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1.
《Applied Geochemistry》2000,15(9):1369-1381
Thirty-eight samples of stream sediments draining high-grade metamorphic rocks in the Walawe Ganga (river) Basin, Sri Lanka, were analysed for their REE contents, together with samples of metamorphic suites from the source region. The metamorphic rocks are enriched in light REE (LREE) compared to heavy REE (HREE) and are characterised by high La/Lu ratios and negative Eu anomalies. The chondrite-normalised patterns for these granulite-grade rocks are similar to that of the average post-Archaean upper crust, but they are slightly enriched with La and Ce. The REE contents of the <63-μm fraction of the stream sediments are similar to the probable source rocks, but the other grain size fractions show more enriched patterns. The <63-μm stream sediments fraction contains lower total REE, more pronouncd negative Eu anomalies, higher EuN/SmN and lower La N/LuN ratios relative to other fractions. The lower La N/LuN ratio is related to the depletion of heavy minerals in the <63-μm fraction. The 63–125-μm and 125–177-μm grain size fractions of sediments are particularly enriched in LREE (average ΣLREE=2990 μg/g and 3410 μg/g, respectively). The total HREE contents are surprisingly uniform in all size fractions. However, the REE contents in the Walawe Ganga sediments are not comparable with those of the granulite-grade rocks from the source region of the sediments. The enrichment of REE is accounted for by the presence of REE containing accessory mineral phases such as zircon, monazite, apatite and garnet. These minerals are derived from an unknown source, presumably from scattered bodies of granitic pegmatites.  相似文献   

2.
Geochemical variations in stream sediments (n = 54) from the Mahaweli River of Sri Lanka have been evaluated from the viewpoints of lithological control, sorting, heavy mineral concentration, influence of climatic zonation (wet, intermediate, and dry zones), weathering, and downstream transport. Compositions of soils (n = 22) and basement rocks (n = 38) of the catchment and those of <180 μm and 180–2000 μm fractions of the stream sediments were also examined. The sediments, fractions, soils and basement rocks were analyzed by X-ray fluorescence to determine their As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc, Fe2O3, TiO2, MnO, CaO, P2O5 and total sulfur contents. Abundances of high field strength and ferromagnesian elements in the sediments indicate concentration of durable heavy minerals including zircon, tourmaline, rutile, monazite, garnet, pyriboles, and titanite, especially in <180 μm fractions. The sediments show strong correlation between Ti and Fe, further suggesting presence of heavy mineral phases containing both elements, such as ilmenite and magnetite. The basement rocks range from mafic through to felsic compositions, as do the soils. The river sediments lack ultrabasic components, and overall have intermediate to felsic compositions. Elemental spikes in the confluences of tributary rivers and high values in the <180 μm fractions indicate sporadic inputs of mafic detritus and/or heavy minerals to the main channel. Al2O3/(K2O + Na2O) and K2O/Na2O ratios of the sediments and LOI values of the soils correlate well with the climatic zones, suggesting intense weathering in the wet zone, lesser weathering in the intermediate zone, and least weathering in the dry zone. Low Sr and CaO contents and Cr/V ratios in stream sediments in the wet zone also suggest climatic influence. Fe-normalized enrichment factors (EFs) for As, Pb, Zn, Cu, Ni and Cr in stream sediments in the main channel are nearly all <1.5, indicating there is no significant environmental contamination. The chemistry of the sediments, rocks and the soils in the Mahaweli River are thus mainly controlled by source lithotype, weathering, sorting, and heavy mineral accumulation.  相似文献   

