Heavy-mineral distribution in modern and ancient bay deposits,Willapa Bay,Washington, U.S.A.

Analysis of heavy-mineral distribution in modern sediments of Willapa Bay, Washington, indicates a dominance of two mineralogic assemblages, one with approximately equivalent amounts of hornblende, orthopyroxene and clinopyroxene, the other dominated by clinopyroxene. The hornblende-orthopyroxene-clinopyroxene suite is derived from the Columbia River, which discharges into the ocean a short distance south of the bay. The clinopyroxene suite is restricted in modern sediments to sands in rivers flowing into the bay from the east. The heavy-mineral distributions suggest that sand discharged from the Columbia River, borne north by longshore transport, and carried into the bay by tidal currents accounts for most of the sand within the interior of Willapa Bay.Three heavy-mineral assemblages are present in the surrounding Pleistocene deposits; two of these are identical to the modern assemblages described above. These heavy-mineral assemblages reflect the relative influence of tidal and fluvial processes on the Late Pleistocene deposits; their relative influences are consistent with those inferred on the basis of sedimentary structures and stratigraphic relations in about two-thirds of the samples examined. The anomalies can be explained by recycling of sand from older deposits. The persistence of the two heavy-mineral assemblages suggests that the pattern of estuarine sedimentation in Late Pleistocene deposits closely resembled that of the modern bay.The third heavy-mineral suite, dominated by epidote, occurs in a few older Pleistocene units. On the north side of the bay, the association of this suite with southwest-directed foresets in crossbedded gravel indicates derivation from the northeast, perhaps from an area of glacial outwash. The presence of this suite in ancient estuarine sands exposed on the east side of the bay suggests that input from this northerly source may have intermittently dominated bay deposition in the past.     

Holocene sedimentary cycle and heavy-mineral distribution,Romagna-Marche coastal plain,Italy

The heavy-mineral composition, facies and age of Holocene clastic sediments of the coastal plain and Adriatic beaches of the Romagna-Marche regions are examined in order to evaluate the relative influence and distribution, during a complete sedimentary cycle regression—transgression, of the sediments supplied by local Apenninic rivers versus sediments supplied by a more distal source, the Po River, debouching at the northwestern corner of the Adriatic Sea.A fairly clear result is that during the Pleistocene regression, sediments supplied by the Po River were transported a long distance southeastward along the basin. The transgressive Holocene sea mostly reworked and redistributed the sediments of the invaded coastal plain. Mineral composition of these marine deposits remains therefore that of the Po sediments.During the standstill of the sea level that followed the transgression, a lateral supply from the rivers draining the eastern slope of the Northern Apennines had a greater influence and the mineralogic composition of the prograding coastal sands began to differentiate, reflecting the composition of sediments supplied by a more local source.     

Geochemical studies in the Indian Pass and Picacho Peak Bureau of Land Management Wilderness study areas, imperial county, Southern California

The U.S. Geological Survey has conducted geochemical studies in the Indian Pass (CDCA-355), 124 km², and Picacho Peak (CDCA-355A), 23 km², Wilderness Study Areas (WSA's) as part of a program to evaluate the mineral resource potential of designated areas in the California Desert Conservation Area. These two WSA's are of particular interest because they lie within a region which has intermittently produced significant quantities of Au since the mid-1800's, and is currently the site of much exploration activity for additional Au resources. Within a 15-km radius of the WSA's, there is one actively producing gold mine, a major deposit which began production in 1986, and one recently announced discovery. In the reconnaissance geochemical surveys of the two WSA's - 177 μm (-80 mesh) stream sediments, heavy-mineral concentrates from stream sediments, and rocks were prepared and analyzed.Four areas of possible exploration interest were identified within the WSA's. The first area is characterized by anomalous W and Bi in nonmagnetic heavy-mineral concentrates, and is underlain primarily by the Mesozoic Orocopia Schist which has been intruded by monzogranite of Oligocene age. Alteration and mineralization appear to be localized near the intrusive contact. The mineralized rock at the surface contains secondary Cu and Fe minerals where the monzogranite intrudes the metabasite horizons of the Orocopia Schist and scheelite where the monzogranite intrudes marble within the Orocopia Schist.The second area is characterized by anomalous As, Sb, Ba, B, and Sr in nonmagnetic heavy-mineral concentrates and by anomalous As in - 177 μm stream sediments. Geologically, this area is underlain by metasedimentary and metavolcanic rocks of Jurassic(?) age; a biotite monzogranite of Jurassic(?) age; and Tertiary volcanic and hypabyssal rocks composed of flows, domes, and tuffs of intermediate to silicic composition. All these rock types are cut by a set of north-south-striking normal faults. The anomalies in the heavy-mineral concentrates are believed to be related to silica-clay alteration observed in the vicinity of some of these faults.     

