Gold Enrichment Research Articles

Separation and Enrichment of Gold in Water, Geological and Environmental Samples by Solid Phase Extraction on Multiwalled Carbon Nanotubes Prior to its Determination by Flame Atomic Absorption Spectrometry.

This study proposes the application of multi-walled carbon nanotubes as a solid sorbent for the preconcentration of gold prior to its flame atomic absorption spectrometry determination. Extraction was achieved by using a glass column (15.0 cm in length and 1.0 cm in diameter). Quantitative recoveries were obtained in the pH range of 2.5-4.0; the elution step was carried out with 5.0 ml of 1.0 mol/L HNO3 in acetone. In the ligand-free study, variables such as pH, eluent type, sample volume, flow rates, and matrix effect were examined for the optimum recovery of gold ions. The gold ions were able to be pre-concentrated by a factor of 150 and their LOD was determined to be 1.71 μg/L. In order to evaluate the accuracy of the developed method, addition-recovery tests were applied for the tap water, mineral water, and sea water samples. Gold recovery studies were implemented using a wet digestion technique for mine and soil samples taken from various media, and this method was also applied for anodic slime samples taken from the factories located in the Kayseri Industrial Zone of Turkey.

Multi-stage enrichment processes for large gold-bearing ore deposits

A review of previous studies of the world's large hydrothermal gold deposits indicates that the largest deposits tend to show complicated parageneses where multiple gold enrichment events and processes have been involved in the deposit generation. These observations suggest that multistage processes may even be a requirement for the formation of large deposits. In some deposits (e.g. Witwatersrand, Boddington Cadia, Sukhoi Log or Carlin) the different enrichment processes occur millions of years apart. In others, such as many large porphyry deposits, the different stages are much closer in time. In many deposits, particularly sedimentary-hosted deposits, early diffuse enrichment occurs within a particular province that is then upgraded by more focused processes (e.g., Sukhoi Log; Kalgoorlie). The presence of this early diffuse enrichment could explain the tendency for gold deposits to cluster into camps.This model has important implications, as the presence or absence of multiple gold events could be used to discriminate, at the exploration and feasibility stages, between small deposits with single stage ore genesis and more complicated deposits with multistage enrichment and the potential for larger gold endowment.

Native gold from volcanic gases at Tolbachik 1975–76 and 2012–13 Fissure Eruptions, Kamchatka

Abstract Aggregates and euhedral crystals of native gold were found in sublimates formed during New Tolbachik Fissure Eruption in 2012–2013 (NTFE). Gold-bearing sublimate samples were taken from a red-hot (690 °C) degassing fracture in the roof of an active lava tunnel 1.5 km from active Naboko cinder cone in May 2013. The gas condensate collected at 690 °C in this site contains 16 ppb Au, 190 ppb Ag and 1180 ppm Cu compared to 3 ppb Au, 39 ppb Ag and 9.7 ppm Cu in the condensate of pristine magmatic gas sampled at 1030 °C. The 690 °C volcanic gas is most likely a mix of magmatic gas and local snow buried under the lava flows as indicated by oxygen and hydrogen isotope compositions of the condensate. The lower-temperature gas enrichment in gold, copper and chlorine is resulted from evaporation of the 690 °C condensate during forced gas pumping at sampling. Native gold was also found in fumarolic encrustations collected from caverns in basalt lava flows with temperature up to 600 °C in June 2014, in a year after eruption finished. The native gold precipitation in newly formed Cu-rich sublimates together with the well known gold occurrences in cinder cones of 1975–1976 Large Tolbachik Fissure Eruption manifest a transport capability of oxidized volcanic gas.

