Au Deposits Research Articles

Textural characterization of unidirectional solidification textures related to Cu–Au deposits and their implication for metallogenesis and exploration

Ten rock samples with unidirectional solidification textures (USTs) from nine Asian and Australian porphyry and intrusion-related Au deposits and prospects were investigated in terms of mineralization sequences, textural and paragenetic relationships, melt and fluid inclusion inventory, cathodoluminescence, and trace elements of UST quartz. Despite general similar structural appearance of the investigated UST sequences, each UST locality has its own specific textural features. We provide a structural classification of observed USTs that distinguishes equigranular and heterogranular USTs, which are subdivided according to the UST layer structure into straight, wavy, or contorted/brain-like type. In addition, there is the group of disturbed USTs comprising incompletely developed UST layers, and ductile- or brittle-fragmented UST sequences. Ti-in-quartz-based calculations revealed crystallization temperatures of UST quartz between 590 and 770 °C. Primary silicate melt inclusions found in four samples indicate that quartz crystallized initially from a silicate melt. In five samples, fluid inclusion “boiling” assemblages were observed, documenting brine-vapor phase separation due to pressure drop. In one case, the “boiling” assemblage is in primary inclusions and in the other cases the assemblages have been trapped in pseudosecondary inclusions in a later stage. Primary brine inclusions without coeval vapor-rich inclusions were identified in one sample only. Our results permit UST formation in either an open-system setting where fluids exsolved from UST crystallization are periodically released by fracturing (pressure drops) after the formation of each individual UST layer, or closed-system setting where the exsolved fluids are trapped because of lack of fracturing as long the UST sequence forms. Here, the closed-system model is introduced for the first time to describe the formation of layered quartz UST sequences in porphyry systems where the open-system setting seems to be not applicable. Nonetheless, quartz UST sequences are very important exploration vectors for porphyry systems. Six out of the ten UST samples are high-grade with up to 12 g/t Au. However, in the investigated cases, the Au mineralization postdates UST formation. At Oyut Ulaan and Tampang, the UST quartz formation is co-genetic with major Cu–Fe sulfide mineralization, whereas in the other localities the UST formation predates the sulfide mineralization. In any case, UST layer sequences clearly functioned as an outer carapace and, thus, as a chemo-physical trap of passing mineralizing fluids entering the UST structure and may offer a vector to the main stockwork mineralization, which might occur 100 to 200 m above the UST zone.

Metallogeny in the Bangong–Nujiang belt, central Tibet, China: A review

The Bangong–Nujiang metallogenic belt consists of scattered Tethyan oceanic blocks, mainly distributed underneath the margins of the Qiangtang and Lhasa terranes in central Tibet. A new world-class metallogenic belt has been reported in this region recently, based on the geological mapping and ore deposit prospecting over the last two decades. It currently comprises inferred resources of 30 Mt Cu and 500 t Au, together with several Cr–Ni, Fe, and W (Mo) resources, forming a significant potential area for future mineral exploration. These metals are mainly hosted in porphyry copper, skarn copper, skarn iron, orogenic gold, quartz-vein tungsten, and ophitic chromite deposits. The mineral deposits in the Bangong–Nujiang metallogenic belt have been widely recognized in different localities, including the southern edge of the southern Qiangtang block, part of the north Lhasa block, and even part of the central Lhasa block, indicating they were formed in variable geological settings, from the initial opening, subduction, and collision to the extension of the Bangong–Nujiang Ocean. Specifically, five major tectonic events contributed to mineralization, including the stage 1 (240–165 Ma) initial opening of the Bangong–Nujiang Ocean, stage 2 (165–145 Ma) oceanic subduction, stage 3 (145–100 Ma) close of the ocean, stage 4 (100–65 Ma) continent–continent collisional orogenesis, and stage 5 (65–0 Ma) post-orogenesis. At stage 1, Cr–Ni deposits were formed during the initial opening of the ocean; porphyry–epithermal Cu (Au), skarn Fe, and minor orogenic Au deposits were formed at stage 2 and stage 3; a younger pulse of a few porphyry–skarn Cu ± Mo and orogenic Au deposits were formed during stage 4; finally, W(Mo) deposits were generated in stage 5. In general, porphyry Cu systems, orogenic Au, and skarn Cu polymetallic deposits that occurred in the subduction and post-collision settings related W(Mo) deposits have the most potential for future exploration. An in-depth investigation of several scientific problems, such as addressing the tectonic setting, magmatism, and metallogeny of this region and genetic linkage of these deposit preservations to plateau uplift, is essential for the future success of exploration in the Bangong–Nujiang metallogenic belt.

