Au Sb Research Articles

ChemInform Abstract: Isothermal Section of the Ce—Au—Sb System at 870 K.

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Thermodynamic description of the Au–Ge–Sb ternary system

The Ge–Sb binary system has been thermodynamically assessed using the CALPHAD method based on the available experimental information from the published literature. The solution phases, including liquid, fcc_A1(Au), diamond_A4(Ge) and rhombohedral_A7(Sb), were modeled as substitutional solutions, of which the excess Gibbs energies were expressed by a Redlich–Kister polynomial. The intermetallic compound AuSb 2 was assumed to have no ternary solubility because of the lack of experimental information. Combined with previous assessments of the Au–Ge and Au–Sb binary systems, the thermodynamic description of the Au–Ge–Sb ternary system has been performed. The liquidus projection and several vertical sections of this ternary system have been calculated, which are in reasonable agreement with the reported experimental data.

Metallogenesis of the Tibetan collisional orogen: A review and introduction to the special issue

Mineral deposits associated with continental collision are abundant in many orogenic systems. However, the metallogenesis of collisional orogens is often poorly understood, due to the lack of systematic studies on the genetic links between collisional processes and ore formation in collisional orogenic belts. This paper reviews the key metallogenic settings and resultant collision-related ore deposits in the Tibetan Orogen, created by Indo-Asian collision starting in the early Cenozoic. The resulting synthesis leads us to propose a new conceptual framework for Tibetan metallogenic systems, which may aid in deciphering relationships among ore types in other comparable collisional belts. This framework includes three principal metallogenic epochs in the Tibetan orogen, and metallogenesis in: (1) a main-collisional convergent setting (∼ 65–41 Ma); (2) a late-collisional transform structural setting (∼ 40–26 Ma); and (3) a post-collisional crustal extension setting (∼ 25–0 Ma), each forming more than three distinct types of ore deposits in the Tibetan orogen. The main-collisional metaollognesis took place in a convergent setting, i.e., a collisional zone, characterized by collision-related crustal shortening and thickening, associated syn-peak metamorphism and two distinct magmatic series (Paleocene–Eocene crust-derived low- fO 2 granitoids generated by crustal anatexis and Eocene high- fO 2 granitoids formed by MASH processes at the base of the Tibetan crust). Metallogenesis during this period formed Sn–W–rare metal deposits related to the low- fO 2 granitoids, skarn-hosted Cu–Au polymetallic deposits related to high- fO 2 granitoids, and orogenic-type Au deposits formed by CO 2-dominant metamorphic fluids. Late-collisional metallogenesis occurred mainly in a transform structural setting dominated by Cenozoic strike-slip faulting, shearing, thrust systems, and associated potassic magmatism in eastern Tibet, and formed the most economically-significant metallogenic province in the orogen. Four significant ore-forming systems are recognized in the transform zone: porphyry Cu–Mo–Au systems associated with potassic adakitic melts and controlled by Cenozoic strike-slip faults; orogenic-type Au systems related to large-scale left-slip ductile shearing; REE-bearing systems associated with lithospheric mantle-derived carbonatite–alkalic complexes; and Zn–Pb–Cu–Ag systems related to basinal brines and controlled by Cenozoic thrust structures and subsequent strike-slip faults developed in the Tertiary foreland basin. Post-collisional metallogenesis occurred in a crustal extension setting, characterized by lithospheric mantle thinning or delamination at depth, crustal shortening at a lower structural level and synchronal extension at shallower levels. The resulting ore-forming systems include: (1) porphyry Cu–Mo ore systems related to high-K adakitic stocks derived from the newly-formed thickened mafic lower-crust; (2) vein-type Sb–Au ore systems controlled by the south Tibetan detachment system (STDs) and the metamorphic core complex or thermal dome intruded by lecuogranite intrusions; (3) hydrothermal Pb–Zn–Ag ore systems controlled by the intersections of N–S-striking normal faults with E–W-trending thrust faults; and (4) spring-type Cs–Au ore systems related to geothermal activity driven by partial melting of the upper crust. Associated ore deposits lie mostly within the mid-Miocene Gangdese tectono-magmatic belt, in which the scavenging role of fluids derived from evolved magma systems or dewatering of rift basins, and finally discharging at intersections of the orogen-transverse and -parallel faults are extremely important for formation of the low-temperature hydrothermal deposits. Based on the synthesis of deposits in the Tibetan orogen and comparison with the metallogenesis of other orogenic systems, a more complete classification for these collision-related deposits can be proposed.

