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1.
The Cenozoic lacustrine basin of Anloua (Adamaoua, Cameroon), is filled with the weathering products of the surrounding rocks. In order to determine the origin of these sediments, multi-element, especially trace element, analysis of their < 2 μm fraction was undertaken. The chemical compositions of these fractions were compared with those of the rocks surrounding the basin by multivariate statistical analysis and also graphically. A principal component analysis allowed the most discriminant elements to be selected, and a hierarchical ascending classification (HAC) made with these elements divided the samples into groups. Some of these groups contain < 2 μm fractions of both the lake sediments and surrounding rocks. These sediments are determined as deriving by weathering from the surrounding rocks. Some of the other groups contain no samples from the surrounding rocks and their petrographic origin cannot be given. A graphic comparison of the relative amounts of the elements (determined by HAC) from the < 2 μm fractions of the various groups and the rocks surrounding the basin allowed attribution of a petrographic origin for the various sediments analysed by using the fit between the curves.
The clay mineralogy, the amounts of Ti, Zr, and heavy minerals in the < 2 μm fractions and their distribution in the area studied, as well as the presence of angular quartzite pebbles in some sediments gives an indication of the paleogeography of the Anloua basin. There was deltaic input towards the north and a high energy input, possibly torrential, to the west. 相似文献
2.
Mark E. Hodson 《Chemical Geology》2002,190(1-4):91-112
Far-from-equilibrium batch dissolution experiments were carried out on the 2000–500, 500–250, 250–53 and 53–2 μm size fractions of the mineral component of the B horizon of a granitic iron humus podzol after removal of organic matter and secondary precipitates. The different size fractions were mineralogically and chemically similar, the main minerals present being quartz, alkali and plagioclase feldspar, biotite and chlorite. Specific surface area increased with decreasing grain size. The measured element release rates decreased in the order 53–2>>>2000–500>500–250>250–53 μm. Surface area normalised element release rates from the 2000–500, 500–250 and 250–53 μm size fractions (0.6–77×10−14 mol/m2/s) were intermediate between literature reported surface area normalised dissolution rates for monomineralic powders of feldspar (0.1–0.01×10−14 mol/m2/s) and sheet silicates (100×10−14 mol/m2/s) dissolving under similar conditions. Element release rates from the 53–2 μm fraction (400–3000×10−14 mol/m2/s) were a factor of 4–30 larger than literature reported values for sheet silicates. The large element release rate of the 53–2 μm fraction means that, despite the small mass fraction of 53–2 μm sized particles present in the soil, dissolution of this fraction is the most important for element release into the soil. A theoretical model predicted similar (within a factor of <2) bulk element release rates for all the mineral powders if observed thicknesses of sheet silicate grains were used as input parameters. Decreasing element release rates with decreasing grain size were only predicted if the thickness of sheet silicates in the powders was held constant. A significantly larger release rate for the 53–2 μm fraction relative to the other size fractions was only predicted if either surface roughness was set several orders of magnitude higher for sheet silicates and several orders of magnitude lower for quartz and feldspars in the 53–2 μm fraction compared to the other size fractions or if the sheet silicate thickness input in the 53–2 μm fraction was set unrealistically low. It is therefore hypothesised that the reason for the unpredicted large release rate from the 52–3 μm size fraction is due to one or more of the following reasons: (1) the greater reactivity of the smaller particles due to surface free energy effects, (2) the lack of proportionality between the BET surface area used to normalise the release rates and the actual reactive surface area of the grains and, (3) the presence of traces quantities of reactive minerals which were undetected in the 53–2 μm fraction but were entirely absent in the coarser fractions. 相似文献
3.
