Textural and geochemical alternations in Late Cenozoic Bahamian dolomites

Dolomitized intervals of a core from San Salvador Island, Bahamas, exhibit variations of two texturally and geochemically distinct end-members. In the Pliocene section of the dolomitized interval, the two end-members alternate in a pattern that may reflect originally and/or diagenetically modified depositional facies. Formerly mud-free intervals, locally capped by exposure surfaces are massive crystalline, mimetic dolomites (CM). Muddier sediments are replaced by friable microsucrosic dolomites (MS). CM and MS dolomites also differ in porosity (< 10% vs > 30%), permeability (< 10 md vs > 100 md), mol% MgCO₃ (44–9 vs 47–7) mol%), oxygen isotopic composition (1–7 vs 2–7‰) and strontium content (241 vs 106 ppm). These data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types. Differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sediments and rocks.     

贵州泥堡金矿床构造蚀变体岩相学及地球化学特征

贵州泥堡金矿构造蚀变体(SBT)为产出于茅口组(P_2m)和龙潭组(P_3l)或峨眉山玄武岩(P_3β)之间沉积间断面-不整合界面附近的一套硅质蚀变岩石组合。采用光学显微镜及ICP-MS研究SBT样品岩相学及元素地球化学特征,结果显示,SBT矿石样品中主要矿物有石英、黄铁矿、褐铁矿、萤石、白云石和辉锑矿等,蚀变类型主要为黄铁矿化、白云石化和硅化,微观结构主要为砂状、岩屑-凝灰碎屑结构、交代结构等,构造主要有浸染状、块状、角砾状、条带状和脉状构造。SBT微量元素标准化曲线以Au、As、Sb、Hg、Te的强烈富集,Li、Sc、Cr的亏损和Cd、Ta的富集为特征。稀土元素CI球粒陨石标准化配分模式图表现为轻稀土富集的右倾型,LREE/HREE为6. 98～19. 91,"四分组"效应明显,重稀土分馏不明显,解释为受热液作用强烈; SBT微量元素标准化图及稀土元素配分曲线均表现出与围岩相似,表明继承了原岩的元素组成;δEu为0. 80～1. 84,显示Eu从明显负异常到明显的正异常;δCe为0. 72～1. 25,显示Ce从明显负异常到弱正异常,认为流体来源于深部或至少经历过对富含斜长石源区的水-岩反应,而不是含矿地层的改造热液。     

Recent and Pleistocene beach/dune sequences,western Australia

Wave-dominated sandy shores occur along much of the coast of Western Australia. Despite local variations there is a characteristic distribution of lithofacies (corresponding to different geomorphic zones). Five lithofacies are recognised: (1) trough-bedded sand/gravel; (2) laminated sand; (3) laminated/bubble sand; (4) laminated/disrupted sand; and (5) aeolian cross-stratified sand.The trough-bedded sand/gravel lithofacies is being deposited in the shallow shoreface below LWL. The laminated sand and laminated/bubble sand lithofacies are sands with gravel layers being deposited on the foreshore swash zone; extensive bubble (or vesicular) sand is common towards HWL especially in berms. The laminated/disrupted sand lithofacies is being deposited on the backshore between HWL and storm water levels and consists of horizontally layered to homogeneous sands with storm debris, especially wood, weed and floatable skeletons (e.g. Sepla and Spirula). The aeolian cross-stratified sand lithofacies is forming in beach ridge/dune areas and consists of fine sands with large-scale, generally landward-dipping forests; soils and rootlets are common.Recognition of these lithofacies within a sedimentary sequence enables reconstruction of gross shoreline conditions in terms of wave and eolian environments, tidal and storm heights, and palaeogeography. Each of these lithofacies with their characteristic features is recognised in Pleistocene sequences in Perth Basin. The Pleistocene sequences fit a model of coastal progradation with the trough-bedded sand/gravel lithofacies at the base and the aeolian sand lithofacies at the top. The value of such a stratigraphic sequence, however, extends beyond the Pleistocene.     