Regio-selective growth of plates on Au nanorods

Morphological control is extremely difficult during the second-stage growth on existing nanoparticles. Here, we show that it is possible to combine nanorod and nanoplate morphologies by using partially encapsulated polystyrene-block-poly(acrylic acid) shells as the mask. Au nanoplates were grown selectively from the exposed surface of the Au nanorod, with consistent orientation for each nanorod. As such, we achieve three levels of synthetic control simultaneously: 1) the polymer mask focuses the Au deposition onto a few sites, so that the nanoplates could extend out prominently; 2) the longitudinal extension of the nanorod could be shifted to the traverse growth of nanoplates by using iodide ions as the inhibitor; 3) the nanoplates grow from only one of the twin planes because of the dynamic competition via the depletion sphere mechanism.

The Birkachan Epithermal Au–Ag Deposit in the Kedonsky Middle Paleozoic Volcanic Belt (Russian Northeast)

The Birkachan Epithermal Au–Ag Deposit in the Kedonsky Middle Paleozoic Volcanic Belt (Russian Northeast)

Scheelite trace element compositions: A robust new tool for mineral exploration

Scheelite is a widespread accessory tungstate mineral that contains a variety of trace elements. Scheelites from the major types of magmatic-hydrothermal mineral deposits (86 deposits, including quartz-vein and greisen, porphyry, skarn and orogenic Au deposits) have been collected and analyzed using laser ablation-inductively coupled plasma mass spectrometry (3270 data, including 584 analyses). Discriminant analysis using Mo, Sr, Nb, La, Ce, Eu, Dy, Yb, Y, Ta, Pb, (Ce/Yb)N, (La/Sm)N, (Gd/Yb)N, Eu anomaly, Ce anomaly, (La/Yb)N, LREE/HREE, Y/Ho and Mo + Sr + Nd + rare-earth elements (REE) + Y + Ta + Pb reveals that scheelites from different mineral deposits can be distinguished. Scheelites from quartz-vein and greisen deposits have high total contents of trace elements that partition onto the Ca sites (Mo, Sr, Nd, REE, Y, Ta, Pb). Scheelites from the porphyry W ± Cu ± Mo deposits, which have been studied in this work, are characterized by low Mo and positive Eu than scheelites from porphyry Cu ± Mo deposits. This indicates that large scale tungsten deposits are related to reduced hydrothermal fluids, in contrast to the oxidized fluids associated with porphyry Cu ± Mo deposits. Scheelites from skarn W ± Sn ± Mo deposits typically exhibit a wide range of Y/Ho ratio and a higher sum of the elements Mo, Sr, Nd, REE, Y, Ta, and Pb compared to that from the skarn Cu ± Mo ± W deposits, indicating that increased amount of fluorine probably leads to large-scale W, Mo, and Sn mineralization during magmatic-hydrothermal processes. Scheelites from the greisen W-Sn polymetallic deposits tend to have more restricted Sr compositions and Eu anomalies than those from the quartz vein W-Sn deposits, as well as relatively high Gd, Tb, Dy and Ho contents, which coincide with extensive hydrothermal alteration, indicating intense fluid-rock interaction during mineralization. Using Discriminant projection analysis (DPA), scheelites from many ore deposits can be readily separated from each other. This study demonstrates that heavy-mineral scheelite grains collected during regional geochemical surveys are effective in identifying specific types of buried mineral deposits in potential mineralization area. Scheelite geochemistry with DPA provides an effective guidance for mineral exploration.

Halogen characteristics and their implications for geochemical exploration in the Qujia gold deposit, Laizhou, Jiaodong

