Geochemistry of native gold from deposits of various types

The ICP-MS-analyzed 490 samples of lode and placer gold allowed us to detect a wide spectrum of trace elements in native gold from the most abundant gold-polysulfide-quartz, low-sulfide gold-quartz, and gold-silver deposits from various gold provinces and districts of the Russian Federation. The typical set of elements in gold of each deposit type has been determined. The distribution of REE, rare alkali and alkaline earth metals in native gold is estimated for the first time, and a geochemical index has been proposed to typify gold occurrences. Compositional differences of native gold are also caused by regional metallogenic features of gold provinces. The results obtained make it possible to determine a type of lode and placer gold occurrences at early stages of geological exploration.     

黑龙江省砂金的粒度及形态特征

对黑龙江省７９个砂金及３个岩金样品进行了粒度及形态研究，多数样品的平均粒径在０．３￣０．６ｍｍ之间，以正态分布为主，分选系数和磨圆度表明多为远距离搬运的砂金，通过金粒总体形态比较，将形态划分为４类，其中第一类为岩金，第二类为有利于在其附近寻找原生金矿床的残积金。     

Does the primary deposit affect the biogeochemical transformation of placer gold and associated biofilms?

Placer gold particles derived from epithermal deposits display distinct morphological and compositional features compared with particles from mesothermal systems. Here, it is hypothesized that the chemical composition of primary gold derived from different deposit types is a principle factor affecting the composition of resident biofilm communities as well as the transformation of placer particles. Gold particles were collected from placers originating from the epithermal system at the Eisenberg, Germany's largest primary gold deposit. For comparison, placer gold from mesothermal sources was studied. Morphological differences due to mechanical transport and physical reshaping were not observed. Biogeochemical gold/silver dissolution and gold re-precipitation were evident on epithermal particles and they accentuate the silver-fabrics and gold-rich clusters. In contrast, on mesothermally derived gold particles these processes led to the development of gold-rich rims via the formation of nano-porous secondary gold. Microprobe- and laser ablation mass spectrometric analyses of polished whole particle mounts confirmed differences in gold/silver content/distribution and trace metal content between particles from epi- and mesothermal sources, respectively. On particles from all sites nano- and micro-particulate gold is associated with polymorphic layers. These are composed of microbial cells, extracellular polymeric substances (EPS) and clay-sized minerals. Multivariate statistical analyses shows a significant difference between biofilm communities from epi- vs. mesothermally derived particles, which is linked to the chemical composition of the primary gold. While a number of key-species capable of gold transformation, e.g., Cupriavidus sp., Geobacter sp. and Rhodoferax sp., were detected on particles from both sources, higher numbers of organisms with the potential for gold solubilization, precipitation and detoxification were associated with particles from the epithermal sources. A range of species involved in gold transformation, i.e., Arthrobacter spp., Delftia sp., Shewanella sp., and Stenotrophomonas spp., were detected only on epithermally derived placer gold. This indicates the communities are sensitive to differences in gold/silver and possibly trace metal-cycling, resulting from differences in their content, distribution and mobilization behaviour in epi- vs. mesothermally derived particles. Ultimately, this study shows that the chemical composition of the primary deposit strongly influences the biogeochemical transformation of placer gold and the composition of associated biofilms, whereas physical transformations appear to be largely unaffected.     

Problem of intermediate collectors of gold

Analysis of conglomerates at the Kamenskoye goldfield, eastern Zabaykal'ye, shows that some of them are auriferous and are typical “interceptors” (transit collectors) of gold, depending on their petrographic composition and the environmental variables, and that three new variants of the lode-placer sequence are recognizable: a) lode → deluvial placer → interceptor → alluvial placer, (b) lode → interceptor → alluvial → placer, c) interceptor → alluvial placer. – V.P. Sokoloff.     

