Provenance and weathering history of Mesoproterozoic clastic sedimentary rocks from the basal Gulcheru Formation,Cuddapah Basin

Geochemical analysis for the Mesoproterozoic clastic sedimentary rocks of the basal Gulcheru Formation of the Cuddapah Basin in the Gugudu-Dadithota-Parnapalle-Palkonda region (extending atleast 40 km) have been performed to highlight their provenance and weathering history. The low K₂O/Al₂O₃ ratios of the representative samples points to the recycled nature of sediments and illite clay in the sediments indicate an overall cold climate (low chemical weathering degree). All the provenance diagrams indicate a mixed source of the Gulcheru sediments with felsic dominancy and Upper continental crust (UCC) signature.     

河南省荥巩煤田大峪沟矿段深部铝土矿床地质特征及找矿方向

荥巩煤田大峪沟矿段深部铝土矿床是最近找矿突破发现的一个大型铝土矿床。该铝土矿床成矿时代为早二叠世,成矿物源主要为古陆铝硅酸岩风化物,少量为中奥陶统马家沟组碳酸盐岩风化产物,成矿环境为潮坪-沼泽环境,由长期的沉积间断、特定的岩相古地理环境、构造变形和表生风化作用共同控制着矿床的形成;上石炭统本溪组是铝土矿形成的含矿岩系;海湾泻湖相沉积环境是铝土矿床形成的有利条件;下古生界奥陶系、寒武系古岩溶侵蚀面是铝土矿形成的有利场所,古岩溶侵蚀面洼地及漏斗是铝土矿的定位空间。大峪沟矿段铝土矿床成因模式属于"古陆风化+碎屑和化学沉积";成矿机制为机械和化学分异。含矿岩系底部铁质风化壳/铁质黏土岩为碳酸盐岩风化形成,铝(黏)土矿来源于铝硅酸盐岩。大峪沟深部铝土矿床的发现为河南省寻找深部铝土矿床指明了方向。     

Lithogenic concentrations of trace metals in soils and saprolites over crystalline basement rocks: A case study from SW Nigeria

In this study, an assessment of the lithogenic concentrations of trace metals in soils and saprolite over basement rock units in Ibadan, SW-Nigeria is presented in respect of bedrock types and geochemical controls on the weathering-associated release of trace metals. Consequently, soil, weathered and fresh rock samples from the Precambrian Basement of SW Nigeria were collected from three different bedrock units within Ibadan metropolis and subjected to mineralogical and geochemical analyses. The analytical results revealed major proportions of oxides in the range of 18–20% Al₂O₃, 2–6% Na₂O and 1–6% K₂O for weathered profiles over granite-gneiss and pegmatite units, compared to 2–3% Al₂O₃, <0.5% Na₂O and <1.0% K₂O over schist-quartzite. For the trace elements, weathered profiles on granite-gneiss and schist-quartzite settings exhibit similar enrichment trends (enrichment factor, EF l) for most of the trace elements, unlike the pegmatite bedrock. However, enrichments are relatively greater in the top soil unit compared to the intermediate saprolite unit, especially for Pb, Ni, Zn, Cr, Co, Rb, Sr and Ba, a situation attributed to leaching and redistribution within the weathered profiles through pedogenetic process and percolating groundwater.Furthermore, the estimated weathering indices using Ruxton Ratio (RR = {SiO₂/Al₂O₃}) and Chemical Index of Alteration (CIA = 100{Al₂O₃/[Al₂O₃ + CaO + Na₂O + K₂O]}) revealed RR of 2.9–3.7 and CIA of 54–73% for granite-gneiss and pegmatite units, implying medium levels of weathering, compared to RR of 30.8–35.5 and CIA of >60% for schist-quartzite units, which suggest weak chemical weathering. Also, the estimated high percentage loss, especially for Pb, Rb, Sr, Ba relative to the bedrocks, shows that the trace elements can be mobilized within the weathering profiles even at a low degree of chemical weathering. Such weathering-induced release of trace metals is of environmental significance as natural lithogenic input sources and as background reference for future monitoring of possible human/anthropogenic impacts.     