3.
《Applied Geochemistry》2000,15(5):629-646
Stream waters and sediments draining a gossan tailings pile at the Murray Brook massive sulphide deposit were collected to investigate Au mobility. Weathering of the massive sulphides at Murray Brook during the Late Tertiary period resulted in the concentration of Au in the gossan cap overlying the supergene Cu and unoxidized massive sulphide zones of the deposit. The gossan was mined between 1989 and 1992, and Au and Ag were extracted using a cyanide vat leach process. Although stream sediments prior to mining had Au<5 ppb (the detection limit), sediments collected in 1997 had Au contents ranging up to 256 ppm with values up to 6 ppm more than 3 km downstream from the deposit. Dissolved Au contents were similarly anomalous, up to 19 μg/L and in excess of 3 μg/L 3 km downstream. The elevated Au contents in the waters and sediments are interpreted to reflect complexation of Au (as Au(CN)2−) by cyanide hosted within the gossan tailings pile. Precipitation recharges through the tailings pile with groundwater flow exiting to Gossan Creek. Degradation of cyanide along the flow path and within Gossan Creek allows colloidal Au to form via reduction of Au(I) by Fe2+, consistent with SEM observations of Au as <1 μm subrounded particles. In the surface waters, the majority of the Au must be in a form <0.45 μm in size to account for the similarity in Au contents between the <0.45 μm and unfiltered samples. The very elevated stream sediment Au values close to the headwaters of Gossan Creek near the tailings indicate that upon exiting to the surface environment, Au(CN)2− complexes are rapidly destroyed and Au removed from solution. However, the high Au<0.004 μm/Autotal in the headwaters and the extended Au dispersion in Gossan Creek waters and sediments suggest that Au(CN)2− complexes persist for the full length of Gossan Creek. The decrease in aqueous Au which is less than 0.004 μm indicates that Au is converted from a complexed form to a colloidal form with increasing distance downstream, consistent with dissolved NO3− contents which decrease from 5210 μg/L near the headwaters to 1350 μg/L at the lower end of the stream.  相似文献   

4.
This study compares the geochemical response of stream sediments and adjacent vegetation samples, with variations in drainage catchment lithology and the occurrence of mineralisation, within a 14,000-km2 block of the northeastern region of New South Wales, Australia. The area contains a range of lithologies within a Devonian–Permian accretionary complex and Mesozoic sedimentary basin, as well as a wide range of mineral deposits. Sampling was designed to confine each sub-catchment to a single lithological group (mafic and ultramafic rocks, acid intrusives, volcanics, metasedimentary and sedimentary rocks or alluvium). Leaves of over 20 genera, dominated by (Allo-)Casuarina, Eucalyptus, Acacia, Callistemon and Melaleuca, and the <250-μm fraction of the stream sediments were analysed by INAA. The uptake of most trace elements varied between genera, with Callistemon displaying the highest median As and La contents and Eucalyptus the highest Co contents. The stream sediment and vegetation geochemistry reflect both hydromorphic and mechanical dispersion within sub-catchments, with regional patterns dominant over local influences. The vegetation appears to be influenced to a greater extent by hydromorphic dispersion, as indicated by differences in the ratio of leaf to sediment Cr concentrations in sub-catchments draining serpentinites and basalts. Although most known mineral deposits in the region produced anomalies in at least one medium, there was little correlation between the trace element concentrations of the vegetation and stream sediments on a site-by-site basis. A number of Au targets were only detected on the basis of the biogeochemistry and others were only reflected in the stream sediment geochemistry. In general, vegetation displayed more extensive dispersion trains away from mineralisation than did the stream sediments. Differences in the response of the two sampling media suggest their joint use in exploration or environmental surveys to maximise the probability of detecting mineralisation.  相似文献   

5.
Arsenic-bearing stream sediments enter the Upper Isle River, an Au mining-influenced basin (France), by the discharge of mining sites, tailings runoff and weathering of mineralized veins in granites and gneiss. Some fresh ochreous As-rich deposits on the river banks and in floodplains are identified as additional As-rich point sources (As between 0.07 and 6.5 wt.%). The <63 μm fraction of stream sediments contains elevated As bulk concentrations, ranging from 160 to 890 mg/kg, compared to the geochemical background (70 mg/kg on average). It is also enriched in Cd, Hg and W. Spatial variations of these trace elements show 3 significant increases corresponding to the 3 drained mining districts. They decrease down river but are still enriched 30 km downstream of the mining districts due to downstream transportation. Three types of trace element-bearing phases have been identified as: (i) detrital primary sulfides, with high in situ As percentages (up to 43.7 wt.%). They also carry significant amounts of W according to the differences in chemical compositions of the total and light <63 μm fractions. These sulfide particles do not show any sign of alteration in the oxygenated stream sediments; (ii) Secondary Fe–Mn oxyhydroxides,some with very high in situ As2O5 concentrations (up to 59.8 wt.%) and with about 40% of the total Cd composition. They occur as fresh precipitates in the river banks and floodplains and as discrete particles in stream sediments and (iii) Al–Si fine-grained phases. Their major element composition is highly variable with in situ As2O5 concentrations ranging between 430 and 5020 mg/kg. This type of solid phase is also the major carrier of Hg.  相似文献   

