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.     

Distribution and dispersion of gold in point bar and pavement sediments of the Huai Hin Laep, Loei, northeastern Thailand

Gold anomalies in drainage sediments are often erratic, reflecting both the nugget effect and hydraulic effects whereby gold is concentrated at favorable sites along a stream. This study investigates these factors in a stream in northeastern Thailand.Bulk sediment samples, consisting of approximately 40 kg of −12 mm material, were collected from bar and pavement sites along an 8 km study reach. Samples were wet sieved into eight size fractions. The five fractions between 425 μm and 53 μm were then processed to obtain heavy mineral concentrates (SG > 3.3). Gold content of all size and density fractions finer than 425 μm was determined by fire assay-atomic absorption.Concentrations of Au in the heavy mineral concentrates typically range from 10,000 to 50,000 ppb (maximum 198,000 ppb), whereas the corresponding light mineral fractions and the −53 μm fraction generally contain <5 ppb gold. Within the heavy mineral fractions concentrations of Au generally increase downstream away from their supposed source and are higher at pavement than at point bar sites. Variations in abundance of gold between point bar sites can be related to stream characteristics (such as width, velocity and bed roughness) that are indicative of changing energy conditions and of the ability of the stream to winnow light minerals from its bed.The estimated median number of gold particles in the heavy mineral concentrates increase from less than one in the 212–425 μm fraction to about three in the 53–106 μm size range. However, because of dilution by the light mineral and −53 μm fractions, the probability of a 30 g analytical sub-sample containing a particle of gold is so low that in thirteen out of sixteen −149 μm sediment samples no gold was detected. Insofar as this results from dilution by large quantities of −53 μm sediment, failure of conventional sieved sediment samples to reliably detect the anomaly is probably a consequence of increased erosion caused by deforestation and land usage.Heavy mineral concentrates from pavement and other high energy sites are more reliable than conventional sediment samples for detecting gold anomalies of the Huai Hin Laep type. A low sample density is adequate but, because anomaly contrast may increase downstream, careful interpretation is required.     

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.     

Size, form and composition of gold grains in glacial drift in Ilomantsi, eastern Finland

Gold grains were studied in 102 till and gravel samples from an area with Au anomalies in the fine fraction of till. The form and size of gold nuggets were studied under the microscope, and the composition of selected grains was determined on a microprobe. The number of visible nuggets per sample was less than 10 per 81. In size, the nuggets were between 0.07 and 1 mm. Most of them were indefinite but flat in form, although idiomorphic gains were also observed. In composition, the gold grains in bedrock differed somewhat from those in till and gravel. Some of the nuggets found in gravel and in till were probably secondary in origin. The possibility that all the grains are not primary is emphasized in the interpretation of geochemical results.     

Quaternary stratigraphy and geochemistry at the Owl Creek Gold Mine, Timmins, Ontario, Canada

The Owl Creek Gold Mine is located in Hoyle Township, approximately 18 km northeast of Timmins, Ontario, Canada. The open-pit mine exposes a sequence of altered and mineralized mafic tholeiitic volcanics bounded to the north and south by greywacke and argillite. Gold occurs in the free state in quartz veins, often with graphite, and as fine gold on surfaces of, and within fractures in, pyrite.The study was designed to determine the distribution and distance of transport of Au in overburden down-ice from subcropping Au mineralization. This required an understanding of the glacial history of the area.The Quaternary stratigraphy at Owl Creek was studied and sampled by means of 17 sonic and 15 reverse-circulation overburden drill holes near the open pit, and several overburden exposures in the open-pit walls. Nonmagnetic heavy-mineral concentrates (specific gravity >3.3) were made from the <2000 μm (−10 mesh) fraction of all overburden samples from the drill hole and section sampling. The heavy-mineral concentrates were analyzed for Au by neutron activation. A till pebble lithology study was done on the >2000 μm (−10 mesh) fraction of the sonic drill core.Our stratigraphic studies indicate that there were three major Wisconsinan (Weichselian) ice advances and one minor, late readvance in the Timmins area. The transport and deposition of sediments comprising the “Oldest”, “Older”, Matheson and Cochrane stratigraphic “packages” (oldest to youngest) are related to three ice advances and one readvance which moved towards 240° ± 10°, 150° ± 5°, 170° ± 5° and 130° ± 5°, respectively.Geochemically anomalous levels of Au in the overburden define two dispersal trains down-ice of the Owl Creek Gold Mine. One, in the “Older” lodgement till, is 400–500 m long. The other in Matheson ablation and waterlain tills, is approximately 700 m long.The till pebble lithology study showed that pebble counting can be used to approximate bedrock contacts, but may not necessarily identify the source rock type of the matrix.     

