while the weaker CO adsorption contributes to the better anti-poisoning ability. On the PdAu(1 0 0) surface, Pd atoms play as the active sites while Au strengthens the activity and poisoning tolerance of the neighboring Pd. The synergistic effect of PdAu is explained by the d-band theory. The upshifted d-band center suggests that the Pd atoms on the PdAu(1 0 0) surface are more active than those on the Pd(1 0 0) surface. Our theoretical study reveals the mechanism of CH3OH dehydrogenation on the PdAu(1 0 0) surface, which is in favor of the further optimization of the promising non-platinum PdAu catalysts for DMFCs.

Revisiting the conformational adsorption of L‐ and D‐cysteine on Au nanoparticles by Raman spectroscopy

AbstractUnderstanding the physical mechanisms of thiolated molecules adsorption on metal surfaces has required copious research, particularly on Au–cysteine systems due to the affinity of sulfur molecules to gold surfaces, as well as the interesting structural modifications that this strong interaction induces and the peculiar optical, chiroptical, and electronic properties of Au(SR) systems. Here, we present vibrational experimental data on the adsorption of L‐ and D‐cysteine on small gold nanoparticles (<2 nm) by means of Raman spectroscopy. L‐ and D‐cysteine molecules adopt the same strained conformation upon adsorption on colloidal gold nanoparticles, regaining structure due to the stabilization that the gold nanoparticle induces on the cysteine, reflected in the recuperation of vibrational bands from their polymorphically distinctive crystalline forms. Through the analysis of Raman vibrational modifications after adsorption, we found experimental evidence that confirms a stabilized cysteine conformation locating the carboxyl group in the antiposition (PC isomeric rotamer) for both molecules. This result is supported by extensive density functional theory (DFT) calculations and simulated Raman spectra, considering zwitterionic cysteine adsorbed on a Au34 cluster, emulating experimental nanoparticle sizes. Our Raman spectroscopy experimental and DFT results determine one of the oxygen atoms of the carboxyl group as a second adsorption site after the sulfur atom, confirming that independent of its polymorphism and enantiomerism, zwitterionic cysteine interacts with gold nanoparticles through the thiol group and the carboxyl group as adsorption sites.

A knowledge-driven way to interpret the isometric log-ratio transformation and mixture distributions of geochemical data

When extracting geo-information from geochemical data, it is essential to consider the compositional nature and geological signatures of the data. Isometric log-ratio transformation (ILR) produces an orthonormal basis of geochemical data and accounts for the compositional nature of the data. However, it still often remains difficult to interpret ILR-transformed variables because of the lack of a geological basis for a data-driven approach; therefore, it is necessary to find some geological knowledge-based criteria to help enable a more understandable interpretation following ILR transformation. Characterized by certain elements and elemental ratios, the chronological order of geological processes can be used to construct interpretable ILR transformation. This concept was applied to extract geo-information from stream sediment geochemical data in the Duolong mineral district, Tibet, China. Furthermore, the expectation-maximization (EM) algorithm modified by a minimum message length criterion (MML) was employed to investigate mixture distributions of the geochemical data. In the study area, mafic rocks with high concentrations of Cr and Ni were emplaced earlier than the porphyry Cu–Au deposits which have stable Cu/Au ratios. Based on these criteria and hierarchical cluster analysis, sequential binary partition was constructed among the Cu, Au, Cr, and Ni concentrations. The ILR-transformed variables follow either a bi-normal or tri-normal distribution, the subpopulations of which were interpreted as fingerprints inherited from mafic magmatic processes, Cu–Au hydrothermal systems, and later geological processes, respectively. The high-average subpopulation of ILR-transformed Cu corresponds to anomalies associated with the porphyry and epithermal Cu–Au systems, and two areas are predicted to have high Cu–Au mineralization potential. This study demonstrates that geological knowledge-driven ILR transformation is a promising way to efficiently extract geo-information from geochemical data.

Insights on landscape geochemistry and mineral exploration in the Fraser Range, Albany-Fraser Orogen, Western Australia

The Albany-Fraser Orogen is an emerging recently opened mineral exploration province in Western Australia due to the recent discoveries of the Tropicana-Havana Au system in 2005, and the Nova-Bolli...