Sediment geochemistry of coastal environments,southern Kerala,India: implication for provenance

Late Quaternary sediments representing the floodplain, estuary and offshore environments of southern Kerala were investigated to infer provenance. The grain size reveals the dominance of sand to silty clay, clay to clayey silt and clayey silt in the floodplain, estuary and offshore sediments, respectively. The chemical index of alteration (CIA) values and A-CN-K plot attributes to high, moderate and low weathering in floodplain, estuary and offshore regions, respectively. The SiO₂/Al₂O₃ values lesser than Post-Archean Australian Shale indicate low to moderate maturity for the estuarine and offshore sediments. The geochemical immaturity indicates its derivation from low to moderately weathered source rocks. The major and trace elemental ratios and discriminant function diagrams attribute that the sediments were derived from intermediate to felsic source rocks. The enrichment of Cr and Ni concentration in the sediments compared to the upper continental crust, related to the contribution of orthopyroxenes, weathered from charnockite and garnets from the granulite terrain, respectively.     

Provenance,tectonic setting and source-area weathering of the lower Cambrian sediments of the Parahio valley in the Spiti basin,India

The geochemical study of siliciclastic rocks from the Lower Cambrian of Parahio Valley has been studied to describe the provenance, chemical weathering and tectonic setting. The K₂O/Al₂O₃ ratio and positive correlation of Co (r=0.85), Ni (r=0.86), Zn (r=0.82), Rb (r=0.98) with K₂O reflects that the presence of clay minerals control the abundances of these elements and suggests a warm and humid climate for this region. The chondrite normalized REE pattern of the samples is equivalent to upper continental crust, which reflects enriched LREE and flat HREE with negative Eu anomaly. The tectonic setting discriminant diagram log[K₂O/Na₂O] vs. SiO₂; [SiO₂/Al₂O₃] vs. log[K₂O/Na₂O]; [SiO₂/20] – [K₂O+Na₂O] – [TiO₂+Fe₂O₃+MgO] indicates transitional tectonic setting from an active continental margin to a passive margin. The discriminant function plot indicates quartzose sedimentary provenance, and to some extent, the felsic igneous provenance, derived from weathered granite, gneissic terrain and/or from pre-existing sedimentary terrain. The CIA value indicates low to moderate degree of chemical weathering and the average ICV values suggests immature sediments deposited in tectonically active settings. The A–CN–K diagram indicates that these sediments were generated from source rocks of the upper continental crust.     

Geochemical Signatures of Karlad Lake Sediments,North Kerala: Source Area Weathering and Provenance

In the present study, the lake floor sediments of the Karlad lake, located at higher elevation in Wayanad region of north Kerala, were analyzed for textural characteristics, organic matter, calcium carbonate, major oxides and trace elements. This study was carried out to infer the chemical composition, provenance and intensity of chemical weathering of the source rocks in the lake catchment area. Textural studies signify that lake floor sediments are predominantly as clays (38.75%) followed by sand (36.36%) and silt (25.19%) fractions. The C/N ratio of the lake sediments signify that the sediments are both autochthonous and allochthonous in origin. The major oxides average content reveals the order of abundance as follows; SiO₂ > Al₂O₃ > Fe₂O₃ > TiO₂ > MgO > CaO > K₂O > P₂O₅ > Na₂O > MnO. Moreover, the various weathering indices such as Chemical Index of Alteration (CIAAvg. 93.5%), PlagioclaseIndex of Alteration (PIA- Avg. 95.6%) and Chemical Index of Weathering (CIW- Avg. 95.76%) suggest an intense chemical weathering of the source area. The A-CN-K diagram is also corroborating the same. Various provenance discrimination diagrams reveal that the sediments are derived from the mafic source rocks.     