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

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.     

Lithology and trace-metal content in till in the kuhmo granite-greenstone terrain, eastern Finland

Rocks of the Archaean Kuhmo greenstone belt and the fine fraction (− 63 μm) of 236 till samples were analysed for Co, Cr, Cu, Mn, Ni, Pb, Zn and Fe mainly with the AAS method. The influence of bedrock upon the lithology and geochemistry of till is clear, and can be seen immediately at the proximal contact. Expected trace-metal contents for the fine fraction of till were calculated using the average metal contents of the rock types and the proportions of these rock types in the till (2–6 cm pebbles). The expected values were then compared with the measured values in the fine fraction of till at the same locality. This approach seems effective for lithological comparison between various grain-size fractions of till.In general, the measured metal contents are clearly lower than expected. This indicates that the proportion of the greenstone material in the fine fraction of the till is smaller than in the coarse fraction. Correlations between measured and expected values of individual trace metals as well as their correlation with the different rock types are low, in most cases indicating inconsistent variation in the lithology of the coarse and fine fraction of the till. Good positive correlations between the number of pebbles of mafic metavolcanics and contents of Cu, Co, Mn and Fe in the fine fraction of till show that the influence of these rocks on the trace-element geochemistry of the tills in the study area is most significant as might be expected.     

Till geochemistry in the ribbed moraine area of Peräpohjola,Finland

The characteristics of ribbed moraines, with an emphasis on till geochemical prospecting, were studied in the area of Peräpohjola, southern Finnish Lapland. Percussion drilling, test pits and trenches were used for till sampling. The samples were partially extracted and then analysed using ICP-AES or GFAAS methods in the geolaboratory of the Geological Survey of Finland. The distribution of Au and Cu in fine fraction (<0.06 mm) of till in both horizontal and vertical dimensions showed that the uppermost part of the moraine ridges contained the highest metal contents. In the coarser fractions of till (0.06–0.5 and >2 mm), the anomalies were located on the distal side of the ribbed moraine ridges, down-ice from mineralized bedrock. Geochemical patterns together with fresh pyrite grains in till heavy mineral concentrates indicate a short glacial transport distance of the mineralized debris. This conclusion is also supported by the presence of a large proportion of local rock fragments and boulders in the uppermost till unit and at the surface, which is a result of glacial quarrying during the ribbed moraine formation.     

Enrichment of As and S in the finest fractions of crushed rock samples — an experimental study simulating the formation of till

To simulate the behavior of As and S in the formation of till, the contents of these elements were determined in the total material, the −0.064 mm fraction and the 0.064–0.5 mm fraction of crushed rock samples. For most of the studied rock types there was a conspicious enrichment of As and a much less prominent enrichment of S in the finest fraction. Accordingly, in the formation of till the mere mechanical crushing of rock material can create enrichment of the derived till.     