Petrographic and geochemical characterization of coal from Talcher coalfield,Mahanadi Basin,India

In the present study an attempt has been made to characterize the coals of Talcher coalfield employing petrographic and geochemical techniques on a large number of coal samples. Pillar coal samples were collected from all the six workable coal seams, which occur in the Karharbari (Seam-I) and the Barakar (Seams II, III, IV, V and IX) formations.     

Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, northeastern Kentucky

Sequences of laminated limestones found within thin Carboniferous carbonate strata of northeastern Kentucky were studied to determine their origin and palaeo-environmental significance. These laminated zones are strikingly similar to Holocene and Pleistocene surficial calcareous crusts (caliche) profiles that occur in various parts of the world. Carboniferous laminated carbonates are associated with shallow marine carbonate units, palaeokarst, and overlying palaeosol zones. A typical laminated profile ranges in thickness from 1 to 2 m and contains brecciated, light olive-grey to brown micrite that lacks distinctive bedding. Structures and textures common in most profiles include: (1) calcareous and silicious laminae (laminae form diffuse, alternating light and dark bands that generally parallel bedding but often fill fractures and vugs within the rocks); (2) particles (allochems, and micrite and microspar fragments) coated by brown microcrystalline calcite; (3) brecciated texture; (4) circular to elliptical fossil moulds (occur in sinuous patterns and fill fractures within the rocks); (5) large and small scale fracture patterns. Subaerial weathering and vadose diagenesis of carbonate mud banks or islands is suggested as a mechanism for the formation of these Carboniferous calcareous crust profiles. These ‘crusts’ formed by a combination of solution (karsting), brecciation, and soil development that transformed an exposed marine biomicrite (‘host’ rock) into a porous subsoil rubble. Laminated ‘crusts’ and coated particles developed as the result of dissolution and reprecipitation of CaCO₃ and SiO₃ from the soil and carbonate bedrock. Carboniferous laminated carbonates in northeastern Kentucky are often referred to as ‘algal limestones’ because of their superficial similarity to some modern and ancient algal structures. This study, however, reveals numerous characteristics that can only be explained by diagenesis in a subaerial environment.     

Petrographic and geochemical study of Jurassic dolostones from Lebanon: Evidence for superimposed diagenetic events

The Jurassic rock sequence in Lebanon is characterized by pervasive dolomitization (thickness > 1000 m). Two distinct dolostones are recognized within this rock succession: fine-to-medium crystalline seepage-reflux grey dolostone and coarse-crystalline hydrothermal beige dolostone. In this contribution, field, petrographic, and geochemical investigations on a dolostone ‘tongue’, occurring in Late Jurassic carbonates in central Lebanon, are discussed. The dolostone ‘tongue’ consists predominantly of the beige Late Jurassic hydrothermal dolostones. During and/or after the deposition of the overlying continental sandstones (Early Cretaceous), meteoric water percolated through the sand layers and into the underlying beige dolostones. This resulted first in dolomite intracrystalline dissolution, and then in the precipitation of oxides/hydroxides within the pore space. Finally, the precipitation of ferroan dolomite cement — from reduced fluids during a new stage of burial — took place. This study attempts to explain how superimposed diagenetic events result in a single pervasive dolostone body.     

Stone line profiles: Importance in geochemical exploration

In vast tropical rain forest areas, weathering profiles are commonly characterized by a “stone line” overlain by a brown-yellow loose-clay horizon. Concordant with the topographic surface, such a stone line may be traced continuously over considerable distances. It is typically composed of coarse fragments of lithorelics, debris of laterite as Fe-oxides nodules, corroded quartz, gibbsitic aggregates, …, embedded in a clayey matrix. These materials cover the saprolitic weathering profile which is typically a few tens of metres thick.The origin of stone lines has given rise to much controversy and are still widely misunderstood. A broad range of processes, allochthonist or autochthonist, have been put forward in the literature. The findings in this paper conclude that these weathering profiles result from chemical leaching and differential movement between the matrix and the coarse fragments which accumulate by downward migration. Accumulation takes place at the lower limit of rain water impregnation and forms the stone line, whereas leaching and homogenization of fine material occur throughout the upper water-impregnated horizon. Although the materials of the loose-clay horizon and of the stone are extensively altered, the relics are chemically rather well recognizable.According to the above hypothesis, stone line weathering profiles should thus be mostly residual. The main aspects of geochemical dispersion processes of some stone line profiles in Gabon are presented as examples. These show that:

1. (a) The vertical redistribution of some major elements in the profiles, accumulation (Fe₂O₃, Al₂O₃, SiO₂) or leaching (K₂O, MgO, CaO, SiO₂,..) are different from the bedrock composition;

2. (b) In some situations, it is possible to characterize the bedrock by using groups of trace elements such as V, Ni,.. for basic rocks or Ba, Sr,.. for gneisses for instance; the contrasts obtained can be smoothed in comparison with results from deeper in the profile.

3. (c) The persistence of geochemical anomalies arising from mineralization, throughout the weathering profile, up to the main sampling media, the surface soil. A “mushroom” dispersion pattern can be recognized where the foot of the mushroom corresponds to the element dispersion pattern can be recognized where the foot of the mushroom corresponds to the element dispersion in the saprolite and the bedrock, with the top of the mushroom being partly in the stone line and partly in the loose clay horizon.

Such a dispersion pattern has two consequences on exploration: (1) the spreading out of the surficial signal favoring the identification of anomalies during follow-up on a relatively wide spaced grid; and (2) at the same time, a reduction of the extension of the signals by dilution and leaching according to the weathering process; therefore, relatively low anomaly contents must be taken into account in exploration.Thus, anomalies arising from stone line profiles tend to be well-dispersed, but of weak magnitude, and represent in situ transfer from the parent rock.     

Petrographic and geochemical characterization of Archaic‐Hellenistic tableware production at Solunto,Sicily

A selected assortment of Archaic‐Hellenistic tableware samples from Solunto, a Phoenician‐Punic site located 20 km east of Palermo (Sicily), has been subjected to thin‐section petrography and chemical analysis (XRF). In this settlement several ceramic kilns remained operative over a long time period (7th to 3rd century B.C.). The main goal of this analytical study is to distinguish the ceramics manufactured locally from regional and off‐island imports. Analytical results were matched to similar data concerning local natural clay sources and to coeval tableware productions from other sites in the same area. The ceramic pastes used by the ancient craftsmen of Solunto in the case of this class of pottery could be differentiated clearly by their petrochemical characteristics. We conclude that ceramics were locally produced far beyond satisfying just internal consumption needs, indicating interaction of Solunto with neighboring Greek colonies, indigenous people, and Phoenician‐Punic colonies of Sicily. © 2009 Wiley Periodicals, Inc.     

Development of a cliff-top dune indicated by particle size and geochemical characteristics: Rubjerg Knude, Denmark

Particle size and geochemical data have been used to investigate the development of a large cliff-top dune at Rubjerg Knude, located on the western coast of Jutland, Denmark. Textural parameters and geochemical ratios provided useful indicators of the dune sediment provenance and mode of evolution of the dune. The dune sediments themselves showed no significant spatial particle size trends and reflect a number of processes, including grainfall, wind-ripple migration and avalanching (grainflow), which formed a high proportion of the deposits on both the stoss and lee sides of the present dune. Fine grainfall sediments, which have accumulated to form a sandplain in the lee of the dune, show fining and improved sorting with distance, and extend more than 2 km downwind of the dune crest. Comparison of the textural and geochemical data from Rubjerg Knude and other locations on the Jutland coast indicates that, although there is a contribution of sand to the dune from local marine sources, the main source of sand to the cliff-top dune and sand plain sediments has been provided by the wind erosion of the underlying cliffs, which are composed of Weichselian age sandy glaciofluvial and glaciolacustrine deposits. Optically stimulated luminescence dating indicated an apparent age for the sand at the base of 274 ± 14 years. If this date is reliable, it suggests that accumulation of the aeolian sand in this area began within approximately the last 300 years. Map and photographic evidence indicate that the modern high dune only began to form after 1885, apparently associated with an acceleration in the rate of coastal cliff retreat.     