Effects of thermal annealing on electrical characteristics of Cd/CdS/n-Si/Au–Sb sandwich structure

In general, at the metal–semiconductor contacts, interfacial layers have been fabricated by different methods such as molecular beam epitaxy, metal organic chemical vapor deposition, sputtering and vacuum evaporation. However, all of these techniques have encountered various difficulties in the deposited films. Instead of these methods, since Successive Ionic Layer Adsorption and Reaction (SILAR) method is simple, fast, sensitive, and less costly to prepare interfacial layer, we have first employed this method in order to prepare Cd/CdS/n-Si/Au–Sb sandwich structure. For this reason, the CdS thin film has been directly formed on n-type Si substrate by means of SILAR method. The Cd/CdS/n-Si/Au–Sb sandwich structure has demonstrated clearly rectifying behaviour by the current–voltage ( I– V) curves studied at room temperature. In order to observe the effect of the thermal annealing, this structure has been annealed at temperatures from 50 to 300 °C for 3 min in N 2 atmosphere. The characteristic parameters such as barrier height, ideality factor and series resistance of this structure have been calculated from the forward bias I– V characteristics as a function of annealing temperature with different methods. The values of n, Φ b and mean R s of the initial Cd/CdS/n-Si/Au–Sb sandwich structure were found to be 2.31, 0.790 eV and 1.86 kΩ respectively. After annealing at 300 °C, these values were changed to 1.89, 0.765 eV and 0.48 kΩ. It has been seen that the barrier height, ideality factor and series resistance have slightly changed with increasing annealing temperature up to 300 °C.

Gold in the Brunswick No. 12 volcanogenic massive sulfide deposit, Bathurst Mining Camp, Canada: Evidence from bulk ore analysis and laser ablation ICP─MS data on sulfide phases

The 329-Mt Brunswick No. 12 volcanogenic massive sulfide deposit (total resource of 163 Mt at 10.4% Zn, 4.2% Pb, 0.34% Cu, and 115 g/t Ag) is hosted within a Middle Ordovician bimodal volcanic and sedimentary sequence. Massive sulfides are for the most part syngenetic, and the bulk of the sulfide ore occurs as a Zn–Pb-rich banded sulfide facies that forms an intimate relationship with a laterally extensive Algoma-type iron formation and defines the Brunswick Horizon. Zone refining of stratiform sulfides is considered to have resulted in the development of a large replacement-style Cu-rich basal sulfide facies, which is generally confined between the banded sulfide facies and an underlying stringer sulfide zone. Complex polyphase deformation and associated lower- to upper-greenschist facies regional metamorphism is responsible for the present geometry of the deposit. Textural modification has resulted in a general increase in grain size through the development of pyrite and arsenopyrite porphyroblasts, which tend to overprint primary mineral assemblages. Despite the heterogeneous ductile deformation, primary features have locally been preserved, such as fine-grained colloform pyrite and base and precious metal zonation within the Main Zone. Base metal and trace element abundances in massive sulfides from the Brunswick No. 12 deposit indicate two distinct geochemical associations. The basal sulfide facies, characterized by a proximal high-temperature hydrothermal signature (Cu–Co–Bi–Se), contains generally low Au contents averaging 0.39 ppm (n = 34). Conversely, Au is enriched in the banded sulfide facies, averaging 1.1 ppm Au (n = 21), and is associated with an exhalative suite of elements (Zn–Pb–As–Sb–Ag–Sn). Finely laminated sulfide lenses hosted by iron formation at the north end of the Main Zone are further enriched in Au, averaging 1.7 ppm (n = 41) and ranging up to 8.2 ppm. Laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) analyses of pyrite (n = 97) from the north end of the Main Zone average 2.6 ppm Au and range from the detection limit (0.015 ppm) to 21 ppm. Overall, these analyses reveal a distinct Au–Sb–As–Ag–Hg–Mn association within pyrite grains. Gold is strongly enriched in large pseudo-primary masses of pyrite that exhibit relict banding and fine-grained cores; smaller euhedral pyrite porphyroblasts, and euhedral rims of metamorphic origin surrounding the pyrite masses, contain much less Au, Sb, Ag, As, and Sn. Arsenopyrite, occurring chiefly as late porphyroblasts, contains less Au, averaging 1.0 ppm and ranging from the detection limit (0.027 ppm) to 6.9 ppm. Depth profiles for single-spot laser ablation ICP-MS analyses of pyrite and arsenopyrite display uniform values of Au and an absence of discrete microscopic inclusions of Au-bearing minerals, which is consistent with chemically bonded Au in the sulfide structure. The pervasive correlation of Au with Sn in the Zn–Pb-rich banded sulfide facies suggests similar hydrothermal behavior during the waxing stages of deposition on the seafloor. Under high temperature (>350oC) and moderate- to low-pH conditions, Au and Sn in hydrothermal fluids would be transported as chlorocomplexes. An abrupt decrease in temperature and aH2S, accompanied by an increase in fO2 and pH during mixing with seawater, would lead to the simultaneous destabilization of both Au and Sn chlorocomplexes. The enrichment of Au in fine-grained laminated sulfides on the periphery of the deposit, accompanied by sporadic occurrences of barite and Fe-poor sphalerite, supports lower hydrothermal fluid temperatures analogous to white smoker activity on the flanks of a large volcanogenic massive sulfide system. In lower temperature (<350oC) and mildly acidic hydrothermal fluids, Au would be transported by thiocomplexes, which exhibit multifunctional (retrograde–prograde) solubility and a capacity to mobilize Au to the outer parts of the sulfide mound. The sluggish nature of this low-temperature venting together with larger variations in ambient fO2 could lead to a sharp enrichment of Au towards the stratigraphic hanging wall of massive sulfide deposits.