R. Beeson 《Journal of Geochemical Exploration》1984,22(1-3)
Partitioning of Cu, Pb, Zn, U, As and Mo between the minus 70 μm and minus 200 μm fractions of stream sediments from arid and semi-arid terrains is examined in the light of published case histories supplemented by new data. The advantages of selecting a particular fraction for routine sampling in such arid environments are assessed in terms of five criteria: (1) homogeneity of background population; (2) definition of threshold; (3) absolute element abundance levels; (4) contrast between anomalous and background populations; and (5) length of dispersion train. The most homogeneous background population distributions and improved definition of the threshold between background and anomaly occur in the very fine, minus 70 μm fraction of stream sediments for the majority of elements, in particular for Zn, Cu, U and As. Data for Pb and Mo do not consistently favour either size fraction in the case histories studied. Increased abundance levels of elements which are normally close to the analytical detection limit (U, Mo, As) occur most frequently in the minus 70 μm fraction, although Cu, Pb and Zn levels are commonly higher in the coarser fraction. In addition the finer size fraction better defines the anomalous population and provides the longer dispersion trains for Cu, Pb, Zn, U and As in the majority of case histories.The data examined indicate that the minus 70 μm fraction provides more useful information, in many instances, than the minus 200 μm fraction. The evidence suggests that problems expected with the use of the fine fraction — dilution through the abundance of wind blown material, and insufficient fine sediment — do not restrict the use of this fraction in stream sediment surveys in arid terrains. 相似文献
4.
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. 相似文献
5.
The grain size composition of detrital sediments in Hole U1438A from the Amami Sankaku Basin(ASB) in the northwest of the Philippine Sea since the last 350 ka was analyzed. The result shows that the mean grain size of the detrital sediment is about 13.1 μm, ranging from 0.04 to 160 μm. The grain size distribution displays a four-peak pattern and positive skewness. Four independent grain size components were separated by using Weibull distribution function. The ultra-fine component varies from 0.04 to 0.9 μm, with a size mode at about 0.3 μm, which may be genetically related to marine authigenetic clay. The fine-grained fraction ranges from 0.2 to 32 μm, with a size mode at about 3.5 μm, and slightly coarser than the eolian dust of the North Pacific. We argued that this fraction was mainly derived from Asian dust. The coarse-grained and ultra-coarse-grained fractions show distinct size mode at about 10 μm and 40 μm, and range from 0.3 to 90 μm, and from 3 to 160 μm respectively. Both the coarse and ultra-coarse components represent volcanic materials which were mainly derived from the ridges and islands around ASB. The variation of the ratio of environmentally sensitive size population 1.3~2.2 μm/28~40 μm was similar with the ratio of fine-sized component (Asian dust) and coarse-sized component (volcanic material) (0.9~3 μm/>10 μm), showing higher value during glacial period than that during interglacial, which is also identical with the variation of the mass accumulation of eolian dust in the North Pacific and Chinese Loess Plateau, and grain size in Chinese Loess Plateau. The increased ratio responded to the enhanced aridity of Asian continent and strengthened East Asia Winter Monsoon (EAWM)/westerly during glacial period. We argued that the increase of eolian fraction was driven by the enhanced aridity of Asian continent and strengthened East Asia Winter Monsoon (EAWM)/westerly during glacial period. Therefore, the ratio of 0.9~3 μm/>10 μm can be used as a proxy of the increased aridity and enhanced atmospheric circulation of Asian continent. These results suggest that the grain size composition of the detrial sediment in the ASB can be used to reconstruct the history of Asian aridity and atmospheric circulation. 相似文献
6.