Fusion reactions in the Be9 + Au197 system above the Coulomb barrier

The cross sections of complete fusion and incomplete fusion for the $ ^{9} $Be + $ ^{197} $Au system, at energies not too much above the Coulomb barrier, were measured for the first time. The online activation followed by offline $\gamma$-ray spectroscopy method was used for the derivation of the cross sections. A slightly higher value of ICF/TF ratio has been observed, compared to other systems reported in the literature with $ ^{9} $Be beam. The experimental data were compared with coupled channel calculations without taking into account the coupling of the breakup channel, and experimental data of other reaction systems with weakly bound projectiles. A complete fusion suppression of about 40\% was found for the $ ^{9} $Be + $ ^{197} $Au system, at energies above the barrier, whereas the total fusion cross sections are in agreement with the calculations.

Rhenium in Molybdenite: a Database Approach to Identifying Geochemical Controls on the Distribution of a Critical Element

Molybdenite is the world’s principal source of rhenium (Re), a critical element in multiple high-tech applications. However, the Re contents in molybdenite vary by orders of magnitude on scales ranging from single grains to whole deposits. In order to better understand the systematics of this variation and what geochemical factors control molybdenite Re concentration, and hence overall Re resources, we examine global patterns in molybdenite Re contents through a compilation of > 3000 measurements of Re in molybdenite from > 700 mainly ore-bearing moderate- to high-temperature hydrothermal systems of different types. Our results are similar to but expand on those of earlier studies. Rhenium concentration in molybdenite has a lognormal distribution and varies systematically with type of geologic system, intrusive lithology, and Mo grade. The lowest-Re molybdenite occurs in greisens (geometric mean 1 ppm ± a multiplicative standard deviation of 9), quartz vein-hosted W-Sn deposits (2 ± 5 ppm), unmineralized granites and granodiorites (12 ± 8 ppm), intrusion-related deposits (24 ± 8 ppm), and porphyry W-Sn deposits (16 ± 11 ppm). Rhenium is most enriched in molybdenites from volcanic sublimates (23,800 ± 5 ppm), with skarn Fe and Au (560 ± 5 ppm and 540 ± 3 ppm respectively) and porphyry Cu and Cu-Au deposits next (470 ± 4 and 430 ± 7 ppm respectively). Among porphyries, skarns, and quartz vein-hosted deposits, Re is most highly concentrated in molybdenites from Cu and Au systems and its concentration decreases systematically through Cu-Mo, Mo, Sn, and W deposits. In nearly all cases, molybdenites from systems associated with intermediate igneous rocks contain more Re than molybdenites from systems of the same type with more felsic rock associations. The disparity between Re contents of molybdenite in felsic and intermediate systems is largest for porphyries, quartz vein-hosted, and skarn deposits and is near zero for subeconomic or barren granite and granodiorite Mo systems; felsic intrusion-related deposits have slightly higher molybdenite Re than their equivalents associated with intermediate intrusions. In most systems, molybdenite Re content does not correlate with metal grade, but may have an inverse correlation with Au grade in intrusion-related deposits (based on a small number of data points) and does exhibit a strong inverse correlation with deposit Mo grade. Dilution of Re through larger amounts (higher deposit grades) of molybdenite explains about 40% of this correlation, but the relative enrichment of Re in molybdenite from low-Mo deposits must also reflect some selective enrichment of Re/Mo in porphyry Cu systems compared to porphyry Mo systems. We found no evidence for secular increase or other systematic temporal variation in molybdenite Re content. The data regarding the use of molybdenite Re content as a proxy for mantle influence are ambiguous. Nearly all observed empirical correlations can be traced back to differences in redox state and sulfide concentration, the two geochemical factors identified here and by previous experimental work as the controlling influences on Re mobility under hydrothermal conditions. Hydrothermal systems with reducing conditions (W- and Sn-rich) tend to have low molybdenite Re even though compiled whole-rock data indicate that their source rocks have as much or more Re as those of more oxidized systems (e.g., Cu-rich). Vapor-phase exsolution, crustal assimilation, and mixing with external fluids may all enrich molybdenite Re concentrations in individual deposits and deposit types, but their extent and importance in overall hydrothermal concentration of Re is uncertain. Thus, it appears that the available molybdenite Re resource in an ore deposit largely depends on how the deposit’s redox and sulfidation conditions have varied over time and space during the timespan of hydrothermal activity. Oxidized, high-sulfide conditions tend to concentrate Re in molybdenite, whereas reducing conditions tend to leave Re dispersed at low concentrations in the bulk rock.