Vanadium in magnetite gabbros and its behaviour during lateritic weathering,Windimurra Complex,Western Australia

Shephards Discordant Zone is a 500–600 m thick interlayered sequence of deformed, altered and metamorphosed magnetite metagabbro and about 50 layers or lenses of magnetitite (> 80–90% magnetite). The sequence shows progressive magmatic fractionation upwards: Ti and Ti/Fe increase, and V, V/Ti and Cr decrease upwards in magnetite and in whole‐rock compositions. The main magnetite‐rich sequence (about 400 m thick) is deeply weathered, with 40 m of saprolite showing vertical zonation of weathering minerals due to progressive weathering. Magnetitites (average 1% V₂O₃) are resistant to weathering and show little chemical change, but magnetite gabbros (average 0.27% V₂O₃) are extensively weathered and show progressive loss of Ca, Na, Mg and S. Plagioclase, magnetite (1.37% V₂O₃), chlorite (up to 0.35% V₂O₃), actinolite, epidote and minor sulfides in unweathered rocks weather to kaolinite, hematite, goethite and minor vermiculite, ilmenite remaining largely unaffected. Vanadium is essentially immobile during weathering and is unaffected during weathering of magnetitites (1% V₂O₃), but is slightly depleted during weathering of magnetite gabbros (0.23% V₂O₃).     

Geochemistry of Proterozoic Radioactive Arkoses in Khammam District,Andhra Pradesh,India: Evidence for K-rich Evolved Granitoid Upper Crust in the Late Archaean

Geochemical studies on radioactive arkoses (43–153 ppm U and 387–862 ppm Th) of the Proterozoic Pakhal Supergroup from Bangaruchilka, Khammam district, Andhra Pradesh, India, indicate that their gross major and trace element chemistry reflect their mineral composition. Chemically, arkoses are rich in silica (83% to 88% SiO₂) and potassium (3% to 5% K₂O), with consistently high Al₂O₃/Na₂O (36 to 50) and K₂O/Na₂O (18 to 25) ratios, which indicate that they are chemically mature sediments. The arkoses also show higher concentrations of Ti, V, Cr, Ni, Cu, Y, Zr, Nb, La and Pb.The values (60% to 68%) of chemical alteration index (CIA) of studied arkoses are moderate, and indicate that the source rocks have undergone lesser degree of chemical weathering. Tectonic setting discriminate plots of Fe₂O₃ (total)/MgO vs. TiO₂ and Al₂O₃/SiO₂, and K₂O/Na₂O vs. SiO₂ and SiO₂/Al₂O₃ indicate that the Bangaruchilka arkoses represent the sediments that were deposited in passive continental margin (PM), which is further supported by association of platformal type of sediments (quartzites and phyllites) with them. Unlike middle Archaean sedimentary rocks, the studied arkoses are depleted in Na₂O, MgO and CaO, and distinctly enriched in SiO₂ and K₂O. These geochemical features match with post-Archaean clastic sediments, which argues for the involvement of late Archaean granitic crust in supply of detritus of studied arkoses. Enrichment of potassium alongwith abundant microcline and elevated concentrations of Y, Zr, Nb, U, Th, La, etc. in arkoses indicate K-rich evolved granitoid upper crust in the provenance during the late Archaean-early Proterozoic period.Dominance of mechanical weathering over chemical weathering favoured arkose formation, and also transfer of radio-elements with clastic sediments into the Pakhal basin. After sedimentation, uranium seems to have been remobilised from the rocks of Archaean-Pakhal system, consequent to post-sedimentation structural disruptions, and concentrated along suitable structural loci. Therefore, it is likely to encounter significant concentration of uranium in close proximity of unconformable Archaean and Pakhal contacts and tectonic zones, thereby making Pakhal basin and its environs a suitable terrain to search for concealed uranium mineralisation.     