The organic matter of recent deltaic sediments cored in the Mahakam delta, East Kalimantan, has been studied before and after physical fractionation into sands >50 μm, silts 5–50 μm and clays <5 μm. Both the lipid and non-lipid components have been investigated.Weight, carbon and nitrogen fractionation budgets were used to define three types of samples, depending on coarse particle contributions to the total amount of organic matter: a = a first type with more than 50% of the O.M. in the coarse particles, high C/N ratios and O.M. content, b = an intermediary type with medium C/N ratios and O.M. content, each fraction having quite the same O.M. content, c = a third type with less than 5% of the whole O.M. in the sands and the lowest C/N ratios and O.M. content.Concerning the global organic characteristics of the fractions, a systematic increase of C/N ratios occurs when going from clays to sands; the finer the fraction is, the more nitrogenous the compounds are. This enrichment in nitrogen is related to a persistent high rate of hydrolysable material either for argillaceous organic matter-poor sediments or for the clay fractions of all types of samples. Conversely, the type (a) coarse sediments, in particular the sandy components were resistant to acid hydrolysis with burial.Concerning the geochemical markers signatures of granulometric fractions, the distribution patterns of n-alkanes and n-fatty acids are characterized by the predominance of high molecular weight compounds >C22. Carbon preference index (CPI) values are higher in the sands and silts, reflecting their enrichment in continentally-derived vegetation debris. For type (c), the fractionation revealed markers of microbial activity within the clay fractions. For all types of samples, we observed an increase with burial of the n-alkane and n-fatty acid concentrations, particularly in the clay fractions, suggesting possibly a better preservation and/or affinity of lipids with the finest fractions. 相似文献
7.
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. 相似文献
8.
The distribution of Si, Al, Fe, Mn, Cu, Zn, Ni and Cr in different grain-size fractions and geochemical association of Fe,
Mn, Cu and Zn with <63-μm size fraction of bed sediments of Damodar River has been studied. In general, concentrations of
heavy metals tend to increase as the size fractions get finer. However at two sites, near mining areas, the coarser particles
show similar or even higher heavy metal concentrations than finer ones. The higher residence time and/or presence of coarser
particles from mining wastes are possibly responsible for higher metal content in the coarser size fractions. The chemical
fractionation study shows that lithogenic is the major chemical phase for heavy metals. Fe and Mn are the major elements of
the lithogenic lattice, constituting 34–63% and 22–59%, respectively, of total concentrations. Fe-Mn oxide and organic bound
fractions are significant phases in the non-lithogenic fraction. The carbonate fraction is less significant for heavy metal
scavenging in the present environment and shows the following order of abundance Zn>Cu>Mn>Fe. The exchangeable fraction of
the Damodar sediments contains very low amounts of heavy metals suggesting poor bioavailability of metals.
Received: 18 August 1998 · Accepted: 1 December 1998 相似文献
9.
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. 相似文献
10.
W.F. Ruddiman G.A. Jones T.-H. Peng L.K. Glover B.P. Glass P.J. Liebertz 《Sedimentary Geology》1980,25(4):257-276
Bioturbation is generally not a strongly size-dependent or shape-dependent process in deep-sea sediments. Different sizes of ash (11–250 μm) deposited during a single eruption are usually mixed about equally, although in a few cores we found minor tendencies toward greater mixing of finer ash. Platey volcanic ash and spherical microtektites initially deposited within a few centimeters of each other in the sediment column are mixed with roughly comparable intensity. Radiocarbon dating of three sizes of CaCO3 near the top of one box core produced ages within the analytical error; this also suggests no size dependency in the mixing process. Another core showed age differences opposite in sense to those that would be predicted if the finer fractions were more intensely mixed than the coarser fractions.The sizes examined (11–250 μm) encompass most of the sedimentary components used in paleoclimatic analyses of deep-sea cores. From these findings, we infer that significant artificial lead/lag offsets in paleoclimatic signals will not be created by size-selective or shape-selective mixing. Mixing models calibrated on one sediment size fraction should be generally applicable to other sizes and kinds of material. 相似文献
11.
