Lateritic Duricrust Research Articles

Regolith mapping using airborne gamma-ray spectrometry in central Brazil

In deeply weathered terrains, with high geological diversity and scarce outcrops, the lateritic regolith features support mineral research. Predictive regolith mapping uses spatial modeling and airborne gamma-ray spectrometric data to locate weathered residual products, such as the ferruginous lateritic duricrusts found in tropical regions. Although the regolith materials exhibit qualitative similarities in their gamma-ray spectrometric responses in RGB (KThU) images, the Boolean and fuzzy approaches highlighted differences that allows for mapping the regolith weathering residual products on a wide geodiversity protoliths. Two inference systems were used, one identified the ferruginous lateritic duricrusts and oxisols derived from the felsic/pelitc and mafic rocks types (TP test procedures), and the other identified solely those derived exclusively from mafic rocks (TPM test procedures). Cutoff values for the conjunction eTh/K ∩ eU/K ∩ eTh ∩ K (TP-4) using Boolean logic, provided the most accurate regolith map (accuracy = 95%) among the various tests carried out and indicate that in 28.6% of the total study area, there are lateritic duricrust and oxisols relative to rocks, saprolites, mottled horizons and other soils type. The eTh cutoff value > 4.4 ppm was used to exclude mafic rocks and associated soils, and the K value < 1.3% to exclude rocks with eTh/K and eU/K values similar to the lateritic duricrusts and oxisols. For mapping the lateritic duricrusts derived exclusively from the mafic rocks the conjunction of eTh ∩ eU/K ∩ eTh/K ∩ K ∩ eU (TPM-3) fuzzified by the large membership function was the most efficient. The eU variable with values < 1.21 ppm helps improve the effectiveness of this procedure. The generated predictive map indicated that in about ∼12% (657.9 km2) of the total study area, there are lateritic duricrusts and oxisols derived from mafic rocks. Therefore, a successful method of mapping vast areas from complicated lateritic regolith mosaic derived from a large geodiversity is to apply specific plugins for data fuzzification, supported by observations from field outcrops.

Unraveling Parent Rock and Mineral Influences in Tropical Weathering Profiles: REE, Nd and Sr Isotopic Geochemistry

This study aims to investigate the effects of parent rock and minerals on lateritic weathering. The study presents X-ray diffraction (XRD), whole-rock geochemistry, and Nd-Sr isotopic data for examining two profiles, 10 and 12 m thick, respectively, that illustrate the regional tropical weathering status in the Midwest of Brazil. The profiles, developed from metasedimentary and sedimentary rocks, are constituted by saprolite, mottled horizon, lateritic duricrust, and oxisol. Across the profiles, the minerals controlling the weathering geochemistry are muscovite, microcline, quartz, kaolinite, hematite, goethite, and gibbsite. Red and yellow zones in the saprolite and mottled horizon as well as the lateritic duricrust with breccia/fragmental, pisolitic, and oolitic textures make profile 1 more complex. In contrast, profile 2 has an oxisol that mantles the homogeneous vermiform lateritic duricrust. Fe2O3, accumulated during surface weathering, is a potent element in the geochemical profile control since it forms the harder goethite to hematite lateritic duricrust, bearing most of the trace elements (As, Cu, Cs, Pb, Sc, Sr, Th, U, V, and Zn) with similar ionic radii and electrovalence. The LREE have affinity for the elements of the Fe2O3 group of the lateritic duricrust. On the other hand, the K2O group together with Zr and TiO2 e in the phyllite, saprolite, and mottled horizon of profile 1, are associated with the HREE. Additionally, in profile 2, the HREE are mostly associated with the Al2O3 group and the residual minerals in the oxisol. The indication that REE is associated with phosphates, zircon, rutile/anatase, cereanite, and muscovite/illite, which have variable weathering behavior, caused the REE fractionation to occur across and between the profiles. Despite the REE fractionation, the ƐNd(0) values along the profiles consistently maintain the signature of the parent rock. Muscovite and microcline weathering, in profiles 1 and 2, respectively, control the decrease in 87Sr/86Sr signatures of both profiles and the distinct radiogenic ratios. The development of lateritic duricrust in both profiles indicates a similar weathering intensity, although the gibbsite–kaolinite predominance in the oxisol of profile 2 highlights a geochemical reorganization under humid conditions, as well as near-intense soluble silica leaching.

