Anomalous Gold Concentrations Research Articles

Behavior of organic matter and gold under conditions of folding deformations

He article provides a brief overview of publications on natural organic matter (OM) and its transformation at various stages of the sedimentary rocks conversion. It is demonstrated, that the changes in the composition, structure, and properties of OM were accompanied by changes in the concentration of metals, including gold. The OM and gold behavior under conditions of folding deformations was examined by the authors on the example of the formation of zones of intense folding in large gold-mining regions of Russia (Central Kolyma and Baikal-Patom). Plastic deformations of sedimentary rocks are associated with active migration of gold in the composition of soluble OM components. The squeezing and injection structures that arise during the folding ensured the natural multidirectional movement of OM of the lowest viscosity into the areas of decompaction of sedimentary rocks. This migration resulted in anomalous concentrations of gold and OM in the locking parts of the folds. This process was intensely manifested in the terrigenous-shale rock sequence within the Dekdekan zone of folded dislocations (Central Kolyma region), where gold-bitumen mineralization was identified, represented by a large lens-shaped deposit 500–800 × 2500 m in size. The only metal concentrator here is oil-like bitumen forming very fine dissemination in the rock.

Anomalous Gold Concentrations in Hypersaline Wetland Sediments (Laguna Honda, South Spain) Caused by Nanoparticles Used in Agricultural Practices: Environmental Transformation.

Illite-rich sediments from the Laguna Honda wetland, an eutrophicated hypersaline wetland with waters enriched in Mg and Ca surrounded by olive groves in the Guadalquivir Basin River (South Spain), are polluted by elevated concentrations of gold (up to 21.9 ppm) due to agricultural practices. The highest gold contents appear in the shore sediments of the lake, where up to 20 µm homoaggregates of fused gold nanoparticles (AuNp) are found. Small nanoaggregates of up to six fused gold nanoparticles and very few isolated nanoparticles around 1 nm in size can also be observed to form heteroaggregates of AuNp-mica, especially in the deeper sediments in the central part of the wetland, where Au concentrations are lower (up to 1.89 ppm). The high nanoparticle concentration caused by the inappropriate application of pesticides favors nanoparticle collision in the wetland's Mg- and Ca-rich waters and the fast coagulation and deposition of Au homoaggregates in the gold-rich shore sediment of the lake. The interaction of gold nanoparticles with the abundant illite particles in the wetland's hypersaline waters promotes the simultaneous formation of low-density Au-illite heteroaggregates, which are transported and deposited in the less-rich-in-gold sediments of the central part of the lake. The small sizes of the isolated AuNp and AuNp-fused contacts of the aggregates suggest modifications in the original nanoparticles involving dissolution processes. The presence of bacterial communities resistant to heavy metal stress (Luteolibacter and Maricaulis), as well as the activity of sulfate-reducing bacteria (SRB) and particularly sulfur-oxidizing bacteria (SOB) communities from the shore sediments, favored the high-Eh and low-pH conditions adequate for the destabilization and transport of AuNp.

Fe-oxides in jasperoids from two gold districts in Nevada: Characterization, geochemistry, and (U-Th)/He dating

This study examines the whole-rock geochemistry, Fe-oxide texture and crystal morphology, and Fe-oxide (U-Th)/He date variation of jasperoids, as well as the relationships between these characteristics, in order to evaluate jasperoids as an indicator for gold mineralization. Jasperoid samples with a range of textures, appearances, and expected gold concentrations were collected from two gold districts in Nevada: the northern Carlin trend (Gold Quarry Mine), and the Battle Mountain district (Marigold Mine; Battle Mountain prospect). In our jasperoid samples, whole-rock concentrations of Au are positively correlated with As, Ag, Pb, Sb, U, W, and Tl. The following Fe-oxide textures are associated with anomalous gold concentrations: disseminated, submicron crystals; feathery and acicular crystal morphologies; concentric zonation from hematite to goethite; and botryoidal textures. Calculated closure temperatures for the (U-Th)/He system in observed jasperoid Fe-oxide morphologies within our dataset range from 26 to 136 °C, indicating variable susceptibility to He diffusion at near-surface conditions. Fe-oxide (U-Th)/He dates range from 29.4 ± 0.5 Ma to 0.11 ± 0.01 Ma, displaying intra- and intersample variation. We attribute the date variation to multiple factors including He implantation, parent isotope exchange, and He loss through closure temperature sensitivity and alpha-ejection. The intra- and intersample variation within the Fe-oxide (U-Th)/He dates limits our ability to make geologic interpretations. Our case study identifies important limitations of the (U-Th)/He method in jasperoids and provides insight on how He date variation might be avoided in future studies.