The ore-forming fluids in the Jiaodong gold deposits are characterized by low to medium salinities and Au-HS complexes are the predominant Au species in the fluid. The role of halogen elements in geochemical exploration for the gold deposits is still unclear. This study takes the Qujia gold deposit as an example to explore the issue. Scatter plots and 3D modeling were conducted based on the geochemical data of 1130 borehole samples. The halogen characteristics are governed by fluid-rock interactions, and rock types and heterogeneity. Nevertheless, the halogen characteristics in the alteration zone along the Jiaojia fault zone are still interesting and appealing. In the mineralized zone along the Jiaojia fault, the F and Cl concentrations generally reduce with a decrease in depth; F anomalies evolved from strong positive anomalies at deep depths to negative anomalies at shallow depths; positive Cl anomalies appeared in the upper part or above the mineralized zone while negative Cl anomalies appear in the middle and lower parts of the mineralized zone. The zonation pattern may be attributed to the preferential replacement of OH− by F− due to the acidic pH and the intense beresitization. The consumption of halogen elements may cause the destabilization of metal-Cl complexes and increase the pH of ore-forming fluid, and the deposition of metal sulfides further results in the destabilization of Au-HS complexes and Au deposition. Therefore, F, Cl, and F/Cl ratio can be jointly used as exploration indicators for the geochemical exploration only if rock types, and characteristics of hydrothermal fluid and alteration are examined in advance.

Fabrication of Chiral Nanostructures through Vibration-Assisted Scratching Combined with Wet Etching for Surface-Enhanced Raman Scattering Substrates

A fabrication technique is proposed based on tip-based ultrasonic vibration-assisted scratching (UVAS) combined with wet chemical etching. UVAS induced a deeper subsurface damage of the machined nanostructure than the conventional scratching (CS), obtaining a high depth-to-width ratio nanostructure after hydrofluoric acid etching. Furthermore, the tip wear for UVAS was lesser than that for CS owing to the small friction of UVAS. Furthermore, periodic nanograting structures, nanodot arrays, and chiral nanostructures were machined by controlling the UVAS trajectory. Moreover, surface-enhanced Raman scattering (SERS) substrates with nanodot arrays and chiral structures were prepared based on the transferred polydimethylsiloxane (PDMS) model after Au deposition. It was found that a large depth-to-width ratio SERS substrate contributed to a high enhancement of the Raman signal. A maximum enhancement factor of 4.03 × 105 could be obtained on an optimal nanodot array SERS substrate when measuring 10–5 mol/L rhodamine 6G (R6G). The circular dichroism (CD) spectra of the chiral nanostructures were revealed through the finite element method simulation. Moreover, the multipole resonance led to the CD enhancement of the chiral nanostructures. Furthermore, the chiral enantiomers (l- and d-cysteines) were tested with the SERS-active chiral nanostructures, which can be recognized using the chiral SERS substrates directly. Thus, the proposed method showed great potential in chiral applications such as chiral sensing and label-free detection.

Facile Preparation of Au–Ag Composite Nanostructure for High-Sensitive and Uniform Surface-Enhanced Raman Spectroscopy

Preparation of a high-sensitive and uniform surface-enhanced Raman spectroscopy (SERS) active substrate structure usually faces complex processes and high costs. Here, porous Au–Ag composite nanostructures that are conventional fabricated by the deposition of a multilayer Au–Ag, annealing, and dealloying process are proposed for high-performance SERS. By annealing at a suitable temperature, nanopores could be firmly distributed on the surface, which serves as hot spots. The electric field distribution was also performed by the finite difference time domain. The experiment results exhibited excellent uniformity and high sensitivity of SERS detection. The enhancement factor of the R6G molecules detected by the SERS substrate reached 1.37 × 107, and the relative standard deviation was as low as 4.9%. The minimum detection concentration of R6G molecules by the Au–Ag composite nanostructures with bottom Au mirror could reach 10−13 M. The proposed Au–Ag composite nanostructures and the fabrication process have great potential in preparation of a high-sensitive and uniform SERS substrate.

The Genetic Link between Iron-Oxide–Apatite and Porphyry Cu–Au Mineralization: Insight from the Biotite–Pyroxene–Zircon Study of the Nihe Fe Deposit and the Shaxi Cu–Au Deposit in the Lower Yangtze Valley, SE China