Decratonic gold deposits: a new concept and new opportunities

Early Cretaceous gold deposits in the North China Craton and the famous Carlin-type gold deposits in Nevada are given the name ‘decratonic gold deposits’ [1]. This is the first new type of ore deposit proposed in the last 20 years. This new type of deposit introduces a concept that may change perspectives and provide fresh opportunities for gold deposit research and exploration. Cratons are stable ancient crustal domains characterized by thick lithospheric mantle roots (>150 km), weak tectonism, and little or no magmatic activity within their boundaries. Since their stabilization, the formation of endogenic mineral deposits in such ancient cratonic blocks is very rare owing to a lack of significant magmatism, deformation, or generation of hydrothermal fluids. Several cratons, however, including the North China, the North American and the Brazilian cratons, have experienced destruction to differing degrees [1]. The eastern part of the North China Craton was extensively destructed in the Early Cretaceous [2]. Meanwhile, gold deposits with a total reserve of 4000– 5000 tons formed there within a few millionyears (130–120Ma), coevalwith craton destruction [1]. Similarly, the western part of the North American Craton was destabilized in theCenozoic and that destructionmay be responsible for the famous Nevada Carlin-type gold deposits that host a total of over 6000 tons of gold reserves [1,3]. It is suggested that all these deposits nowbe termed ‘decratonic type’ gold deposits, deposits formed by magma-derived fluids under extensional tectonics conditions associated with craton destruction [1]. The model further postulates that the westward subduction of the western Pacific Plate in the Early Cretaceous and the eastward subduction the eastern Pacific Plate in the Cenozoic were responsible for these twomajor gold mineralization events. Dehydration of the subducted and stagnant slabs led to continuous hydration andmetasomatism of the overlying mantle wedges and the enrichment of gold and other chalcophile elements. Subsequent partial melting of the mantle wedge produced ore-forming fluids (Fig. 1) [1]. Gold deposits in the North China Craton have been classified into two types: ‘altered-rock’ type and ‘quartzvein’ type. The altered-rock-type gold deposits are characterized by disseminated and stockwork goldmineralization, which is usually controlled by brittle or ductile to brittle fault zones. These types of deposits have commonly been intensively overprinted by hydrothermal alteration and suffered later brittle deformation. The quartz-vein-type gold deposits are hosted in the Archean amphibolite facies rocks or earlier granitoids and typically have large proportions of visible native gold. In contrast, the Carlin-type gold deposits in Nevada typically contain invisible gold hosted in fine-grained sulfides disseminated in Paleozoic marine sedimentary rocks.The North China Craton and Carlin gold deposits are seemingly entirely different, but both types are genetically related to craton destruction. Carlin-type gold deposits are abundant in Nevada but have not been found in the North China Craton. The decratonic gold deposit model ascribes the contrast in mineralization styles to host rocks, controlling structures, and ore-forming depths [1]. Decratonic gold deposits in both the North China Craton andNevada formed within a relatively short time interval (<10Ma). In both areas, the deposits become progressively younger toward the subduction zones. Such temporal trends may plausibly be explained by subduction retreat or rollback of the subducted slab at rates of 8.8 and 3.3 cm y−1 for the western and eastern Pacific plates, respectively [1]. Craton destruction starts deep in the mantle with mantle-derived melts and fluids, implying that decratonic gold deposits may extend to great depths. This suggests newopportunities for future discoveries. It is proposed that gold mineralization belts extending approximately NNE in the eastern and the western portions of the Eastern Block of the North China Craton are the most favorable areas for future exploration [1]. ‘Decratonic gold deposit’ is a new term. It will inevitably face reactions that will affirm or challenge it, and that may improve or refute it. Major questions are: the (paleo)-Pacific plate has subducted westward underneath the whole of east China followed by slab rollback during three periods since the Jurassic [4,5].Why did goldmineralization in the North China Craton take place mainly within a short period in the Early Cretaceous? Episodes of tectonic extensions in the North China Craton also have lasted for long periods. What made the Early Cretaceous so different? If craton destruction was the controlling factor, then what made craton destruction unique? Alternatively, are craton

Compilation of West African mineral deposits: Spatial distribution and mineral endowment

The West African Craton is highly endowed in minerals, and their spatial and temporal distribution varies from single to multi-phase mineralization events. They are broadly related to three major tectono-metallogenic elements and formed during distinct mineral epochs: (1) In both Archean Shields (Kénéma-Man and Reguibat) and Paleoproterozoic domains (Baoulé-Mossi, Eglab). These are characterized by giant iron ore deposits that formed between ca. 2.5–2.3Ga, nearly all gold, porphyry copper, lead–zinc and sedimentary manganese ore that developed between 2.2 and 2.1Ga, and primary diamonds that formed between two intervals at ca. 2.2–2.0Ga and in the Mesozoic. (2) Across Pan-African and Variscan belts. These are distinguished by major Precambrian IOCG's, copper–gold that formed at ca. 2.1Ga and approximately 680Ma, and Neoproterozoic sedimentary iron ore and phosphate deposits. (3) Within intracratonic and coastal basins. These include the development of Cenozoic lateritic bauxites over Mesozoic dolerites, Tertiary/Quaternary mineral sands deposits, oolitic iron ore and sedimentary phosphate deposits. Geological, spatial and temporal correlations using the multi-commodity West African Mineral Deposit Database highlight that gold and non-gold commodities formed in multiple phases. This commenced in the Liberian Orogeny (2.9–2.8Ga) with the enrichment of iron ore, nickel sulphides, diamonds and gold in the earth's crust. The pre-Eburnean or Tangaean–EoEburnean–Eburnean I Event yielded gold, and the major Eburnean Orogeny yielded gold, iron ore, manganese, diamonds, magmatic nickel sulphides, copper–gold, lead–zinc, and REE minerals. Throughout the Pan-African event sedimentary manganese deposits, lead–zinc, REE minerals, sedimentary phosphates, and again gold were formed. Primary diamonds and magmatic nickel sulphides are related to the break-up of Gondwana, followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin.