Statistical evaluation of stream sediment geochemical data from Tchangue-Bikoui drainage system, Southern Cameroon: a regional perspective

ABSTRACT Stream sediments play a significant role in geochemical exploration by identifying possible sources of anomalous element concentration. In this study, we use a multi-element association analysis to investigate the concentration of gold and associated elements in stream sediment samples from the Tchangue-Bikoui drainage area, Southern Cameroon. The main river and its tributaries were sampled, panned for gold grain recovery and the 100 μm size fractions analyzed for Au and associated elements by inductively coupled plasma atomic emission spectrometry (ICP-AES). A total of 50 samples were collected defining an area of ~145 km2 and 20 samples selected for geochemical analysis for a suite of 20 elements. Gold grades estimated by the panning and weighing technique rarely exceed 0.45 g/t and generally erratic while the Au concentrations from chemical analysis range from 0.01ppb to 4.00ppb. R-mode factor analysis produced a five-factor model accounting for 62.4% of the total variance. The Ni-Au-V-Sr association indicated gold mineralization while the others reflect lithologic control. Mo-Cr-Hg-Ag-As-Zr and Pb-Cu show no spatial relationship to Au and point to a typical primary mesothermal sulphide mineralization in the area which can be further investigated. Anomalous gold concentrations clustered mostly in the southeastern part downstream of River Tchangue defining a potential primary gold target.

Magmarizm and Ore Occurrences of the Kakheti and Tusheti Regions, Greater Caucasus, Georgia Republic

Kakheti and Tusheti regions (Eastern Graeter Caucasus) of the Georgia Republic are mainly underlain by highly deformed Lower-Middle Jurassic shales, sandstones, and volcaniclastic rocks, intruded by numerous intrusive bodies of various composition. All units contain a variety of ore mineral deposit types and their surrounding zones of hydrothermal alteration. Continued geological and metallogenic researches in the past 30 years have led to important new information regarding the evolution of this region. Three main stages of magmatic activity are now clearly distinguished. Oldest magmatism comprises Early Jurassic events, related to extensional tectonism, which ranged from rhyolitic, through dacitic and andesitic, and to basaltic magma. Additional extensional processes (in Bajocian) were characterized by intrusion of a gabbro and diorite dike system into recently deposited late Early Jurassic-early Middle Jurassic formations. The final magmatic event was associated with Middle Jurassic folding and uplift during which multiphase diorite plutons were emplaced. Related intense hydrothermal activity was responsible for important polymetallic mineralization, with more than 100 recognized outcropping ore occurrences. A detailed study of 11 of these ore occurrences has indicated anomalous concentrations of gold, thorium, yttrium, cobalt, cadmium, and bismuth. In addition, several new and potentially significant ore mineral occurrences were discovered.

The Asosa region of western Ethiopia: a golden exploration opportunity

Gold mining has a legendary history in Ethiopia, with Ethiopian mines providing gold to the ancient Egyptian empire and possibly even King Solomon's Mines and the Queen of Sheba. Today, gold occurs in the Pan‐African age schist belt and Tertiary basaltic lavas of the Asosa region of Benishangul‐Gumuz, western Ethiopia. There is widespread artisanal gold production in the Asosa region, with moderate activity by small numbers of local workers producing relatively large amounts of gold. There is a strong relationship between predominantly shear zone‐hosted gold deposits and the Kuluck shear zone, suggesting a structural control on gold accumulation. Gold is also commonly associated with secondary sulphide mineralization and magnetite alteration in Asosa rocks. The source of the gold is thought to be the sub‐volcanic intrusions generated during the subduction of the oceanic crust at the trench island‐arc system, akin to what we see today in epithermal and porphyry type gold systems. Historical workings and anomalous gold concentrations in rock, soil and stream sediments point to a potentially significant untapped gold resource in the Asosa region of western Ethiopia.