Styles of lode gold mineralization contributing to the placers of the Indian River and Black Hills Creek,Yukon Territory,Canada as deduced from microchemical characterization of placer gold grains

Between 1978 and 2009, approximately 430,000 oz of placer gold were obtained from the Indian River and Black Hills Creek, which equates to roughly 20% of the production for the entire Yukon Territory during that period. The area is unglaciated, exposure is poor, and there are few known lode gold occurrences present. The technique of microchemical characterization of placer gold grains has been applied to illuminate the style(s) of source mineralization and their relationship to placer gold from the Klondike gold district immediately to the north. A total of 2,613 placer gold grains from 22 localities were characterised in terms of the Au, Ag, Cu, and Hg content of their alloy and associated suite of opaque mineral inclusions. A combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the Klondike. Gold type 3b (8–25% Ag) is the main component of the placers in lower Dominion Creek but is augmented and eventually replaced by type 3a gold (10–40% Ag) in placers in the main Indian River valley, probably through erosion of gold-bearing veins in the valley floor. Type 4 gold exhibits highly variable Ag which may contain Hg to a maximum of 11 wt.%. This gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides, tellurides, sulfotellurides, and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization. Placer gold in drainages radiating from Eureka Dome exhibits various proportions of types 3 and 4 gold depending on location, but type 3 gold forms the major component in Black Hills Creek and northerly flowing tributaries of the Indian River with the exception of Eureka and Montana creeks. Type 5 gold is found only in placers in the middle and lower Indian River. It is distinguished by slightly elevated (0.05–0.17%) Cu in the gold alloy, together with low (5–9%) Ag contents. Inclusions of Bi minerals, Cr-bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion-related source. Candidates for such a source include undiscovered lode occurrences associated with Cretaceous age intrusions to the south of the Indian River, or deformed Cu-Au (−Mo) porphyry occurrences which are known to be present in the same area. This analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from Eureka Creek to the larger placers of the Indian River is minor. Consequently, the placer gold inventory of the Indian River is primarily orogenic in origin. Similarly, the characterization of placer gold in Blackhills Creek strongly suggests an orogenic source. This study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern Klondike goldfield. This information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration.     

Gold resource potential of the eastern Siberian Platform: Placers and their feeding sources

The data on typomorphism of placer gold and its localization in the eastern Siberian Platform have been integrated for the first time. The alluvial, pseudolode, and eolian morphology of gold determines the origin of placers, which is explained not only by hydrodynamic but also by eolian processes. The recent and older alluvial (Au-bearing reservoir rocks in the second case), as well as eolian (unconventional type), genetic types of placers are recognized in the given territory. Precambrian and Mesozoic stages of ore formation are distinguished by the set of typomorphic attributes of placer gold (chemical composition, microinclusions, internal structure, etc.). On the basis of distinguishing features of placer gold, Precambrian lode deposits of great depth spatially related to outcrops of the basement have been forecasted for the first time and classified into low-sulfide gold-quartz, Au-bearing banded iron formation, Au-Cu porphyry, and Au-PGE types. The inferred shallow-seated gold-silver and gold-sulfide-quartz disseminated deposits occur locally and are related to the Mesozoic tectonomagmatic reactivation. The elaborated methods and approaches to the study of placer gold typomorphism in the eastern Siberian Platform also could be helpful in providing insights into placer origin, and forecasting and prospecting gold deposits on the East European Platform, which is close to the Siberian Platform in geological evolution.     

Effect of physical and biogeochemical factors on placer gold transformation in mountainous landscapes of Switzerland

In surface environments, placer gold particles are (re)shaped by physical, chemical and biogeochemical processes. This study hypothesizes that in modern landscapes combinations of physical and biogeochemical factors determine placer gold transformation and the composition of biofilm communities living on particle surfaces. Morphology, composition and biofilm communities of placer particles from eight locations in Switzerland were analyzed. Sites reflect a variety of modern-day placer environments in mountainous terrains, ranging from eluvial sites in close proximity to primary sources, to alluvial sites associated with both slow- and fast-moving streams, and to distal glacio-fluvial and depositional lake sediments. Particles are progressively physically and biogeochemically transformed with increasing distance from source. Variable degrees of rounding, gouging and scratching from physical reshaping due to transport and sedimentation processes are evident especially on particles from high energy environments. Surface textures indicative of biogeochemical gold/silver dissolution are common. In low energy/depositional environments and with increasing distance from source overgrowths of gold nano- and micro-particles and aggregates are abundant. These form micro-crystalline layers of secondary gold on particle surfaces covering them to depths of tens of micrometers. Biofilm communities across all sites are dominated by Proteobacteria (669 Operational Taxonomic Units (OTUs), 49.8%), especially β‑Proteobacteria. Other important phyla include Acidobacteria, (172 OTUs, 9.7%), Bacteroidetes (222 OTUs, 11.6%), Cyanobacteria (26 OTUs, 13.7%), and Planctomycetes (125 OTUs, 7.1%). Canonical analysis of principal coordinates demonstrates significant links of community assemblages to biogeochemical transformation and landscape position of collected particles. Increasing specialization towards metallophilic phyla was recorded on particles collected with increasing distance from source, suggesting that Au toxicity resulting from progressing biogeochemical transformation affected bacterial communities. This study ultimately shows that placer gold transformation and biofilm composition within modern landscapes is highly dependent on the interplay of physical and biogeochemical factors.     