Cenozoic Strike-Slip Tectonics and Structural Controls of Porphyry Cu-Mo and Epithermal Deposits During Geodynamic Evolution of the Southernmost Lesser Caucasus, Tethyan Metallogenic Belt

Abstract The Zangezur-Ordubad mining district of the southernmost Lesser Caucasus is located in the central segment of the Tethyan metallogenic belt and consists of porphyry Cu-Mo and epithermal Au and base metal systems hosted by the composite Cenozoic Meghri-Ordubad pluton. Ore-hosting structures and magmatic intrusions are predominantly confined to a central N-S–oriented corridor 40 km long and 10 to 12 km wide, located between two regional NNW-oriented right-lateral faults, the Khustup-Giratagh and Salvard-Ordubad faults. The anatomy and kinematics of the main fault network are consistent with dextral strike-slip tectonics controlled by the NNW-oriented Khustup-Giratagh and Salvard-Ordubad faults. Dextral strike-slip tectonics was initiated during the Eocene, concomitantly with final subduction of the Neotethys, and controlled the emplacement of the Agarak, Hanqasar, Aygedzor, and Dastakert porphyry Cu-Mo and Tey-Lichkvaz and Terterasar epithermal Au and base metal deposits. The Eocene structures were repeatedly reactivated during subsequent Neogene evolution in transition to a postsubduction geodynamic setting. Ore-bearing structures at the Oligocene world-class Kadjaran porphyry Cu-Mo deposit were also controlled by dextral strike-slip tectonics, as well as porphyry mineralization and its epithermal overprint hosted by an early Miocene intrusion at Lichk. Eocene to early Miocene dextral strike-slip tectonics took place during NE- to NNE-oriented compression related to Paleogene Eurasia-Arabia convergence and subsequent Neogene postcollision evolution. Paleostress reconstruction indicates major reorganization of tectonic plate kinematics since the early Miocene, resulting in N-S– to NW-oriented compression. Early Miocene epithermal overprint at the Kadjaran porphyry deposit and left-lateral reactivation of faults and mineralized structures are linked to this late Neogene tectonic plate reorganization.

A Sea Trial of Air-Lift Concept Artificial Upwelling in the East China Sea

AbstractArtificial upwelling (AU), as one of the geoengineering tools, has received worldwide attention because of its potential ability to actualize ocean fertilization in a sustainable way. The severe challenges of AU are the design and fabrication of a technologically robust device with structural longevity that can maintain the function in the variable and complex hydrodynamics of the upper ocean. In this work, a sea trial of an air-lift concept AU system driven by self-powered energy was carried out in the East China Sea (ECS; 30°8′14″N, 122°44′59″E) to assess the logistics of at-sea deployment and the durability of the equipment under extremely complex hydrodynamic conditions from 3 to 7 September 2014. Seawater below the thermocline layer was measured to be uplifted from approximately 30 m to the euphotic layer with a volumetric upwelling rate of 155.43 m3 h−1 and total inputs of 2.8 mol h−1 NO3−, 0.15 mol h−1 PO43−, and 4.41 mol h−1 SiO43−. A plume formed by cold, saline deep ocean water (DOW) was tracked by a drifting buoy system with a mixing ratio of 37%–51% DOW at the depth of 18–22 m, which conforms to the simulation results. During the AU’s application, disturbance in the vertical hydrological structure could be observed. However, diatom (Skeletonema costatum) blooming from somewhere in the outer ECS floated to the sea trial region on the second day after the AU’s application, which makes it hard to strip off the biochemical effects of AU from the effects of S. costatum bloom.