Evidence for exsolution of Au-ore fluids from granites crystallized in the mid-crust, Archaean Louis Lake Batholith, Wyoming

Abstract The Louis Lake Batholith is contemporaneous with and sits adjacent to Archaean lode gold deposits and minor copper stockwork vein systems of the South Pass Greenstone Belt, Wyoming Province, and may thus have contributed fluid into these hydrothermal systems. The batholith is granodiorite to granite, lacks extreme fractionation, and contains evidence for early formation and later replacement of an immiscible early sulphide phase. Mafic enclaves contain anomalous concentrations of gold and copper, suggesting that metals were introduced from in-mixed mafic magma. Apatite SO 3 and Cl values imply relatively early exsolution of sulphur- and chlorine-bearing fluids during fractionation. Exsolution of possibly immiscible high- and low-salinity fluids is implied from fluid inclusion petrography, even though crystallization pressures were greater than 3 kbar. Low salinity inclusions share affinities with those of orogenic gold systems, but contain notably higher Pb and Zn concentrations. High salinity inclusions have similar elemental compositions to brines of intrusion-centred ore systems. It is proposed that sulphide melt separated early during magma fractionation, scavenging Au and Cu, but became later destabilized, possibly because of increased f O 2 , releasing scavenged metals. Metals were transported from the pluton in sulphur- and chlorine-bearing hydrothermal fluids of low and high salinity, with Au being preferentially transported in the low-salinity fluid. Supplementary material: Whole Rock Geochemical data are available at http://www.geolsoc.org.uk/SUP18751 .

Geochemical Dispersion of Gold in Stream Sediments in the Paleoproterozoic Nyong Series, Southern Cameroon

Twenty-five stream sediment samples were collected in the Ngovayang area of southern Cameroon over a 1100 km2 area for heavy mineral fraction geochemical survey. The heavy mineral fraction of the stream sediments was analyzed for 45 trace elements. The majority of samples showed high Au concentrations though the anomalies were erratic. The geochemical data were processed using statistical and spatial analytical methods. R-mode Factor analysis produced a four-factor model which accounted for 77.02 % of the total variance in the data with the following metal associations: U-Th-Pb-W, Nb-Ta-Co-V, Au-Hf and Cu. The Au-Hf association indicated gold mineralization while the others reflect lithologic control. Uranium, Pb, Co, Ni and Ag show an almost uniform distribution within the Nyong Series indicating essentially the lithology. Anomalous gold concentrations clustered mostly in the northeastern part of the study area defining a potential primary gold target.

Source of anomalous gold concentrations in termite nests, Moolart Well, Western Australia: implications for exploration

The Moolart Well gold deposit lies in the Duketon Greenstone Belt in the Western Australian Goldfields in an area that has seen nearly 150 years of Au exploration with limited success due to the transported cover masking deposits. Here, the site displays no anomaly indicative of underlying mineralization within surface soils. Termites have the ability to burrow to the subsoil and contribute to the development of soil profiles through bioturbation. This study discusses the use of mounds formed by the termite Tumulitermes tumuli from a site where shallow ferruginised palaeochannel sediments with secondary Au enrichment overlays deeper primary mineralization. A series of samples from termite nests and surrounding soil were sampled along a transect from background areas to over mineralization. Various fractions of these samples were analysed with ICP-MS/AES. Tumulitermes tumuli is able to bring sub-surface mineralized material to the surface from 1 to 4 m depth. Termite mounds over mineralization and soil immediately adjacent to the mounds display an Au anomaly in both <250 and >250 µm fractions. Very high concentrations (>5000 ppb) were found in >2000 µm fractions in nests over mineralization as a result of vertical transport of anomalous pisolitic gravels by termites. These results suggest termite-driven local soil heterogeneity and termite mounds being a consistent geochemical and mineralogical sample medium for the discovery of ore deposits beneath weathered cover and shallow sediments.

