Au Deposits Research Articles

Electroplating-based engineering of plasmonic nanorod metamaterials for biosensing applications

Sensing lower molecular weight in a diluted solution using a label-free biosensor is challenging and requires a miniaturized plasmonic structure, e.g. a vertical Au nanorod (AuNR) array-based metamaterials. The sensitivity of a sensor mainly depends on transducer properties and hence for instance, the AuNR array geometry requires optimization. Physical vapour deposition methods (e.g. sputtering and e-beam evaporation) require a vacuum environment to deposit Au, which is costly, time-consuming, and thickness-limited. On the other hand, chemical deposition, i.e. electroplating deposit higher thickness in less time and at lower cost, becomes an alternative method for Au deposition. In this work, we present a detailed optimization for the electroplating-based fabrication of these metamaterials. We find that slightly acidic (6.0 < pH < 7.0) gold sulfite solution supports immersion deposition, which should be minimized to avoid uncontrolled Au deposition. Immersion deposition leads to plate-like (for smaller radius AuNR) or capped-like, i.e. mushroom (for higher radius AuNR) structure formation. The electroplating time and DC supply are the tuning parameters that decide the geometry of the vertically aligned AuNR array in area-dependent electroplating deposition. This work will have implications for developing plasmonic metamaterial-based sensors.

Zircon constraints on gold enrichment in a high-silica porphyry, Yixingzhai region, north of the Trans-North China Orogen

The Hewan granite porphyry is a newly discovered, highly evolved Au deposit in north China. Two types magmatic zircon are found in the porphyry. Type I is CL-dark and only occurs in the core of some zircons. Type II is CL-bright and mantles Type I zircon or occurs as individual grains. The two Types are close in age, i.e. 138.0 ± 2.2 Ma for Type I and 135.9 ± 2.1 Ma for Type II. Type I zircon is enriched in Y, U, Th, Nb, Ta, P, REE, while Type II zircon is relatively depleted in these elements, which reflects a reverse in the magma evolution. Whole-rock geochemistry and the presence of xenocrystic zircons (2522 ± 18 Ma), combined with εHf(t) (−20.2 to −22.3) and δ18O (5.3 to 6.6 ‰) for Type I zircons, suggests that the Hewan granite porphyry was mainly derived from the melting of the surrounding older, country rock gneiss. The higher εHf(t) (−16.5 to −19.4) and lower δ18O (5.0 to 6.0‰) of Type II zircon indicates that Type II zircon formed during infiltration of a later fluid. This later fluid also had a higher oxygen fugacity (Ce/Ce* Z = 248 to 3663) and a higher H2O activity. We propose that the Hewan granite porphyry originated from the shallow melting of regional gneisses due to heat from underplating mafic magmas related to a subduction event. Type I zircons, which formed during this event where later mantled or totally replaced by Type II zircon due to an influx of Cl-, S-, and Au-bearing fluids from these crystallizing underplating mafic magmas. The high F and pH in this shallow magma promoted decoupling of AuCl− in the upwelling fluids, which allowed for the deposition of elemental Au.

Gold endowment and unloading along pathway for giant gold mineralization: Insights from spatiotemporal variations of in-situ pyrite geochemistry and gold fineness from the Jiaodong gold deposits, north China Craton