Origin of Listwanite in the Luobusa Ophiolite, Tibet, Implications for Chromite Stability in Hydrothermal Systems

AbstractListwanite from the Luobusa ophiolite, Tibet, forms a narrow, discontinuous band along the eastern part of the southern boundary fault. We undertook a detailed petrographic and geochemical study to understand the mineral transformation processes and the behaviour of major and trace elements during listwanite formation. Three alteration zones characterized by distinct mineral components and texture are recognized and, in order of increasing degree of alteration, these are: zoneIII is rich in serpentine minerals; zoneII is rich in talc and carbonates; and zoneI is mainly composed of carbonates and quartz. Geochemical data for the three alteration zones show significant modification of some major and trace elements in the protolith, although some oxides show linear correlations with MgO. Gold mineralization is recognized in the Luobusa listwanite and may signify an important target for future mineral exploration. Gold enrichment occurs in both zoneI and zoneII and is up to 0.91 g/t in one sample from zoneI. We show that CO2‐rich hydrothermal fluids can modify both the occurrence and composition of chromite grains, indicating some degree of chromite mobility. Low‐Cr anhedral grains are more easily altered than high‐Cr varieties. The compositions of chromite and olivine grains in the listwanite suggest a dunite protolith.

Witwatersrand gold deposits formed by volcanic rain, anoxic rivers and Archaean life

The Witwatersrand gold deposit is the largest in the world. Thermodynamic calculations show that such rich accumulations of gold could be linked to abundant volcanism, primitive life and the oxygen-free atmosphere of the Archaean. The Witwatersrand Basin in South Africa is one of the best-preserved records of fluvial sedimentation on an Archaean continent1. The basin hosts the worlds biggest gold resource in thin pebble beds, but the process for gold enrichment is debated2,3,4,5. Mechanical accumulation of gold particles from flowing river water is the prevailing hypothesis2,6, yet there is evidence for hydrothermal mobilization of gold by fluids invading the metasedimentary rocks after their burial3,4,7. Earth’s atmosphere three billion years ago was oxygen free8, but already sustained some of the oldest microbial life on land9. Here I use thermodynamic modelling and mass-balance calculations to show that these conditions could have led to the chemical transport and precipitation of gold in anoxic surface waters, reconciling the evidence for fluvial deposition with evidence for hydrothermal-like chemical reactions. I suggest that the release of sulphurous gases from large volcanic eruptions created acid rain that enabled the dissolution and transport of gold in surface waters as sulphur complexes. Precipitation of the richest gold deposits could have been triggered by chemical reduction of the dissolved gold onto organic material in shallow lakes and pools. I conclude that the Witwatersrand gold could have formed only during the Archaean, after the emergence of continental life but before the rise of oxygen in the Earth’s atmosphere.

Structural and Chemical Characterization of Placer Gold Grains: Implications for Bacterial Contributions to Grain Formation