冯坪子砂金矿成矿条件研究

陇南地区白龙江下游冯坪子位于西秦岭复杂构造带中,广泛发育碧口群绿片岩系,为含金初始矿源岩,第四纪以来,白龙江流域长期隆起剥蚀,对矿金运移富集提供了良好的地质环境,断陷带、构造谷负向部位成为砂金储集场所,冯坪子河漫滩采样分析结果表明,深度增加金品位增高,3.5～5m,7.5～11m为金富集段。     

中国金成矿的大地构造背景

指出中国金成矿的三个大地构造特征，中国第一个岩金成矿高潮的地层时代（古元古代）晚于全球第一个岩金成矿高潮的地层时代（新太古代）；中国第二个岩金成矿高潮地层时代（中生代）早于全球等二个岩金成高潮的地层时代（新生代）；全球有一个围绕北极圈的砂金成矿区，中国另有一个青藏高原砂金成矿区。     

Mapping of prospectivity and estimation of number of undiscovered prospects for lode gold,southwestern Ashanti Belt,Ghana

In the southwestern part of the Ashanti Belt, the results of fractal and Fry analyses of the spatial pattern of 51 known mines/prospects of (mostly lode) gold deposits and the results of analysis of their spatial associations with faults and fault intersections suggest different predominant structural controls on lode gold mineralisation at local and district scales. Intersections of NNE- and NW-trending faults were likely predominantly involved in local-scale structural controls on lode gold mineralisation, whilst NNE-trending faults were likely predominantly involved in district-scale structural controls on lode gold mineralisation. The results of the spatial analyses facilitate the conceptualisation and selection of spatial evidence layers for lode gold prospectivity mapping in the study area. The applications of the derived map of lode gold prospectivity and a map of radial density of spatially coherent lode gold mines/prospects results in a one-level prediction of 37 undiscovered lode gold prospects. The applications of quantified radial density fractal dimensions of the spatial pattern of spatially coherent lode gold mines/prospects result in an estimate of 40 undiscovered lode gold prospects. The study concludes finally that analysis of the spatial pattern of discovered mineral deposits is the key to a strong link between mineral prospectivity mapping and assessment of undiscovered mineral deposits.     

Determination of trace elemental signatures in placer gold by laser ablation–inductively coupled plasma–mass spectrometry as a potential aid for gold exploration

It is possible that placer gold retains the trace element composition (`signature') of the parent lode. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) offers sensitive detection limits of 59 elements with sub-millimetre sample spot sizes, ideal for signature analysis of placer. Here, the optimum laser parameters, analytical precision and reproducibility of elemental signatures are determined in placer nuggets by infrared LA–ICP–MS. The highest precision was offered by Q-switched ablation at a relatively low energy level, using Au as an internal standard; these parameters gave median precision values of 33–82% RSD for trace elements in nuggets. In addition to major constituents such as Ag, Cu and Hg, placer from several rivers world-wide commonly contained a wide range of trace elements including, in some cases, unusual REE patterns. At least two distinct signatures could be distinguished in placer from most of the rivers examined, while multiple examples of certain signatures were also seen. Tracking these signatures back to their original tributaries may offer a means of identifying the number and approximate location of deposits within a river watershed, thereby significantly aiding exploration.     