Distribution and dispersion of gold in glacial till associated with gold mineralization in the Canadian shield

The results described relate to an investigation into the nature of Au dispersion in glacial till, undertaken to identify optimum search techniques for use in exploration for Au mineralization.The diversity of Au mineralization, in terms of the host rock lithologies, mineralogy and grain size of the Au, would be expected to give rise to differences in the secondary response in the associated overburden. Common exploration procedures involve the analyses of the heavy-mineral fraction or a particular size fraction of the tills. However, having regard to the expected variable response of Au in associated glacial till, attributed to variations in primary mineralization, effective exploration requires that the methodology employed is capable of locating all types of Au mineralization.Bulk till samples were collected from various sites associated with the Owl Creek deposit near Timmins and the Hemlo deposits. Grain size analyses were carried out on the till samples and on the heavy-mineral concentrates. The concentration of the Au in the various fractions was determined by Instrumental Neutron Activation Analysis.Preliminary results allow a number of provisional conclusions to be drawn:

1. (1) Grain size analysis of the −2 mm fraction of tills indicates that the silt and clay fraction constitutes 20–50%, whereas, in contrast, the equivalent heavy-mineral concentrates are dominantly composed of the coarser −500 + 63 μm material.

2. (2) The amount of Au present in the heavy-mineral concentrates of tills represents only a minor proportion of the total Au in the original till samples. In addition, the proportion of the total Au recovered in the heavy-mineral concentrate varies from 4 to 15%. Both factors indicate that caution is necessary in interpreting the significance of heavy-mineral Au data.

3. (3) Examination of the size distribution of Au within the heavy-mineral concentrate indicates that the majority of the Au is contained in the −125 μm fraction.

4. (4) The concentration factor (the original sample weight divided by the heavy-mineral concentrate weight) varies up to 7-fold between samples due presumably to the differing proportions of heavy minerals. Hence, in Au deposits of equivalent economic significance this gives rise to varying Au concentrations in heavy-mineral concentrates according to the quantity of heavy minerals present. Significant interpretation can only be achieved by re-expressing the Au contents of heavy-mineral concentrates in terms of the absolute amount of Au in heavy-mineral concentrates.

5. (5) A comparison of the heavy-mineral concentrates produced by different laboratories indicates marked differences in the weight of the heavy-mineral concentrate, the Au concentration of the heavy-mineral concentrate, the total weight of Au in the heavy-mineral concentrate and the size distribution of the Au in the heavy-mineral concentrate.

6. (6) Analysis of the −63 μm silt and clay size fraction indicates anomalous Au contents within this fraction of the tills collected from Owl Creek and Hemlo, extending over 500 m down-ice from mineralization at Hemlo.

7. (7) Analysis of the −63 μm silt and clay size fraction is suitable for the detection of fine-grained Au deposits that are not amenable to detection on the basis of heavy-mineral concentrate analyses.

8. (8) The analysis of the silt and clay fraction reduces the sample representativity problems associated with analyzing coarser fractions.

9. (9) A comparison of the Au distribution in heavy-mineral concentrates and the −63 μm fraction of till down-ice from the Owl Creek deposit indicates broadly similar dispersion patterns.

In conclusion, although the results are based on relatively few samples, their consistency permits some general conclusions to be drawn. The silt and the heavy-mineral concentrate analyses provide different information and in view of the diversity of exploration targets and surface environments exploration reliability can be increased by analyzing both the −63 μm silt and clay fraction and the heavy-mineral concentrate.     

Seasonal variation of gold and arsenic in biogeochemical samples from a disseminated gold deposit in the Northern Cordillera