Hydrothermal fluid evolution and ore genesis in the Arghash epithermal gold prospect, northeastern Iran

The Arghash epithermal gold prospect is located in the Sabzevar zone near the northern margin of the Central Iranian Microcontinent. The prospect includes six gold vein systems, hosted mostly by Lower-Middle Eocene intermediate to felsic volcanic and intrusive rocks with chemical compositions characteristic of continental arc magmas. Wall rocks are variably altered to clay minerals and subordinate carbonates, chlorite, and epidote, with intense alteration confined within 1–5m of the veins. Mineralization consists of fracture fillings with local occurrences of hydrothermal breccias, and disseminations and veinlets in the immediate wall rocks. The veins consist of quartz, carbonates, minor sulfides, and gold. Pyrite is the main sulfide mineral in the hypogene ore in all vein systems except Au-VI where abundant stibnite occurs intergrown with quartz and minor pyrite; base metal sulfides are rare. Four generations of pyrite were identified: (1) disseminated euhedral to anhedral pyrite (Py-I), with up to 430ppm Au; (2) framboidal pyrite (Py-II) with concentric As-poor and As-rich bands and up to 960ppm Au; (3) arsenian pyrite overgrowths (Py-III) occurring on euhedral to anhedral Py-I grains, with up to 1980ppm Au; and (4) fracture-filling, late-stage pyrite (Py-IV) that is anhedral and barren.Homogenization and ice-melting temperatures of fluid inclusions vary from 186 to 306°C and −0.1 to −3.2°C in quartz, and from 169 to 287°C and 0.0 to 2.3°C in calcite, respectively. Laser combustion analyses indicate a narrow range of δ34S values for Pyrites I–III, between −5.8‰ and +0.1‰, consistent with a magmatic source for sulfur. Pyrite IV is highly enriched in 34S (δ34S=+8.9‰ to +23.7‰), and may reflect a contribution from a source enriched in 34S, such as evaporites. The δ34S values for two stibnite samples from the stibnite-rich vein (−18.8‰ and −14.4‰) sharply contrast with those of the pyrite, suggesting a different sulfur, and possibly metal source, or strong fractionation. The δ13C values for calcite are near 1‰, which is typical of marine carbonates. The calculated δ18O values for the hydrothermal fluid in equilibrium with quartz range from +5.5‰ to +7.1‰, and the calculated δD values of the fluid in equilibrium with illite range from −48‰ to −57‰.The fluid inclusion and stable isotope data suggest that the fluids experienced a complex history of prolonged water/rock interaction, boiling, and mixing. Evidence for boiling is shown by quartz pseudomorphs after bladed calcite and coexisting vapor-rich and liquid-rich fluid inclusions. Water/rock interaction is supported by the occurrence of sulfides and anomalous concentrations of gold in the altered wall rocks adjacent to veins. The alteration and ore mineralogy, textures, fluid inclusion data, and calculated fluid isotopic composition are more consistent with the described characteristics of low-sulfidation epithermal deposits.

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

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

Use of a sensitive analytical method and the silt-clay (< 53 μm) fraction of stream sediments in exploration for gold in northern Thailand

Heavy mineral concentrates (SG ⪢ 3.3) from the Huai Hin Laep, a tributary of the Huai Kho Lo River in northern Thailand, contain strongly anomalous concentrations of gold. In contrast, the gold content of the associated < 149 μm and <53 μm fractions of the sediment is generally less than the 5 ppb detection limit obtained by a conventional fire assay-atomic absorption spectrometry method. To test for the presence of a gold anomaly at concentrations < 5 ppb, we have used an aqua-regia digestion followed by an Amberlite XAD-8 column preconcentration technique that, when used with a spectrometer that enables full display of the analytical spectrum to optimize baseline analysis, gives a detection limit of 0.1 ppb Au. Gold content of the < 53 μm sediment fraction ranges from 1.0 to 3.1 ppb compared to concentrations that typically range from 1000 to more than 100,000 ppb in the heavy mineral concentrates. However, despite gold concentrations in the sediment being several orders of magnitude lower than those in the heavy mineral concentrates, the downstream dispersion patterns are similar, with gold concentrations increasing at high energy sites and downstream away from the assumed source. These results identify the presence of a greatly diluted gold anomaly in the < 53 μm fraction of the sediments and suggest that transport and deposition of this fine grained gold is controlled by the same sedimentological factors that control the behavior of gold in the sand-size range, between 53 μm and 425 μm. Most important from an exploration standpoint, is that by using a sufficiently sensitive analytical method, meaningful gold dispersion patterns can be recognized at concentrations below 5 ppb.