胶东脉型金矿成矿深度与矿体延深的模型研究

主体与岩浆成矿热液有关的、产于矿脉中的金矿体,称脉型金矿。本文以成矿深度作为脉型金矿的分类依据,胶东地区可分为中深和浅成两类脉型金矿。从成矿深度模型中,获得中深脉型金矿成矿深度的理论值主要为２．４６～３．８ｋｍ,浅成的为０．２２～２．４９ｋｍ。从矿体延深模型中,获得中深脉型金矿的延深理论值主要为０．４０２～１．００２ｋｍ,浅成的为０．１６４～０．５５８ｋｍ。论述了成矿深度与矿体延深模型的理论值为正相关,说明成矿深度的研究有重要意义。     

Typomorphic features of placer gold of Vagran cluster (the Northern Urals) and search indicators for primary bedrock gold deposits

The Vagran placer cluster is located on the eastern slope of Northern Urals. During > 100 years of gold mining history approximately 40 tons of gold have been extracted from the placer deposits.Bedrocks of the region consist of high metamorphic Upper Proterozoic and Paleozoic terrigeneous, terrigeneous-volcanogenic and igneous rocks. Gold placer deposits are mostly alluvial genesis deposits and of Quaternary to Oligocene (?) age. The alluvial deposits consist of gravel with pebbles, boulders, and sandy clay covered by sandy silt and a soil layer. The thickness of the alluvial sequence is usually 5–10 m and reaches 18 m in the main watercourses of the third order. Nearly all of the alluvial sediments are gold bearing but concentrations of economic importance prevail in the bottom part of the sequence above the bedrock.There are four different types of gold particles: (I) rounded and well-rounded particles of high fineness and homogeneous inner structure, (II) rounded to sub-rounded high fineness particles with a pure gold rim developed over a core, (III) crystallomorphic (idiomorphic) high fineness with a homogeneous inner structure, and (IV) irregular angular and subangular particles of medium fineness with a significant content of Ag (10–40 wt.%) and elevated Hg (up to 1.15 wt.%).The first type is prevalent and comprises up to 65% of the total gold particles; it is uniformly distributed throughout the territory. There are features with initially complicated dendritic and laminar shaped particles which were rounded during transportation. The second and third types have a propensity for zones of the inherited erosion–tectonic depressions. Apparently, types I, II and III are related with orogenic mesothermal gold-sulfide-quartz mineralization; the differences of these types depend on the primary zonation of ore bodies and supergenic transformation of the alloys. They were connected with middle-depth ore bodies of an orogenic gold-sulfide-quartz formation. The fourth type is evident of nearby transportation from primary sources and a short duration of supergenic influence. It is controlled by a zone of NW-SE orientation, diagonal to the main structures of Ural Fold Belt.The plot of Au content vs coefficient of heterogeneity (ratio of the Au content in the core and in the rim of the grains) is the distinguishing factor between the four types of gold grains both by primary hypogenetic characteristics and supergenetic features.No corresponding lode occurrence of gold-sulfide-quartz mineralization has been identified to date in this region. Placer gold concentrations are related to the intermediate hosts of the Mesozoic-Cenozoic surfaces of the Ural peneplain uplift in the Oligocene and eroded in Miocene-Quaternary time. This factor determines the widespread distribution of placer gold in the territory of the Vagran cluster.The large, Carlin-type Vorontsovsk gold deposit is located 60 km south-east from the Vagran area. It has a shallow erosional level, small size of native gold, and its distal location from the placer deposits makes it an unlikely primary source for the Vagran placers. However, mineralization of this type of deposit is noted within the cluster.Gold of the fourth type nearly resembles the gold of the Vorontsovsk deposit and, apparently, the source is related to the same hydrothermal mineralization event. ICP MS analyses of the quartz-sulfide lodes in the floor of gold-bearing valleys revealed a gold content of 2.0–6.9 g/t in the zone of type IV distribution. Therefore, gold of the fourth type can be used as an indicator for the exploration of primary bedrock mineralization. The geological setting and typomorphic features of this placer gold shows that the primary gold mineralization is similar to the Vorontsovsk deposit and within the zone of distribution of the placer gold of the fourth type.     