An orientation survey was conducted over the Quartz Mountain, Oregon, hot-spring type disseminated gold deposit to address three questions critical to successful exploration of the area: What is the relationship between bedrock geology and the trace-element content of trees and soils; do these relationships change seasonally; are these relationships similar in the two tree species which discontinuously blanket the area?Twig, needle and wood samples were collected four times during the year from both ponderosa pine (Pinus ponderosa) and white fir (Abies concolor). Soils were collected once. All samples were analyzed for Au and As and the wood samples were analyzed also for Sb.Arsenic was the best pathfinder element, having little analytical, spatial, or seasonal variation in the twig and needle samples. Anomalous levels were 150 μg/g (ppm) As in pine and 30 μg/g in fir. Gold showed analytical variation of 20–30%, sample duplicate variation up to 90%, spatial changes, and seasonal variation ranging from winter-summer background [10–20 ng/g (ppb) Au] to anomalies of 300–400 ng/g (ppb) Au in pine and 100 μg/g (ppb) Au in fir in the spring and fall. Antimony could not be completely evaluated as a pathfinder element because it was only determined in wood samples.Needles and twigs from both species proved to be viable sample media for exploration of the area. Wood was not a suitable medium because of low, erratic values, perhaps due in part to analytical technique. Needles had 2 to 20 times higher As concentrations than did twigs. Twigs had a slightly higher Au content [25 ng/g (ppb)] than did needles. The pine samples were higher in both elements than were the fir samples. The data, normalized by species, could be treated as one homogeneous data base.The soil Au and As data outlined the mineralization clearly with a central Au anomaly [100 ng/g (ppb) and greater] surrounded by As anomalies [100 μg/g (ppm) and greater] over a distance of 594 m. Neither spring nor fall tree Au anomalies were coincident with the soil Au anomaly. Consistent throughout the year, tree As anomalies coincided with the soil As anomalies, but covered a smaller area. Both the Au and As anomalies in trees appeared to be related to bedrock contact zones rather than to the soil the trees were growing in.     

Alluvial gold in Kalimantan, Indonesia: A colloidal origin?

Extensive placer gold deposits occur in Quaternary palaeochannels in the Ampalit and Cempaga Buang drainage basins near Kasongan, Central Kalimantan, Indonesia. Broad interfluvial Pleistocene terraces surrounding the drainages also contain significant amounts of placer gold. The “channel” and “terrace” gold deposits together cover an area in excess of 180 km².Exploration for gold at Ampalit followed traditional alluvial methods, and assumed that mechanical and gravitational factors were the principal mechanisms affecting gold accumulation and concentration. Gold was considered to be physically reworked from terraces and redeposited in the present drainage channels or underlying, laterally displaced, palaeochannels. However, a comparison of gold grains from the Ampalit drainage channel and adjacent terraces indicates that gold grains in the drainage channels are possibly of colloidal origin and not mechanically transported to their present domain.The morphology of gold grains from the Ampalit drainage is compared with grains from adjacent terraces, using scanning electron microscopy and atomic force microscopy. The latter procedure allows for nondestructive analysis to give true three dimensional surface topography down to nanometre resolution. Evidence from force microscopy supports a colloidal origin for gold aggregates.The average purity of gold produced from the Ampalit dredging operation is 970 fine (970/1000) and the majority of examined gold grains extracted from beneath the drainage channel have a purity of 998 fine. This unusually high degree of purity suggests a selective dissolution/aggregation mechanism. We propose that gold is transported downward and laterally in groundwater percolating through the terrace sands and gravels to the current drainage as a humic acid-stabilised colloid. As the pregnant groundwater migrates toward the present drainage channels, it encounters a sleep chemical gradient and the colloid aggregates. The aggregation process occurs near clay zones within palaeochannel sands and gravels beneath the present river sediments. Here the colloidal aggregates form small grains (usually <1 mm) of extremely high purity.Today, the inherently unstable gold deposits at Ampalit are in a state of both aggregation and dissolution. Identification of low energy zones, location of point bar lags in palaeo and recent drainage channels, and the extrapolation of areas of low energy are not solely valid as exploration guides for gold accumulation in this environment. Exploration methods that also recognise ground water migration and composition, and the geochemical controls of dissolution and aggregation, will improve the ability to identify ore accumulations, thus contributing to improved economics of mining these deposits.     