6.
《Applied Geochemistry》2001,16(3):339-350
This study on the application of multi-element relationships in stream sediments to mineral exploration in the Walawe Ganga Basin presents one of the first of its kind in Sri Lanka. In order to determine the nature of these sediments, multi-element analysis of selected grain size (<63 μm, 63–125 μm, 125–177 μm and 177–250 μm) fractions was undertaken. The chemical composition of these fractions were compared with those of the upper continental crust.Some elements, notably Zr, Hf, Th, U, Ce and La, show very high enrichment factors (e.g. 54 for Zr) as compared to the upper crustal abundance. These are presumably associated with heavy minerals such as zircon, rutile and monazite found in abundance in the stream sediments of the Walawe Ganga Basin.Principal Component Analysis (PCA) of the geochemical data show that multi-element relationships could be effectively used to delineate target areas for mineral exploration. With the use of PCA, this study reveals that areas with associations of calc-silicate/marble and charnockitic rocks are probable source regions for mineral occurrences, particularly in the axial regions of anticlines.  相似文献   

7.
《Applied Geochemistry》1999,14(5):621-633
Forms of Pb, Zn and Cd in the different size fractions (<2 μm, 2–53 μm and >53 μm) of waste dumps, stream sediments and surrounding soils from a former Au mine in Korea, were investigated chemically by sequential extraction analysis and mineralogically by XRD and analytical SEM, so as to clarify the relationships between chemical and mineralogical forms. Total concentrations for the waste dumps and the stream sediments range from 655 to 2920 mg/kg for Pb, 565 to 1191 mg/kg for Zn, and 24.4 to 71.4 mg/kg for Cd, while those for the surrounding soils do not exceed the natural background levels. Direct observations on the heavy mineral fractions of the waste dumps and the stream sediments indicates that the primary sphalerite is still the main pool of the Zn and Cd, while a large part of the primary galena has been changed into a carbonate-bound form. This is in a good agreement with the partitioning of chemical forms in the coarse fractions, in which most of the Zn (75.3 to 79.4% for the waste dumps) and Cd (54.8 to 60.1% for the waste dumps) are associated with the oxidizable form, while most of the Pb (68.8 to 71.0% for the waste dumps) is in the acid (NaOAc)-extractable form. On the other hand, the partitioning of metal forms in the clay fraction is characterised by the highest proportion of the reducible form for all metals (56.6 to 73.8% for Pb, 60.2 to 68.4% for Zn, and 27.1 to 36.8% for Cd in the waste dumps), suggesting precipitation of easily to moderately reducible oxides and hydroxides from the other forms during weathering. With the increase of pH, the dramatic changes of the acid-extractable Pb, the oxidizable Zn and Cd in the coarse fractions, and the exchangeable form, especially for Cd in the clay fraction indicate that pH is the prime factor controlling the partitioning of heavy metals.  相似文献   

8.
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   

9.
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   

10.
The abandoned Kilembe copper mine in western Uganda is a source of contaminants, mobilised from mine tailings into R. Rukoki flowing through a belt of wetlands into Lake George. Water and sediments were investigated on the lakeshore and the lakebed. Metal associations in the sediments reflect the Kilembe sulphide mineralisation. Enrichment of metals was compared between lakebed sediments, both for wet and dry seasons. Total C in a lakebed core shows a general increment, while Cu and Co decrease with depth. The contaminants are predominant (> 65%) in the ≤ 63 μm sediment size range with elevated Cu and Zn (> 28%), while Ni, Pb and Co are low (< 18%) in all the fractions. Sequential extraction of Fe for lakeshore sediment samples reveals low Fe mobility. Relatively higher mobility and biological availability is seen for Co, Cu and S. Heavy metal contents in lake waters are not an immediate risk to the aquatic environment.  相似文献   