Residual-eluvial gold placers in northeastern Russia (Dal’nii placer)

We consider the formation of the Dal’nii (Dal’nyaya) eluvial gold placer (Bol’shoi Anyui ore–placer district, western Chukchi Peninsula), related to the Dal’nii (Dal’nee) gold-bearing porphyry Mo–Cu occurrence. The Dal’nii placer is located within the transition between the Kur’ya Ridge and Anyui basin, which has been relatively stable at the recent (Pliocene–Quaternary) tectonic stage. Minor recent uplift has determined the slight denudation of interfluves, the leading role of eluvial processes in the formation of a loose cover on them, and the preservation of the relict matter of pre-Pliocene chemical-weathering crusts (including the oxidized zones of orebodies) in present-day eluvium. The Dal’nii placer consists of relict weathering-crust placers altered by recent eluvial processes in different degrees. Therefore, it is relatively rich in metal, whereas the primary lode contains mainly fine-sized gold, which is almost not released from ore under periglacial lithogenesis in present-day interfluves. We suggest calling this genetic type of placers “residual-eluvial.” The primary lodes being highly eroded (during the formation of residual concentrations, which serve as an intermediate reservoir for these placers), residual-eluvial placers or their parts might not be directly related to specific orebodies at the present-day level of erosional truncation.     

On the stability of gold-silver alloys in the weathering environment

Thermodynamic analysis of the stability conditions for gold-silver solid solutions reveals that the silver component is unstable with respect to chlorargyrite and silver sulfate in most weathering environments. As a consequence supergene (secondary) gold and gold found in modern river placers (detrital and authigenic) tends to be pure. Impure, detrital placer gold is subject to natural purification processes as evidenced by silver depleted, spongey grain margins. In contrast, under hydrothermal conditions the silver content of primary gold can be controlled by equilibrium reactions with argentite.     

The Ventersdorp Contact placer: a gold-pyrite placer of stream and debris-flow origins from the Archaean Witwatersrand Basin of South Africa

Five local varieties of the Ventersdorp Contact placer at the East Driefontein Gold Mine, Carletonville, are distinguished by clast-type assemblage, reflecting discrete provenances. The placer further comprises six lithofacies which are not restricted to specific provenance-varieties. The six lithofacies are: massive, matrix-supported conglomerate (facies Gms): massive, or crudely bedded, clast-supported conglomerate (facies Gm); channel-based conglomerate (facies Gt); single clast layers (facies Glag); horizontally stratified sandstone (facies Sh); and trough cross-stratified sandstone (facies St). Facies Gms represents debris-flow deposits, whereas the other facies are stream deposits. Lithofacies and provenance-varieties are complexly interrelated in local environments, which are themselves complexly related to geomorphic elements of a degradation surface. The maximum thickness of the placer is 7 m. The placer was deposited on a hard, i.e. non-alluvial, bedrock surface, suggesting that it is a pediment mantle. It owes its preservation to a capping of conformably overlying basalts. Economic gold mineralization is present in all the conglomerate facies and all the provenance-varieties. Gold concentrations are high in gravel-bar deposits and channel fills but are not restricted to these environments. A major feature of gold concentration is its variability between provenance-controlled varieties. The gold concentration of a specific primary source is interpreted to be a major control on gold concentration in the final deposit. The origin of the placer as a pediment mantle, and its subsequent preservation by lava-flows, suggests that there was little potential for selective reworking of heavy minerals. Nevertheless, the Ventersdorp Contact placer on the Carletonville Goldfield is one of the richest gold placers in the Witwatersrand Basin.     

