Secondary Au Research Articles

Complexity in the Au–Ag–Hg system: New information from a PGE (‘osmiridium’) concentrate at Waratah Bay, Victoria, Australia

AbstractAu–Hg–Ag phases have been described from a variety of metallogenic orebodies and the placer deposits derived from them. In many documented placer deposits, the phases typically occur intergrown as ‘secondary’ rims to primary Au–Ag grains. The origin of these rims has been ascribed to supergene redistribution reactions during deposition or to the effects of amalgamation (i.e. use of mercury) during mining for gold. Difficulties in determining compositions and crystal structures on such a small scale have made full characterisation of these phases problematic. This paper describes a new occurrence of these phases, found by accident during investigation of a historical concentrate of ‘osmiridium’ containing a number of gold grains from beach sands at Waratah Bay, in southern Victoria, Australia. The phases occur as rims to gold grains and are intergrown on a scale of tens of micrometres or less. Application of electron microprobe analysis (EPMA) and limited electron back-scattered diffraction (EBSD) was required to characterise them. These techniques revealed the presence of the approved mineral weishanite (Au–Hg–Ag) and a phase with compositional range Au2Hg–Au3Hg surrounding primary Au–Ag (electrum) containing trace amounts of Hg. EBSD analysis showed weishanite is hexagonal P63/mmc and Au2Hg to be hexagonal P63/mcm. Comparison with published data from other localities (Philippines, British Columbia and New Zealand) suggests weishanite has a wide compositional field. Textures shown by these phases are difficult to interpret, as they might form by either supergene processes or by reaction with anthropogenic mercury used during mining. However, in the absence of any historical evidence for the use of mercury for gold mining at Waratah Bay, we consider the formation of the Au–Hg phases is most probably due to supergene alteration of primary Au–Ag alloy containing small amounts of Hg. In addition to revealing some of the reaction sequences in the development of these secondary Au–Hg–Ag rims, this paper illustrates methods by which these phases can be more fully characterised and thereby better correlated with the Au–Hg synthetic system.

Double pseudo-polymeric complexes of [Au(S2CNR2)2][TlCl4], R2 = (CH2)5, (CH2)6: preparation, structural organization, and thermal behavior (low-temperature reduction and crystallization of gold)

The capability of thallium(I) alkylenedithiocarbamates to bind gold(III) from solutions in 2 M HCl was studied. Double complexes of [Au{S2CN(CH2)5}2][TlCl4] and [Au{S2CN(CH2)6}2][TlCl4] were preparatively isolated as individual forms of gold(III) binding into the solid phase. In the crystalline state, the ionic structural units of the complexes are combined together due to the secondary Au···S, Tl···S and S···Cl bonds that results in complicated supramolecular architectures. Thermal behavior of compounds was studied using the simultaneous thermal analysis technique; both reduced elemental gold and thallium(I) chloride were identified as the final thermolysis products.

Discriminating Between Primary and Secondary Au Events in a Paragenetically Complex Archean Lode-Gold Deposit, Wawa Gold Corridor, Ontario, Canada

AbstractThe Wawa gold corridor, located in the Michipicoten greenstone belt of the Superior province, Canada, comprises Au-bearing shear zones that crosscut the 2745 Ma Jubilee stock and that evolved during protracted deformation (D1-D3). Numerous generations of sulfide minerals crystallized before, during, and after these deformation events, and gold is associated with D1 arsenopyrite, D2 pyrite, and Bi-Te phases and chalcopyrite in assemblages that crosscut D3 veins. Observations of porosity and inclusions in D1 arsenopyrite and D2 pyrite suggest these sulfides underwent coupled dissolution-reprecipitation reactions. By coupling these textural observations with trace element analysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), it is evident that such replacement reactions generated gold from Au previously in solid solution. Conversely, textural relationships among paragenetically late gold and Bi-Te minerals are consistent with the precipitation of these phases from Bi-rich polymetallic melts. Mass balance calculations involving comparisons of the mass of Au in sulfides and the total mass of Au in their associated host rocks indicate that only D1 arsenopyrite contained enough Au to account for whole-rock Au content and overall deposit grade. Although D1 arsenopyrite is less volumetrically abundant than the various pyrite generations present in the deposit, it is often replaced by the later pyrite types, which is compatible with higher initial volumes of arsenopyrite than what is presently observed. It is concluded that the D1 Au + arsenopyrite event was the principal Au-mineralizing event in the Wawa gold corridor and that the other gold-bearing assemblages (i.e., gold + D2 pyrite, gold + Bi-Te phases + chalcopyrite) largely represent secondary mobilization of this primary enrichment. Given that LA-ICP-MS sulfide chemistry is regularly used in orogenic Au research, the approach outlined herein to assess the relative impact of distinct Au- and sulfide-mineralizing events could easily be applied to the study of other Au deposits in which complex hydrothermal parageneses are recognized.