11.
The Batu Hijau porphyry copper-gold deposit, Sumbawa Island, Indonesia   总被引：4，自引：0，他引：4  
The Batu Hijau porphyry Cu---Au deposit lies in southwest Sumbawa Island, Indonesia. It is a world-class porphyry Cu deposit in an island are setting, and is typical of this deposit type in most features, including igneous association, morphology, hydrothermal alteration and mineralisation style.The region was not previously recognised as a porphyry Cu province; disseminated Cu sulphides were first recognised in float samples in southwest Sumbawa in 1987. Associated stream sediment sampling identified a broad area of anomalous Au and Cu in an area of greater than 5 km2 around Batu Hijau, including 169 ppb Au in BLEG samples and 580 ppm Cu in stream silts 1 km from the deposit. Mineralisation in bedrock at surface contains > 0.1 wt % Cu and > 0.1 ppm Au over an area of 0.6 km × 1.2 km, including a zone 300 m × 900 m containing > 0.3 wt % Cu. Areas with elevated Mo (> 30 ppm) form a distinctive annulus around this Cu-rich zone.Batu Hijau mineralisation is hosted in a tonalite intrusive complex, and diorite and metavolcanic wallrocks. There are no post-mineralisation igneous intrusions or breccia pipes within the deposit. The main tonalite intrusion forms a stock in the centre of the deposit, where it generally displays intensely pervasive potassic (biotite with magnetite-quartz) alteration and hosts most of the higher grade mineralisation. Younger tonalite dykes intruding the centre of this stock are generally less altered and mineralised than the older tonalite.The core zone of potassic alteration grades outward into extensive propylitic alteration (chlorite-epidote), with both variably overprinted by widespread fracture controlled intermediate argillic alteration (sericite-chlorite), and minor phyllic (sericite-pyrite) and sodic (albite) alteration. Argillic (sericite-kaolinite) and advanced argillic (kaolinite-alunite-pyrophyllite) assemblages occur near surface.Copper and Au grades within the orebody show a positive correlation with quartz stockwork intensity, although disseminated Cu sulphides are also common. Chalcopyrite and bornite are the principle hypogenal minerals, with minor chalcocite. Oxidation extends to a depth of 5 m to 85 m below surface across the deposit, and is underlain by weak supergene mineralisation. Drill testing of the deposit down to 650 m below surface reveals a single cylindrical to conical orebody of 334 million tonnes grading 0.8 wt % Cu and 0.69 gm per tonne Au; the depth extent of mineralisation is unknown.  相似文献   

12.
《Applied Geochemistry》2006,21(9):1613-1624
Ingestion of soil is a common behaviour in young children as a means of exploring their surroundings. Much attention has been given to remediation of point-source polluted sites with regard to potential health risks for children. However, because of diffuse pollution and long-range atmospheric deposition, soil contaminant levels are generally increased in urban areas compared to their rural counterparts, even in areas located away from any point sources of pollution. Intake of urban soil can thereby result in significant amounts of the child’s daily metal intake. In the present study, soil samples were collected from 25 playgrounds around urban Uppsala, Sweden and analysed for contents of Al, As, Fe, Cr, Cu, Cd, Hg, Mn, Ni, Pb, W and Zn. Prior to aqua regia digestion, the samples were wet-sieved in order to separate soil particle fractions representing deliberate (<4 mm) and involuntary (<50 μm) soil ingestion by children, as well as a third size fraction of 50–100 μm representing soil that is easily transported by suspension. While the metal and As contents in the 50–100 μm fraction were similar to those of the <4 mm fraction, the <50 μm fraction had metal and As contents on average one and a half times higher than those of the <4 mm fraction. The metal and As contents correlated negatively with the sand content in both particle size fractions <4 mm and 50–100 μm, suggesting a general decrease in metal and As content with increasing sand content. However, a positive correlation was found between sand content and the metal and As contents of the finest fraction (<50 μm), suggesting that when the sand content is high, the bulk of the sorbed elements are on the finest particles. The difference between metal and As contents in the different size fractions was greater in the soil sample with the highest sand content than in the sample with the lowest sand content. This implies that texture is a significant factor in metal and As distribution in soils with moderate metal and As contents, when the number of binding sites associated with small particles is low. Tolerable daily intake (TDI) values for Pb and As were exceeded at all sites, and at two sites for Cd, for children with pica behaviour. A high ingestion rate of mainly small particles could also result in the TDI value for Pb being exceeded at 10 sites and that for As at one site. This study also found that soil analysis by the procedure recommended by Swedish authorities accurately represents the metal intake from deliberate soil ingestion, whereas involuntary soil ingestion of mainly small particles could result in metal intakes which are up to twice as high.  相似文献   