Lithium isotopic composition and concentration of the upper continental crust

The Li isotopic composition of the upper continental crust is estimated from the analyses of well-characterized shales, loess, granites and upper crustal composites (51 samples in total) from North America, China, Europe, Australia and New Zealand. Correlations between Li, δ⁷Li, and chemical weathering (as measured by the Chemical Index of Alteration (CIA)), and δ⁷Li and the clay content of shales (as measured by Al₂O₃/SiO₂), reflect uptake of heavy Li from the hydrosphere by clays. S-type granites from the Lachlan fold belt (-1.1 to -1.4‰) have δ⁷Li indistinguishable from their associated sedimentary rocks (-0.7 to 1.2‰), and show no variation in δ⁷Li throughout the differentiation sequence, suggesting that isotopic fractionation during crustal anatexis and subsequent differentiation is less than analytical uncertainty (±1‰, 2σ). The isotopically light compositions for both I- and S-type granites from the Lachlan fold belt (-2.5 to + 2.7 ‰) and loess from around the world (-3.1 to + 4.5‰) reflect the influence of weathering in their source regions. Collectively, these lithologies possess a limited range of Li isotopic compositions (δ⁷Li of −5‰ to + 5‰), with an average (δ⁷Li of 0 ± 2‰ at 1σ) that is representative of the average upper continental crust. Thus, the Li isotopic composition of the upper continental crust is lighter than the average upper mantle (δ⁷Li of + 4 ± 2‰), reflecting the influence of weathering on the upper crustal composition. The concentration of Li in the upper continental crust is estimated to be 35 ± 11 ppm (2σ), based on the average loess composition and correlations between insoluble elements (Ti, Nb, Ta, Ga and Al₂O₃, Th and HREE) and Li in shales. This value is somewhat higher than previous estimates (∼20 ppm), but is probably indistinguishable when uncertainties in the latter are accounted for.     

火山岩风化壳储层发育模式——以三塘湖盆地马朗凹陷石炭系火山岩为例

中国的火山岩油气勘探近年来进展非常快,并不断在许多盆地发现了优质火山岩储层,其中风化壳型储层作为非常重要的火山岩储集体类型而倍受重视。三塘湖盆地马朗凹陷石炭系风化壳型储层储集空间以溶蚀孔洞缝为主,储层物性非常好。通过火山岩油气勘探中的野外露头观察、钻井取心、镜下薄片鉴定,及主量元素、微量元素等分析化验资料,并结合火山岩岩石学特征和物性特点,初步建立了该区的风化壳储层的发育模式,将火山岩风化壳储层在垂向上自上而下划分为五个带: ①最终分解产物带; ②水解带; ③淋滤带; ④崩解带; ⑤未风化带(母岩)。淋滤带储集物性最好,该区风化淋滤是改善储层的关键。     

Chemical weathering of monsoonal eastern China: implications from major elements of topsoil

Major element compositions of 36 bulk samples and 41 clay samples, which were obtained from 47 topsoils collected in monsoonal eastern China, were investigated with conventional wet chemistry and X-ray fluorescence (XRF) spectrometry, respectively. Based on major element analyses, the mobility of major elements and latitudinal distributions of SiO₂/Al₂O₃ ratio, chemical index of alteration (CIA), chemical index of weathering (CIW) and weathering index of Parker (WIP) were analyzed. Meanwhile, the suitability of these chemical weathering indices to topsoils in monsoonal eastern China and its controls were discussed.These investigations indicate that Na, K, Ca, Mg, and Si are relatively depleted, while Mn, P, Fe and Ti are relatively enriched in topsoils of the study area by comparison with their contents in the upper continent crust (UCC), and that alkali metal (Na, K) and alkaline earth metal (Ca, Mg) elements are generally easier to be depleted from their parent materials than other major elements during chemical weathering. The latitudinal distributions of CIA, CIW and WIP show that they are suitable to both bulk and clay samples, but SiO₂/Al₂O₃ is only suitable to clay samples, not suitable in bulk ones. All these investigations indicate a significant dependence of grain-size in major element abundance and latitudinal distributions of SiO₂/Al₂O₃, CIA, CIW and WIP, but parent rock type has little effect on them, except its impact on the latitudinal distribution of WIP in clay samples. The significant grain-size dependence probably indicates the presence of unaltered minerals in bulk samples, thus we suggest that clay samples are more suitable to investigating chemical weathering of sediments on continents than bulk samples. The trivial effect of parent rock type probably indicates a relatively uniform chemical weathering on various parent rocks. Correlation analyses indicate that climate is the dominant control of chemical weathering of topsoils in the study area, and the significant latitude effect indicated by the spatial distributions of chemical weathering indices actually reflect the climate control on chemical weathering of topsoils.Chemical weathering indices actually reflect the integrated weathering history in the study area. Besides the dominant control of climate, other factors like tectonics, parent rock, biology, landform and soil depth and age might also have some effect on the chemical weathering of topsoils in the study area, which needs further research.     