Different ore deposit types may evolve from a common magmatic-hydrothermal system. Establishing a genetic link between different deposit types in an ore cluster can not only deepen the understanding of the magmatic-hydrothermal mineralization process but can also guide exploration. Both the Nihe iron-oxide–apatite (IOA) deposit and the Shaxi porphyry Cu–Au deposit in the Lower Yangtze Valley, Anhui, Southeast China, formed in the Luzong Cretaceous volcanic basin at ~130 Ma. We examined a temporal–spatial and potential genetic link between these deposits based on stratigraphic lithofacies sections, biotite and clinopyroxene mineralogical chemistry, zircon chronology, Hf isotopes, and trace elements. Stratigraphy, petrology, mineralogical chemistry, and available fluid inclusion results support that the emplacement depth of the Nihe ore-related porphyry is shallower than that of the Shaxi porphyry. The magmatic zircon and hydrothermal zircon from Nihe provided U–Pb ages of 130.6 ± 0.7 Ma and 130.7 ± 0.7 Ma, respectively. The magmatic zircon U–Pb age (130.0 ± 0.8 Ma) of Shaxi overlaps with its molybdenite Re–Os age (130.0 ± 1.0 Ma). The agreement between the mineralization and porphyry emplacement ages of Nihe and Shaxi indicates a temporal coincidence and supports a possible genetic link between the two deposits, considering their close spatial relationship (in the same ore district, 15 km). The zircon Hf isotopes and trace elements support the evolution of both deposits from an enriched lithospheric mantle, although the Shaxi deposit may have experienced contamination of the Jiangnan-type basement. Both deposits lie above the fayalite-magnetite-quartz buffer, but the Nihe magmatic zircons are of lower temperature and less oxidized than that of Shaxi. The much higher Eu/Eu* and Yb/Dy values of zircons from Shaxi are likely caused by the suppression of early plagioclase crystallization and the prevalence of amphibole fractionation, thus indicating more hydrous content of the Shaxi ore-related magma. Additionally, the Shaxi ore-related porphyry has higher zircon Hf concentrations, suggesting that the porphyry Cu–Au deposit has experienced a greater degree of magma fractionation. Our study highlights that the Nihe IOA deposit and the Shaxi porphyry Cu–Au deposit have a common magma source, while different extent of crust contamination, magma oxidation state, hydrous content, and degree of magma fractionation collectively result in the two distinct ore deposits. This possible genetic link suggests a great potential of porphyry Cu–Au-PGE mineralization in the Middle–Lower Yangtze River metallogenetic belt, especially in the deep part of the IOA district in the Luzong Cretaceous volcanic basin.

Gold, antimony and mercury ore formation and metallogenic link in the Sandu-Danzhai area, Jiangnan Orogen, SW China

The turbidite-hosted Au-Sb-Hg deposits in the Sandu-Danzhai area are distributed along the southwestern marginal fault of the Jiangnan Orogen, yet any genetic link among the Au, Sb and Hg mineralization and their respective ore-forming process remain unclear. Here, we investigate the Paiting Au(-Hg) and Miaolong Au-Sb deposits and present a new metallogenic model based on detailed petrographic observations, and in-situ trace element (EPMA and LA-ICP-MS) and sulfur isotope (fs-LA-MC-ICPMS) analyses on sulfide minerals. Our results show that the ore formation in the Sandu-Danzhai metallogenic belt (SDMB) can be divided into Au (pyrite + arsenopyrite + dolomite), Au-Sb (pyrite + arsenopyrite + stibnite + quartz), Sb (stibnite + native antimony + barite + dolomite + quartz) and Hg (cinnabar + calcite) stages, based on mineral paragenetic and vein crosscutting relationships. Trace element compositions suggest that progressive As enrichment in pyrite is accompanied by increasing Au, Sb and Cu contents, implying that these elements may have been leached from the same source via fluid-rock reactions. Sulfur isotope compositions of the sulfates and sulfides in the Au-Sb-Hg ores reveal that the sulfur was originated from thermal-chemical sulfate reduction (TSR) of marine sulfates in the Cambrian ore-bearing strata. Thus, we propose that deep metamorphic fluids (as evidenced from the high fluid CO2 and CH4 contents) may have promoted ore-material (e.g., Au, Sb, As, Hg) mobilization in the metamorphic basement and/or Cambrian strata. As for the metal transport and precipitation mechanism, fluid-rock interaction may have decreased the ore-fluid As content and caused auriferous pyrite precipitation. Meanwhile, changes in temperature and oxygen-sulfur fugacity may have significantly decreased Sb solubility, leading to extensive stibnite precipitation. Cinnabar associated with calcite was likely deposited via carbonate dissolution in the Cambrian strata in late hydrothermal (waning) stage. Overall, we propose that the Au-Sb-Hg deposits in the SDMB are best classified as orogenic type, and the metallogeny was controlled by boundary faults of the Jiangnan Orogen.