晚第四纪钱塘江下切河谷充填物物源特征*

对晚第四纪钱塘江下切河谷内SE2孔沉积物重矿物和稀土-微量元素进行系统分析,并与邻区长江沉积物相应特征进行对比。结果显示: (1)晚第四纪钱塘江下切河谷充填物中河床(U5)、河漫滩(U4)和古河口湾(U3)沉积物的重矿物组合为赤褐铁矿-磁铁矿-锆石-白钛石-锐钛矿;微量元素中的铁族元素、大离子亲石元素和高场强元素与平均大陆上地壳(UCC)相近,且大离子亲石元素和大部分高场强元素与北美页岩(NASC)相近,在Th/Co-Cr/Th投点图中靠近钱塘江物源区,远离现代长江沉积物投点区;稀土元素球粒陨石和NASC标准化曲线与钱塘江沉积物特征相似,不同于现代长江沉积物。(2)近岸浅海(U2)和现代河口湾(U1)沉积物的重矿物组合为绿帘石-辉石-角闪石-石榴石-榍石-金红石-电气石;微量元素具有与UCC较为相近或富集的铁族元素,大离子亲石元素和高场强元素相对亏损,且与NASC相比,铁族和高场强元素明显亏损,大离子亲石元素亏损较弱,在Th/Co-Cr/Th投点图中主要落于长江和钱塘江来源沉积物之间;稀土元素在球粒陨石和NASC标准化曲线中,配分模式与现代长江沉积物极为相似。(3)U5、U4和U3单元具有与U1和U2单元不同的沉积物来源: 前者母岩主要为长英质中酸性火成岩,为古钱塘江提供; 而后者沉积物主要来源于中基性物源区,由长江提供,同时上游钱塘江也提供了部分沉积物。可见,长江沉积物自近岸浅海沉积时期开始大量进入到钱塘江下切河谷内,为钱塘江下切河谷的沉积环境演化提供了重要物质基础。     

Major and trace element anomalies in stream sediments of the Teign Valley Orefield

A detailed geochemical and sedimentological study of twenty-two bulk stream sediments in the Teign Valley of southwest Devon shows the influence of lithology, mineralization and contamination on such cumulative samples. K, Cu and Ni in stream sediments emerge as potential indicators of lithology. Ba and S values, as well as barytes concentrations, demonstrate the contribution of Ba, Zn and Pb mineral lodes, whilst Fe, S and heavy-mineral percentages emphasize that of Fe lodes and contamination. As, Pb and Zn anomalies are observed within Fe-oxide crusts giving evidence of Fe in the role of scavenger. High Mn values are attributed to precipitations of the metal derived largely from impregnations of the Culm.     

长江三角洲晚第四纪地层沉积物源特征及其对季风气候变化的响应

物源分析可以反演河流系统演化过程中,构造运动、气候变化与海平面升降等因素的影响。选择长江三角洲现代、中全新世和末次盛冰期3个代表性时代的沉积物样品,进行重矿物组合、锆石U—Pb年龄谱系等分析,以揭示不同气候条件下长江三角洲河口沉积物源的变化。与末次盛冰期相比,中全新世沉积物中榍石、锆石含量较低,重矿物GZi指数较大,古元古代锆石含量较高,但白垩纪、新元古代锆石含量较低。这些特征表明中全新世长江河口沉积物有更多源自上游地区的物质,这种变化可能与流域受夏季风影响的降雨量变化有关。初步分析认为,末次盛冰期研究区季风很弱,降雨量和径流量均下降,上游物质较少被搬运至河口;全新世季风加强后,降雨量和径流量均上升,此时河口沉积物更多源自上游,尤其是在中全新世夏季风最强盛期。     

Heavy-mineral variability in miocene marine sediments in Denmark: a combined effect of weathering and reworking

Heavy-mineral analyses have been performed on marine sandy and silty deposits of Miocene age from the western part of Denmark. The heavy-mineral associations are dominated by epidote and amphibole, but the mutual distribution of these minerals is variable. Studies of surface textures show a large, but variable, proportion of the grains to be corroded. In a few cases the surface textures are indicative of post-depositional processes; however, most of these may have existed prior to deposition. It is suggested that the variations in heavy-mineral distribution and surface textures result from mixing of freshly supplied, unweathered detritus from Fennoscandia, with weathered detritus from the same provenance. This weathering took place during a temporary deposition in fluvial systems between the source area and the North Sea Basin. The relation to transgressive and regressive phases is discussed.     