Post-collisional Sb and Au mineralization related to the South Tibetan detachment system, Himalayan orogen

More than 50 Sb–Au deposits and occurrences have recently been found in an E–W-striking structural–thermal dome zone, related to the South Tibetan detachment system (STDS) in the Himalayan orogen. They are mainly distributed around the thermal domes intruded by mid-Miocene leucogranite bodies, and show a metallic zoning varying from Au, Sb–Au, to Sb mineralization from the domes outwards. At least three mineralization styles are recognized, i.e., Sb-, Sb–Au-, and Au-styles of deposits, all hosted in the Mesozoic, Tethyan passive continental-margin sequences. The Shalagang-style Sb deposits, controlled by N–S-striking normal faults and E–W-striking bedding faults, are usually composed of quartz–stibnite veins with open-space filling structure. Low homogenization temperatures (135–367 °C) and low salinities (0.5–12.6 wt.% NaCl equiv.), as well as highly variable calculated δ 18O H2O values (− 11.5–12.3‰) and very low δD values (− 140 to − 166‰), suggest an ore-forming fluid dominated by geothermal or meteoric water. The Mazhala-style Sb–Au deposits were controlled by small-scale bedding faults and their intersections with N–S-striking normal faults. They consist of gold-bearing quartz–stibnite veins with minor disseminated ores, and are associated with silicification, sericitization, chloritization and carbonatization. CO 2-rich fluid inclusions in Au-bearing gangue quartz, calculated δ 18O H2O values (5.4–11.5‰) and measured δD values (− 72 to − 119‰) for precipitation of gangue quartz suggest a mixed ore-forming fluid containing both meteoric and magmatic waters. Langkazi-style Au deposits, controlled by detachment faults and normal faults along the margin of the thermal dome, consist of Au-bearing quartz veins and disseminated ores with intensive silicification, chloritization and sericitization. Gangue quartz from Au-bearing quartz veins yielded a limited range of δ 18O H2O values (1.8–8.2‰) and δD values (− 52 to − 83‰), suggesting an ore-forming fluid dominated by magmatic water. The genetic links among these deposits, the granite-intruded thermal domes and ore-controlled fault systems suggest a mid-Miocene metallogenic epoch. The spatial zonation in metallic associations, and the variation in compositions and sources of ore-forming fluids indicate that these Sb and Au deposits resulted from a post-collisional dome-centered geothermal system, driven by mid-Miocene granite magmas related to the STDS. Rb–Sr and S isotopic data indicate that the scavenging of the convective geothermal fluids through Mesozoic permeable strata play an extremely significant role in concentrating metallic Sb and minor Au, and forming Sb, Sb–Au, and Au deposits in southern Tibet.