New insights into thermal processes of metal deposits on h-BN/Rh(1 1 1): A comparison of Au and Rh

In this paper the thermal properties of Au and Rh deposits are compared on hexagonal boron nitride (h-BN) “nanomesh” prepared on Rh(111), applying STM, XPS, low energy ion scattering (LEIS), LEED, and DFT. At room temperature, both metals essentially follow Volmer-Weber (3D) growth. Upon subsequent annealing, agglomeration (sintering), intercalation, and desorption are competing surface processes for both metals. For the more reactive Rh, we suggest an additional encapsulation mechanism: between 600 K and 750 K, fragments of decomposed h-BN diffuse locally from the bottom onto the metal clusters covering them partially. STM data indicates that agglomeration of gold nanoparticles proceeds faster compared to rhodium. At higher temperatures (∼1050 K–1100 K), all non-desorbing gold atoms diffuse below h-BN, even for large initial coverages. On the other hand, for larger Rh deposits (≥5 ML), the outermost layer always contains Rh. Accumulation of gold at the interface between h-BN and Rh(111) significantly enhances the thermal stability of h-BN, attributed to the lower reactivity of Au in the decomposition of h-BN compared to Rh(111). At elevated substrate temperatures, intercalation of individual adatoms takes place during deposition, which requires higher temperatures for rhodium due to its slower diffusion and higher probability of nucleation.

Geologic and Geochemical Features of the World-Class Archean Windfall Intrusion-Related Au Deposit, Abitibi Subprovince, Canada

Abstract The Neoarchean Windfall gold deposit, hosted in the Urban-Barry greenstone belt of the Abitibi subprovince (Quebec, Canada), represents an emerging and significant Au deposit with a resource of 7.4 Moz of Au. It is hosted in 2717 Ma bimodal volcanic rocks that are cut by several generations of calc-alkaline quartz-feldspar porphyry dikes separated into (1) a 2697.6 ± 2.6 Ma group spatially related to Au mineralization and (2) a 2697.6 ± 0.4 Ma group that truncates the earlier dikes and the Au mineralization. The Au zones are structurally controlled and localized to faults and fractures proximal to the contacts of the early quartz-feldspar porphyry dikes; these zones form thin, subvertical, and elongate lenses plunging 35° east-northeast. Gold mineralization, present as both free gold and inclusions in pyrite, occurs (1) in gray quartz veins and stockworks with pyrite and subordinate carbonate and tourmaline and (2) in pervasive to patchy sericite-silica-pyrite-carbonate ± tourmaline ± fuchsite alteration zones. The Au mineralization and associated hydrothermal alteration, along with all the host rocks that include postmineralization intrusions, are overprinted by D2 deformational features that include a penetrative fabric, shear zones, and associated folds. The spatial and temporal association of the quartz-feldspar porphyry intrusions with the Au mineralizing event at the Windfall gold deposit, along with its elemental association (Ag, As, Sb, S, Se, Bi, Te, ± Zn, Cu, Pb, Mo, W), suggests an intrusion-related model and contrasts with the more abundant orogenic gold deposits in the Abitibi greenstone belt. This interpretation has important implications both locally and regionally for Au exploration in Archean greenstone terranes.

Au-Ag-Se-Te-S Mineralization in the Maletoyvayam High-Sulfidation Epithermal Deposit, Kamchatka Peninsula

The Maletoyvayam high-sulfidation (HS) epithermal Au-Ag deposit is one of the numerous hydrothermal deposits of the Kamchatka volcanogenic belt, consisting of two main associations: Au-rich (Ag-free) and Ag-bearing. The first one derived from acidic solutions, whereas the second assemblage crystallized from moderately dilute solutions, with both occurring at high oxygen fugacity. The Au-rich association contains the most atypical gold chalcogenides of the Au-Se-Te-S system, which are characterized by Se-S and Te-Se substitutions, e.g., a complete series from maletoyvayamite to tolstykhite Au3(Se,S)4Te6; a series of auroselenide Au(Se1.00–0.64S0.36–0.00); a combined series of gachingite Au(Te,Se) and unnamed Au(Se,Te): Au(Te0.80–0.40Se0.20–0.60). Meanwhile, in the second Ag-bearing assemblage, sulfides of the Au-Ag type prevails, e.g., petrovskaite AuAgS, miargyrite (Ag,Au)(Sb,As)S2, uytenbogaardtite Ag3AuS2, fischesserite Ag3AuSe2 with Au-Ag substitution, and tolstykhite. The Se/S ratio, of the second association, decreases while increasing the Ag concentration in the ore-forming system, including Au-Ag substitutions. The Au content in miargyrite (Au,Ag)SbS3 reaches up to 0.48 apfu, suggesting the existence of a new mineral phase of composition AgAuSb2S6. Au oxide complexes, in both associations, are represented by either a mixture of redeposited gold and Fe-Sb oxide or a homogeneous (Au,Sb,Fe)2O3 composition. These oxides are formed by replacement of calaverite. The ore mineralization of this HS deposit is considered unique due to the special conditions of the ore-forming environment, such as acidic solutions, high oxygen fugacity, and log fSe2 above −5.7; all contributed to the formation of AuSe phases.