内蒙古呼伦贝尔盟北部砂金矿成矿规律

呼伦贝尔盟北部地区由于新构造运动含金地质体长期风化剥蚀，促使金的活化。河流及其2－3级支流发育，流域广，水流充足，有利于砂金矿的迁移、富集。现代的乐山丘陵、宽阔沟谷有利于砂金的保存。该区已发现数十个砂金矿，多为河谷型，主要含金层为含粘土砂层或砂砾层。砂金富集与河谷地貌关系密切，河床弯曲的凸岸、变异部位、关门咀、迎门山等地貌是砂金富集有利部位。该区金直接来源于加里东和华力西花岗岩。结合金的来源、矿床的分布特征和富集规律，指出了该区今后砂金找矿方向。     

Unravelling the formation histories of placer gold and platinum-group mineral particles from Corrego Bom Successo,Brazil: A window into noble metal cycling

Gold and platinum-group-metals (PGM) are cycled through Earth’s environments by interwoven geological, physical, chemical and biological processes leading to the trans/neoformation of metallic particles in placers. The placer deposit at Corrego Bom Successo (CBS, Brazil) is one of the few localities worldwide containing secondary gold- and PGM-particles. Placer gold consists of detrital particles from nearby hydrothermal deposits that were transformed in the surface environment. Processes that have affected these particles include short-distance transport, chemical de-alloying of the primary gold‑silver, and (bio)geochemical dissolution/re-precipitation of gold leading to the formation of pure, secondary gold and the dispersion of gold nanoparticles. The latter processes are likely mediated by non-living organic matter (OM) and bacterial biofilms residing on the particles. The biofilms are largely composed of metallophillic β- and γ-Proteobacteria. Abundant mobile gold and platinum nanoparticles were detected in surface waters, suggesting similar mobilities of these metals. Earlier hydrothermal processes have led to the formation of coarsely-crystalline, arborescent dendritic potarite (PdHg). On potarite surfaces, biogeochemical processes have then led to the formation of platinum- and palladium-rich micro-crystalline layers, which make up the botryoidal platinum‑palladium aggregates. Subsequently potarite was dissolved from the core of many aggregates leaving voids now often filled by secondary anatase (TiO₂) containing biophilic elements. The presence of fungal structures associated with the anatase suggests that fungi may have contributed to its formation. For the first time a primary magmatic PGM-particle comprising a mono-crystalline platinum‑palladium-alloy with platinum‑iridium‑osmium inclusions was described from this locality, finally defining a possible primary source for the PGM mineralisation. In conclusion, the formation of modern-day placer gold- and PGM-particles at CBS began 100s of millions of years ago by magmatic and hydrothermal processes. These provided the metal sources for more recent biogeochemical cycling of PGEs and gold that led to the trans/neoformation of gold- and PGM-particles.     

秦岭贵金属矿查勘剖析

秦岭横亘中国大陆中部,东西绵延1300km,地跨豫、鄂、皖、陕、甘、青、川七省,面积约400000km²,它是我国著名的"中部多金属成矿带".金银矿产为重要的优势资源,矿床星罗棋布.迄今为止,已探明金矿床145处,其中砂金矿床44处,岩金矿床101处;银矿床24处.该区金、银矿床成矿类型繁多,具有得天独厚的聚矿条件,其中小秦岭享有"第二金库"的美称.遐迩闻名的"陕甘川金三角"地区的成矿受巨型帚状构造控制,可与全球诸多巨型-超巨型金矿之聚矿条件媲美.本区金、银矿产成矿规律明显,具有极其理想的成矿条件,是进一步查勘巨型-超巨型金矿的最佳地区     

玻利维亚铜金锡铁主要矿床类型及资源潜力

玻利维亚的铜矿规模以中小型为主,主要成矿类型为红层型(砂岩层)、沉积相关脉状型、VMS型和IOCG型等。金矿规模以中小型为主,主要成矿类型有火山成因浅成热液型、与深成岩相关的脉状矿床、造山型矿床和砂金矿。锡矿发育众多大型、超大型矿床,成矿类型以玻利维亚型多金属脉状矿床和与长英质深成岩相关的脉状矿床为主,少量砂锡矿。铁矿以El Mutún超大型BIF型铁锰矿著称。西科迪勒拉和玻利维亚高原有重要的浅成低温热液贵金属资源潜力;东科迪勒拉北部主要为钨、锡、金、锑资源,中部为锡、银、金、锑资源,南部有金、锑、银、铅、锌潜力;次安第斯带南部有银-锌资源潜力;查科-贝尼平原带有广泛的砂金矿资源;前寒武纪克拉通内金、铂、镍、钽、铜和铁锰资源潜力丰富。