Evaluation of leaching parameters for a refractory gold ore containing aurostibite and antimony minerals: Part I – Central zone

A cyanidation study was conducted on a mild refractory gold ore sample from the Central zone of Clarence Stream Property, owned by Freewest Resources Canada, to develop a leaching strategy to extract gold. Gold, at a grade of 8.00 g/t, is present as native gold, electrum and aurostibite. The ore also contains 2.8% pyrrhotite, together with several antimony minerals (0.8% berthierite and gudmundite, 0.18% native antimony and stibnite). It also exhibits weak preg-robbing properties with 0.16% organic carbon. Aurostibite, a gold antimony compound, is particularly known to be insoluble in cyanide solution. The antimony dissolves in cyanide solution to form antimonates, which retards gold dissolution. Industrial practice of extracting gold from aurostibite generally consists of producing a flotation concentrate, which is leached in a pipe reactor at low alkalinity and high oxygen pressure with about 20 g/L cyanide. The proposed new approach is efficient and allows the extraction of gold directly from an ore at atmospheric pressure and a low cyanide concentration at pH 10.5. The effects of grinding, pre-treatment, lead nitrate, kerosene and cyanide concentrations have been investigated. The maximum gold extraction obtained on the ore was 87.9% using 800 ppm NaCN, 500 g/t lead nitrate, 30 g/t kerosene, DO (dissolved oxygen) 10 ppm and pH 10.5 in 168 h. The associated cyanide consumption was 1.3 kg/t. The additions of lead nitrate and kerosene increased gold extraction. In comparison to a P 80 of 74 μm, a P 80 of 30 μm significantly increased gold extraction. Gold in solid solution in gudmundite and arsenopyrite was believed to be responsible for the un-leached fraction until mineralogical analysis of hydroseparation concentrates of leach residues showed that most of the un-leached gold occurs as aurostibite, either as locked grains in sulphides/sulpharsenides or as grains with passivation rims of an Au–Sb–O phase. Coarse gold was also found. Gold extraction was not sensitive to cyanide concentration from 250 to 1200 ppm NaCN and high pH was detrimental. Decreasing the cyanide concentration reduced the cyanide consumption from 1.39 to 0.85 kg/t. The removal of coarse gold using a Knelson concentrator and a Mosley table prior to leaching increased the gold extraction to 90.4% (leach residue at 0.77 g/t).

Isothermal section of the Ce–Au–Sb system at 870 K

Phase equilibria in the Ce–Au–Sb system were derived from X-ray diffraction data and, optionally, from scanning electron microscope analysis of annealed at 870 K alloys. The structures of compounds were investigated using X-ray single crystal and powder diffraction methods. Five ternary compounds were found to exist within the investigated concentration region (0–50 at.% Ce). New U 3Cu 2Sb 3-type Ce 3Au 2Sb 3 compound ( a = 4.5966(4) Å, c = 24.005(2) Å, space group P6 3 /mmc, no. 194) was discovered and studied by X-ray single crystal diffraction. The existence and crystal structures of following compounds were confirmed at 870 K: the ZrCuSi 2-type CeAu 1− x Sb 2, UHg 2-type Ce(Au 1− x Sb x ) 2 and Y 3Au 3Sb 4-type Ce 3Au 3Sb 4. For the reported earlier UHg 2-type Ce(Au 1− x Sb x ) 2 and ZrCuSi 2-type CeAu 1− x Sb 2 compounds the homogeneity regions were established from annealed at 870 K alloys: 0.19 ≤ x ≤ 0.28 and 0 ≤ x ≤ 0.21 respectively. At 870 K, the structure of CeAuSb compound was found to be related to the CaIn 2-type from X-ray powder diffraction data. Moreover, the single crystal X-ray diffraction studies of the as-cast CeAu 1− x Sb 2 for x = 0.118(2) and x = 0.162(4) are given in present paper as well.