An ionic gold(III)–zinc(II) pseudo-polymeric compound of [H3O][Au{S2CN(CH2)5}2]3[ZnCl4]2: Synthesis, supramolecular architecture and anti-tuberculosis activity

An ionic Au(III)-Zn(II) dithiocarbamato-chlorido compound of [H3O][Au{S2CN(CH2)5}2]3[ZnCl4]2 (1) has been synthesised by the reaction of binuclear zinc(II) pentamethylenedithiocarbamate with a H[AuCl4]/2.0 M HCl solution. The obtained complex was successfully characterised by FT-IR spectroscopy, elemental analysis and single-crystal X-ray diffraction. There are six ionic moieties in the unit cell of 1: (i) three dithiocarbamato-gold(III) cations, one of which is structurally inequivalent to the other two; (ii) two symmetrically related tetrachloridozincate(II) anions and (iii) one hydronium cation that is disordered over four positions with equal occupancies of 0.25. The compound exhibits a rather complicated supramolecular structure, including pseudo-binuclear gold-rich [Au2(S2CNPm)4]2+ cations and {[ZnCl4]2}4– counter anions, which are stabilised by secondary Au···S and Cl···Cl interactions, respectively. In turn, the former pseudo-binuclear formations involving centrosymmetric mononuclear cations of [Au(S2CNPm)2]+ give the pseudo-polymeric cationic chains of (···[Au2(S2CNPm)4]2+···[Au(S2CNPm)2]+···)n; while the latter, pseudo-binuclear zinc(II) anions, {[ZnCl4]2}4–, as the double linkers combine the neighbouring cationic chains yielding a two-dimensional pseudo-polymeric network. The results on the biological activity of 1in vitro against the non-pathogenic strain Mycolicibacterium smegmatis (MIC 0.05 µg/disc) and virulent strains Mycobacterium tuberculosis H37Rv (0.14–0.19 µg/mL) and CN-40 (0.11–0.14 µg/mL) indicate a high efficiency of the studied Au(III)–Zn(II) compound against mycobacteria.

Binding of Gold(III) from Solutions by the [Ag6{S2CN(CH2)6}6] Cluster: Synthesis, Thermal Behavior, and Self-Organization of the Supramolecular Structure of the Double Complex [Au{S2CN(CH2)6}2]2[AgCl2]Cl\xb72CHCl3 (Role of Secondary Au\u22c5\u22c5\u22c5Cl, Ag\u22c5\u22c5\u22c5S, and Cl\u22c5\u22c5\u22c5Cl Interactions)

The capability of silver(I) cyclo-hexamethylenedithiocarbamate to concentrate gold(III) from solutions characterized by a high level of salinity (5.15 M NaCl) into the solid phase has been established. The double chloroform-solvated Au(III)–Ag(I) complex [Au{S2CN(CH2)6}2]2[AgCl2]Cl·2CHCl3 (I) was preparatively isolated as an individual form of binding of [AuCl4]– anions. The composition of the ionic structural units of compound I indicates that gold(III) binding from a solution to the solid phase is accompanied by the complete redistribution of the HmDtc ligands between the coordination spheres of Ag(I) and Au(III). Complex I characterized by IR spectroscopy, simultaneous thermal analysis, and X-ray structure analysis (CIF file CCDC no. 2051654) exhibits the supramolecular structure containing two oppositely charged pseudo-polymeric subsystems. Complex cations [Au{S2CN(CH2)6}2]+ and anions [AgCl2]– (in a ratio of 2 : 1) form a complicatedly organized cation-anionic pseudo-polymeric ribbon ({[Au(HmDtc)2]⋅⋅⋅[AgCl2]⋅⋅⋅[Au(HmDtc)2]}+)n due to secondary interactions Ag⋅⋅⋅S (3.2613 Å) and Au⋅⋅⋅Cl (3.2765 Å). The pseudo-polymeric ribbon consists of two rows of cations and a row of anions. The outer-sphere chloride ions combine the solvate chloroform molecules by two equivalent hydrogen bonds Cl⋅⋅⋅H–C yielding anion-molecular triads [Cl3CH⋅⋅⋅Cl⋅⋅⋅HCCl3]–, which are involved in the formation of the supramolecular ribbon due to the secondary Cl⋅⋅⋅Cl interactions (3.4058 Å) between the nonequivalent chlorine atoms of the nearest solvate molecules. The study of the thermal behavior of complex I makes it possible to determine the character of thermolysis and conditions for the quantitative regeneration of bound gold.