Gold grains collected from the Rio Saldaña River, Colombia were hundreds of micrometers in size and discoid-ellipse in shape. Fourteen of 63 grains contained an iron oxyhydroxide coating that occurred as ca. 50 to 100 nm thick lamina while thicker coatings were comprised of colloids 200 nm to 4 μm in diameter. Bacterial-size casts were observed throughout the thicker iron oxyhydroxide coating and intuitively represent relic impressions of bacterial cells. The surface textures of gold grains were generally smooth with surficial depressions or crevices containing detrital material colonized by bacteria. Focus Ion Beam (FIB) milled cross-sections demonstrated that the detrital material contained nanophase gold particles. Biofilm attached to this detrital material contained ca. 2 to 3 nm colloidal gold attached to exopolymeric substances. Cross sections of grains revealed solid cores with vesicular voids near the grain edge including a bacterial-size cast interpreted to be a permineralized bacterial cell. Synchrotron-based elemental mapping indicated that grains contained heterogenously distributed Ag and Cu. While strong Ag and Cu signals (relative to Au) were detected in the core, a stronger Au signal occurred at the edge of grains demonstrating enriched rims of secondary gold. The preservation of bacterial casts and biofilms associated with secondary gold structures at the surface of grains suggest that bacteria may contribute to gold enrichment and growth in this placer environment. Bacteria, occurring on the surface of 13 of 25 gold grains, were enriched by “inoculating” individual grains into separate test tubes containing R2B growth medium. Enriched growth of bacteria on gold grain surfaces demonstrated preferential attachment onto detrital material within creviced regions. The dominant bacteria from these enrichments were transferred to solid R2A medium to obtain pure isolates. The isolates were identified as one of four bacterial species: Nitrobacter sp. 263, Shewanella sp. YM-8, Sediminibacterium sp. B2-10-2 and sp. I-32 based on 16S ribosomal DNA sequencing.

Regional setting and characteristics of the Neoproterozoic Wadi Hamama Zn–Cu–Ag–Au prospect: evidence for an intra-oceanic island arc-hosted volcanogenic hydrothermal system

The Wadi Hamama area is a volcanogenic Zn–Cu–Au–Ag prospect. It is hosted by a Neoproterozoic bimodal-mafic sequence, which comprises basalt, dacite and rhyolite along with volcaniclastic rocks. The rocks have a low-K tholeiitic affinity and are enriched in large ion lithophile elements over high field strength elements, which indicated their formation in an intra-oceanic island arc tectonic setting. The area was intruded by a tonalite–trondhjemite body, which has an intra-oceanic island arc affinity and later by diorite, which has a cordilleran-margin geochemical affinity. These rock units were intruded by post-tectonic granite dykes, which have a within-plate geochemical signature. There is a quartz-carbonate horizon extending along the contact between the basalt and the volcaniclastic rocks, mainly banded and lapilli tuffs. This horizon is of exhalative origin and is underlain by a mushroom-shaped alteration zone extending from the horizon down to the massive basalt. The footwall alteration is characterized by a silica-rich core surrounded by a thick chlorite sheath. Both the quartz-carbonate horizon and the footwall-altered rocks enclose historical trenches and pits. Sulfide-rich core samples are enriched in Zn, relative to Cu, and in Ag, which indicates the low-temperature nature of the hydrothermal system. The prospect was affected by supergene processes, which led to the widespread occurrence of secondary copper minerals and gold enrichment relative to the leached base metals, especially Zn. The prospect formed through a limited rifting of an intra-oceanic island arc which resulted in the formation of a small-scale volcanogenic Zn–Cu–Ag–Au prospect.

Gold-copper nano-alloy, "Tumbaga", in the era of nano: phase diagram and segregation.

Gold–copper (Au–Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core–shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations.

Meso- and Neoarchaean geological history of the Bjørnesund and Ravns Storø Supracrustal Belts, southern West Greenland: Settings for gold enrichment and corundum formation

To understand the Meso- and Neoarchaean geological history of the Bjørnesund region, southern West Greenland (ca. 62–63° N), we have undertaken an integrated study combining field observations with U–Pb zircon geochronology by LA-ICP-MS, metamorphic petrology, and microstructural analyses. The Bjørnesund region links the southern part of the Fiskenæsset anorthosite complex to the well-preserved metavolcanic rocks of the Ravn Storø Supracrustal Belt and the Bjørnesund Supracrustal Belt, so the development of this region is of key interest to unravelling the Archaean geology of the entire region. In this study, we couple the observed phases of gold-enrichment and corundum formation to the geological history.The leucogabbroic rocks of the Bjørnesund region, which are part of the Fiskenæsset complex, intruded at ca. 2.95Ga into the tholeiitic and calc-alkaline metavolcanic rocks of the two studied belts. Following isoclinal folding, the rocks were intruded by diorite (2.92Ga) and granodiorite (2.91 and 2.88Ga), which are now texturally orthogneisses. The area was affected by three major phases of folding; F1 (undated), F2 at 2.85–2.83Ga, and F3 at 2.71–2.70Ga. The former deformation event is associated with the gold-enrichment in alteration zones in shear folds in metavolcanic amphibolite near the contact with the orthogneiss. Intrusion of granitic sheets during the F3 deformation triggered the formation of corundum, which is locally of gem quality, at the contact between leucogabbroic and ultramafic rocks. Peak metamorphic conditions were calculated as 580–630°C and 6–4kbar at 2.85–2.83Ga. Later retrogression in the southern part of the area at ca. 500–450°C and 4.5–3kbar, and extensional fault reactivation overprinted some of the older structures.