Fine gold grains inside the limonite in the supergene Shangmanggang gold deposit, SW China: Implications for gold mobilization and mineral exploration

Supergene Au deposits have attracted economical attention because of their increased grades and more environmentally leaching. Understanding the supergene processes responsible for the Au mobilization in the surface environments is important to determine the location of concealed Au deposits. In this study, we recognized and sampled three Au-rich spatial zones in the Shangmanggang deposit in southwestern China: the hypogene, semi-oxidized, and red clay zones. The microscopic and chemical analyses of Au-bearing minerals show that (i) gold is mostly present as lattice-bound Au+ in the pyrite within the hypogene zone and (ii) numerous fine Au grains, mostly 100 × n nm to 1 × n μm in size, with a granular, spindle, or irregular shape, have been widely observed in the semi-oxidized and red clay zones, mainly inside the fissures and cavities of limonite. When exposed to an oxidizing environment, primary pyrite in the hypogene zone is gradually oxidized into limonite. With S diffusing out of the limonite, the Au in pyrite becomes rapidly activated and also diffuses into the environment as Au(S2O3)23− with a neutral or weakly alkaline pH in karst terrains. When meteoric water containing Au(S2O3)23− infiltrated the regolith profile, Au+ was reduced to elemental Au and precipitated in the limonite cavities and fissures due to their difference in the pH with the soil solution, which is an important insight into the formation of Au grains inside the limonite in the regolith. These results could provide a reference to better understand Au mobilization in the immature laterite in SW China, as well as the exploration of a potential hypogene Au deposit covered by the lateritic profile.

Surface Wrinkling with Memory for Programming Adhesion and Wettability

Modulating surface wrinkling is important for a variety of engineering applications. It has been known for more than two decades that the wavelength of surface wrinkles occurring in a metal film–soft polymer system scales linearly with the deposited film thickness. In the current experimental study of ultrathin gold film (0.2–8 nm) deposition on polydimethylsiloxane (PDMS), an unexplored thickness-dependent wrinkling phenomenon is found. By manipulating the deposition sequence as a degree of freedom for tailoring surface topography, we discovered a morphology memory effect where the wrinkle evolution in the subsequent deposition step inherits the surface pattern already formed in the previous step. Moreover, a stepwise deposition targeting 1 nm thick film can lead to 1 order of magnitude higher surface roughness than the one in the continuous deposition. By programming the sequences within 8 nm Au deposition, a surface strain map varying drastically from 0.2% to 27% is realized. Instructed by the strain map, we show the great potentials of tailored wrinkles in alternating surface wettability, enhancing surface Raman scattering, and on-demand tuning of surface adhesion.

Origin of Disseminated Gold-Sulfide Mineralization from Proximal Alteration in Orogenic Gold Deposits in the Central Sector of the Yana–Kolyma Metallogenic Belt, NE Russia