Over 5000 tonnes of gold were accumulated in the Jiaodong Peninsula at the early Cretaceous, the sources and critical ore-forming processes of which are still controversial. Here, we conducted comprehensive in-situ textural, elemental and isotopic analyses on pyrite and gold grains from the different mineralization styles, hydrothermal stages and depths for three representative gold deposits (Linglong, Xiadian and Jiangjiayao) from the northwestern Jiaodong Peninsula, with the aim to exactly constrain the sources and ore-forming processes. Three groups of δ34Spyrite were identified in the studied deposits, of which the Group 1 shows stable and low δ34S (4.8–7.6‰) throughout the hydrothermal evolution while the Group 2 has elevated δ34S (6.0–12.8‰) coupled with increasing As and Au concentrations. The Group 3 shows moderate δ34S (5.2–9.8‰) accompanied by high Al and Si concentrations. The Group 1 suggests a low-δ34S primary auriferous fluid derived from a mantle-related source, whereas the Group 2 indicates the primary auriferous fluid leaching additional S, As and Au from the Precambrian metasedimentary rocks along pathway. The Group 3 is petrographically coupled with numerous silicate inclusions, indicating enhanced fluid-rock interaction between the auriferous fluids and the granitic wallrcoks. Gold fineness decreases from the early to the late hydrothermal stages, combined with increasing deposition of polymetallic sulfides, suggesting an increase of pH resulting in efficient Au deposition. Gold fineness and Bi concentrations in pyrite decrease with shallowing, suggesting that cooling affected Au deposition as well. Bismuth-Te-S minerals were identified in the deep depths, combined with the well correlated Bi and Ag, implying that Bi-Te complexes might play a role in transporting Au and Ag at high-temperature conditions. The above results corroborate that multiple sources and ore-forming processes were responsible for the giant gold mineralization, of which the gold endowment from Precambrian wallrocks needs to be further concerned.

Argentopolybasite, Ag16Sb2S11, a new member of the polybasite group

AbstractThe new mineral argentopolybasite, ideally Ag16Sb2S11, was found at the Kremnica Au–Ag epithermal deposit, Žiar nad Hronom Co., Banská Bystrica Region, Slovakia (type locality), Šibeničný vrch near Nová Baňa, Žarnovica Co., Banská Bystrica Region, Slovakia (cotype locality) and the Arykevaam epithermal Au–Ag deposit, Anadyr’ District, Chukotka Autonomous Okrug, Russian Federation (cotype locality). At the Kremnica deposit argentopolybasite was found as discrete, well-developed (pseudo)hexagonal tabular crystals up to 4 mm in size or as complex crystalline aggregates and groups up to 5 mm in size in cavities of quartz. It is associated with pyrargyrite, polybasite, stephanite, miargyrite, rozhdestvenskayaite-(Zn), argentotetrahedrite-(Zn), naumannite, gold and pyrite. Argentopolybasite is dark grey to black, with a black streak and metallic to opaque lustre. The Mohs hardness is ~3. It is brittle with no observable cleavage and with a conchoidal fracture. The calculated density is 6.403 g⋅cm–3. In reflected light, argentopolybasite is grey, with no observable bireflectance and very weak pleochroism. It shows moderate anisotropy in crossed polarisers with weak greenish and green–blue tints. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the IMA are (Rmin/Rmax, %): 30.3/31.0 (470 nm), 28.8/29.3 (546 nm), 28.1/28.6 (589 nm) and 27.4/27.8 (650 nm). The empirical formulae (based on 29 apfu) are, Kremnica: (Ag15.94Cu0.18)Σ16.12(Sb1.40As0.61)Σ2.01(S10.60Se0.25Cl0.03)Σ10.88, Nová Baňa: Ag16.30(Sb1.74As0.22)Σ1.96(S10.69Cl0.04)Σ10.73 and Arykevaam: (Ag15.54Cu0.38)Σ15.92(Sb1.56As0.51)Σ2.07S11.01. The ideal end-member formula for argentopolybasite is Ag16Sb2S11. Argentopolybasite is trigonal, space group P321, a = 15.0646(5) Å, c = 12.2552(5) Å, V = 2408.61(15) Å3 and Z = 2. The seven strongest powder X-ray diffraction lines are [dobs in Å, (I), hkl]: 12.169, (40), 001; 3.162, (100), 041; 3.045, (54), 004; 2.881, (45), 042; and 2.4256, (28), 421. The crystal structure of argentopolybasite from Kremnica, refined to Robs = 0.0741 for 2804 observed reflections, confirmed that the atomic arrangement is isotypic to that of the other members of the polybasite group and it is isostructural with argentopearceite.