Crystal structure, solid-state 13C and 15N NMR characterisation, chemisorption activity and thermal behaviour of new mercury(II) dipropyldithiocarbamate: Binuclear, pseudo-binuclear and heteronuclear complexes of [Hg2(PrDtc)4], [Hg(PrDtc)2]2 and [Au(PrDtc)2]2[Hg2Cl6

Two polymorphs of a new mercury(II) N,N-dipropyldithiocarbamate represented by centrosymmetric binuclear and pseudo-binuclear molecules, [Hg2(PrDtc)4] (1) and [Hg(PrDtc)2]2 (2), have been isolated, identified using heteronuclear (13C, 15N) CP-MAS NMR and structurally characterised by single-crystal X-ray diffraction analysis. There are two pairs of inequivalent PrDtc ligands playing either terminal chelating or tridentate bridging structural functions in the former compound; while the latter complex comprises two pairs of structurally inequivalent chelating PrDtc ligands. The reaction of freshly precipitated mercury(II) N,N-dipropyldithiocarbamate (HgPrDtc) with a [AuCl4]−/2 M HCl solution results in the formation of the ionic complex [Au(PrDtc)2]2[Hg2Cl6] (3). There are two inequivalent centrosymmetric cations, [Au(PrDtc)2]+ (‘A’ and ‘B’), and a binuclear centrosymmetric anion, [Hg2Cl6]2− in the structure of 3. In the cationic part of the complex, each of the gold(III) cations has two pairs of the relatively weak inequivalent secondary Au···S bonds with two neighbours, therefore forming linear supramolecular cationic chains (⋯‘A’⋯‘B’⋯‘A’⋯‘B’⋯)n. To study the thermal behaviour of the compounds, simultaneous thermal analysis was also performed. The formation of HgS and reduced elemental gold was established during the thermolysis of 1/2 and 3, respectively.

Dissolved organic matter-mediated reduction of ionic Au(III) to elemental Au nanoparticles and their growth to visible granules

The biogeochemical transformation of gold (Au), i.e. its dissolution and re-precipitation, is critical in supergene transport of Au and formation of Au granules. Besides biogenic reduction, the formation Au granules can also be driven by chemical processes. Previous studies have showed the formation of Au nanoparticles (AuNPs) from ionic Au(III) can be mediated by dissolved organic matter under sunlight. In this letter, we further demonstrated that these AuNPs can further slowly (in years) grow into visible Au granules. Different sized nano-flower and fractal dendrite-like branched gold structures (from tens of nanometres to over 100μm) were observed in the Au granule sample. This growth of AuNPs into visible Au granules may play a critical role in the supergene mineralization and enrichment of secondary Au and drive the biogeochemical cycle of Au.

Binding of gold(iii) with silver(i) dipropyldithiocarbamate: supramolecular self-assembly (role of secondary Au…S and Ag…S bonds) and thermal behavior of the ionic-polymer complex ([Au(S2CNPr2)2][AgCl2])n

The double ionic-polymer complex ([Au(S2CNPr2)2][AgCl2])n (1) was prepared as an individual fixation form of gold(III) from NaCl solutions with silver(I) dipropyldithiocarbamate and was characterized by single-crystal X-ray diffraction and 13C magic-angle spinning (MAS) NMR spectroscopy. The structure of 1 comprises two nonequivalent centrosymmetric complex cations [Au(S2CNPr2)2]+ (A and B) and the discrete linear anion [AgCl2]–. Gold(III) cations are linked by pairs of unsymmetrical secondary Au…S bonds to form linear supramolecular chains (…A…B…)n. Neighboring cations are additionally linked by [AgCl2]– anions via secondary Ag…S and Cl…S bonds, the anions being involved in the overall stabilization of the supramolecular structure. The cation–anion interactions lead to a distortion of the linear configuration of the [AgCl2]– anion. The character of thermolysis of 1 accompanied by quantitative regeneration of bound Au and Ag was established by simultaneous thermal analysis.