The Yana–Kolyma metallogenic belt, NE Russia, is a world-class gold belt with resources numbering ~8300 tons of gold. The belt is localized in the central part of the Verkhoyansk–Kolyma orogen, formed by a collage of diverse terranes. The Tithonian-to-Early-Cretaceous orogenic gold deposits are hosted in a sequence of Permian–Triassic and Jurassic clastic rocks and altered Late Jurassic andesite, dacite, granodiorite, trachyandesite, and trachybasalt dykes. High-fineness gold (800–900‰) in quartz veins and invisible gold in disseminated arsenian pyrite-3 (Py3) and arsenopyrite-1 (Apy1) are present in ores. Here, we present new data about microtextures; the chemical composition and stable sulfur isotopes of auriferous pyrite-3 and arsenopyrite-1 from proximal alterations in sediment-hosted (Malo–Taryn, Badran, Khangalas); and intrusion-hosted (V’yun, Shumniy) orogenic Au deposits in the central sector of the Yana–Kolyma metallogenic belt to better constrain the ore-forming process and tracking their evolution. Detailed petrography defined the following generations of pyrite: syn-sedimentary/diagenetic Py1, metamorphic Py2 and hydrothermal Py3, and Apy1. Hydrothermal Py3 and Apy1 are localized in the proximal pyrite–arsenopyrite–sericite–carbonate–quartz alteration in ore zones and make a major contribution to the economic value of the veinlet-disseminated mineralization with “invisible” gold in the orogenic deposits of the Yana–Kolyma metallogenic belt. Electron microprobe analysis (EMPA) of Py3 in both types of deposits shows concentrations of As (up to 3.16 wt%), Co, Ni, Cu, Sb, and Pb. Py3 in intrusion-hosted orogenic gold deposits reveals elevated concentrations of Co (up to 0.87 wt%), Ni (up to 3.52 wt%), and Cu (up to 2.31 wt%). The identified negative correlation between S and As indicates an isomorphic substitution of sulfur by As1−. Py3 from igneous rocks is characterized by a high degree of correlation for the pairs Fe2+→ Co2+ and Fe2+→ Ni2+. For hydrothermal Apy1, Co (up to 0.27 wt%), Ni (up to 0.30 wt%), Cu (up to 0.04 wt%), and Sb (up to 0.76 wt%) are typomorphic. According to atomic absorption spectrometry, the concentration of Au in Py3 reaches 159.5 ppm; in Apy1, it reaches 168.5 ppm. The determination of the precise site of the invisible gold within Py3 and Apy1 showed the predominance of solid-solution Au+ in the crystal lattice. The values of δ34S in Py3 and Apy1 (from −6.4 to +5.6‰, mean value of about +0.6‰), both from sediment-hosted and from intrusion-hosted deposits, display a relatively narrow range and are characteristic of the hydrothermal ore stage. Our analytical results showed no systematic differences between the chemical and stable sulfur isotope compositions of both auriferous pyrite-3 and arsenopyrite-1 from the proximal alteration in sediment-hosted (Malo–Taryn, Badran, Khangalas) and intrusion-hosted (V’yun, Shumniy) orogenic Au deposits, indicating that the primary source of sulfur, gold, and mineralizing fluids was likely from subcrustal and metamorphic systems in the Late-Jurassic-to-Early-Cretaceous Verkhoyansk–Kolyma orogen.

Low-cost, mass-producible nanostructured surface on flexible substrate with ultra-thin gold or silver film for SERS applications

Surface Enhanced Raman Spectroscopy has emerged as a powerful analytical technique for fingerprint recognition of molecular samples with high sensitivity. The Surface Enhanced Raman Scattering (SERS) effect has been extensively studied for the past few decades. However, only recently the commercialization of portable Raman spectrometers has taken SERS a step closer to real-world applications. Swift and convenient testing of analytes for point-of-care, environmental as well as food quality control and safety applications, is very lucrative. This can be realized with the use of low-cost, mass producible and environmentally friendly SERS active substrates in combination with portable Raman spectrometers. In this study, we demonstrate one approach to accomplish such a SERS-active substrate using nanostructured latex coated paperboard as a base substrate. The nanostructure is accomplished by applying a reverse gravure coater in combination with a short-wavelength infrared (IR) heater. The whole process is easily up-scalable. The SERS functionality is then obtained by physical vapor deposition of an ultra-thin layer of Au or Ag. The surface nanostructure was confirmed by atomic force microscopy, showing an additional nanoscale graininess after the deposition of Au or Ag. The successful metal deposition was confirmed by X-ray photoelectron spectroscopy and deposition homogeneity was also analyzed. To confirm the SERS effect, two model compounds; crystal violet and rhodamine 6G were tested in the concentration range of 1–1000 μM. The results confirmed that the nanostructured, flexible, paper-based substrate can perform as a SERS-active substrate with negligible background noise.