Size distribution of gold in some soils associated with selected gold mineralization in Canada and the United States of America

Samples were collected near known gold mineralization from anomalous and background soils developed on glacial till in British Columbia and Ontario, and in residual soils in Nevada, Utah and the Yukon Territory. After wet sieving to five size fractions finer than 2000 mm, and separation of heavy minerals (S.G. > 3.3) for the − 212 + 106 μm and − 106 + 53 μm fractions, gold concentrations in each fraction were determined by fire assay-atomic absorption.In all cases more than 60% of the gold resides in the − 53 μm fraction. For the residual soils most of the remaining gold is associated with the light mineral fractions and only a negligible proportion resides in the heavy minerals. Estimates of the numbers of gold particles and subsampling variability suggest that gold, in both heavy and light mineral fractions, is largely present as inclusions of fine gold. For exploration purposes, optimum sample representativity would be obtained with the − 53 μm fraction. However, because this fraction is a major component of the soils, use of a coarser size fraction (e.g., − 212 μm) will only slightly reduce sample representativity. Also, because most of the gold is associated with the − 53 μm and light mineral fractions, use of heavy mineral concentrates offers no significant advantage and in some cases would result in anomalous conditions being overlooked.     

Gold dispersion in Andean desert environments (Atacama, Chile)

The mechanism of Au dispersion in sediments of mountainous desert environments has been studied in two different areas of the Chilean Andes. The San Pedro de Cachiyuyo placer (ca. 1800 m a.s.l.) consists of alluvial fans and ephemeral stream sediments deposited in a stable piedmont. The primary Au source is Cu–Au-bearing hydrothermal tourmaline breccia. The relief of this area is low (1850 m) and the average annual rainfall is ca. 20 mm. The La Coipa area (ca. 4000 m a.s.l.) is characterized by a rugged relief. The annual average rainfall is ca. 100 mm. The sedimentary deposits are less sorted than in San Pedro de Cachiyuyo and consist of ephemeral stream alluvium and slope deposits formed in a periglacial environment. The primary Au sources are two volcanic-hosted epithermal precious metal deposits. At San Pedro de Cachiyuyo, the halo is less than 1 km in length and the highest Au concentration are observed at the break in slope between the hillside and the piedmont. In the La Coipa area, Au was only detected by chemical analysis in the 125–63 μm and <63 μm fraction; however, the dispersion halo extends for over 10 km from the source. In both cases the geochemical signal of gold was strongest in the fraction <63 μm.     

Solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions

An experimental study of the solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions was conducted at temperatures from 200° to 350 °C and at saturated vapor pressure. A 500-mL Bridgemantype pressure vessel constructed of titanium, and equipped with a motor-driven magnetic stirrer was employed. The pH and the oxidation state were buffered by the coexistence of H₂S/HS^–/SO _inf4 ^sup2– . The pH at temperature was calculated to be in the range 5.91–9.43, and S was 0.3–2.2 m. Under the experimental conditions, the measured solubility of gold is about two to three orders of magnitude greater than that of either platinum and palladium, and the measured solubility of platinum is, in general, approximately equal to that of palladium, in molal units. The solubilities are found to be in the range: platinum 4–800 ppb, palladium 1–400 ppb, and gold 2–300 ppm. The solubility data can be modeled adequately using the following reactions: Au+H₂S+HH^–=Au(HS) ₂ ^– +1/2H₂ (K₁₄); PtS+HS^–+H⁺=Pt (HS) ₂ ⁰ (K₁₅); PdS+HS^–+H⁺=Pd (HS) ₂ ⁰ (K₁₆); PtS₂+H₂=Pt (HS) ₂ ⁰ (K₂₁).With equilibrium constants determined as follows (errors represent two standard deviations): Preliminary measurements of the solubilities of metallic Pt, Pd and Au as hydroxide complexes were also conducted using a second titanium pressure vessel, at temperatures of 200° to 350 °C and vapor saturation pressure, with pH and the oxidation state controlled or buffered by adding known amounts of NaOH and H₂ gas. The concentration of NaOH was in the range 0.01–1.3 m, and the partial pressure of H₂ at 200 °C was 62–275 bars, initially. Under the temperature and pressure conditions of these experiments, the solubility of platinum in 1 m NaOH solution is less than 100 ppb, that of palladium is less than 10 ppb and that of gold is less than 0.2 ppm; and in 0.01 m NaOH solutions, both Pt and Pd solubilities are less than 1 ppb. These data indicate that the contributions of hydroxide complexes to the total solubilities in the bisulfide runs, where the pH was in the range of 5.9–9.4, are negligible. The concentrations of both Pt and Pd as bisulfide complexes in the Salton Sea geothermal system predicted using the stability constants determined in this work agree very well with those values measured by McKibben et al. (1990). This calculation strongly suggests that the PGE are transported in moderately reducing, near neutral hydrothermal fluids as bisulfide complexes, as is gold. However, the much lower maximum solubility of the PGE relative to gold severely constrains models of re genesis, and may explain the relative rarity of hydrothermal PGE deposits compared to the relative abundance of hydrothermal Au deposits.     