Size-Dependent Electrochemical Metal Growth Kinetics

This work explores the electrochemical deposition (ECD) of Au onto chemically preformed Au nanoparticles (NPs) as a function of Au NP size. The growth potential and current depend strongly on the size of the preformed Au NPs (seeds) due to different Au NP size-dependent kinetics of the growth process. Electrochemical reversibility depends on the Au NP size, scan rate, metal precursor concentration ([AuCl4–]), and Au NP coverage. The Au NPs need to reach a critical total surface area to have enough active area to cause the ECD over the entire electrode to be electrochemically reversible and operate under linear diffusion mass transfer-limited conditions. The smaller Au NPs require less total surface area to reach this condition because they have higher activity per surface area, which is not fully understood but could be due to a greater percentage of defects on their surface, such as edge and corner atom sites.

A microscopic view on the electrochemical deposition and dissolution of Au with scanning electrochemical cell microscopy – Part I

The nucleation and dissolution of gold nanoparticles (NPs) on a glassy carbon substrate have been studied with a local electrochemical approach based on Scanning Electrochemical Cell Microscopy (SECCM). By performing hundreds of locally-resolved voltammograms, a distribution of responses for nucleation was found when scanning multiple regions of the substrate, addressing experimentally the statistical nature of the nucleation and growth process at the micro-scale. Moreover, the confinement of the electrochemical cell to the SECCM meniscus enables us to resolve a diversity of events during the electrochemical dissolution of previously electrodeposited NPs, characterized as current spikes during the voltammetric scans. These current events do not manifest in the macro-scale due to the averaging of the electrochemical response over the large electrode area and reveal previously untapped information from the electrochemical nucleation, growth, and dissolution of supported metal nanoparticles. This approach opens up new opportunities for the rational design (electrodeposition) of functional nanostructured materials and the evaluation of their durability under electrochemical polarization (resistance to electrodissolution). The ability to study these considering the heterogeneous nature of the supports and the differences within nanomaterial ensembles is essential for applications in electrochemical conversion and storage.

Deformation and mineralizing fluid circulation of gold-silver-bearing quartz veins in the Yushishan gold deposit, Altyn Mountain, northwestern China

Understanding the coupled process of deformed microstructure and fluid evolution of ore veins therefore provides insight into processes that influence mineralization distribution and its mechanism. Gold is often developed in quartz veins in many gold deposits. This paper presents detailed evidence for solid deformation, hydrothermal fluid, and mineralization in a fault-controlled gold-silver-bearing quartz vein from the Yushishan gold deposit, Altyn Mountain, NW China. Macro-and microstructure combined quartz crystal preferred orientation (CPO) unravel the ductile–brittle transition deformation of quartz veins formed at the deep crustal level ca. 9 ∼ 13 km and the deformation temperature ranging between 180 ∼ 381 °C. The homogenization temperatures and salinities of fluid inclusions change slightly, while the trapping pressure of fluid inclusions has a large fluctuation of ca. 125 ∼ 275Mpa. HO isotopes and the composition of fluid inclusions decipher the source of ore-forming fluid mainly related to the regional magmatic-metamorphic event. Combined with the fluid immiscibility that is evidenced by different types of fluid inclusion coexistence characteristics and the pH increase induced by CO2 degassing in an inhomogeneous fluid system, it is suggested that an obvious fluid overpressure and rapid decompression occur during Au deposition from the quartz vein. The seismically fractured opening and healing of rocks during ductile–brittle deformation controls the formation of the mineralization and fluid circulation. There has positive feedback between the fluid overpressure evolution, strain localization and mineralization through time. The channelized and circulated fluid flow triggered extensive veining, mineral growth, and connections with fluid reservoirs.

Optimization of NH2-UiO-66/TiO2/Au composites for enhanced gas-phase CO2 photocatalytic reduction into CH4

Here we report on the optimization of NH2-UiO-66/TiO2/Au (ca. 1.5 wt%) composite photocatalysts applied to gas phase CO2 photocatalytic reduction in presence of water as reducing agent by varying NH2-UiO-66/TiO2 ratio and pH during synthesis. It is shown that 10 wt% NH2-UiO-66/TiO2/Au (at pH=7) composite leads to the best cumulated CH4 production rate of 136 μmol/gcatalyst with 70% electronic selectivity over 5 h of continuous test. This composite exhibits the best compromise between MOF surface area and thus CO2 adsorption sites, visible light photons absorption capacity and a large interface contact area, which ensured preferable metal(Zr)-to-metal(Ti) charge carrier. Au deposition by impregnation/chemical reduction in also required to perform CO2 photoreduction, and may presumably act as electron traps, co-catalyst or surface plasmon resonator.