Geochemistry and geochronology of chemical weathering,Butler Hill Granite,Missouri

Major element, Rb, Sr and Sr isotope abundances were examined in an ancient (Cambrian) weathering zone in the Butler Hill Granite, Missouri. Most of the major elements behaved predictably and systematically as a result of weathering intensity. MgO and the alkalis (Na₂O and K₂O) show highly erratic variations in both fresh and weathered rocks, suggesting considerable initial heterogeneity in the composition of biotite and feldspar. Rb predictably follows the behaviour of K during weathering (showing erratic variations similar to K), whereas Sr follows Ca (decreasing steadily as a result of weathering intensity). A method is postulated to date ancient weathering episodes using Rb-Sr data from a sequence of fresh to highly weathered samples. Data from the Butler Hill Granite (1185 m.y.) lend support to the proposed dating technique but show that more detailed sampling of the most intensely weathered regions is needed to produce more accurate results in future studies.     

Geochemistry of major and trace elements and Pb–Sr isotopes of a weathering profile developed on the Lower Cambrian black shales in central Hunan,China

This paper reports a geochemical study on the major and trace elements and Pb–Sr isotopes of a weathering profile developed in the Lower Cambrian black shales in central Hunan (China). Six weathering horizons were identified and sampled vertically throughout the profile. The chemical composition of the profile consists of variable concentrations of the major elements Fe₂O₃, FeO, MnO, MgO, CaO, Na₂O, and P₂O₅ and of less variable concentrations of SiO₂, TiO₂, Al₂O₃, and K₂O. The chemical change caused by weathering is estimated by mass-balance calculations, and the results show that the element mobility is characterised by substantial loss of SiO₂, FeO, CaO, K₂O, Na₂O, LOI, Cr, V, Ba, Cs, Rb, Sr, U, and Th, and moderate loss of Al₂O₃, MgO, Fe₂O₃, Ni, Cu, Pb, Tl, Sn, Sc, Ge and REE (Y). The high field strength elements TiO₂, Sn, Sc, U, Ga, Ge, Zr, Hf, Nb, and Ta were immobile during weathering. The chemical changes and the Pb–Sr isotopic data suggest that four types of chemical reactions occurred: the oxidation of sulphide minerals (e.g., pyrite) and organic carbon (OS), the dissolution of less resistant clinochlore-Ia, calcite, and P-bearing minerals (DL), the dissolution of detrital albite and microcline (DA), and the transformation of clay (TC) minerals (e.g., muscovite and illite–smectite). These chemical reactions then led to two stages of geochemical processes, an early stage of chemical differentiation and a later stage of chemical homogenisation. The chemical differentiation dominated by the OS, DL, and DA reactions, led to the leaching of mobile elements (e.g., MgO, Na₂O, K₂O, P₂O₅, Sr, and REE) and the redistribution of some less mobile elements (e.g., SiO₂ and Al₂O₃). In contrast, the chemical homogenisation, which was caused by TC reactions, led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil. Soils derived from black shales in South China might result from the above two geochemical processes.     