Gold ± Copper Endowment and Deposit Diversity in the Western Tethyan Magmatic Belt, Southeast Europe: Implications for Exploration

Abstract Major Au and Cu deposits in the Western Tethyan magmatic belt formed during two main periods of Cretaceous and Cenozoic magmatism. The Cretaceous deposits are dominantly Cu-Au porphyry, high-sulfidation epithermal, and volcanic massive sulfide deposits, whereas in the Cenozoic Cu is significant only in porphyry systems. However, the Cenozoic contains approximately three times greater total Au endowment (for Au deposits >0.5 million ounces), and also has a greater deposit diversity, including porphyry Au-Cu and Au-only deposits, high-, intermediate-, and low-sulfidation epithermal Au systems, and Au-rich carbonate replacement and sediment-hosted styles. The differences in endowment and deposit styles likely reflect regional-scale tectono-magmatic processes as well as local preservation and emplacement levels. The Cu ± Au endowment of the Cretaceous is consistent with typical subduction-related arc environments and generation of calc-alkaline porphyry to high-sulfidation epithermal systems, whereas Au enrichment related to Cenozoic magmatism appears to be related to high-K calc-alkalic to shoshonitic compositions. In many of the Au-rich Cenozoic magmatic belts, there is geochemical evidence for sourcing subcontinental lithospheric mantle that was previously enriched by Cretaceous subduction-related metasomatism. Additional differences in Au endowment may reflect the preservation of shallow-level systems in the Cenozoic, particularly for the Au-rich Miocene porphyry deposits such as Kışladağ and Bierly Vrch and the Apuseni porphyry Au-Cu deposits. However, in both the Cretaceous and Cenozoic, crustal exposure levels vary across the belt and cannot explain all the differences in Cu and Au endowment. A compilation of exploration discovery methods highlights the importance of historic workings in addition to geochemistry and geology as an initial vector, whereas geophysics has had limited involvement in direct discovery, primarily due to its limited application historically. Geologic models for well-understood systems such as porphyry and proximal epithermal systems provide excellent guides for explorers; however, more distal deposits such as Au-rich carbonate replacement deposits and deposits with poorly constrained models such as sedimentary rock-hosted and intermediate-sulfidation deposits are more challenging for exploration.

The origin of CH4-rich fluids in reduced porphyry–skarn Cu–Mo–Au systems

Although most of the methane on Earth is of biogenic origin, significant abiogenic methane has been identified in a variety of geological environments. Moreover, the presence of methane has been reported recently in fluid inclusions from a number of reduced porphyry deposits. The genesis of this methane, however, remains poorly understood. Here, we present results of a study of magma-exsolved CH4-rich aqueous fluid inclusions in the Seleteguole reduced porphyry–skarn Cu–Mo–Au deposit, NW China. Four types of fluid inclusions, namely intermediate-density two-phase aqueous inclusions, low-density vapor inclusions, brine inclusions and aqueous liquid inclusions, have been distinguished on the basis of the phases present at room temperature. The intermediate-density inclusions were trapped as a single phase and contain 3.8–10.4 mol/kg CH4, corresponding to an oxygen fugacity between ΔFMQ − 1 and ΔFMQ − 0.5. This attests to the remarkably reduced nature of the fluid. The δ13C value of the CH4-hosted fluid inclusions ranges from –29.4‰ to –19.1‰ (PDB), which distinguishes this methane from biogenic methane. Indeed, the high homogenization temperature of the CH4-rich inclusions (~400 °C) implies that the CH4 was exsolved directly from the magma. Given the environment in which the magma was likely generated, i.e., at or immediately above a subducting plate, we propose that the methane was produced by reactions involving organic matter-bearing carbonate rocks in the subduction zone. Although Cu, Mo and Au in most porphyry systems are interpreted to have been transported under oxidizing conditions, our results indicate that reduced aqueous liquids and vapors are also capable of transporting appreciable concentrations of these metals.

Effect of nuclear surface diffuseness on Coulomb excitation and total nuclear reaction cross sections

We investigate the effect of variation of nuclear surface diffuseness, through diffuseness parameter, on the evaluation of survival probability, absorption effects, Coulomb excitation cross section and total nuclear reaction cross section of Kr+80197Au system. The diffuseness parameter is found to be affecting all the above mentioned quantities appreciably. Specifically, the survival probability and Coulomb excitation cross section are found to be decreasing with increase in a while absorption effects and σR are found to be increasing.