Physicochemical modeling of the coupled behavior of gold and silver in hydrothermal processes: Gold fineness, Au/Ag ratios and their possible implications

An estimation of gold fineness deposited from aqueous sulphide–chloride solutions in pyritic ore with varying (Au/Ag) ore has been obtained using thermodynamic modeling (at T = 200–500 °C and P = 1000 bar). Silver sulphide with Au–Ag solid solution or Au–Ag solid solution are only stable under conditions of the Py–Po(–Mt) and Hem–Py (–Mt) buffers depending on Au and Ag concentrations in the ore-forming system. The predominant Au and Ag species in aqueous sulphide–chloride solutions depend on temperature, pH and chloride concentration and these solutions can be grouped as follows: Type I: Acid, high chloride ( m HCl > 0.0l, m NaCl = 1 to 5), high temperature (300–500 °C) solutions in which AgCl 2 − and AuCl 2 − are dominant; Type II: Weakly acidic, near-neutral, medium chloride ( m NaCl ≈ 1), moderate to high temperature (200–400 °С) solutions in which AgCl 2 − and AuHS 0 are dominant; Type III: Near-neutral, high chloride ( m NaCl = 1 to 5), high temperature (300–500 °С) solutions in which AgCl 2 − and Au(HS) 2 − are dominant; Type IV: Alkaline, variable chloride ( m NaOH > 0.01, m NaCl = 0 to 5), moderate to high temperature (200–500 °C) solutions in which Au(HS) 2 − and Ag(HS) 2 − are dominant; and Type V: Acid to near-neutral, low chloride ( m NaCl < 1), low to moderate temperature (< 300 °C) solutions in which AuHS 0 and AgHS 0 are dominant. The acid and near-neutral high- and medium chloride solutions (types I–III) are distinguished by high Ag-carrying capacity and low values of (Au/Ag) aq ≈ 10 − 7 to 10 − 2 . The highest Au-carrying capacity and high (Au/Ag) aq ratios (≈ 10 − 3 to 1) are typical of alkaline solutions of any chloride concentrations (type IV) and low chloride solutions of variable acidity (type V). Data on the physicochemical conditions of Au–Ag mineralization and mineralogical–geochemical characteristics of pyritic ores of hydrothermal gold deposits of different types are summarized. Wide ranges of gold fineness and low (Au/Ag) ore ratios of Au–Ag epithermal, Au–Cu-porphyry and Au–Cu-skarn deposits are related to the participation of acid or near-neutral high- or medium chloride ore-forming solutions (types I–III). High fineness gold and high values of (Au/Ag) ore ratio of Au–Te, Au–As, Au–Sb, Au orogenic and Carlin-type deposits are explained by participation of alkaline (type IV) or low chloride (type V) sulphide solutions. Gold fineness and (Au/Ag) ore ratios in the sulphide ores may be characteristics that determine the type of ore solution and its evolution in the process of hydrothermal deposit formation.

Antimony quartz and antimony–gold quartz veins from northern Portugal

Antimony- and Pb–Sb-quartz veins from the Bragança district, Portugal, are mainly hosted by Silurian phyllites. Antimony–Au-quartz veins from the Dúrico–Beirã region are mainly hosted by a Cambrian schist–metagraywacke complex, as well as Ordovician phyllites and quartzites. The deposits were mostly exploited in the late 19th Century. Mineralogical characteristics and chemical compositions of individual ore minerals are similar in the two areas. First and second generations of arsenopyrite precipitated at 390 and 300 °C, respectively. Berthierite and stibnite are the most abundant Sb-bearing minerals and precipitated between 225 and 128 °C, native antimony at < 200 °C. Drastic fluid cooling is the main cause of mineral precipitation. The Pb isotope compositions of stibnite suggest a homogeneous crustal source of lead, from the metasedimentary sequences, for Sb, Pb–Sb and Sb–Au deposits in both areas, which is consistent with the findings for comparable mineralizations elsewhere in Europe. Remobilization of Pb is related to Variscan metamorphism and deformation.

Calculation from the current–voltage and capacitance–voltage measurements of characteristics parameters of Cd/CdS/n-Si/Au-Sb structure with CdS interface layer grown on n-Si substrate by SILAR method

The CdS thin film has been directly formed on n-type Si substrate to form an interfacial layer between cadmium (Cd) and n-type Si with Successive Ionic Layer Adsorption and Reaction (SILAR) method. An Au–Sb electrode has been used as an ohmic contact. The Cd/CdS/n-Si/Au–Sb structure has demonstrated clearly rectifying behaviour by the current–voltage ( I– V) curves studied at room temperature. The characteristics parameters such as barrier height, ideality factor and series resistance of Cd/CdS/ n-Si/Au–Sb structure have been calculated from the forward bias I– V and reverse bias C −2–V characteristics. The diode ideality factor and the barrier height have been calculated as n = 2.06 and Φ b = 0.92 eV by applying a thermionic emission theory, respectively. The diode shows non-ideal I– V behaviour with an ideality factor greater than unity that can be ascribed to the interfacial layer, the interface states and the series resistance. At high current densities in the forward direction, the series resistance ( R s) effect has been observed. The values of R s obtained from d V/d(ln I)– I and H(I)– I plots are near to each others ( R s = 182.24 Ω and R s = 186.04 Ω, respectively). This case shows the consistency of the Cheung′s approach. In the same way, the barrier height calculated from C −2 -V characteristics varied from 0.698 to 0.743 eV. Furthermore, the density distribution of interface states ( N ss) of the device has been obtained from the forward bias I– V characteristics. It has been seen that, the N ss has almost an exponential rise with bias from the mid gap toward the bottom of conduction band.

Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal–As–Sb–Au mineralisation during Hercynian strike-slip deformation

In the Saint-Aubin-des-Châteaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Gres armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After the massive sulphidation stage (Fe–As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb–Sb–Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right-lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390–350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300–275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-Proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest that large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones, which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralisation of Saint-Aubin-des-Châteaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits.

Experimental Study of the Phase Diagram of the Ag—Au—Sb Ternary System.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Rare earth element geochemistry of the Woxi W–Sb–Au deposit, Hunan Province, South China

The Woxi W–Sb–Au deposit in Hunan, South China, is hosted by Proterozoic metasedimentary rocks, a turbiditic sequence of slightly metamorphosed (greenschist facies), gray-green and purplish red graywacke, siltstone, sandy slate, and slate. The mineralization occurs predominantly (> 70%) as stratabound/stratiform ore layers and subordinately as stringer stockworks. The former consists of rhythmically interbedded, banded to finely laminated stibnite, scheelite, quartz, pyrite and silty clays, whereas the latter occurs immediately beneath the stratabound ore layers and is characterized by numerous quartz + pyrite + gold + scheelite stringer veins or veinlets that are typically either subparallel or subvertical to the overlying stratabound ore layers. The deposit has been the subject of continued debate in regard to its genesis. Rare earth element geochemistry is used here to support a sedimentary exhalative (sedex) origin for the Woxi deposit. The REE signatures of the metasedimentary rocks and associated ores from the Woxi W–Sb–Au deposit remained unchanged during post-depositional processes and were mainly controlled by their provenance. The original ore-forming hydrothermal fluids, as demonstrated by fluid inclusions in quartz from the banded ores, are characterized by variable total REE concentrations (3.5 to 136 ppm), marked LREE enrichment (La N/Yb N = 28–248, ∑LREE/∑HREE = 16 to 34) and no significant Eu-anomalies (Eu/Eu ⁎ = 0.83 to 1.18). They were most probably derived from evolved seawater that circulated in the clastic sediment pile and subsequently erupted on the seafloor. The bulk banded ores are enriched in HREE (La N/Yb N = 4.6–11.4, ∑LREE/∑HREE = 3 to 14) and slightly depleted in Eu (Eu/Eu ⁎ = 0.63 to 1.14) relative to their parent fluids. This is interpreted as indicating the influence of seawater rather than a crystallographic control on REE content of the ores. Within a single ore layer, the degree of HREE enrichment tends to increase upward while the total REE concentrations decrease, reflecting greater influence and dilution of seawater. There is a broad similarity in chondrite-normalized REE patterns and the amount of REE fractionation of the banded ores in this study and exhalites from other sedex-type polymetallic ore deposits, suggesting a similar genesis for these deposits. This conclusion is in agreement with geologic evidence supporting a syngenetic (sedex) model for the Woxi deposit.

Experimental study of the phase diagram of the Ag–Au–Sb ternary system

The phase diagram of the Ag–Au–Sb ternary system has been assessed using X-ray diffraction analysis, electron-probe micro-analysis and differential scanning calorimetry. Four isopletic sections were studied: 80, 70 and 10 at.% Ag and 10 at.% Au. Three ternary invariant reactions were noted. This study is part of an international program, COST 531, about lead-free soldering materials.