Palaeoweathering characteristics of an intrabasaltic red bole of the Deccan Flood Basalts near Shrivardhan of western coast of India

An intrabasaltic red bole horizon is studied for its weathering characteristics with respect to the underlying and overlying basalts. The study indicates that all the three units have been considerably weathered; the red bole unit, however shows some distinctive characteristics. The red boles show a higher cation exchange capacity (CEC) and lower sodium adsorption ratio (SAR) and organic carbon (OC) as compared to the weathered basalts. The lower values of Al₂O₃, TiO₂ and Fe₂O₃(T) in red boles indicate their lesser weathering than the underlying and overlying basalts, which is further corroborated by the weathering intensity measured by the indices like chemical index of alteration (CIA) and statistical empirical index of chemical weathering (W). It is also evident that the red bole samples show more retention of original mafic and felsic components. While K₂O exhibits an erratic behaviour, the MgO and CaO do not show much leaching in red boles. Lesser leaching and salinity in the red boles is indicated by the higher values of calcification and lower values of salinization. The SiO₂–Al₂O₃–Fe₂O₃ plot indicates that red bole samples are close to the basalt field, while the weathered upper basalt is more kaolinized than the weathered lower basalt. These observations reveal that the post-formational weathering processes have least affected the original palaeoweathering characters of the red bole horizon and hence the intrabasaltic palaeosols (weathering horizons) can effectively be used to constrain the palaeoweathering and palaeoclimates during the continental flood basalt episodes in the geologic past.     

Provenance, tectonics and source weathering of modern fluvial sediments of the Brahmaputra–Jamuna River, Bangladesh: Inference from geochemistry

This present study describes the elemental geochemistry of fluvial sediments in the Kurigram (upstream) to Sirajganj–Tangail (downstream) section of the Brahmaputra–Jamuna River, Bangladesh, with the aim of evaluating their provenance, weathering and tectonic setting. Petrographically, the sediments are rich in quartz (68%), followed by feldspars (8.5%) and lithic grains (7%). The bulk sediment chemistry is influenced by grain size. Concentrations of TiO₂, Fe₂O₃, MgO, K₂O, P₂O₅, Rb, Nb, Cr, V, Y, and, Ce, Th and Ga slightly decrease with increasing SiO₂/Al₂O₃ and grain size, suggesting clay matrix control. In contrast, concentrations of CaO, Na₂O, Sr and Pb increase with increasing SiO₂/Al₂O₃ and grain size, suggesting residence of these substances in feldspar. Decrease in Zr as grain size increases is likely controlled both by clay matrix and heavy minerals. In addition, heavy minerals' sorting also influences Ce, Th, Y and Cr abundances in some samples. The sediments are predominantly quartzose in composition with abundant low-grade metamorphic and sedimentary lithics, low feldspars and trace volcanic detritus, indicating a quartzose recycled orogen province as a source of the sediments. Discriminant diagrams together with immobile element ratio plots show that, the Brahmaputra–Jamuna River sediments are mostly derived from rocks formed in an active continental margin. Moreover, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate derivation of the sediments of Brahmaputra–Jamuna River from felsic rock sources of upper continental crust (UCC). The chemical indices of alteration suggest that Brahmaputra–Jamuna River sediments are chemically immature and experienced low chemical weathering effects. In the A–CN–K ternary diagram, most of the samples close to the plagioclase–K-feldspar join line and to the UCC plot, and in the field of various lithologies of Higher Himalayan Crystalline Series, suggesting that rocks in these series are likely source rocks. Therefore, the elemental geochemistry of the Brahmaputra–Jamuna River sediments is controlled mostly by mechanical breakdown of lithic fragments and subsequent preferential attrition of muscovite > albite > quartz.     