Pulverized coals from eleven power plants burning Central Appalachian coal blends and eight power plants burning Illinois Basin coal blends were studied in order to assess the petrographic nature of industrial-scale coal grinding. All coals were high volatile bituminous. Coals were wet screened at 100 (150 μ), 200 (75 μ), 325 (about 40 μ), and 500 (about 25 μ) mesh. Petrographic analysis of the whole coals and size fractions consisted of a combined maceral and microlithotype analysis. Microlithotype analysis, in particular, provides a reasonable approximation of the whole-particle composition at the scale of utility coal pulverization. In the size fractions, duroclarite, the most abundant trimaceral microlithotype, is most abundant in the coarsest fraction and least abundant in the finest fraction. Vitrite, the most abundant monomaceral microlithotype, exhibits the opposite trend. Duroclarite becomes more enriched in vitrinite towards the finer sizes. The partitioning of microlithotypes and the partitioning of macerals within the microlithotypes is indicative of the relative brittle nature of vitrite compared to the hard-to-grind trimaceral microlithotypes. Increased vitrinite in duroclarite is an indication that the microlithotype within the particular size fraction is more brittle than relatively vitrinite-depleted duroclarite in coarser fractions. The relative grindability of microlithotypes will, in turn, impact combustion efficiency. 相似文献
12.
Atsushi Noda Hajime Katayama Tsumoru Sagayama Kazuya Suga Yasuhito Uchida Kenji Satake Kohei Abe Yukinobu Okamura 《Sedimentary Geology》2007,200(3-4):314-327
A combined approach of field geology and numerical simulation was conducted for evaluating the tsunami impacts on the shelf sediments. The 2003 Tokachi-oki earthquake, M 8.0, that occurred on 25 September 2003 off southeastern Hokkaido, northern Japan, generated a locally destructive tsunami. Maximum run-up height of the tsunami waves reached 4 m above sea level. In order to estimate the tsunami impacts on shallow marine sediments, we compared pre- and post-tsunami marine sediments in water depths of 38–112 m in terms of grain size, sedimentary structure, and microfossil content. Decreases of fine fractions, especially finer than very fine sand, which led to coarsen the mean grain size, were detected in the inner shelf of the northern part of the study area. Foraminiferal assemblages also changed in the coarsened sediments. On the other hand, the other shelf sediments largely unchanged or slightly fined. We also simulated the tsunami wave velocity and direction, and grain size entrained by the modeled tsunami. The numerical simulation resulted in that the 2003 tsunami could transport very fine sand in water depths shallower than 45–95 m at the northern part of the study area. This is comparable with the actual grain-size changes after the tsunami had passed. However, some storms and tidal currents might also be possible to stir the surface sediments after the pre-tsunami survey, so we could not conclude that the grain-size changes had been caused only by the tsunami. Nevertheless, a combined approach of sampling and modeling was powerful for estimating the tsunami impacts under the sea. 相似文献
13.
E. Molinaroli S. Guerzoni G. De Falco A. Sarretta A. Cucco S. Como S. Simeone A. Perilli P. Magni 《Sedimentary Geology》2009,219(1-4):196-207
A comparison was made of shallow water sediments from the Lagoon of Venice (LV) and the Lagoon of Cabras (LC), comparing depositional environments and exploring the relationships between hydrodynamics and sedimentological parameters. The two water bodies are very different in size (LV: 360 km2; LC: 22 km2), and the sediments predominantly consist of silty-clay (LV: Mz ≈ 26 μm; LC: Mz ≈ 6 μm). However, there are large differences between the two lagoons with respect to sand (LV: mean 19%; LC: mean ~ 3%) and clay (LV: mean 20%; LC: mean 45%) contents. The Lagoon of Venice (mean depth ~ 1 m) can be considered a tidal basin, whereas the Lagoon of Cabras (mean depth ~ 2 m) has the character of a coastal lake in which wind is the main hydrodynamic forcing factor. A comparison of sediment grain-size distributions with water circulation patterns in different parts of the lagoons highlighted some interesting differences. Grain-size analyses of samples reveal a deficiency of particles around 8 μm in the LC, which is interpreted as reflecting the transition between cohesive flocs/aggregates and non-cohesive coarser silt particles, while the transition limit in the LV is ~ 20 μm. Thus, particles are cohesive below 8 μm in the LC and below ~ 20 μm in the LV. This is probably because of the differences in the clay/silt ratio, which is much lower in the LV (~ 0.3) than in LC (~ 1), conferring a “silt-dominated network structure” on most of the LV sediments.The hydrographical data used were root mean square velocity (RMSV) and water residence time (WRT), computed under the main forcing conditions. The results show a general correlation between RMSV and sortable silt in the LC, and between RMSV and coarser sediments (63–105 μm) in the LV. Some significant differences between the lagoons were detected in the degree of correlation between WRT and grain size. Root mean square velocity (~ 7 cm s− 1 in the LV and ~ 3 cm s− 1 in the LC) was a greater forcing factor in the LC than in the LV. Conversely, WRT, which is on average ~ 16 days in the LV and ~ 19 days in the LC, has more influence in the LV. This study highlights the usefulness of comparing environments with different hydrodynamic energies, e.g., tidal and/or wind-driven currents, to elucidate and thereby improve our understanding of the processes governing the spatial distribution of sedimentological features, the transport mechanisms of sediments, and the relationship between them. The results demonstrate that the approach outlined in this study has the potential to provide a universal hydro-sedimentological classification scheme. 相似文献
14.