Orogenic Au deposits with atypical metal association (Cu, Co, Ni): Insights from the Pohjanmaa Belt, western Finland

The Laivakangas Au-Cu metallogenic area is characterised by orogenic Au deposits with both Au-only and with atypical (Au ± Cu, Co, Ni) metal associations. Here we study and compare four examples to better constrain the parameters controlling enrichment in base metals in addition to Au. We selected two typical Au-only deposits, the Laivakangas and the Huhta deposits and two orogenic Au deposits with atypical metal association, the Jouhineva Au-Cu-Co-Ag and the Kurula Au-Co deposits. All four deposits record multiple successive mineralisation events with local variations in their respective metal association. Two auriferous mineralisation events are identified, (1) a ubiquitous As-Au-(Co, Ni) event close to peak metamorphism (620–430 °C) where Au occurs either as invisible Au in arsenides or as inclusion in arsenopyrite; (2) a later Cu(-Au)-rich sulfide event on the retrograde path where Au locally occurs as free, native grains along with chalcopyrite. From S isotope studies of the sulfide and sulfarsenide minerals and relations between the deposits and surrounding rocks, we propose that the variation in metal association of the ore fluid is linked to the diversity of lithologies involved in metamorphic fluid production. Multi-event hydrothermal mineralisation and relatively reduced redox conditions appear critical to increase the Au endowment in a deposit and to introduce atypical metals. Results of this study provide a new comprehension of the variability of metal association in orogenic Au deposits of the Laivakangas Au-Cu metallogenic area and elsewhere.

Magnetic Signature of Gold Deposits: Example from Um Garayat Region, South Eastern Desert, Egypt

Magnetic methods of exploration have proved to be efficient and have potential in the gold mineralization industry. New magnetic processing technologies aid in improving the process of interpretation and gold opportunity identification. In this work, we show the possible application of combined digital magnetic filters to explore new gold mineralization localities with application to a well-known Au mineralization zone. Um Garayat (UG) region, southeastern desert, Egypt, is an ancient example of a potential area for gold mining. Modern analysis showed that other types of mineral concentrations are present. So, exploiting the magnetic signature of the area for future investment is of great interest. The old UG gold mine is characterized by volcanic and tectonic features such as faulting and folding that affect the arc sedimentary rock sequence of repeated deformation stages. A thorough geophysical effort has been carried out around the old gold mine in the UG area to explore the extension of mineralized ore deposits. A detailed geophysical survey using magnetics was carried out in this study together with the available aeromagnetic data. Field data sets on appropriate sites were measured, processed, and evaluated by suitable software. High magnetic anomalies were detected based on grid filter analysis and contact occurrence maps as marked as possible ore deposits after satisfying the geologic conditions for gold formation. An integrated understanding of attained results revealed that the new possible ore deposits are related directly to fault and fracture zones in the shape of lenses of variable thickness in this zone. Results show that newly detected mineral occurrences in the UG area are also controlled by major faults and hydrothermal solution enrichment along fault zones at a depth ranging from 20 to 70 m. Favorable fault/joint mineralized places were located. The relationship between the Au-quartz vein's strike direction compared with magnetic anomaly lineament analysis was studied. Search for new sources of Au and other mineral deposits in addition to quartz veins is needed as pockets of accumulated mineral-rich rock fragments are deposited in drainage wadis and fault/joint zones because of hydrothermal solution enrichment.