The geochemical evolution of the Pleistocene Lake Lisan-Dead Sea system

The geochemical history of Lake Lisan, the Pleistocene precursor of the Dead Sea, has been studied by geological, chemical and isotopic methods.Aragonite laminae from the Lisan Formation yielded (equivalent) Sr/Ca ratios in the range 0.5 × 10^?2?1 × 10^?2, Na/Ca ratios from 3.6 × 10^?3 to 9.2 × 10^?3, $\text{δ}{}^{18}\text{O}{}_{\mathrm{PDB}}$ values between 1.5 and 7%. and $\text{δ}{}^{13}\text{C}{}_{\mathrm{PDB}}$ from ?7.7 to 3.4%..The distribution coefficient of Na⁺ between aragonite and aqueous solutions, $\text{λ}{}^{\mathrm{A}}{}_{\mathrm{Na}}$, is experimentally shown to be very sensitive to salinity and nearly temperature independent. Thus, Na/Ca in aragonite serves as a paleosalinity indicator.Sr/Ca ratios and $\text{δ}{}^{18}\text{O}$ values in aragonite provide good long-term monitors of a lake's evolution. They show Lake Lisan to be well mixed, highly evaporated and saline. Except for a diluted surface layer, the salinity of the lake was half that of the present Dead Sea (15 vs 31%).Lake Lisan evolved from a small, yet deep, hypersaline Dead Sea-like, water body. This initial lake was rapidly filled-up to its highest stand by fresh waters and existed for about 40,000 yr before shrinking back to the present Dead Sea. The chemistry of Lake Lisan at its stable stand represented a material balance between a Jordan-like input, an original large mass of salts and a chemical removal of aragonite. The weighted average depth of Lake Lisan is calculated, on a geochemical basis, to have been at least 400, preferably 600 m.The oxygen isotopic composition of Lake Lisan water, which was higher by at least 3%. than that of the Dead Sea, was probably dictated by a higher rate of evaporation.Na/Ca ratios in aragonite, which correlate well with $\text{δ}{}^{13}\text{C}$ values, but change frequently in time, reflect the existence of a short lived upper water layer of varying salinity in Lake Lisan.     

Petrographic and geochemical arguments for hydrothermal formation of the Ouenza siderite deposit (NE Algeria)

The Ouenza siderite deposit is located proximal to evaporitic diapirs of Triassic age. Mineralization occurs mainly in Aptian neritic limestones which host important iron concentrations (120–150 MT) and minor Pb, Zn, Cu, Ba and F occurrences. The iron ore consists of iron carbonate minerals which have been oxidized partially to hematite. Fine-grained ankerite and siderite replace limestones, whereas sparry ankerite and siderite were emplaced in veins. Limited variation in the chemical and isotopic compositions of ankerite and siderite were observed, which indicate that they precipitated from the same fluid. Stable isotope compositions (δ¹⁸O and δ¹³C) of iron carbonates and limestones allow estimation of the isotopic composition of the mineralizing fluid and precipitation temperature: δ¹⁸O = 7.5‰ SMOW, T = 100–120 °C. Later deposition of Pb, Zn, Cu, Ba and F minerals is controlled by fractures oriented NE–SW and SE–NW. Fluid inclusion studies of quartz yield salinities of 18–22 wt.% equivalent NaCl and homogenization temperatures between 150 and 180 °C. These values are similar to those of Mississippi Valley type deposits which are associated with basinal brines. Received: 4 January 1996 / Accepted: 17 July 1996     

Petrographic and geochemical characteristics of organic matter associated with stream sediments in Trail area British Columbia,Canada