Orientation studies were carried out by the BRGM mission in Saudi Arabia between 1982–1986 to investigate the dispersion of gold in soils and wadi sediments and to define the optimum sample medium around eight prospects in the central Arabian pediplain. A comparison of gold distribution in several sieved fractions with various pedologic horizons shows that the distribution of gold changes abruptly from the coarser sizes near the source to the finest sizes 150 m downstream.The coarse fraction recommended by previous workers is not representative and yields erratic results in the specific environment of the pediplain. Gold is enriched and more homogeneously distributed in the minus 80 μm fraction of the skeletal soils and wadi sediments.Regional geochemical survey can be effective using a sensitive analytical method for gold and the minus 80 μm fraction of the brown gravelly sediment with a minimum density of 2 samples per km2. Geochemical halos in the 30–50 ppb range indicate gold mineralization 500 to 1000 m upstream, depending on relief.In a first follow-up stage, continued use of the same size of the upper argillaceous layer with a regular reg sampling grid gives more contrasted and more extensive anomalies than using a coarse material. These anomalies may be slightly offset by the present arid erosion. A second follow-up stage is then recommended at a closer grid and sampling the brown blocky layer below the surficial reg pavement. Again, the use of the finest fraction at this exploration stage has given the best probability of finding a blind gold target.Optical determination of gold on a nonmagnetic fraction of heavy panconcentrate is not recommended, because nugget growth is practically absent in the present arid conditions and flour-sized gold particles are lost in desliming samples. 相似文献
15.
Radiocaesium (137Cs) dispersion and Cs+ fixation were studied in the sediments collected from the lagoon systems of “Ria de Aveiro”. The Cs+ sorption was tested for the fine mica grains and for the < 2 μm clay fractions extracted from silty clays. The Cs+ exchange is found strongly onto mica-rich fractions than smectite-rich fractions. The distribution coefficient increases if the silty material is constituted by rich-mica clay fractions or if the non-clay minerals are removed from the silty-clay material. The samples studied behave as multisite ion exchange, where Cs+ engages in ion-exchange reactions with hydrated cations on planar sites on expansible layer silicates. Higher concentrations of the 137Cs were found associated with mica-rich silty clays. The 137Cs ranges from 3.2 to 3.9 Bq kg− 1 in the < 38 μm fractions and from 2.9 to 3.3 Bq kg− 1 in the < 64 μm fractions. 相似文献
16.
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. 相似文献
17.
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. 相似文献
18.