Geochemistry of trace element in carbonaceous sediments from recent seas and oceans

Published and original data on the contents of 50 elements in carbonaceous sediments from seas (Black, Baltic, and Caspian) and fertile oceanic shelves (shelves of Namibia, Peru, Chile, and California) are generalized. The comparison of these results with the average composition of ancient carbonaceous shales reveals both similarities and differences in the distribution of indicator trace elements (Re, Hg, Se, Ag, Cd, Tl, U, Mo, As, Ni, and Zn). Correlation coefficients were analyzed to determine the characteristic element associations. It is established that oceanic carbonaceous sediments are closer in composition to carbonaceous shales than their marine analogues, which is related to the differences in sedimentation conditions, including hydrological, hydrochemical, and biogeochemical factors. The role of anoxic environment in the accumulation of chalcophile elements in sediments is estimated. The comparison of the contents of some mobile trace elements in oceanic water and carbonaceous sediments of modern oceans demonstrates that the water composition affects the composition of carbonaceous sediments and can be used for deciphering the composition of ancient ocean water.     

Modern river discharge and pathways of supplied material in the Eurasian Arctic Ocean: evidence from mineral assemblages and major and minor element distribution

Clay-mineral, heavy-mineral, and elemental distributions in sediments from the Arctic Ocean and the adjacent Laptev and Kara seas can be attributed to the geology of the hinterland and the transport of terrigenous material by rivers onto the shelves. Kara Sea sediments are characterized by increased contents of smectite and elevated Ni/Al-, Ti/Al-, and Cr/Al ratios. In the western Laptev Sea sediments are enriched in smectite and clinopyroxene and increased in Ti/Al-, Cr/Al-, and Ca/Al ratios. The composition of the sediments reflects suspended matter input from the large trap basalt of the Putoran Mountains. The eastern Laptev Sea sediments display increased illite and amphibole contents as well as a chemical composition similar to average shale. This composition is due to the discharge from the Lena and Yana rivers, which drain a large catchment area consisting of sedimentary Mesozoic and Paleozoic rocks. Material from the eastern Laptev Sea is transported by ocean currents and sediment-laden sea ice along the Transpolar Drift into the central Arctic Ocean. This is indicated by similar values of Ti/Al-, Cr/Al-, Rb/Al-, and K/Al ratios as well as increased concentrations of amphibole and illite, determined in sediments from the Lomonosov Ridge. A minor input from the Beaufort Sea into the central Arctic Ocean is suggested from increased Ca/Al ratios and increased contents of opaque minerals.     

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.     

Compositional variability of Saalian till in The Netherlands and its origin

Variations in composition between and within Saalian till types of The Netherlands are demonstrated with reference to grain-size distribution, fine-gravel petrography and heavy-mineral composition. Several factors explain different aspects of the compositional variations. Firstly, a compositional layering results from differences in the amount of local material that has been assimilated in successive till hands. Secondly, a petrographic stratification is present in the erratic components, which is most strikingly expressed by the occurrence of flint-poor till overlying till rich in flints. This feature is interpreted to have been inherited from an englacial debris stratification which is related to the distribution of source rock types in the up-glacier area. Finally, a predominantly lateral variation in till composition is present, reflected best in the assemblages of the indicator pebbles, resulting from deposition by ice streams of different source and direction of flow.     

Correlation of the tertiary geosynclinal sediments of the Surma Valley,Assam, and Tripura State (India)

The Tertiary geosynclinal sediments of the Surma Valley and Tripura State range in age from Eocene to Mio-Pliocene. They are classified into those of the Disang, Barail, Surma and Tipam groups which respectively represent sediments belonging to geosynclinal, flysch, early molasse and late molasse stages. These sediments are devoid of diagnostic and persistent faunal and palynological assemblages. Frequent lithofacial variations render lithologic criteria unsuitable for regional correlation. Heavy-mineral suites, however, appear diagnostic and they, along with lithologic criteria, can be utilised for local and regional correlations.The mode of evolution of the heavy-mineral suites is discussed. Appearance of marker heavy minerals in successively younger sequences is concluded to be a reflection of successive cycles of positive tectonism in the provenance. The study brings out that the bulk of the sediments were derived from the Shillong Plateau. Contributions from older sedimentaries are also concluded.     