Reflecting on Gold Geomicrobiology Research: Thoughts and Considerations for Future Endeavors

Research in gold (Au) geomicrobiology has developed extensively over the last ten years, as more Au-bearing materials from around the world point towards a consistent story: That microbes interact with Au. In weathering environments, Au is mobile, taking the form of oxidized, soluble complexes or reduced, elemental Au nanoparticles. The transition of Au between aqueous and solid states is attributed to varying geochemical conditions, catalyzed in part by the biosphere. Hence, a global Au-biogeochemical-cycle was proposed. The primary focus of this mini-review is to reflect upon the biogeochemical processes that contribute to what we currently know about Au cycling. In general, the global Au-biogeochemical-cycle begins with the liberation of gold-silver particles from a primary host rock, by physical weathering. Through oxidative-complexation, inorganic and organic soluble-Au complexes are produced. However, in the presence of microbes or other reductants—e.g., clays and Fe-oxides—these Au complexes can be destabilized. The reduction of soluble Au ultimately leads to the bioprecipitation and biomineralization of Au, the product of which can aggregate into larger structures, thereby completing the Au cycle. Evidence of these processes have been “recorded” in the preservation of secondary Au structures that have been observed on Au particles from around the world. These structures—i.e., nanometer-size to micrometer-size Au dissolution and reprecipitation features—are “snap shots” of biogeochemical influences on Au, during its journey in Earth-surface environments. Therefore, microbes can have a profound effect on the occurrence of Au in natural environments, given the nutrients necessary for microbial metabolism are sustained and Au is in the system.

Reactivity of Silver(I) Diethyldithiocarbamate and Ion-Polymeric Complexes {[Au(S2CNEt2)2][AgCl2]}n and {[Au(S2CNEt2)2]2[AgCl2]Cl·2H2O}n: Synthesis, Supramolecular Structures, and Thermal Behavior

New ion polymeric gold(III)–silver(I) complexes, {[Au(S2CNEt2)2][AgCl2]}n and {[Au(S2CNEt2)2]2[AgCl2]Cl·2H2O}n have been synthesized and characterized by X-ray diffraction data and IR spectra. The structure of the first complex is represented by [Au{S2CNEt2}2]+ complex cations and polymeric silver chloride anion ([AgCl2]–)n; the second complex consists of isomeric cations, linear [AgCl2]– anions, chloride ions, and water molecules. Gold cations are involved in zigzag (AuAuAu angle 99.06°) and linear polymeric chains via secondary Au···S bond pairs. Thermal decomposition of the complexes with regeneration of gold and silver has been studied by simultaneous thermal analysis (STA).

Binding of Gold(III) from Solutions with Thallium(I) Dibutyldithiocarbamate: Synthesis, Supramolecular Self-Organization, and Thermal Behavior of the Complex ([Au{S2CN(C4H9)2}2][TlCl4])n

The binding of gold(III) from solution in a 2 M HCl with thallium(I) dibutyl dithiocarbamate leads to the formation of ion-polymeric complex ([Au{S2CN(C4H9)2}2][TlCl4])n, which was studied by (13С, 15N) MAS NMR spectroscopy and X-ray diffraction analysis. In the complex comprising the nonequivalent cations [Au{S2CN(C4H9)2}2]+ (A and B) and anions [TlCl4]–, the supramolecular self-assembly is provided by secondary bonds Au···S and S···Cl. The former are involved in the formation of isomeric binuclear cations [А···А] and [В···В] that build the (···[А···А]···[В···В]···)n polymeric chain; the latter selectively combine the thallium(III) anions and the dimeric cations. Thermolysis of the complex is accompanied by gold recovery and release of TlCl.

Nanoparticle Immobilization for Controllable Experiments in Liquid-Cell Transmission Electron Microscopy.