Geochemistry and origin of hydrothermal apatite in Carlin-type Au deposits, southwestern China (Gaolong deposit)

Apatite can record the evolution of magmatic–hydrothermal ore systems, but it is unclear whether it is suitable for investigating low-temperature, sediment-hosted hydrothermal ore systems. To fill the knowledge gap. we investigated the Gaolong Carlin-type Au deposit in the Guangxi Zhuang Autonomous Region, southwestern China (Youjiang Basin). Results show that the hydrothermal apatite occurs in quartz–sulfide ores, has an intact crystal form and homogeneous texture, and is F-rich (2.77–3.67 wt%) and Cl-poor (<0.01 wt%). The apatite is enriched in rare earth elements (REEs; ΣREE = 739–2481 ppm) and has middle-REE-enriched chondrite-normalized REE patterns with positive Eu anomalies (δEu = 1.35–2.00). These REE features were inherited from a reducing hydrothermal system in which anion complexation was dominant. The apatite has high Sr contents (up to 0. 9 wt%) and relatively uniform initial 87Sr/86Sr ratios (0.709055–0.709208), suggesting that Sr was leached or derived from the deep continental crust. In situ U–Pb dating of 156.8 ± 8.3 Ma for apatite shows that the Late Jurassic–Early Cretaceous was an important time for the formation of Carlin-type Au deposits in the Youjiang Basin. The Au mineralization was associated with far-field tectonism and deeply sourced hydrothermal ore-forming fluids. Hydrothermal apatite can be used to fingerprint low-temperature, sediment-hosted hydrothermal ore systems.

Fluid partitioning between veins/fractures and the host rocks in Carlin-type Au deposits: a significant control on fluid-rock interaction and Au endowment

Fluid partitioning between veins/fractures and the host rocks in Carlin-type Au deposits: a significant control on fluid-rock interaction and Au endowment

Contribution of Triassic Tectonomagmatic Activity to the Mineralization of Liziyuan Orogenic Gold Deposits, West Qinling Orogenic Belt, China

The Western Qinling orogenic belt (WQOB) is one of the most important prospective gold districts in China, with widely distributed Indosinian intermediate–acidic intrusions. The Liziyuan Au deposit is a representative orogenic deposit in the northern WQOB, hosting several sections spatially associated with igneous rocks. The Au deposit is hosted by meta-sedimentary volcanic rocks of the Cambrian–Ordovician Liziyuan Group and the Tianzishan monzogranite. Two periods, including five stages of mineralization, are recognized in this area: an early metamorphic mineralization period (PI), including quartz–pyrite (Stage I) and banded quartz–polymetallic sulfide (Stage II) veins, and a later magmatic mineralization period (PII) including quartz–K-feldspar–pyrite–molybdenite veins (Stage III), quartz–polymetallic sulfide–chlorite ± calcite veinlets and stockwork (Stage IV), and late calcite–quartz veinlets (Stage V). Geochronological studies indicate a SHRIMP zircon U-Pb age of 236.1 Ma for the Tianzishan monzogranite, and our published ages of ore-bearing diorite porphyrite of the Suishizi section and granite porphyry of the Jiancaowan section being 213 and 212 Ma, respectively. Pyrites formed in association with PI and PII mineralization have well-defined Rb–Sr ages of 220 ± 7.5, 205.8 ± 8.7, and 199 ± 15 Ma, with close temporospatial coupling between mineralization and magmatism. The δ18O and δD values of fluid inclusions in Stage IV auriferous quartz veins range from −0.03‰ to +5.24‰ and −93‰ to −75‰, respectively, suggesting that mineralizing fluid was likely of magmatic origin. Three distinct ranges of δ34S values are identified in the studied sections (i.e., 7.04‰–9.12‰, −4.95‰ to −2.44‰, and 0.10‰–3.08‰), indicating a source containing multiple sulfur isotopes derived from magmatic and metamorphic fluids. The Liziyuan Au deposit is thus likely an orogenic deposit closely related to magmatism. Geochemical characteristics indicate that Tianzishan monzogranite is adakitic and was derived from thickened lower crust during Triassic orogenesis. The ore-bearing diorite porphyrite and granite porphyry formed in a post-collision extensional setting. Together with previous geological and geochemical data, our results indicate that the Liziyuan orogenic Au deposit was formed by early collisional–compressional metamorphism and late post-collision extensional magmatic fluids related to the evolution of the WQOB.