A regional geochemical and mineralogical study aimed at investigating the mineralization in the western-part of the Walawe Ganga (river) Basin in Sri Lanka is represented in this paper. The river basin is the 3rd largest in the country and has within it a boundary zone between two geologically different crustal blocks, which are marked by granulitic grade rocks and amphibolite grade rocks. Size fractions of stream sediments (< 63 μm; 63–125 μm; 125–177 μm and 177–250 μm) developed on the granulite-grade metamorphic terrain have been analysed at their source for their mineralogical and selected element compositions. Thirty-eight (38) sediment samples and 15 representative probable parent rock samples were chemically analysed giving special emphasis to the High Field Strength trace Elements (HFSE) including the Rare Earth Elements (REE). The granulite grade rocks in the study area is geochemically similar to that of post Archean upper crust. However the stream sediments developed from the high-grade rocks during the intense weathering, are markedly enriched with HFSE and REE. The enrichment of HFSE and LREE is accounted for by the presence of HFSE- and REE- rich accessory mineral phases such as zircon, monazite, apatite, garnet and rutile in the sediments. In some samples, the content of heavy minerals contributes as much as 50 wt. %. These minerals act as a source of elements in the sediments. However, extreme hydraulic sorting of HFSE- and REE-bearing minerals during the sediment deposition cannot be expected within a short distance from near the sources except from a mineralized occurrence. Therefore, the higher enrichment of these elements presumably indicates occurrences of scattered mineral sources such as highly differentiated granites and associated pegmatites within the Walawe Ganga drainage basin. These granitic pegmatites are probably intruded during or soon after the main granulite-facies metamorphic event and similar events are seen in other terrains of East-Gondwana. 相似文献
19.
为探讨构造稳定地区(如黄土高原)和构造活跃地区(如青藏高原东缘)粉尘沉积物中磁化率(SUS)与粒度的相关性及其对环境事件的指示意义,本次研究分析了黄土高原蓝田剖面黄土—古土壤样品和青藏高原东缘湖相沉积样品的粒度和磁化率记录。黄土高原黄土—古土壤沉积SUS与2~10 μm粒度组分最强正相关,青藏高原东缘湖相沉积的SUS与2~10 μm粒度组分最强负相关,反映2~10 μm粒度组分为黄土高原和青藏高原乃至亚洲干旱—半干旱地区连续稳定敏感的背景沉积组分。黄土高原黄土—古土壤沉积的SUS与32~63 μm粒度组分最强负相关,青藏高原东缘湖相沉积的SUS与32~63 μm粒度组分最强正相关,反映32~63 μm粒度组分不仅是黄土高原尘暴事件沉积的敏感指标,也是青藏高原东缘湖相沉积记录的地震事件敏感指标。SUS与粒度组分的相关性在青藏高原东缘地区地震事件层开始部分高于结束部分,也较好地反映地震事件为研究区添加新鲜沉积物后随地形地貌恢复逐步减少的过程。SUS与粒度组分相关性也受当地物源的影响。 相似文献
20.
为探讨构造稳定地区(如黄土高原)和构造活跃地区(如青藏高原东缘)粉尘沉积物中磁化率(SUS)与粒度的相关性及其对环境事件的指示意义,本次研究分析了黄土高原蓝田剖面黄土—古土壤样品和青藏高原东缘湖相沉积样品的粒度和磁化率记录。黄土高原黄土—古土壤沉积SUS与2~10 μm粒度组分最强正相关,青藏高原东缘湖相沉积的SUS与2~10 μm粒度组分最强负相关,反映2~10 μm粒度组分为黄土高原和青藏高原乃至亚洲干旱—半干旱地区连续稳定敏感的背景沉积组分。黄土高原黄土—古土壤沉积的SUS与32~63 μm粒度组分最强负相关,青藏高原东缘湖相沉积的SUS与32~63 μm粒度组分最强正相关,反映32~63 μm粒度组分不仅是黄土高原尘暴事件沉积的敏感指标,也是青藏高原东缘湖相沉积记录的地震事件敏感指标。SUS与粒度组分的相关性在青藏高原东缘地区地震事件层开始部分高于结束部分,也较好地反映地震事件为研究区添加新鲜沉积物后随地形地貌恢复逐步减少的过程。SUS与粒度组分相关性也受当地物源的影响。 相似文献