We demonstrate that silanization can control the adhesion of nanostructures to the SiN windows compatible with liquid-cell transmission electron microscopy (LC-TEM). Formation of an (3-aminopropyl)triethoxysilane (APTES) self-assembled monolayer on a SiN window, producing a surface decorated with amino groups, permits strong adhesion of Au nanoparticles to the window. Many of these nanoparticles remain static, undergoing minimal translation or rotation during LC-TEM up to high electron beam current densities due to the strong interaction between the APTES amino group and Au. We then use this technique to perform a direct comparative LC-TEM study on the behavior of ligand and nonligand-coated Au nanoparticles in a Au growth solution. While the ligand coated nanoparticles remain consistent even under high electron beam current densities, the naked nanoparticles acted as sites for secondary Au nucleation. These nucleated particles decorated the parent nanoparticle surface, forming consecutive monolayer assemblies of ∼2 nm diameter nanoparticles, which sinter into the parent particle when the electron beam was shut off. This method for facile immobilization of nanostructures for LC-TEM study will permit more sophisticated and controlled in situ experiments into the properties of solid-liquid interfaces in the future.

Biogeochemical cycling of gold: Transforming gold particles from arctic Finland

(Bio)geochemical cycling of gold (Au) has been demonstrated in present-day (semi)-arid, (sub)-tropical and temperate environment. Hereby biofilms on Au-bearing mineral- and Au-particle surfaces drive Au dispersion and reconcentration, thereby (trans)forming the particles. However, it is unknown if biogeochemical cycling of Au occurs in polar environments, where air temperatures can reach −40 °C and soils remain frozen for much of the year. Therefore, placer Au-particles, soils and waters were collected at two placer mining districts in arctic Finland, i.e., the Ivalojoki and Lemmenjoki goldfields. Sites were chosen based on contrasting settings ((glacio)-fluvial vs. glacial-till deposits) and depths (surface to 5 m below current surface). Gold particles were studied using a combination of tagged 16S rRNA gene next generation sequencing and electron microscopic/micro-analytical techniques. Across all sites a range of Au-particle morphologies were observed, including morphotypes indicative of Au dissolution and aggregation. Elevated Au concentrations indicative of Au mobility were detected in placer particle bearing soils at both districts. Typically Au-particles were coated by polymorphic biofilm layers composed of living and dead cells embedded in extracellular polymeric substances. Intermixed were biominerals, clays and iron-sulfides/oxides and abundant secondary Au morphotypes, i.e., nano-particles, micro-crystals, sheet-like Au, branched Au networks and overgrowths and secondary rims. Biofilms communities were composed of Acidobacteria (18.3%), Bacteroidetes (15.1%) and Proteobacteria (47.1%), with β-Proteobacteria (19.5%) being the most abundant proteobacterial group. Functionally, biofilms were composed of taxa contributing to biofilm establishment, exopolymer production and nutrient cycling, abundant taxa capable of Au mobilization, detoxification and biomineralization, among them Cupriavidus metallidurans, Acinetobacter spp. and Pseudomonas spp., were detected. In conclusion, these results demonstrate that placer Au-particle transformation and Au dispersion occur in cold, arctic environments. This corroborates the existence of biogeochemical Au cycling in present-day cold environments.

A Polymorphic Gold(III) Complex Comprising a Multifunctional Triazolylpropanamide Ligand

Reaction of HAuCl4·4H2O with the multifunctional ligand 3‐(1H‐1,2,4‐triazol‐1‐yl)propanamide (1, TPA) afforded the polymorphic gold(III) complex AuCl3(TPA) (2) in 75 % yield. Thermal decomposition of the latter revealed the hydrochloride [HTPA]Cl (3). Complex 2 forms a stable monoclinic and a metastable triclinic modification. The crystal structures of both modifications feature infinite supramolecular assemblies formed by secondary Au···Cl bonding interactions and hydrogen bonding.

Multiple isomerization of structural units in ion-polymeric heteronuclear gold(III)–zinc(II) complex ([Au{S2CN(C4H9)2}2]2[ZnCl4])n: Chemisorption-based synthesis, supramolecular structure (self-organization of long-period cation–cationic polymer chains), and thermal behavior