Au/TiO2 thin film with ultra-low content of gold: An efficient self-supported bifunctional electrocatalyst for oxygen and hydrogen evolution reaction

Electrochemical water splitting has triggered much attention for the cost-effective production of hydrogen (H2) to alleviate the energy crises around the globe. Searching for efficient and scale able electrode materials to efficiently catalyze both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) at low overpotential is direly needed. Herein we report a scalable method to develop a thin film of Au@TiO2 on fluorinated tin oxide (FTO) with ultra-low content of Au (0.573 µg/cm2), which can dramatically sensitize the mesoporous TiO2 for OER and HER in alkaline medium (1 M KOH). The thin film was developed via spin coating with layer-by-layer deposition of TiO2 and Au at optimum concentration and temperature. The catalyst needed a small overpotential of 200 and 240 mV for the HER and OER, revealing a sustainable behavior for more than 60 h, mainly depending on the nature of the thin film, uniformly distributed small particles of Au, and their oxidation states/orbital occupancy. It is anticipated that sensitization of thin film of mesoporous TiO2 with an ultrasmall amount of Au could simultaneously realize the role of synergistic interaction of Au and TiO2, the oxidation state of Au0/1+ and self-supported/free standing nature of electrodes which are indeed very appealing to facilitate the mass and charge transport during energy intense water splitting process. We believe that such a facile and scalable pathway could provide a guiding principle to rationally design more efficient electrode material with minimum use of precious metals for the viable electrochemical water splitting processes.

Evidence for transpression during formation of the Candelaria Punta del Cobre IOCG -district and regional implications

The youngest and best exposed Iron Oxide Cu–Au (IOCG) deposits currently recognized are found in the coastal belt of the Andes. Their formation has been attributed to back-arc extension or transtension associated with the convergent Andean margin. Here we document transpressional deformation synchronous with Cu mineralization in the Candelaria-Punta del Cobre district, the largest IOCG district of the Andean belt. A northwest-southeast shortening direction is recorded by north-northwest sinistral strike-slip faults that host mineralization, northwest dikes, and north-northeast compressive structures. Batholith emplacement synchronous with mineralization formed a north-northeast oriented foliation zone parallel to the intrusive contact and associated folds in the host rock sequences that face inwards towards the intrusive contact. Age constraints indicate that transpressional deformation in the Cretaceous arc, at least locally, begun earlier than previously documented, and IOCG mineralization may have spanned the transition from extension to the initial phase of compression.

The Birkachan Epithermal Au–Ag Deposit in the Kedonsky Middle Paleozoic Volcanic Belt (Russian Northeast)

The Birkachan deposit in the Middle Paleozoic Kedonsky Volcanic Belt (KVB) is unique in Northeast Russia for its stockwork epithermal Au–Ag ores. The deposit is located in the northeastern part of the Gurnik volcanotectonic depression, in the Kedonsky segment of the KVB. The deposit is localized in the lying side of a large sublatitudinal thrust fault, due to which it was overlain in the Jurassic–Early Cretaceous by allochthon rocks and, therefore, was slightly eroded, as evidenced by the preserved fragments of the argillisite cap. The ore-bearing Gurnik sequence is composed of felsic tuffs and lavas with intercalations of ignimbrites. The main ore-bearing zone of the Birkachan deposit is 4.5 km long and 200–300 m wide consists of several echelon-like vein zones of northeast strike, falling to the southeast at angles of 55°–70°. Most of the ore bodies are linear stockworks. Single, lenticular rich ore bodies represented by mineralized breccias have been found at deep horizons. In the section, the system of ore bodies of the Birkachan deposit forms a fan-shaped structure. The main textures of ores are vein-disseminated, breccia, and rhythmically banded. The ores are enriched in comparison with the upper continental crust in a rather narrow range of elements (Au, Ag, Sb, As, Mo, W, and Li) and are poor in rare-earth elements, among which light lanthanides predominate. The sulfide grade of ores is 0.1–0.5%. The vein minerals in ores are dominated by quartz, sericite, and siderite. Among the ore minerals, pyrite dominates, with fahlore, native gold and chalcopyrite, and minerals of the series acanthite → Se–acantite → naumannite, pearceite, and Se–pearceite being less common. The average fineness value of native gold is 643‰. The parameters of mineral-forming solutions correspond to typical medium-temperature fluids of epithermal low-sulfidized deposits: homogenization temperature of 93–291°С, salt concentration of 0.2–7.0 wt % eq. NaCl, and fluid density of 0.71–0.99 g/cm3. The main indicators of fluid composition are CO2/CH4 = 15.8–23.6, Na/K = 2.3–4.3, and K/Rb = 2007. The prospects for increasing the reserves of Au and Ag are associated with further study and exploration of the flanks and deep horizons of the deposit.