13.
Sixteen hundred stream sediments (<150 μm fraction) collected during regional geochemical surveys in central and SW Nigeria have high median and maximum concentrations of Zr that exceed corresponding Zr concentrations found in stream sediments collected from elsewhere in the World with similar bedrock geology. X-ray diffraction studies on a sub-set of the analysed stream sediments showed that Zr is predominantly found in detrital zircon grains. However, the main proximal source rocks (Pan-African ‘Older Granites’ of Nigeria and their Proterozoic migmatitic gneiss country rocks) are not enriched in zircon (or Zr). Nevertheless, U–Pb LA-ICP-MS dating with cathodoluminescence imaging on detrital zircons, both from stream sediment samples and underlying Pan-African ‘Older Granites’ confirms a local bedrock source for the stream sediment zircons. A combination of tropical/chemical weathering and continuous physical weathering, both by ‘wet season’ flash flooding and ‘dry season’ unidirectional winds are interpreted to have effectively broken down bedrock silicate minerals and removed much of the resultant clay phases, thereby increasing the Zr contents in stream sediments. The strong correlation between winnowing index (Th/Al) and Zr concentration across the study area support this interpretation. Therefore, ‘anomalous’ high values of Zr, as well as other elements concentrated in resistant ‘heavy’ minerals in Nigeria’s streams may not reflect proximal bedrock concentrations of these elements. This conclusion has important implications for using stream sediment chemistry as an exploration tool in Nigeria for primary metal deposits associated with heavy minerals.  相似文献   

14.
The Bell Springs deposit is a bulk-tonnage, low-grade gold deposit, formed in a hot-spring environment, that is hosted by middle Miocene weakly peralkaline, high-silica rhyolite ash flows and rheomorphic tuffs. Ore grade mineralization over about 460 × 460 m was controlled by northeast- and northwest-trending structures. Sixty-nine soil samples collected along a traverse across the Bell Springs deposit, screened into six mesh size fractions, +10, −10+35, −35+80, −80+120, −120+200, and −200, were treated by low-detection-limit acid digestion/organic extraction procedures, and analyzed by ICP or graphite furnace techniques for Au, Ag, As, Bi, Cd, Cu, Ga, Hg, Mo, Pb, Sb, Se, Te, Tl, and Zn. Twenty-five stream-sediment samples were collected from a drainage crossing mineralization and from a nearby tributary. Sediment size fractions of −200 mesh and −10+200 mesh were analyzed by the same procedures used for soils. In addition, analyses of bulk leachable gold via cyanide leach method (BLEG) was done on bulk −10 mesh sediments.Anomalous Au, Ag, As, Sb, and Mo in all soil size fractions revealed the underlying ore. Areas with anomalous Sb and As were about three times larger than areas with anomalous Au and Ag. Analyses of fine-fractions of stream sediments provided the best-defined Au anomalies in drainages. A multi-element factor calculated by multiplying Au, As, Sb, and Mo reinforced anomalous Au in drainages and correlated well with Bell Springs mineralization.  相似文献   