Chemisorption of gold(III) from solutions in 2 M HCl with freshly precipitated binuclear zinc dithiocarbamate [Zn2{S2CN(C4H9)2}4] resulted in the formation of a polymeric heteronuclear gold(III)–zinc(II) dithiocarbamato-chlorido complex ([Au{S2CN(C4H9)2}2]2[ZnCl4]) n (I), which was characterized by MAS 13C NMR, X-ray diffraction (CIF file CCDC no. 1526616), and simultaneous thermal analysis. Compound I isolated on a preparative scale was found to have a highly intricate supramolecular structure composed of 13 centrosymmetric and non-centrosymmetric isomeric complex cations, [Au{S2CN(C4H9)2}2]+, with 24 structurally non-equivalent BuDtc ligands, and six isomeric [ZnCl4]2– anions. The isomeric gold(III) cations perform different structural functions. Four and six cations are involved in the formation of two sorts of long-period cation–cationic chains (via pair non-valence secondary Au···S bonds): (···A···B···C···D···C···B···) n and (···F···G···H···I···J···K···) n . The discrete E, L, and M cations and the [ZnCl4]2– complex anions are located alongside of the polymer chains and do not take part in the secondary interactions. According to simultaneous thermal analysis, thermolysis of I includes destruction of the dithiocarbamate moiety with reduction of gold to the metal in the cation and liberation of zinc chloride with partial conversion to ZnS in the anion.

Principles of supramolecular polymeric chain formation in heteronuclear gold(III)–iron(III) complexes ([Au(S2CNR2)2][FeCl4]) n (R = C3H7, iso-C3H7): Chemisorption synthesis, structural organization, and thermal behavior

Polymeric gold(III)–iron(III) dithiocarbamate–chloride complexes of the ionic type are synthesized by the chemisorption binding of gold(III) with freshly precipitated iron(III) dipropyl and di-iso-propyl dithiocarbamates from solutions of H[AuCl4] in 2 M HCl. Heteropolynuclear complexes of the compositions ([Au{S2CN(C3H7)2}2][FeCl4])n (I) and ([Au{S2CN(iso-C3H7)2}2][FeCl4])n (II) are preparatively isolated as individual forms of gold(III) binding. The structural organization of the complexes is established by X-ray diffraction analysis (CIF files CCDC no. 1480802 (I) and no. 1480806 (II)). The structures of compounds I and II are characterized at the supramolecular level by the presence of two types of polymeric chains, the methods of formation of which differ substantially. Compound I contains the following structural units: four structurally nonequivalent centrosymmetric complex cations [Au{S2CN(C3H7)2}2]+ (A, B, С, and D) and two complex anions [FeCl4]– related to each other as conformers. Two independent cation-cationic linear polymeric chains (···А···В···)n and (···С···D···)n are formed in the structure of complex I due to pair relatively weak secondary interactions Au···S (nonvalent type) between the adjacent complex cations. The structure of compound II is characterized by zigzag cation-anionic chains (···[Au{S2CN(iso-C3H7)2}2]+···[FeCl4]–···)n in the formation of which the secondary interactions Au···Cl play the determining role. The thermal behavior of complexes I and II is studied by simultaneous thermal analysis. The thermal destruction process includes the thermolysis of the dithiocarbamate moiety of the complexes and [FeCl4]− with the reduction of gold(III) to the metal, the liberation of FeCl3, and the partial transformation of the latter into Fe2O3. In both cases, the final products of the thermal transformations of the studied compounds are elemental gold and Fe2O3.

The dynamics of gold in regolith change with differing environmental conditions over time

Significant Au discoveries are becoming less common because the remaining prospective, underexplored areas are obscured by transported cover. At Moolart Well (Western Australia), secondary Au deposits hosted in transported pisolitic ferricrete and saprolite are overlain by younger transported cover. Here, we show how Au has been, and is being, dispersed and concentrated in these deposits and the overlying younger transported cover and biota during the evolution of the landscape. We identified coarse (>400 µm), Ag-rich, primary, angular Au accumulated residually along with some precipitated, Ag-poor ( Acacia aneura ) and termite mounds indicates active dispersion.

Intermolecular Aurophilic versus Intramolecular Au⋅⋅⋅N Secondary Interactions in Two‐Coordinate Gold(I) Selenolate Complexes

AbstractA series of gold(I) selenolates viz., [Au(Sepy‐2)(PPh3)] (1), [Au(Sepy‐4)(PPh3)] (2) (py=pyridyl), [Au(Sepyz‐2)(PPh3)] (3) (pyz=pyrazinyl), [Au(Sepym‐2)(PPh3)] (4) (pym=pyrimidyl) and [Au{Sepym(Me2‐4,6)‐2}(PPh3)] (5) {pym‐2‐(Me2‐4,6)=4,6‐dimethyl‐2‐pyrimidyl} have been synthesized and structurally characterized. The gold atom acquires a two‐coordinate linear geometry with Au−Se distance of ∼ 2.41 Å. The existence of aurophilic interactions are determined by the nature of substituent on selenium. It was observed that the complexes derived from selenolate ligand with pyridyl moiety (one hetero‐N atom in the ring) could exhibit aurophilicity; whereas, the complexes derived from ligands containing pyrazinyl and pyrimidyl moieties (two hetero‐N atoms) are devoid of such interaction but show secondary Au⋅⋅⋅N interaction. The complex [Au(Sepy‐4)(PPh3)] shows polymorphism and the two polymorphs differ in the relative orientations of linear molecules. There are competing secondary intramolecular Au⋅⋅⋅N interaction which plays crucial role on intermolecular aurophilic interaction and overall structure. Theoretical calculations at DFT and MP2 levels have been carried out to rationalize the experimental findings.