Anomalous gold contents in brown coals and peat in the south-eastern region of the Western-Siberian platform

Abstract One hundred twenty-two samples of Jurassic and Paleogene brown coals and 1254 peat samples from the south-eastern region of the Western-Siberian platform were analyzed for gold by the neutron-activation method. Mean content of Au in Jurassic coals is 30 ± 8 ppb, in Paleogene coals is 10.6 ± 4.8 ppb, and in peat is 6 ± 1.4 ppb. Concentrations of gold as high as 4.4 ppm were found in coal ash and 0.48 ppm in the peat ash. Coal beds with anomalous gold contents were found at Western-Siberian platform for the first time. Negative correlation between gold and ash yield in coals and peat and highest gold concentrations were found in low-ash and ultra-low-ash coals and peat. Primarily this is due to gold's association with organic matter. For the investigation of mode of occurrence of Au in peat the bitumen, water-soluble and high-hydrolyzed substances, humic acids, cellulose and lignin were extracted from it. It was determined that in peat about 95% of gold is combined with organic matter. Forty to sixty percent of Au is contained in humic acids and the same content is in lignin. Bitumens, water-soluble and high-hydrolyzed substances contain no more than 1% of general gold quantity in peat. The conditions of accumulation of high gold concentrations were considered. The authors suggest that Au accumulation in peat and brown coals and the connection between anomalous gold concentrations and organic matter in low-ash coals and peat can explain a biogenic–sorption mechanism of Au accumulation. The sources of formation of Au high concentration were various Au–Sb, Au–Ag Au–As–Sb deposits that are abundant in the Southern and South-Eastern peripheries of the coal basin.

Electrodeposition of Sb on Au(1 0 0) at underpotentials: structural transition involving expansion of the substrate surface

We report an in situ scanning tunneling microscopic (STM) study of a structural transition of antimony on Au(1 0 0). Sb electrodeposition on Au(1 0 0) at underpotentials involves two successive stages: the reduction of irreversibly adsorbed Sb(III) and underpotential deposition of Sb. In the reduction stage, the Sb adlayer appears disordered on the surface. A remarkable structural transition takes place when potential is brought to initiate the underpotential deposition. An ordered structure of 2 −2 1 2 is observed accompanied by releasing of Au islands. Reversing of potential recovers the disordered state of the adlayer in a few minutes. Both the coverage and potential-dependent of Sb–Au interaction are responsible for the structural transition. A Sb-chemisorbed type of surface compound with a stoichiometric form of AuSb 2 is suggested. Together with the expansion of the Au(1 0 0) surface layer, this type of structural transition is unusual in the metal underpotential deposition.

A comparative study of APS minerals of the Pacific Rim fold belts with special reference to south American argillaceous deposits

Aluminum-phosphate-sulfate (APS) minerals are of particular interest to exploration geologists in search of nonmetal deposits emplaced in volcanic or subvolcanic environments. These minerals are often more sensitive to changes in the physicochemical conditions during mineralization than phyllosilicates and thus can shed some light on the evolution of argillaceous deposits that may be of supergene or hypogene origin. The APS-bearing argillaceous deposits under study are located in El Salvador, Peru, Chile and Indonesia and occur in volcanic and pyroclastic rocks of the circum-Pacific Rim fold belts. Some sediment-hosted kaolin deposits from Chile where APS minerals were not found and volcanic-hosted Sb–Au deposits from Bolivia, which contain massive alunite as the only alteration mineral, were also discussed to connect this research to neighboring realms such as sedimentary host rocks and metallic deposits. The mineral assemblages in volcanic and pyroclastic rocks consist of alunite, woodhouseite, and crandallite solid solution series with Fe disulfides, Fe oxide hydrates, 7, 10 and 14 Å sheet silicates, siliceous compounds, zunyite and topaz. The APS-bearing argillaceous deposits may be subdivided on the basis of their facies patterns (first-order-classification) and mineral zonation (second-order-classification). Facies patterns in the deposits vary with the distance of mineralization from the feeder channel system. A detailed study of these patterns provides an overview of temperature regime across the entire district of argillaceous mineralization. The APS-bearing argillaceous mineralization under consideration correspond to the following alteration patterns: advanced and intermediate argillic alteration, silicification, and sericitization, which is common to epithermal and porphyry-type ore deposits. The mineral zonation of APS mineral assemblages may be correlated with equivalent APS mineralization in ore deposits. The fourfold subdivision of APS mineralization reflects a decrease in the temperature of formation that controls the accommodation of bivalent (Sr 2+, Ba 2+) and trivalent (Ce 3+) cations in the lattice of APS minerals and is governed by the chemical composition of mineralizing fluids and preexisting element concentration in the volcanic host rocks.