Stability of ternary nanocrystalline alloys in the Pt–Pd–Au system

Binary alloys have frequently been found to exhibit nanostructured states that are stabilized against grain growth when the minority alloying element strongly segregates to grain boundaries, thereby reducing the grain boundary excess energy. This concept has been formalized in several theoretical frameworks that are now used to design stable nanocrystalline binary alloys. In this work, we consider the extension of such a framework to ternary alloy systems. By considering pairwise interactions between atoms in ternary systems, we identify an experimental system, Pt–Au–Pd, and produce combinatorial alloy films in this system to evaluate their stability against grain growth. In several of these nanostructures stability against grain growth up to 400 °C is observed due to the co-segregation of Au and Pd solutes to the grain boundaries. This result is in line with calculations of a grain boundary energy-based estimation of stability after extending the analysis to account for the nontrivial ternary interaction.

Charge-transfer transition in Au-induced quantum wires on Si(553)

The Si(553)-Au system resembles a heteroatomic chain ensemble with a delicate spin-charge interplay. The ordering of the $\ifmmode\times\else\texttimes\fi{}3$ reconstruction vanishes via a phase transition taking place at ${T}_{c}=100$ K. Our directional-dependent surface transport measurements showed that this order-disorder phase transition is not driven by the formation of a charge-density wave, as previously suggested. Instead, at 65 K there is a pronounced increase of the surface-state conductivity along the wires. We attribute this to activated charge transfer between the localized Si dangling bond states and the proximate Au bands revealing a $\ifmmode\times\else\texttimes\fi{}2$ periodicity. Apparently, a quasiorthogonality between the wave functions of the two proximal reconstructions is also responsible for a missing $\ifmmode\times\else\texttimes\fi{}6$ periodicity along the wires. The electronic charge transfer is in agreement with recent band-structure calculations.

Two-stage gold mineralization of the Axi epithermal Au deposit, Western Tianshan, NW China: Evidence from Re–Os dating, S isotope, and trace elements of pyrite

The Axi gold deposit is one of the largest epithermal deposits in northwestern China and is characterized by multistage generations of pyrite associated with gold mineralization. Genetic models for superjacent epithermal Au and porphyry Cu systems involve either contemporaneous or episodic mineralization, implying different metal sources and ore-forming events. Nine pyrite separates from disseminated ores associated with pyrite-sericite-quartz alteration in the Axi deposit yield a Re–Os isochron age of 350 ± 10 Ma (MSWD = 5.1, initial 187Os/188Os (IOs) = 0.20 ± 0.11). This age likely reflects a mixed contribution of two subtypes of disseminated pyrite (Py1) based on zoned pyrite crystals. The isochron age of 353 ± 6 Ma (MSWD = 1.6, IOs = 0.107 ± 0.021, n = 6), excluding samples with pyrite cores, overlaps previous ages for andesite from the Dahalajunshan Formation (ca. 356 Ma), indicating that the early mineralization event occurred shortly after the formation of host rocks (ca. 353 Ma). Pyrite from disseminated ores shows little variation of sulfur isotopic compositions (+ 2.9 to + 4.0‰) and has high Cu, Co, Ni, and V contents, which favors a magmatic-hydrothermal origin. Pyrite grains from gray quartz veins yield a younger Re–Os isochron age of 332 ± 8 Ma (MSWD = 0.22, n = 4). Vein-hosted pyrite shows characteristics of a mantle-derived magma source (IOs: 0.171 ± 0.024; δ34S: − 0.10~+ 3.10‰), which result from the intrusion-driven convective hydrothermal circulation accompanying fluid exsolution and metal remobilization from basement and wallrocks. Our data demonstrate that the Axi gold deposit was formed during two temporally separated hydrothermal events. This model of ore formation may apply to other epithermal-porphyry deposits within the Axi–Tawuerbieke district: disseminated Au and porphyry-style Au ± Cu mineralization occurred shortly after magmatic activity (356–353 Ma) whereas epithermal-style Au mineralization occurred at ca. 332 Ma.