Geological and geochemical characteristics and formation mechanisms of the Zhaishang Carlin-like type gold deposit, western Qinling Mountains, China

Located in the western part of the Min–Li metallogenic belt within the western Qinling Mountains, the Zhaishang gold deposit is a giant Carlin-like disseminated gold deposit discovered recently. The ore deposit is present both in rocks of low grade metamorphic Middle Devonian and Lower Permian clastic formation, which is composed of quartz sandstone, siltstone, calcareous slate and argillaceous limestone. Gold mineralization is strictly controlled by a fault zone. Minerals in ores are quite complex and consist of sulfides, sulfosalt, oxides, sulfate, carbonate, tungstate, telluride, native metals, and polymetallic alloys. The diversity of mineral in the ores and the existence of microscopic visible native gold constitute the outstanding features of the gold deposit.We believe that the mineral source of the ore deposit has a close connection with the host rocks, because all samples show the light of the REE distribution patterns. The S, C, Pb, H and O isotopic compositions show that the ore-forming elements were mainly derived from the country rocks, the underlying rocks, and partially deep sources. Data obtained suggest that the ore-forming fluids were derived predominantly from an active meteoric groundwater system. Mineralization and related alteration have features of a low water/rock ratio. Ore-forming temperatures are estimated to have been in the range from 120 to 240°C. The dissolution of ferruginous limestone in the host rocks and the sulfidation of the dissolved iron by H2S introduced by ore fluids constituted not only the most important depositional mechanisms for the existence of microscopic visible gold grains but also favorable conditions for gold enrichment in the Zhaishang gold deposit.

Ore-forming process of the Huijiabao gold district, southwestern Guizhou Province, China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO2 inclusions, CO2–H2O inclusions, hydrocarbon inclusions, and hydrocarbon–H2O inclusions. The ore-forming fluids are characterized by a H2O+CO2+CH4±N2 system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H2O+NaCl→H2O+NaCl+CO2+CH4±N2→H2O+NaCl±CH4±CO2 with a slight decrease in homogenization temperature and salinity. The δ18O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δDH2O and calculated δ18OH2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ13CPDB and δ18OSMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H2O–NaCl–CO2 fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.

Ore petrography and chemistry of the tellurides from the Dongping gold deposit, Hebei Province, China

The Dongping gold deposit is a mesothermal lode gold deposit hosted in syenite. The ore petrography and chemistry of the tellurides from the alteration zone of the deposit have been studied in detail using optical microscopy, scanning electron microscopy, electron probe micro-beam and X-ray diffraction facilities. The tellurides, consisting mostly of calaverite, altaite, petzite tellurobismuthite and tetradymite, are hosted irregularly in pyrite fractures and voids. In the ore bodies, the species and quantity of tellurides decrease from the top downwards, accompanied with lowering of gold fineness, and the existence of tellurides exhibits a positive correlation with gold enrichment. Mineral paragenesis and chemical variations suggest that during evolution of the ore-forming fluids Te preferably incorporated with Pb to form altaite, followed in sequence by precipitation of petzite, and calaverite when Ag has been exhausted, and the residue fluids were enriched in Au, giving rise to formation of native gold. Calculation with reference of the fineness of native gold coexisting with the tellurides indicates that at 300°C, log f (Te2) varied between −8.650 and −7.625. Taking account of the Au–Ag–Te mineral paragenesis, it is inferred that log ƒ (Te2) varies from −9.12 to −6.43, log ƒ (S2) −11.47 to −8.86. In consideration of the physicochemical conditions for formation of tellurides, with comparison to some known telluride deposits, it is suggested that high log ƒ (Te2) is a key factor for high fineness of native gold as well as precipitation of abundant tellurides.