Fifty-six samples of stream sediments from 12 creeks in the vicinity of Trail, British Columbia, Canada were examined to determine their origin, characterize their organic matter and their relation to natural/geogenic and anthropogenic sources. The samples were initially screened by Rock-Eval® 6 pyrolysis for their TOC, HI, and OI contents and then examined by both reflected (polarized) and fluorescent light microscopy. It was found that organic matter in stream sediments is mostly from natural/biological sources from local vegetation, such as woody tissue, suberin, spores, and pollen, as well as altered natural/biological input from char formed due to forest fires. Anthropogenic organic matter, mostly coke particles, was also found in the stream sediments. The coke particles have anisotropic properties with medium grained texture formed from medium volatile bituminous coal. The occurrence of coke particles is limited to Ryan Creek located close to an area were some small gold, nickel, and lead smelting operations previously occurred. There is no evidence to indicate that the coke particles found in the creek are emitted from the lead and zinc smelter currently in operation in the area. There are no coal-bearing strata in the area that may have a direct input of coal fragments in any of the creeks.     

Syn-sedimentary hydrothermal dolomites in a lacustrine rift basin: Petrographic and geochemical evidence from the lower Cretaceous Erlian Basin,Northern China

Dolomites occur extensively in the lower Cretaceous along syn-sedimentary fault zones of the Baiyinchagan Sag, westernmost Erlian Basin, within a predominantly fluvial–lacustrine sedimentary sequence. Four types of dolomite are identified, associated with hydrothermal minerals such as natrolite, analcime and Fe-bearing magnesite. The finely-crystalline dolomites consist of anhedral to subhedral crystals (2 to 10 μm), evenly commixed with terrigenous sediments that occur either as matrix-supporting grains (Fd1) or as massive argillaceous dolostone (Fd2). Medium-crystalline (Md) dolomites are composed of subhedral to euhedral crystals aggregates (50 to 250 μm) and occur in syn-sedimentary deformation laminae/bands. Coarse-crystalline (Cd) dolomites consist of non-planar crystals (mean size >1 mm), and occur as fracture infills cross-cutting the other dolomite types. The Fd1, Md and Cd dolomites have similar values of δ¹⁸O (−20·5 to −11·0‰ Vienna PeeDee Belemnite) and δ¹³C (+1·4 to +4·5‰ Vienna PeeDee Belemnite), but Fd2 dolomites are isotopically distinct (δ¹⁸O −8·5 to −2·3‰ Vienna PeeDee Belemnite; δ¹³C +1·4 to +8·6‰ Vienna PeeDee Belemnite). Samples define three groups which differ in light rare-earth elements versus high rare-earth elements enrichment/depletion and significance of Tb, Yb and Dy anomalies. Medium-crystalline dolomites have signatures that indicate formation from brines at very high temperature, with salinities of 11·8 to 23·2 eq. wt. % NaCl and T_h values of 167 to 283°C. The calculated temperatures of Fd1 and Cd dolomites extend to slightly lower values (141 to 282°C), while Fd2 dolomites are distinctly cooler (81 to 124°C). These results suggest that the dolomites formed from hydrothermal fluid during and/or penecontemporaneous with sediment deposition. Faults and fractures bounding the basin were important conduits through which high-temperature Mg-rich fluids discharged, driven by an abnormally high heat flux associated with local volcanism. It is thought that differing amounts of cooling and degassing of these hydrothermal fluids, and of mixing with lake waters, facilitated the precipitation of dolomite and associated minerals, and resulted in the petrographic and geochemical differences between the dolomites.     

云南省香格里拉县甭哥碱性杂岩体岩石地球化学特征及其地质意义

云南省香格里拉地区发育一套以甭哥碱性杂岩体为代表的富碱高钾岩浆岩。该岩体SiO_2变化范围较宽,高FeO~T和碱质,低Al_2O_3、MgO和CaO;富集LILE和LREE,相对亏损HFSE并显著亏损Cr、Co、Ni等过渡族元素,同时还具有从弱负异常至强正异常的δEu及δCe,表现出埃达克质与岛弧型火山岩双重特征。岩石地球化学研究表明,甭哥岩体并非单一岩浆经简单结晶分异而来,其岩浆来源具有多样性与多期性,它们主要代表了晚三叠印支期俯冲洋片和地幔楔与下地壳部分熔融及新生代含金云母相的交代富集地幔低程度部分熔融的岩浆作用。其形成和演化大致经历了晚印支期初始岩浆形成和燕山期后续岩浆逐步叠加及新生代喜山期岩浆侵位三个阶段,在整个岩浆作用过程中,伴随有地幔流体交代作用及由此引发的壳幔物质混染现象。而与甭哥碱性杂岩体有关的金成矿作用主要受制于该区多阶段岩浆与含矿地幔流体的相互作用及地幔流体交代叠加混染作用,其金成矿的富矿体可能定位于深部,据此提出甭哥碱性杂岩体具有深部成矿的勘探开发远景。     