Sb–Au-bearing chalcedonies in hot geothermal systems: insights from the jasperoids of Poggio Peloso (southern Tuscany, Italy)

AbstractDetailed characterization was performed on the chalcedonies from the jasperoids of the Pietratonda–Poggio Peloso Sb–Au deposit (southern Tuscany, Italy). The main purpose was to retrieve information on the geothermal fluids that formed the chalcedonies and the source of antimony concentrations. Investigations were performed using optical microscopy, laser ablation inductively coupled plasma mass spectroscopy and X-ray diffraction on both the chalcedonies and the lithotypes cropping out in the area. The results obtained allow the chalcedonies of Pietratonda–Poggio Peloso to be described as a unicum, based on the very high contents of Sb that do not find a comparison in the literature. The textures showed multiple generations of silica that agree well with an environment characterized by multiple injections of mineralizing solutions, bearing variable physicochemical characteristics. The transport likely took place in an alkaline environment, while the acidification of the water may have favoured the precipitation at varying temperatures but not higher than 225 °C. The rocks from which the constituents may have been leached are the hosting carbonates and the surrounding metamorphic rocks. Among the examined rocks, the metamorphic rocks showed the most numerous and significant correspondences with the chalcedonies and were the only ones in which discrete amounts of gold contents were found.

Trace Element Composition of Molybdenite: Deposit Type Discrimination and Limitations

Molybdenite is a common sulfide hosting many trace elements. Trace elements in molybdenite from individual deposits have been widely used to constrain the source and conditions of ore-forming fluids. However, the relationship between the trace element composition of molybdenite and deposit types has not been well investigated from a large dataset. Here, simple statistics and partial least squares–discriminant analysis (PLS-DA) were used to determine whether different types of deposits can be distinguished by trace elements in molybdenite and what factors control the variations in trace element composition based on published laser ablation ICP–MS data. Molybdenite from porphyry deposits is separated from that from quartz veins, greisen Sn–W, granite vein Mo, and granodiorite Mo deposits. The former is characterized by relatively high Re, Cu, Ag, Se, Pb, Bi, and Te contents, whereas the latter has higher Ni, Co, Sn, Sb and W contents. Molybdenite from the quartz vein Au ± W deposits (Au-dominated), and porphyry Cu–Au–Mo (moderate Au) are separated from other deposits without gold due to positive correlations with Au, Sb, Te, Pb, and Bi for the former. Assemblages of Au–Sb–Te–Pb–Bi in molybdenite are thus useful to discriminate as to whether deposits contain gold and the degree of gold mineralization. Higher oxygen fugacity is responsible for the relative enrichment of W in molybdenite from greisen Sn–W deposits, whereas lower oxygen fugacity results in the relative enrichment of Re in molybdenite from porphyry Cu ± Mo ± Au and Mo ± Cu ± Au deposits. There are some limitations to using molybdenite as an indicator mineral because of the complex occurrences of elements in molybdenite, large compositional variations within a specific deposit type, and an imbalanced dataset. To develop molybdenite as an indicator mineral tool, further work should be carried out to overcome these limitations. This study provides an attempt to classify deposit types using molybdenite trace elements and has important implications for ore genesis research and mineral exploration.

Multimodal imaging of cubic Cu2O@Au nanocage formation via galvanic replacement using X-ray ptychography and nano diffraction

Being able to observe the formation of multi-material nanostructures in situ, simultaneously from a morphological and crystallographic perspective, is a challenging task. Yet, this is essential for the fabrication of nanomaterials with well-controlled composition exposing the most active crystallographic surfaces, as required for highly active catalysts in energy applications. To demonstrate how X-ray ptychography can be combined with scanning nanoprobe diffraction to realize multimodal imaging, we study growing Cu2O nanocubes and their transformation into Au nanocages. During the growth of nanocubes at a temperature of 138 °C, we measure the crystal structure of an individual nanoparticle and determine the presence of (100) crystallographic facets at its surface. We subsequently visualize the transformation of Cu2O into Au nanocages by galvanic replacement. The nanocubes interior homogeneously dissolves while smaller Au particles grow on their surface and later coalesce to form porous nanocages. We finally determine the amount of radiation damage making use of the quantitative phase images. We find that both the total surface dose as well as the dose rate imparted by the X-ray beam trigger additional deposition of Au onto the nanocages. Our multimodal approach can benefit in-solution imaging of multi-material nanostructures in many related fields.