Formation of AB2-intermetallics by diffusion in the Au–Sb–Bi system

Binary diffusion couples, in which one single-phased product layer is growing between pure elements, were employed to study the diffusion properties of Au2Bi- and AuSb2-intermetallics at 230 and 330 °C. The position of the Kirkendall-marker plane inside the reaction zones revealed that in this temperature range the minority element is the faster diffuser in the Laves-phase Au2Bi as well as in AuSb2. The concept of integrated diffusion coefficient is used to describe the growth kinetics of the intermetallic compounds. The integrated diffusion coefficient in an intermetallic is related to the tracer diffusivities of the components and the thermodynamic stability of the phases involved in the interaction. The tracer diffusion coefficients were deduced from the interdiffusion experiments. The isothermal cross-section through the ternary phase diagram Au–Sb–Bi at 230 °C was constructed by means of the diffusion couple technique. No ternary phases are found in this system. Both intermetallic compounds Au2Bi and AuSb2 are in equilibrium with the (Sb,Bi)-solid solution. The solubility of Sb in the Laves-phase Au2Bi was found to be negligible. Up to about 10.5 at.% of Bi can be dissolved in the AuSb2-phase, the Bi-atoms substituting Sb in the cubic lattice of AuSb2.

Paleozoic orogenic gold deposits in the eastern Central Andes and its foreland, South America

In the eastern Central Andes and its foreland (6°–34°S), abundant quartz veins emplaced along brittle–ductile deformation zones in Ordovician to Carboniferous granites and gneisses and in saddle-reefs in lower Paleozoic turbidites represent a coherent group of middle to late Paleozoic structurally hosted gold deposits that are part of three major Au (±Sb±W) metallogenic belts. These belts, extending from northern Peru to central Argentina along the Eastern Andean Cordillera and further south in the Sierras Pampeanas, include historical districts and mines such as Pataz–Parcoy, Ananea, Santo Domingo, Yani–Aucapata, Amayapampa, Sierra de la Rinconada and Sierras de Córdoba. On the basis of the available isotopic ages, two broad mineralization epochs have been identified, with Devonian ages in the Sierras Pampeanas Au belt (26° to 33°30′S), and Carboniferous ages for the Pataz–Marañón Valley Au-belt in northern Peru (6°50′ to 8°50′S). The absolute timing of the southeastern Peruvian, Bolivian and northwestern Argentinian turbidite-hosted lodes, which form the Au–Sb belt of the southern Eastern Andean Cordillera (12° to 26°S), is poorly constrained. Field relationships suggest overlap of gold veining with Carboniferous deformation events. The northernmost belt, which includes the Pataz province, is over 160-km-long and consists of sulfide-rich quartz veins hosted by brittle–ductile shear zones that have affected Carboniferous granitic intrusions. Gold mineralization, at least in the Pataz province, occurred a few million years after the emplacement of the 329 Ma host pluton and an episode of molassic basin formation, during a period of rapid uplift of the host units. The two southern belts are associated with syn- to post-collisional settings, resulting from the accretion of terranes on the proto-Andean margin of South America. The Au–Sb belt of the southern Eastern Andean Cordillera presumably formed in the final stages of the collision of the Arequipa–Antofalla terrane and the Sierras Pampeanas Au belt is considered concurrent with the late transpressional tectonics associated with the accretion of the Chilenia terrane. The three Devono–Carboniferous Andean belts are the South American segments of the trans-global orogenic gold provinces that were formed from Late Ordovician to Middle Permian in accretionary or collisional belts that circumscribed the Gondwana craton and the paleo-Tethys continental masses. A paleogeographic map of the Gondwana supercontinent in its Middle Cambrian configuration appears as a powerful tool for predicting the location of the majority of the Paleozoic orogenic gold provinces in the world, as they develop within mobile belts along its border. The three South American belts are sited in the metallogenic continuation of the Paleozoic terranes that host the giant eastern Australian goldfields, such as Bendigo–Ballarat and Charters Towers, with which they share many features. When compared to deposits in the French Massif Central, direct counterparts of the Andean deposits such as Pataz and Ananea–Yani are respectively the Saint Yrieix district and the Salsigne deposit. Considering the ubiquity of the Au (±Sb±W) vein-type deposits in the Eastern Cordillera and Sierras Pampeanas, and the relatively little attention devoted to them, the Devonian and Carboniferous orogenic gold deposits in the eastern section of the Central Andes constitute an attractive target for mineral exploration.