Petrographic and geochemical evidence for hydrothermal evolution of the North Deposit,Mt Tom Price,Western Australia

High-grade iron mineralisation (>65%Fe) in the North Deposit occurs as an E-W trending synclinal sheet within banded iron formation (BIF) of the Early Proterozoic Dales Gorge Member and consists of martite-microplaty hematite ore. Three hypogene alteration zones between unmineralised BIF and high-grade iron ore are observed: (1) distal magnetite-siderite-iron silicate, (2) intermediate hematite-ankerite-magnetite, and (3) proximal martite-microplaty hematite-apatite alteration zones. Fluid inclusions trapped in ankerite within ankerite-hematite veins in the hematite-ankerite-magnetite alteration zone revealed mostly H₂O–CaCl₂ pseudosecondary and secondary inclusions with salinities of 23.9±1.5 (1, n=38) and 24.4±1.5 (1, n=66) eq.wt.% CaCl₂, respectively. Pseudosecondary inclusions homogenised at 253±59.9°C (1, n=34) and secondary inclusions at 117±10.0°C (1, n=66). The decrepitation of pseudosecondary inclusions above 350°C suggests that their trapping temperatures are likely to be higher (i.e. 400°C). Hypogene siderite and ankerite from magnetite-siderite-iron silicate and hematite-ankerite-magnetite alteration zones have similar oxygen isotope compositions, but increasingly enriched carbon isotopes from magnetite-siderite-iron silicate alteration (–8.8±0.7, 1, n=17) to hematite-ankerite-magnetite alteration zones (–4.9±2.2, 1, n=17) when compared to the dolomite in the Wittenoom Formation (0.9±0.7, 1, n=15) that underlies the deposit. A two-stage hydrothermal-supergene model is proposed for the formation of the North Deposit. Early 1a hypogene alteration involved the upward movement of hydrothermal, CaCl₂-rich brines (150–250°C), likely from the carbonate-rich Wittenoom Formation (¹³C signature of 0.9±0.7, 1, n=15), within large-scale folds of the Dales Gorge Member. Fluid rock reactions transformed unmineralised BIF to magnetite siderite-iron silicate BIF, with subsequent desilicification of the chert bands. Stage 1b hypogene alteration is characterised by an increase in temperature (possibly to 400°C), depleted ¹³C signature of –4.9±2.2 (1, n=17), and the formation of hematite-ankerite-magnetite alteration and finally the crystallisation of microplaty hematite. Late Stage 1c hypogene alteration involved the interaction of low temperature (~120°C) basinal brines with the hematite-ankerite-magnetite hydrothermal assemblage leaving a porous martite-microplaty hematite-apatite mineral assemblage. Stage 2 supergene enrichment in the Tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.Editorial handling: B. Lehmann     

Depositional environments of some Pleistocene coastal terrace deposits,southwestern Oregon—case history of a progradational beach and dune sequence

Pleistocene coastal terrace deposits exposed in sea cliffs near Gold Beach, Oregon can be divided into four stratigraphic units: a basal gravelly unit and three overlying sandy units, each with mud beds, a paleosol, or the modern soil in its uppermost part. The gravelly unit consists of gravel and sand in its lower part, sand, in part pebbly or cobbly, in its middle part, and mud and sand in its upper part. Black sand and transported pieces of wood are common in the middle part of the unit, and wood is common in the mud. This unit is interpreted as a progradational deposit including environments ranging from lower forebeach at the base to backbeach flats and streams at the top.The main sandy parts of the sandy units are made up of a crossbedded sand facies, the dominant structure in which is medium-scale crossbedding, and an irregularly bedded sand facies, which is locally pebbly and is dominated by scour-and-fill structures. Deciding between shallow marine and eolian interpretations of the sandy units proved exceptionally difficult until modern analogues were found in the fine details of the internal structures. Largely on the basis of such structural details, the crossbedded sand facies is interpreted as the product of small eolian dunes, and the irregularly bedded sand facies is interpreted as deposits of interdune ephemeral streams, ephemeral ponds, and wet to dry subaerial flats. The mud beds and paleosols at the tops of the sandy units represent times of temporary stabilization of the dune field.     