Dislocation generation, slip systems, and dynamic recrystallization in experimentally deformed plagioclase single crystals

Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5–8.7×10⁻⁷ s⁻¹. The starting material is effectively dislocation-free so that all observed defects were introduced during the experiments. Two samples were shortened normal to one of the principal slip planes (010), corresponding to a “hard” orientation, and one sample was deformed with a Schmid factor of 0.45 for the principal slip system [001](010), corresponding to a “soft” orientation. Several slip systems were activated in the “soft” sample: dislocations of the [001](010) and 110(001) system are about equally abundant, whereas 110{111} and [101] in ( 31) to ( 42) are less common. In the “soft” sample plastic deformation is pervasive and deformation bands are abundant. In the “hard” samples the plastic deformation is concentrated in rims along the sample boundaries. Deformation bands and shear fractures are common. Twinning occurs in close association with fracturing, and the processes are clearly interrelated. Glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation. Grain boundaries of tiny, defect-free grains in healed fracture zones have migrated subsequent to fracturing. These grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization. Nucleation via small fragments can explain a non-host-controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi-brittle to plastic transition.     

The distribution of Hg, Ba, Cu and Zn in the vicinity of cupriferous sulfide deposits, Troodos complex, Cyprus

A geochemical rock- and soil-sampling program was carried out in the vicinity of eight concealed “Cyprus type” deposits, occurring in marginal mafic to intermediate metapillow lavas of the Troodos Ophiolite Complex. The mineralization of massive and stockwork sulfide ore is characterized by the predominance of pyrite, intergrown with less chalcopyrite and minor amounts of sphalerite.Background values of Hg are in the range of 8–12 ppb for soils and 3–6 ppb for surface rocks. Anomaly/background ratios of 10:1 (soils) and 5:1 (rocks) have been found only, where Hg migrated along channels formed by faults cutting shallow-seated mineralization. Here, Hg sometimes shows significant correlations with Cu, Zn, Ba and exceptionally with Co. However in one case an Hg anomaly in soils and surface rocks was detected directly over a deposit. The use of Hg as indicator element for these types of deposits is therefore limited. Buried mineralization may be delineated more distinctly by Cu, Zn and Ba.     

胶东三山岛金矿床黄铁矿显微结构和微量元素特征:对金富集机制的指示

三山岛金矿床是位于胶东金矿集区西北部的超大型破碎带蚀变岩型金矿床.该矿床细致的矿相学及元素地球化学研究尚有不足,限制了对其金富集机制及过程的理解.基于野外地质调查和室内矿相学研究将该矿床划分为4个成矿阶段:石英?绢云母?黄铁矿阶段(I)、石英?金?黄铁矿阶段(I I)、石英?金?多金属硫化物阶段(I I I)和碳酸盐?...     