Mineralogical constraints on the genesis of an alkalic-type epithermal Au-Te deposit: Tuvatu, Fiji

To study the characteristics and genetic constraints on alkalic-type epithermal Au mineralisation, here we use the example of the Tuvatu Au-Ag deposit in Fiji, with an emphasis on detailed, quantitative mineralogy. Tuvatu mineralisation is hosted in a weakly altered potassic monzonite in parallel sided-veins of K-feldspar, biotite, sericite, calcite, and quartz, with epidote-bearing propylitic or sericite-rich selvages. Petrographic study of core and automated SEM-based mineralogical mapping of thin sections have been utilised to update previous parageneses of the deposit. Automated SEM techniques enable identification of small amounts of obscure minerals that form minuscule grains, which would otherwise be very difficult to identify and measure. As a result, our data show that gold fineness is extremely high, with the mean and median Au content of native-Au and Au-Ag alloy being 96.7% and 100% respectively, yet precious-metal tellurides make up the majority of the Au deportment. Tellurides show evidence of multiple phases and zoning with depth. For the first time at Tuvatu, Pt- and Pd-tellurides have been identified. Tuvatu has a number of features in common with alkalic systems elsewhere, including quartz-poor, carbonate-rich veins and alteration, abundant and varied telluride minerals, high gold grades, and Pt-Pd occurrences. We suggest these characteristics are a result of relatively high temperature (250–300 °C) fluids and immiscible semi-metal melts fluxing into the shallow epithermal environment. High pH fluids lead to quartz-poor alteration, but mildly acidic conditions dominate in areas of high fluid flux, where the lower pH causes precipitation of tellurides with quartz. Boiling of the fluids produces zonation of tellurides with depth but leaves relatively subtle textural evidence compared to boiling in most epithermal systems, in common with other quartz poor, carbonate-rich alkalic epithermal deposits around the world.

Lateral variations in the Hosen 8‐2 vein in the Hishikari deposit, Japan: Implications for high Au‐grade zone

AbstractLateral sampling of each blasting interval (~ 2.6 m) along a 79 m strike length was conducted for the Hosen 8‐2 vein on the −5 ML (mining level, meters relative to sea level) of the Main ore zone in the Hishikari epithermal Au deposit, Kagoshima, Japan. The horizontal variation of mineral textures, bulk chemical compositions, and fluid inclusion temperatures and apparent salinity of ore fluids were determined. The major gangue minerals of the quartz veins studied include quartz and adularia with minor amounts of calcite and smectite. The major ore minerals include electrum, galena, and sphalerite with minor pyrite, petzite, and hessite. Electrum commonly occurs in a discrete band with microcrystalline quartz, granular adularia, and smectite. The electrum‐bearing band is followed by tabular adularia, and finally by granular or comb quartz. Mineral textures that indicate high degrees of supersaturation with respect to amorphous silica (presently quartz with microcrystalline/mosaic and feathery textures) and adularia (with tabular, rhombic, and granular textures) were common throughout the vein strike. Bulk chemical analyses indicate that Au grade is positively correlated with Ag, Bi, Pb, and Te contents. A bonanza zone with Au grades up to 10,800 ppm occurs in the central part of the Hosen 8‐2 vein on the −5 ML, and is associated with high adularia content, calculated as adularia / (quartz + adularia). The wide lateral variation in the geochemical composition contrasts with the consistent association of electrum with microcrystalline quartz, granular adularia, and smectite throughout the Hosen 8‐2 vein on the −5 ML. Fluid inclusion microthermometry of primary and pseudosecondary inclusions in quartz and adularia yielded histogram modes of homogenization temperature between 160 and 240°C; the most frequent mode is 200–210°C, with most data within ±10°C from this value. The maximum ice‐melting temperature of most samples excluding late comb quartz is −1.5°C, which is equivalent to an apparent salinity of 2.6 wt% NaCl eq. The wide variation in ice‐melting temperatures and apparent salinities (up to 5.2 wt% NaCl eq.) may be due to dissolved CO2 in the fluids, while the sharp decrease in apparent salinity with temperature decrease indicates CO2 loss due to fluid boiling during vein formation. The intimate association of electrum with microcrystalline quartz that recrystallized from amorphous silica indicate sharp boiling and vapor loss as the primary mechanism of Au deposition in the Hosen 8‐2 vein.