Application of factor analysis and concentration-volume fractal modeling to delineation of 3D geochemical patterns: a case study of the Jinwozi gold field, NW China

Delineation of vertical distribution patterns of elements in overburden over a concealed deposit is essential for determining potential upward migration mechanism of elements and for predicting undiscovered mineral deposits. The aim of this study is to identify the vertical distribution features of elements in the regolith over the Jinwozi gold field, NW China by using factor analysis (FA) and concentration-volume (C-V) fractal modeling based on the data-set generated from 70 overburden drilling boreholes. Firstly, the zones of different gold enrichment states were obtained by the C-V fractal method. Secondly, FA was utilized to define the mineralization-related element association. Finally, the C-V method was applied to the relevant association defined by FA. The results showed that: (1) the C-V fractal method distinguished four enrichment states determined by Au thresholds of 1.6, 7.9 and 39.9 ng/g and by factor scores of −1.14, 0.33 and 1.45; (2) the first factor that describes 41.5% of the common variance has positive loadings in Au, As, Sb and Mo; (3) the zones of both high Au contents and factor scores generally showed horizontally northeasterly distribution along the ductile-shearing alteration belt and vertically top-bottom symmetric distribution in the regolith over the deposit; (4) the rest of the zones determined by the C-V fractal model properly delineated the zones of low and high backgrounds and moderate Au enrichment; and (5) the moderately-enriched zones of the raw Au data are better than those of the factor scores, and the rest of the zones have the similar distributions. These results indicate that: (1) the distribution of elements in the regolith was the product of nonlinear natural processes resulting in an enrichment pattern at the surface over the deposit; and (2) the integration of FA and C-V fractal model is effective for the delineation of 3D geochemical patterns.

Model of formation of gold-hematite-barite mineralization of the Prognoznoe deposit, Northeastern Russia

The results of geological-structural, petrographical, and mineralogical-geochemical studies allowed the authors to offer an original model of formation of the Prognoznoe deposit. The relict shallow basin accumulated clasts of the Archean Au-bearing iron formations and metamafic rocks from the uplifted block of the Khivach inlier of the Precambrian basement of the Omolon craton terrane. As a result, the rounded and semirounded material composes a gentle volcanic sequence of the Middle-Upper Devonian Kedon tuff sandstones and gravelites (red-colored member of ferrous volcanosedimentary rocks) with dispersed clastic and supergene gold. Secondary enrichment and redistribution of gold and iron in zones of the quartz-hematite-barite-carbonate veinlets are related to the intrusion of thick Cretaceous dikes and sills. The areas without dikes and sills are free of economic gold-bearing bodies. An extremely low amount of sulfides is typical of this deposit and of the epithermal deposits of the Kedon volcanic belt in the southern part of the Omolon craton terrane.

Linking high-grade gold mineralization to earthquake-induced fault-valve processes in the Porgera gold deposit, Papua New Guinea

Processes responsible for high-grade gold enrichment are of significant economic interest and have long been elusive. Structural and geophysical models postulate that earthquake-induced fault-valve processes are frequently responsible for gold mineralization in structurally controlled hydrothermal systems, although a comprehensive geochemical record of these processes has been lacking. The Porgera gold deposit (Papua New Guinea) provides an opportunity to examine the complex processes surrounding high-grade Au mineralization in structurally controlled hydrothermal systems. New high-resolution trace element and sulfur isotope data obtained in situ by laser ablation–inductively coupled plasma–mass spectrometry and sensitive high-resolution ion microprobe–stable isotope analysis of pyrite across a well-constrained paragenetic sequence reveal a temporal evolution in fluid chemistry. The data uncover a stratigraphy of repeated high-Au negative δ 34 S and low-Au positive δ 34 S zones within individual pyrite crystals present in the highest grade gold event (stage II). Results provide the first clear geochemical documentation of rapid, complete switching in ore-forming processes, preserved in individual pyrite crystals throughout an evolving hydrothermal system. It is postulated that geochemical variations are attributable to rapid Au deposition during pressure release due to fault failure followed by switching to more reduced rock-buffered (metal poor) fluids from adjacent sediments as the rupture seals. The cyclicity of this process is reflected in the pyrite zonation seen at the Porgera deposit. This study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis and fluid pathways, revealing that deposition of exceptionally high grade ore at the Porgera deposit was the result of multiple events in a fluctuating ore-forming environment.