Relations between stream sediment and source-rock mineralogy,Piney Creek Basin,Wyoming

The areal distribution of stream sediment and source-rock heavy mineralogy was studied to determine the mineralogical relations between source and derived sediment in the Piney Creek drainage basin of Wyoming. Heavy-mineral distribution in the streams was characterized using factor analysis. The results of analyses show that for an actively degrading stream system, the heavy-mineral composition of stream sediment is controlled to a considerable extent by the composition of local sediment sources and by continuous addition of detritus from bank cutting. Only the most common minerals from upstream sources persist as statistically significant variables downstream. In some cases the heavy-mineral mineral composition of stream sediment can be controlled by comparatively few, or volumetrically rare source rocks, even though more abundant source rocks supply a wide range of heavy minerals. Specific drainage configurations such as lakes and proximity of streams to major sediment sources significantly modify sediment mineralogy immediately downstream.     

Mobility of arsenic in West Bengal aquifers conducting low and high groundwater arsenic. Part II: Comparative geochemical profile and leaching study

Aquifer sediments from areas of low- and high-As groundwater were characterized mineralogically and geochemically at a field site in the Nadia district of West Bengal, India. Leaching experiments and selective extraction of the sediments were also carried out to understand the release mechanism of As in the sub-surface. The correlation between measured elements (major, minor and trace) from low- and high-As groundwater areas are only significant for As, Fe and Mn. The borehole lithology and percentage of silt and clay fraction demonstrates the dominance of finer sediments in the high-As aquifer. Multivariate analysis of the geochemical parameters showed the presence of four different mineral phases (heavy-mineral fraction, phyllosilicates/biotite/Fe-oxyhydroxides, carbonates and sulphides) in the sediments. Selective extraction of sediment reveals that amorphous Fe-oxyhydroxide acts as a potential sink for As in the sub-surface. The result is consistent with microbially mediated redox reactions, which are controlled in part by the presence of natural organic matter within the aquifer sediments. The occurrences of As-bearing redox traps, primarily formed of Fe- and Mn-oxides/hydroxides, are also important factors that control the release of As into groundwater at the study site.     

Dispersal of tungsten in glacial drift and humus in Bergslagen, southern central Sweden

Since the late part of the 19th century prospecting for W in the western world has been based mainly on the mechanical concentration of heavy minerals in sediments (panning), followed by visual and chemical analyses of the heavy-mineral fraction. Since the early 1970's, W prospecting in Sweden has been based on the use of heavy-mineral concentrates from till.In tests performed in a scheelite-mineralized area in the Bergslagen area of central Sweden, samples from the A- and C-horizon of the podzolic profile have been collected along grid lines across the ore as well as down-ice along the direction of final glacial transport from the ore. The purpose of the study was to test which sample types should be used in geochemical prospecting for W in the region.The results show that all the sample types studied can be used in geochemical prospecting for W in southern central Sweden. The heavy-mineral concentrate of till is primarily recommended for use at a regional and local scale. Humus is given the lowest priority of the sample types studied because of the poor analytical reproducibility for some samples with high values of W.     

Mineralogical and geochemical analysis of shallow overburden as an aid to gold exploration in southwestern Gaspésie, Quebec, Canada

Mineralogical and geochemical data from shallow overburden surveys are examined to ascertain parameters which govern the distribution of gold in overburden in a 2400-km² area of southwestern Gaspésie, Quebec, Canada.The area is a deeply dissected plateau underlain by faulted and gently folded Siluro-Devonian strata. Complex geomorphic and glacial histories are reflected in the non-glacial character of the landscape, the preservation of very old erosional landforms and extensive variation in the composition and distribution of overburden.Total sample analysis and heavy-mineral studies show that the composition of overburden changes across the area in approximate correspondence with changes in underlying bedrock. Three broad zones related to bedrock and overburden types are delimited. Gold analyses of <250 μm overburden are insensitive to regional variations, with only 15 samples out of 300 registering above the detection limit of 2 ppb. Better contrasts of gold concentrations are obtained from chemical analyses of nonmagnetic heavy-mineral concentrates (NM HMC). Although sample density is low, NM HMC data show anomalies which can be related to particular bedrock and structural settings. Particulate gold was not observed in any of the NM HMC. Gold is associated with secondary iron-oxide phases replacing primary sulphide minerals. High concentrations of gold in NM HMC of overburden collected north of the mouth of the Assemetquagan River support the hypothesis of a local source to the north or northwest for the alluvial gold in the lower 2 km segment of the river.Dilution by far-travelled, shield-derived heavy minerals is by far the most important cause of regional mineral variation. Conversion of gold concentrations in NM HMC to concentrations in total size fraction eliminates some of the erratic behavior of NM HMC data caused by variations in heavy-mineral abundance and corrects for the dilution effect where the proportion of heavy minerals in the far-travelled component of overburden is much greater than in the local component. These calculations suggest a contribution of gold to the background in the fine sand fraction of overburden of 0.07 ppb. Where the diluting component is local, conversion of NM HMC data to total size fraction may or may not correct for differences in heavy-mineral contributions of underlying bedrock, depending on local conditions. Where dilution by far-travelled components is excessive, NM HMC analyses are inadequate to reflect conditions in underlying bedrock.     