Similarities and differences in geological characteristics and sulfur isotope composition and their controlling factors of the Dahu, Linghu and Jinqu Au(Mo) deposits in the Xiaoqinling gold province

小秦岭金(钼)矿省发育大量脉状金(钼)矿床,具有重要经济价值。本次研究选择大湖、灵湖和金渠金(钼)矿床,通过系统的构造-蚀变-矿化研究和金相关黄铁矿原位硫同位素分析,拟查明小秦岭金和钼矿化基础地质异同和硫同位素组成及其控制因素。构造-蚀变-矿化研究表明上述金(钼)矿脉赋矿围岩均为前寒武太华群变质岩,控矿构造主要为近EW向(局部为NW向)剪切带,金和钼矿体发育在同一剪切带不同部位或局部叠加。钼矿脉通常发育钾化和硅化蚀变和辉钼矿和黄铁矿等矿石矿物组合,而金矿脉以绢英岩化蚀变为主,主要发育黄铁矿、多金属硫化物和碲化物等矿石矿物。原位S同位素结果显示大湖矿床钼矿脉中黄铁矿<i>δ</i>³⁴S值为-3.3‰~+3.3‰,而大湖、灵湖和金渠金矿脉中黄铁矿<i>δ</i>³⁴S值为-7.8‰~+8.9‰。大湖和灵湖近EW金矿脉中黄铁矿<i>δ</i>³⁴S值分别为-2.8‰~+0.4‰和-7.8‰~+8.2‰,与大湖金矿脉相比,灵湖金矿脉倾向较缓且发育大量围岩角砾表明上述硫同位素组成可能由差异水力破裂和水岩反应控制。金渠金矿脉近EW和NW向矿脉中黄铁矿<i>δ</i>³⁴S值范围分别为-3.9‰~-3.5‰和0.7‰~8.9‰,其中陡倾矿脉黄铁矿<i>δ</i>³⁴S值较负(-3.9‰~+3.7‰),而缓倾矿脉<i>δ</i>³⁴S值较正(+4.1‰~+8.9‰),这些黄铁矿硫同位素组成差异可归因于不同构造背景控制的差异氧逸度等物理化学条件。本次研究结合前人年代学、流体包裹体等数据指示除了源区属性和成矿路径外,成矿末端的水力破裂和水岩反应等过程在控制小秦岭金和钼成矿异同方面同样扮演重要角色。

Geochronology, magma source, and geochemistry of igneous rocks in the northern Taebaeksan metallogenic region, South Korea

AbstractCretaceous igneous rocks are concentrated in the northern Taebaeksan metallogenic region of South Korea, some of which are related to Fe skarns and/or hydrothermal vein Au–Ag deposits. However, detailed studies on the emplacement age and magma source of igneous rocks supplying this metallogenic area are lacking. In this study, we investigated the emplacement age, magma sources, and geochemical characteristics of seven Cretaceous igneous rocks around the ore deposit, comparing them with previous studies. Zircon U–Pb and mica K–Ar age dating indicated that two magmatism events occurred in the Early Cretaceous (~113.7 ± 0.2 to 104.7 ± 0.5 Ma) and the Late Cretaceous (~85.8 ± 1.1 to 77.6 ± 0.4 Ma). Negative ɛHf(t) values (−2.57 to −22.05 approx.) and the calculated (2.55–1.33 Ga) suggested that the magma source of these igneous rocks was derived from the Proterozoic crust. Whole‐rock geochemical data indicated that the northern Taebaeksan igneous rocks are mostly high‐K calc‐alkaline series, enriched in light rare‐earth elements (LREEs) and large‐ion lithophilic (LIL) elements but depleted in high‐field‐strength (HFS) elements. Although igneous rocks in the northern Taebaeksan metallogenic region have broadly similar magma source characteristics, they can be classified into two groups: one comprises Early Cretaceous intermediate rocks, some of which are related to Au–Ag mineralization, whereas the other includes Late Cretaceous intermediate to felsic rocks, related to Au–Ag or Fe mineralization.