Enhanced H2O2 production over Au-rich bimetallic Au–Pd nanoparticles on ordered mesoporous carbons

The industrial implementation of hydrogen peroxide as a green oxidant requires the development of efficient catalytic systems for its direct synthesis from molecular oxygen and hydrogen. Bimetallic Au–Pd nanoparticles supported on acid pre-treated ordered CMK-3 carbon catalysts show high H2O2 productivities and high H2 selectivities (>99%). Encapsulation of the nanoparticles within the pore network lead to stable small size (∼2nm) particles. This paper reveals that the acid pre-treatment, does not only improve the metal dispersion of metals but most importantly, it promotes their gold enrichment, dramatically decreasing the parallel H2O2 decomposition, which is key for the economic feasibility of the process.

Copper recovery and gold enrichment from waste printed circuit boards by mediated electrochemical oxidation

The present study aims to develop an eco-friendly chemical–electrochemical process for the simultaneous recovery of copper and separation of a gold rich residue from waste printed circuit boards (WPCBs). The process was carried out by employing two different types of reactors coupled in series: a leaching reactor with a perforated rotating drum, for the dissolution of base metals and a divided electrochemical reactor for the regeneration of the leaching solution with the parallel electrowinning of copper. The process performances were evaluated on the basis of the dissolution efficiency, current efficiency and specific energy consumptions. Finally a process scale up was realized taking into consideration the optimal values of the operating parameters. The laboratory scale leaching plant allowed the recovery of a high purity copper deposit (99.04wt.%) at a current efficiency of 63.84% and specific energy consumption of 1.75kWh/kg cooper. The gold concentration in the remained solid residue was 25 times higher than the gold concentration in the initial WPCB samples.

Setting the stage for the genesis of the giant Bendigo ore system

Abstract The Ordovician sediments that host the giant Bendigo saddle reef gold deposits consist of a 3 km-thick sequence of turbiditic sandstones and interbedded siltstones and mudstones. Sedimentological studies suggest the succession formed within a major deep marine channel–levee complex similar to those described from contemporary continental margin to basin plain settings outboard of major river systems (e.g. the Amazon, Mississippi and Congo). Black shales, which are commonly the immediate host rocks to the epigenetic gold reefs, are interpreted to be over-bank deposits or abandoned channel fills, developed adjacent to active channels which were sandstone-dominated and had an incised axial thalweg marked by the coarsest-grained sediments present. Organic carbon content of the black shales at Bendigo varies from 0.2 to 2 wt%, compared with the grey shales, siltstones and sandstones, which vary from 0.05 to 0.2 wt%. Trace elements fall into two main groups: (a) elements that have a linear relationship with aluminium, and are controlled by the detrital clay content (Sn, Ba, Rb, Li, Cs, Mn, Cr and Tl); and (b) elements that show relationships with both aluminium and organic carbon (V, U, Ni, Zn, Cu, Bi, Pb, Se, Ag and Au) and are controlled by both the clay and organic matter content in the carbonaceous shales. The elements in the second group are enriched in the black shale facies. The background gold content of the black shales in the drill holes distal from mineralization averages 8.9 ppb, compared with the sandstones with 1.5 ppb. Most of the gold in the shales is present in diagenetic pyrite and marcasite, which laser ablation inductively coupled mass spectrometer (LA-ICPMS) analyses indicate varies from 5 to 3850 ppb and averages 370 ppb Au. The geochemical data suggest that this syngenetic gold was most likely sourced by erosion of the hinterland, and transported attached to detrital clay particles or as colloidal gold, by a high-volume feeder river system. High Rb/K ratios in the shales support a highly weathered source typical of a giant river system. By analogy with modern systems, following transport into deep marine channel–levee complexes via continental margin canyons, gold and other redox sensitive trace elements were ultimately trapped by reduction, adsorption and complexation with organic matter in the sub-oxic to anoxic over-bank deposits. Oxidation of much of the organic matter during diagenesis released the gold and certain trace elements (Ni, Co, Se, Ag, Cu, Bi, Pb), which became incorporated into diagenetic pyrite. Enrichment of gold in diagenetic pyrite of the black shale facies of the Ordovician turbidites at Bendigo was the first stage in a two-stage process that produced the world-class quartz–gold saddle reef deposits. Supplementary material: Whole rock analyses for sedimentary rocks in drill holes NBD005 and NBD186, Kangaroo Flat Mine, Bendigo, are available at http://www.geolsoc.org.uk/SUP18732