A geochemical record of Late Cenozoic sedimentation history in the southern Netherlands

A sediment-geochemical study was performed on unconsolidated Upper Cenozoic siliclastic sediments from an area in the south of the Netherlands. Glauconite-rich sediments (Breda Fm) show high K contents and low Ba/K ratios. Major shifts in sediment composition as a result of changes in the Rhine system and of shifts between Rhine and Scheldt provenance, as known from heavy-mineral studies, are also recorded in changes in the grain-size-dependent variations between Al, Na and K. Pleistocene Rhine sediments (Tegelen Fm) show higher Na contents than Pliocene Rhine sediments (Oosterhout and Kiezeloöliet Fms) and Scheldt-derived material (Kedichem Fm), probably as a result of larger contents of sodic plagioclase. Scheldt-derived sediments show low K/Al ratios as a result of a smectite-dominated clay-mineralogical composition and low contents of micas, whereas Rhine-derived sediments have high K/Al ratios which reflect an illite-kaolinite-dominated clay mineralogy and higher contents of muscovite. The presence of siderite causes high Fe contents in the Tegelen Fm in the east of the area, suggesting a fresh-water depositional environment. Increased Mg contents in the siderite-bearing sections of the Tegelen Fm and in parts of the Oosterhout and Kiezeloöliet Fms are probably caused by the presence of minor amounts of dolomite. Localized high concentrations of (pyrite-) S are not only found in the marine Oosterhout Fm and the estuarine Tegelen Fm, but also in the fluviatile Kiezeloöliet and Kedichem Fms, which indicates at least minor marine transgressions during their deposition.     

Geochemistry of stream sediments along the western coast of the Gulf of Elat (Aqaba)

One hundred twenty-eight samples of heavy-mineral fractions from sand-size sediments whose mineralogy was previously studied by A. Ayalon, were chosen for this geochemical study. The samples represent the stream sediments along the coastal belt from Elat to Sherm el-Sheikh. This study supplements the geochemical study of A. Zilberfarb and R. Bogoch on the Precambrian terrain of Sinai.The mineral composition of the samples was determined by X-ray diffraction and optical microscopy. In all samples, 14 elements were analyzed: B, Co, Cr, Cu, Ga, Mn, Mo, Nb, Ni, Pb, Sn, Ti, V and Zr. Seven magnetic fractions were separated from twenty of these samples and in these fractions 14 to 20 elements were analyzed, according to the sample. The chemical determinations were mainly by DC emission spectrography.The results were submitted to factor analysis and to association analysis (D. Gill and co-workers). Since the environment is typically arid, there is relatively little chemical weathering. The combined results of both statistical analyses show clearly that three types of source areas can be differentiated: sandstone, metamorphic and granitic terrain. Furthermore, a distinct Mn anomaly was found in the Sherm el-Sheikh area.     

New data on the composition of stable isotopes in glendonites of the White Sea and their genesis

Several samples including host sediments, recent molluscan shells, glendonites, and associated carbonate concretions were subjected to the thorough investigation. The obtained analytical data characterize the unique locality of Holocene and Recent carbonate structures in the White Sea basin. The microscopic examination of glendonites revealed several microtextural types of calcite and the succession of their formation. It is established that they are composed of carbonate material, which was discretely formed due to biochemical destruction of terrigenous sedimentary material. The revealed differences in the carbon isotope composition in different glendonites are likely explained by local geochemical properties, which existed within host sediments, or the asynchronous formation of carbonate structures due to changes in the chemical composition of waters that saturated sediments.     

Modern Bottom Sediments of Lake Onega: Structure,Mineralogical Composition,and Systematization of Rare-Earth Elements

The results of studying the structure and mineral composition of the Upper Holocene bottom sediments of Lake Onega are presented. It is established that there are two types of columns of bottom sediments, for which stratification of layers and formation of Fe–Mn crusts are determined by the diagenesis processes occurring under physicochemical conditions of sediment accumulation according to two scenarios. The distribution spectra of rare-earth elements (REEs) are different in the indicated types of columns, which is apparently attributed to the differences in the oxidation–reduction conditions at the water–sediment interface during the formation of REEs and possibly to the influence of hydrothermal processes.