15.
《Applied Geochemistry》2006,21(11):1969-1985
Gossan Creek, a headwater stream in the SE Upsalquitch River watershed in New Brunswick, Canada, contains elevated concentrations of total Hg (HgT up to 60 μg/L). Aqueous geochemical investigations of the shallow groundwater at the headwaters of the creek confirm that the source of Hg is a contaminated groundwater plume (neutral pH with Hg and Cl concentrations up to 150 μg/L and 20 mg/L, respectively), originating from the Murray Brook mine tailings, that discharges at the headwaters of the creek. The discharge area of the contaminant plume was partially delineated based on elevated pH and Cl concentrations in the groundwater. The local groundwater outside of the plume contains much lower concentrations of Hg and Cl (<0.1 μg/L and 3.8 mg/L, respectively) and displays the chemical characteristics of an acid-sulfate weathering system, with low pH (4.1–5.5) and elevated concentrations of Cu, Zn, Pb and SO4 (up to 5400 μg Cu/L, 8700 μg Zn/L, 70 μg Pb/L and 330 mg SO4/L), derived from oxidation of sulfide minerals in the Murray Brook volcanogenic massive sulfide deposit and surrounding bedrock. The HgT mass loads measured at various hydrologic control points along the stream system indicate that 95–99% of the dissolved HgT is attenuated in the first 3–4 km from the source. Analyses of creek bed sediments for Au, Ag, Cu, Zn, Pb and Hg indicate that these metals have partitioned strongly to the sediments. Mineralogical investigations of the contaminated sediments using analytical scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM), reveal discrete particles (<1–2 μm) of metacinnabar (HgS), mixed Au–Ag–Hg amalgam, Cu sulfide and Ag sulfide.  相似文献   

16.
Tracing sources of aeolian sediment is key to reconstructing earth surface processes in arid areas and interpreting the paleoenvironmental significance of aeolian sequences. However, the difference of geochemical characteristics between different fractions of sediments in the Yarlung Zangbo River Basin (YZRB) is still unclear, so we want to identify which fraction of sediments is more suitable for tracing sediment sources. Considering the long distance between different wide valleys in the YZRB, we wonder whether there is any difference between these wide valleys in terms of geochemical characteristics of sediments. Forty-three surface sediment samples in the YZRB are collected, and the grain-size distributions and the major-element composition for 37 samples and the trace and rare earth element composition for the coarse (75–500 μm) and fine (<?75 μm) fractions of all samples are determined. The results reveal the following: (1) the fine fractions of the deposits contain more environmental information, suggesting that the fine fractions cannot be directly compared between different climate zones for provenance identification and that appropriate coarse fractions, based on the grain-size distribution of the targeted sediments, are more ideal for tracing sediment sources; and (2) geochemical characteristics of various sediment types show spatial heterogeneity. The coarse and fine fractions of the loose sediment samples can be divided into two regional groups based on geochemical characteristics: the Maquanhe zone in the upper reach, the Xigaze, Shannan and Mainling zone in the middle reach, which is consistent with the geological background of the YZRB; (3) aeolian deposits in the YZRB are a local origin and predominantly derived from the adjacent loose sediments, and fluvial sediments in the upper reach contribute little to the aeolian sands in the middle reach.  相似文献   