Major element geochemistry of purple soils/rocks in the red Sichuan Basin, China: implications of their diagenesis and pedogenesis

Major element compositions and chemical weathering features of the purple soils and their corresponding rocks in the Red Sichuan Basin, China were studied in this paper to infer the diagenetic features of the purple rocks and the pedogenetic features of the purple soils. The results showed that all of the rock and soil samples, except those from Yibin, have strongly similar major element compositions and the distribution patterns of their major element compositions are similar to those of the upper continental crust. Chemical index of alteration, chemical index of weathering and Na₂O/K₂O indicate that the purple soils and rocks have similar chemical weathering intensities, whereas the Al₂O₃–CaO* + Na₂O–K₂O (A–CN–K) relationship, suggests that chemical weathering was strong during diagenesis of the purple rocks, but weak during pedogenesis of the purple soils. Variations in major element compositions of the purplish rock samples, except that from Yibin, do not primarily reflect differences in compositions of the sediment source, sediment recycling and potassium metasomatism, and therefore the effects of chemical weathering on the major element compositions were mainly dependent on climate condition and the residence time of material exposed to chemical weathering. Chemical weathering of the purple rocks reached only moderate degrees under the general influence of warm and humid climate during Jurassic and Cretaceous. Warmer and more humid climate conditions partly resulted in stronger chemical weathering of rocks in the southern region of the basin than in the central and western region, whereas shorter residence time of material in upslope position partly resulted in weaker chemical weathering of rocks in the eastern region than in the central and western region. The same climate and stable tectonic setting led to comparable chemical weathering intensities of rocks in the Ziliujing and Jiaguan Formations. Gradually colder and dryer climate from Early Jurassic to Middle Jurassic then to Late Jurassic and shorter residence time in tectonically active setting of Middle Jurassic and late Late Jurassic resulted in the following order of chemical weathering intensity of the purple Jurassic rocks: Ziliujing Formation ? Shaximiao Formation ≈ Suining Formation > Penglaizhen Formation.     

河南伊川石佛寺铝土矿地质特征及控矿因素

河南伊川石佛寺铝土矿是近年在豫西地区龙门-巩义铝土矿成矿带新发现的中型铝土矿床之一.矿床含矿岩系为上石炭统本溪组中下部,铝土矿呈层状产于寒武系崮山组碳酸盐岩古风化剥蚀面,与上覆太原组整合接触.矿床赋存的本溪组从下而上分别由含铁黏土岩和铝土矿层组成.铝土矿体直接顶板为一1煤,厚约0.80～1.60 m;直接底板为高铁铝质...     

福建同安二长花岗岩风化壳中的粘土矿物

二长花岗岩风化壳自下而上划分为原生带、微风化带、弱风化带和强风化带。风化壳中粘土矿物主要为埃洛石,其次为高岭石及少量伊利石。微风化带下部以高岭石为主,往上埃洛石逐渐占优势,并于弱风化带中、上部和强风化带中局部富集,这与当时地下水活动状态有关。高岭石结晶程度随风化程度的增强而提高。在弱酸性介质条件下及水分丰富、溶出条件强烈时,从长石解理面上和长石的“溶蚀”空隙中可以直接形成高岭石或埃洛石,而过渡阶段的伊利石很不发育或不存在。     

陕南镇巴地区中-晚三叠世界线富锂黏土岩的发现及找矿意义

首次报道了扬子板块北缘镇巴地区中-晚三叠世界线黏土岩锂的超常富集,黏土岩产于关岭组(T₂g)/须家河组(T₃x)平行不整合界面,为古风化壳沉积物,Li₂O品位0.08%～0.11%,最高达0.22%,超过了该类型矿产的边界品位(0.06%),界线黏土岩的岩石学特征、矿物组成及岩石成因亟待查明。本文通过XRD分析、TIMA分析以及详细的地球化学研究,查明了界线黏土岩的矿物组成,探讨了黏土岩形成的沉积环境及物质来源,提出了不整合面黏土岩类关键金属矿产的综合找矿方向。研究表明,镇巴地区T₂/T₃界线黏土岩主要由石英、伊利石、高岭石组成,并含有极少量绿泥石、蒙脱石和铝绿泥石。主量元素(SiO₂、Al₂O₃、^TFe₂O₃、TiO₂)特征表明界线黏土岩属于铝土质泥岩,黏土岩CIA值(化学蚀变指数)为85～93,ICV值(成分变异指数)为0...