Late Pleistocene aeolian and fluvial interactions in the development of the Nizzana dune field, Negev Desert, Israel

The interdunal areas in the Nizzana linear sand dune field contain both sandy and silty sediments. A series of trenches was excavated across the interdunal corridor exposing stacked sequences of silty and sandy units which are locally restricted to palaeodepressions. The silty units contain fining upward sequences and are interpreted as overbank deposits from the Nahal Nizzana. Thermoluminescence dating and identification of buried palaeosols indicates that the silt and clay layers were deposited over a period of several thousands of years in the late Pleistocene. The sands between the silt layers have been fluvially reworked and are not primary aeolian deposits. The stacked sequences of fluvial deposits indicate that the palaeodepressions persisted in the landscape for a significant time attesting to long-term stability of the interdunal areas. It also suggests that the linear dunes themselves have not moved laterally during this time despite climatic changes and devegetation. Since the end of the late Pleistocene the Nahal Nizzana has downcut and overbank deposition no longer occurs within the interdunal corridors. The playa deposits today are positive relief features indicating that topographic inversion has occurred and that the interdunal areas are geomorphically active.     

湘东北仁里- 传梓源矿床5号伟晶岩岩相学、地球化学特征及成矿时代

湘东北幕阜山地区由东北向西南,稀有金属矿化种类由单一至复杂,成矿规模逐渐增大,显示出明显的空间规律性。本文选取幕阜山地区西南部岩浆分异演化程度最高、矿化种类最复杂的仁里-传梓源稀有金属矿床,开展典型伟晶岩脉矿物学、地球化学及云母Ar-Ar年代学研究,旨于揭示Be-Nb-Ta-Li-Cs伟晶岩地球化学特征,探讨其与花岗岩围岩成因关系,并限定矿床稀有金属成矿时代。仁里-传梓源矿床5号伟晶岩脉表现出相对富碱(Na_2O+K_2O=5.11%～14.02%,平均9.4%)、过铝质(A/CNK=1.04～2.26,平均为1.4)的特征,微量、稀土元素含量极低,总体上富集Ta、Nb、Hf、Zr等高场强元素(HFSE),相对亏损Ba、Sr等离子亲石元素(LILE),不相容元素Rb、Cs、Nb、Ta、Zr、Hf等的强烈富集表明岩浆分异演化程度极高。矿物学及地球化学特征显示,仁里-传梓源矿床5号伟晶岩脉由边部至核部分异演化程度逐渐升高,但其化学指数与花岗岩围岩相比表现出较明显的突变关系。结合野外接触关系、年代学等证据,推断围岩二云母花岗岩并非稀有金属伟晶岩母岩。仁里-传梓源矿床显示出稀有金属岩浆-热液两阶段成矿的特征,伟晶岩中锂云母Ar-Ar坪年龄为125.0±1.4 Ma,代表了岩浆分异演化晚阶段,近热液体系中稀有金属聚集成矿的时代。     

地物化综合剖面在内蒙古扎鲁特旗榆树屯化探异常区的应用

榆树屯化探异常区位于大兴安岭成矿带中南段,元素富集明显,异常区内中酸性脉岩发育,可见多组NE向和NW向断裂构造,具较强烈的蚀变特征,主要为硅化、褐铁矿化等。通过地物化综合剖面方法在榆树屯化探异常区查证发现,利用综合地物化剖面可以迅速明确异常性质、确定找矿地段,使找矿效率得到明显提高。