17.
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   

18.
The Albany-Fraser Orogen (AFO), southeast Western Australia, is an underexplored, deeply weathered regolith-dominated terrain that has undergone complex weathering associated with various superimposed climatic events. For effective geochemical exploration in the AFO, integrating landscape evolution with mineralogical and geochemical variations of regolith and bedrock provides fundamental understanding of mechanical and hydromorphic dispersion of ore and pathfinder elements associated with the different weathering processes.In the Neale tenement, northeast of the AFO, a residual weathering profile that is 20-55 m thick was developed under warm and humid climatic conditions over undulating Proterozoic sheared granitoids, gneisses, schists and Au-bearing mafic rocks. From the base, the typical weathering profile consists of saprock, lower ferruginous saprolite, upper kaolinitic saprolite and discontinuous silcrete duricrust or its laterally coeval lateritic residuum. These types of duricrusts change laterally into areas of poorly-cemented kaolinitic grits or loose lateritic pisoliths and nodules.Lateritic residuum probably formed on remnant plateaus and was transported mechanically under arid climatic conditions over short distances, filling valleys to the southeast. Erosion of lateritic residuum exposes the underlying saprolite and, together with dilution by aeolian sands, constitutes the transported overburden (2-25 m thick). The reworked lateritic materials cover the preserved silcrete duricrusts in valleys. The lower ferruginous saprolite and lateritic residuum are well developed over mafic and sulphide-bearing bedrocks, where weathering of ferromagnesian minerals and sulphides led to enrichment of Fe, Cu, Ni, Cr, Co, V and Zn in these units. Kaolinitic saprolite and the overlying pedogenic silcrete are best developed over alkali granites and quartzofeldspathic gneisses, which are barren in Au and transition elements, and enriched in silica, alumina, rare earth and high field strength elements.A residual Au anomaly is formed in the lower ferruginous saprolite above a Au -bearing mafic intrusion at the Hercules prospect, south of the Neale tenement, without any expression in the overlying soil (< 20 cm). Conversely, a Au anomaly is recorded in the transported cover, particularly in the uppermost 3 m at the Atlantis prospect, 5 km southwest of the Hercules prospect. No anomalies have been detected in soils using five different size fractions (> 2,000 μm, 2,000-250 μm, 250-53 μm, 53-2 μm and < 2 μm). Therefore, soil cannot be efficiently applied as a reliable sampling medium to target mineralization at the Neale tenement. This is because mechanical weathering was interrupted by seasonal periods of intensive leaching under the present-day surface conditions and/or dilution by recently deposited aeolian sediments which obscure any signature of a potential Au anomaly in soils. Therefore, surface soil sampling should extend deeper than 20 cm to avoid dilution by aeolian sands and seasonal leaching processes. Regolith mapping and the distinction between the residual and transported weathering products are extremely significant to follow the distal or proximal mineralization.  相似文献   

19.
The present work aims to estimate Au-anomalous distribution patterns, the optimum grain size fraction and pathfinder elements for gold placer. The obtained data of analyzed elements (Au, Ag, As, Ba, Cd, Co, Cr, Cu, Fe, Ga, Hg, Li, Mn. Mo, Ni, Pb, Rb, Sb, Se, Sr, Te, Ti, V, Y and Zn) in both the fine (?0.25 mm) and coarse (?1 mm+0.25 mm) grain size fractions of 32 dry stream sediment samples, which have been derived from metamorphosed Island Arc volcanic rocks of Late Proterozoic age, revealed that most of the analyzed elements in both grain size fractions are asymmetrically distributed and did not pass the tests of normality. The coarse grain size fraction appears to be the better size fraction for Au detection and can be considered as the optimum grain size fraction for future application in regional stream sediment surveys. The presence of Au anomalies in the upper part of the stream and beside the mine is either sourced by the main Au-mineralization or new potential extension of the mineralization in the study area. Silver, Cd, Se, Zn and Te can be considered as useful pathfinder elements for Au in the coarse grain size fraction, and they may be used for future geochemical exploration for Au in the area. Recommendation of the authors is pointing to perform lithogeochemical survey in the eastern and western parts of the mine.  相似文献   

20.
The importance of trace metal scavenging by organic matter in geochemical samples was estimated using an alkaline sodium hypochlorite extraction to leach copper, zinc, molybdenum, iron and manganese from a variety of soils, and stream and lake sediments collected on the Nechako plateau, central British Columbia. The reagent oxidizes or dissolves most forms of organic matter, together with any sulphide minerals, to give strongly coloured extracts containing the associated trace elements at a pH where solution of other sample fractions is at a minimum. Metals precipitated due to alkaline conditions are redissolved by a succeeding distilled-water leach (pH 3.0 ± 0.3).A large fraction of the copper, zinc, molybdenum, and manganese held within the organic fraction of the A soil horizon is liberated whereas only minor amounts of copper, zinc, and manganese are released from inorganic soil (B and C) horizons. Molybdenum, however, is relatively soluble in all soils as the molybdate ion. Despite similar concentrations of organic matter in A horizon soils and stream sediments the latter release a lower proportion of their trace element content. Behaviour of the organic fraction of lake sediments varies from lake to lake and there is great variability in the association of copper, zinc, molybdenum and manganese with organic matter even within the same lake.The presence of organic matter in samples subjected to other partial extractions can be a deleterious factor if the organic fraction is not first removed by a hypochlorite extraction.  相似文献   

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