Westerly derived Early Cretaceous gold paleoplacers in the Western Canada foreland basin, southwestern Alberta: tectonic and economic implications

Albian-aged (Early Cretaceous) igneous pebble to cobble conglomerates fill multiple, northeast–southwest-oriented, subparallel channels in the upper Blairmore Group (upper Beaver Mines and Mill Creek formations) of the Rocky Mountain foreland basin, southwestern Alberta and southeastern British Columbia. Paleocurrent data show that the conglomerate was derived from the west. Clast petrography implies a provenance that includes granitoids, mafic volcanics, low-grade metamorphic rocks, and shallow-level (ca. 7 km depth) postmetamorphic quartz veins formed from meteoric fluids. The conglomerate was probably derived from the southern Omineca Belt of British Columbia prior to the rise of the Rocky Mountains. The conglomerate contains detrital gold grains up to 150 μm in diameter, and chemical analyses indicate widespread anomalous gold concentrations (up to 910 ppb Au) in conglomerate matrix. Gold content in these igneous-clast conglomerates excèdes that reported from the richest modern placers in Alberta. Less pronounced but persistent As anomalies (up to 260 ppm) occur also. Postdepositional alteration of conglomerate matrix chemically mobilized Au and As from their detrital source grains and redistributed these elements. Gold enrichment in the igneous-clast conglomerate contrasts strongly with background gold concentrations in the underlying conglomeratic Cadomin Formation. The gold concentrations in the igneous-clast conglomerate demonstrate that paleoplacers derived from the Canadian Cordillera have accumulated in the Western Canada foreland basin, a sedimentary succession previously dismissed as a host for detrital gold.

Distribution and behavior of gold in soils and tills at the Nickel Plate Mine, southern British Columbia, Canada

The Nickel Plate deposit, in which gold occurs as <25 μm blebs associated with arsenopyrite in garnet-pyroxene skarns, is in the subalpine zone near the southern limit of the Thompson Plateau. During the last glaciation the Cordilleran ice sheet moved south-southwest across the deposit and deposited a stony basal till. A dispersion train with anomalous concentrations of gold in tills and soils now extends 2 km down ice from the deposit. Gold contents of samples of humus (LFH horizon) and the −212 μm fraction of mineral soils (A, B and C horizons) was determined by instrumental neutron activation and fire assay-atomic absorption, respectively. Selected samples were examined in detail to determine distribution of gold between different size and density fractions. Despite erratic variability, Au contents of the −212 μm fraction generally decrease from 200–400 ppb close to the mine site to <50 ppb at distal sites. At most sites there is also a twofold increase of gold values down the soil profile. Within samples concentrations of Au in the −420 + 212 μm, −212 + 106 μm, −106 + 53 μm and −53 μm fractions are usually roughly constant. However, because of its abundance, the −53 μm fraction contains more than 70% of the gold. Amenability of gold in this fraction to cyanidation suggests that it is largely free gold. For size fractions > 53 μm the contribution of the heavy mineral (SG > 3.3) fraction to total gold content increases with decreasing grain size. Distribution of gold between size and density fractions is consistent with its release from the bedrock or pre-glacial regolith by glacial abrasion. The bulk of the gold was incorporated into the fine fractions of the till at or close to the source. However, differences between down ice dilution ratios for gold in different heavy mineral size fractions suggest that comminution of host minerals continued to transfer gold to the finer size fractions during glacial transport. For exploration purposes, B and C horizon samples provide the best anomaly contrast. Estimates of the abundance of gold particles in different size fractions indicate that the nugget effect, which causes erratic gold values in the −212 μm fraction, can be avoided by analysis of 30 g of −53 μm material.

Transport of magnetite and gold in Harris Creek, British Columbia, and implications for exploration

Earlier studies have suggested that difficulties sometimes encountered in following up anomalies can result partly from large seasonal variations in gold content of stream sediments. This possibility has been investigated using pit traps installed in a bar on Harris Creek, British Columbia. Sediments caught in the traps in 1988 and 1989 provide data on variations in concentrations of gold and magnetite in bedload as stream discharge changes. These show that anomalous concentrations of gold are only transported for brief periods when increased discharge, caused by snowmelt floods, disrupts the cobble-gravel pavement. For the purposes of exploration geochemistry, these flood sediments, trapped in the voids of the newly formed pavement as the flood peak passes, provide the best chance of recognizing the presence of anomalous concentrations of gold in a catchment. However, their subsequent burial by post-flood gold-poor sediments may cause seasonal variations in gold content of near-surface sediments.