The Archean BIF-hosted Cuiabá Gold deposit, Quadrilátero Ferrífero, Minas Gerais, Brazil

The Cuiabá Gold Deposit is located in the northern part of the Quadrilátero Ferrífero, Minas Gerais State, Brazil. The region constitutes an Archean granite–greenstone terrane composed of a basement complex (ca. 3.2 Ga), the Rio das Velhas Supergroup greenstone sequence, and related granitoids (3.0–2.7 Ga), which are overlain by the Proterozoic supracrustal sequences of the Minas (< 2.6–2.1  Ga) and Espinhaço (1.7 Ga) supergroups.The stratigraphy of the Cuiabá area is part of the Nova Lima Group, which forms the lower part of the Rio das Velhas Supergroup. The lithological succession of the mine area comprises, from bottom to top, lower mafic metavolcanics intercalated with carbonaceous metasedimentary rocks, the gold-bearing Cuiabá-Banded Iron Formation (BIF), upper mafic metavolcanics and volcanoclastics and metasedimentary rocks. The metamorphism reached the greenschist facies. Tectonic structures of the deposit area are genetically related to deformation phases D₁, D₂, D₃, which took place under crustal compression representing one progressive deformational event (E_n).The bulk of the economic-grade gold mineralization is related to six main ore shoots, contained within the Cuiabá BIF horizon, which range in thickness between 1 and 6 m. The BIF-hosted gold orebodies (> 4 ppm Au) represent sulfide-rich segments of the Cuiabá BIF, which grade laterally into non-economic mineralized or barren iron formation. Transitions from sulfide-rich to sulfide-poor BIF are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide-poor portions. The deposit is “gold-only”, and shows a characteristic association of Au with Ag, As, Sb and low base-metal contents. The gold is fine grained (up to 60 μm), and is generally associated with sulfide layers, occurring as inclusions, in fractures or along grain boundaries of pyrite, the predominant sulfide mineral (> 90 vol.%). Gold is characterized by an average fineness of 0.840 and a large range of fineness (0.759 to 0.941).The country rocks to the mineralized BIF show strong sericite, carbonate and chlorite alteration, typical of greenschist facies metamorphic conditions. Textures observed on microscopic to mine scales indicate that the mineralized Cuiabá BIF is the result of sulfidation involving pervasive replacement of Fe-carbonates (siderite–ankerite) by Fe-sulfides. Gold mineralization at Cuiabá shows various features reported for Archean gold–lode deposits including the: (1) association of gold mineralization with Fe-rich host rocks; (2) strong structural control of the gold orebodies, showing remarkable down-plunge continuity (> 3 km) relative to strike length and width (up to 20 m); (3) epigenetic nature of the mineralization, with sulfidation as the major wall–rock alteration and directly associated with gold deposition; (4) geochemical signature, with mineralization showing consistent metal associations (Au–Ag–As–Sb and low base metal), which is compatible with metamorphic fluids.     

The noble metal distribution in the black shales of the Degdekan gold deposit in Northeast Russia

The mineral and chemical composition of the carbon-bearing rocks of the Late Permian Pionerskaya Formation containing the Degdekan gold deposit has been studied. The bulk contents of Au, Ag, Pt, and Pd in the black shales and their light, sulfide, and electromagnetic fractions were determined by electrothermal atomization. The mineral composition and the phase analysis of the rocks were studied using a scanning electron microscope. Gold is present as fine xenomorphic grains of high fineness with an Fe admixture of up to 4 at %, as well as intergrowths of kustelite and electrum. The Au and Pt contents in the black shales and ores vary in a wide range (g/t): Au 0.01–13.12, Pt 0.001–1.34. The highest Au contents (up to 1748 g/t) were noted in the sulfide fraction. The Pt-bearing phases were not found, whereas a Pt content of about 0.61 wt % was determined using an electron microscope in a carbonaceous matrix. The initial rocks have a steady and low Pt content (less than 0.007 g/t). A stable even Au distribution in the studied rocks was established within 1.14–2.46 g/t. The chemical analysis of the soluble fraction of the carbonaceous matter extracted from the black shales showed the presence of Au 0.375, Ag 3.68, Pt 0.147, and Pd 0.052 g/t. It has been concluded that the carbon-bearing rocks of the Pionerskaya Formation play a resource role in the accumulation of noble metals, whereas economic concentrations of the latters are formed in the course of the superimposed metamorphic-hydrothermal processes.