Synthesis, Supramolecular Self-Organization, and Thermal Behavior of Gold(III)–Thallium(III) Heteronuclear Complexes ([Au{S2CN(CH3)2}2][TlCl4])2 and ([Au{S2CN(C2H5)2}2][TlCl4]) n

The interaction of polymeric thallium(I) dimethyl- and diethyldithiocarbamates with [AuCl4]– in 2 M HCl has been studied. Heteropolynuclear complexes ([Au{S2CN(CH3)2}2][TlCl4])2 (I) and ([Au{S2CN(C2H5)2}2][TlCl4]) n (II) have been preparatively isolated from chemisorption systems [Tl2{S2CNR2}2] n –Au3+/2 M HCl (R = CH3, C2H5). These compounds have been characterized by 13C CP/MAS NMR, and their crystal and supramolecular structures have been determined by X-ray crystallography. Basic structural units of compounds I and II are square-planar [Au{S2CNR2}2]+ cations (with S,S'- bidentate coordination of two Dtc ligands to the gold atom) and distorted tetrahedral [TlCl4]–anions. In supramolecular self-organization, the decisive role is played by relatively weak secondary interactions Au⋯S and Au⋯Cl. With the use of simultaneous thermal analysis, the thermal behavior of I and II have been studied, which enabled the elucidation of temperature-induced transformations and identification of TlCl and reduced gold among the thermolysis products.

Occurrences and distribution of “invisible” precious metals in sulfide deposits from the Edmond hydrothermal field, Central Indian Ridge

Here, we report the first documented occurrences of “invisible” gold and silver in seafloor sulfide deposits from an active hydrothermal system on the Central Indian Ridge. A detailed mineralogical and geochemical study of polymetallic sulfides from the Edmond vent field was conducted in order to identify controls on the distribution of precious metals. Bulk samples (N=18) contain up to 18.7ppm Au and 1450ppm Ag, with average concentrations of 2.3ppm Au and 218.9ppm Ag. Among them, several Zn-rich chimney fragments and anhydrite-dominated ore samples have higher contents of precious metals than Fe-Cu-rich massive sulfides and silica-rich hydrothermal precipitates. Native gold grains are mainly associated with sphalerite, anhydrite, barite and Fe-oxyhydroxides. Abundant submicroscopic Au-Ag alloys tend to occur along grain boundaries between Cu-Fe sulfides and tennantite, or close to the rims of Fe-poor sphalerite. In contrast to primary electrum with high Ag/Au ratios, the absence of detectable silver in high-purity gold indicates that secondary Au enrichment has probably occurred after a direct co-precipitation with Zn-rich mineral assemblages upon cooling and mixing of vent fluids with cold seawater. A suite of late-stage Ag-rich phases, including argentotennantite, pearceite and acanthite, occur as crack-filling veinlets and patches in low-temperature fahlores, or as tiny inclusions enclosed by pyrite, chalcopyrite and colloform sphalerite. By using HRTEM combined with HAADF-STEM imaging, we have found out that silver is also present in significant quantities as discrete colloidal nanoparticles in tennantite. Minor native copper is closely associated with altered chalcopyrite, sphalerite and covellite, exhibiting signs of dissolution, recrystallization and reprecipitation. Extensive hydrothermal reworking resulted from a long history of high-temperature venting in this field, together with post-depositional supergene replacement processes (involving oxidation, leaching or coupled dissolution-reprecipitation mechanisms facilitated by a permeable porosity generated in primary Cu-Fe sulfides) are considered to be important for the remobilization and local reconcentration of early-formed precious metals, and may have been responsible for the formation of relatively coarse-grained native gold or silver within recrystallized massive sulfides and chimney debris.