Macrotermes falciger termite mounds as indicators of lithogeochemical anomalies of metals of interest

This study investigated the potential lithogeochemical filiation of Macrotermes falciger termite mound materials and the parent substrate materials sampled in both the Sakania and Tshisenda areas of the Haut-Katanga Province of the Democratic Republic of Congo. These geological areas with predominantly autochthonous cover are characterized by lateritic type profiles. Mineralogical characterization of Macrotermes falciger termite mound materials by infrared on 20–63 μm fractions indicated the presence of secondary minerals specific to tropical weathering. Further mineralogical analysis of 0–20 μm fractions using XRD also identified such primary and secondary minerals in these materials. A multi-element chemical analysis was performed on the <125 μm fractions of the different lithological substrates (granitic rock, feldspathic sandstone, and lateritic duricrust) as well as of the Macrotermes falciger termite mounds. Using log Fe2O3t/K2O vs logSiO2/Al2O3 ratios, all the samples were geochemically classified and lithogeochemical relationships were established. We showed that the yellow, grey and beige termite mounds were characterized by lithogeochemical facies of quartz-rich and slightly altered substrates (granite, arkose…) whereas the red termite mounds had lithogeochemical facies of clay formations associated with iron oxyhydroxides with less quartz. The accumulation of oxyhydroxides (Al, Fe, Mn) and the enrichment of low-mobility trace elements (Zr and Hf) in some mound materials allowed us to identify the Macrotermes falciger material supply zone between the accumulation horizon marked by chemical precipitation phenomena and the horizon of progressive transition to altered bedrock. Carrier phases of metals of interest, such as Cu, Co, Ni, Pb and Zn, were identified through scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) on constitutive particles of Macrotermes falciger termite mounds. These 5 metals of interest were sorted into <63 μm, 20–63 μm and 0–20 μm fractions, then chemically analyzed to reveal their accumulation in the 0–20 μm fractions. These results, provide clear evidence that Macrotermes falciger termite mound materials are ideal for providing information on lithogeochemical anomalies in geochemical prospection.

Acquiring Iron-Reducing Enrichment Cultures: Environments, Methods and Quality Assessments

Lateritic duricrusts cover iron ore deposits and form spatially restricted, unique canga ecosystems endangered by mining. Iron cycling, i.e., the dissolution and subsequent precipitation of iron, is able to restitute canga duricrusts, generating new habitats for endangered biota in post-mining landscapes. As iron-reducing bacteria can accelerate this iron cycling, we aim to retrieve microbial enrichment cultures suitable to mediate the large-scale restoration of cangas. For that, we collected water and sediment samples from the Carajás National Forest and cultivated the iron-reducing microorganisms therein using a specific medium. We measured the potential to reduce iron using ferrozine assays, growth rate and metabolic activity. Six out of seven enrichment cultures effectively reduced iron, showing that different environments harbor iron-reducing bacteria. The most promising enrichment cultures were obtained from environments with repeated flooding and drying cycles, i.e., periodically inundated grasslands and a plateau of an iron mining waste pile characterized by frequent soaking. Selected enrichment cultures contained iron-reducing and fermenting bacteria, such as Serratia and Enterobacter. We found higher iron-reducing potential in enrichment cultures with a higher cell density and microorganism diversity. The obtained enrichment cultures should be tested for canga restoration to generate benefits for biodiversity and contribute to more sustainable iron mining in the region.

The Brazilian Midwest lateritic domains: Regolith features and dynamic

Intracontinental deformations cause the uplift of erosive surfaces and are consequence of lithosphere destabilization processes in cratonic domains. The Atlantic Ocean opening influenced processes 2000 km away from the coast with reflections in the regolith. The Brazilian Midwest regolith is characterised by a stepwise landscape of highlands, mountains, plateaus, inselbergs, canyons, and lowlands that form four morphostratigraphic surfaces. Surface 1 is sustained by quartzite mountains. Surfaces 2 and 3 are mantled by ferruginous lateritic duricrusts. The duricrusts in surface 2 are quartz-hematitic, hardened, columnar, reddish-pink to yellowish-red, have protonodular, fragmental, and pisolitic textures, and reach up to 3 m thick, whereas the duricrusts in surface 3 are more friable, quartz-goethitic, columnar, yellowish-red to orangish-yellow, have a vermiform texture with bioturbated micro-aggregated clay-ferruginous matrix, reaching up to 2 m thick. Surface 4 includes the lower alluvial plains. The purpose of this study was to understand the formation of these regolith surfaces, their relationships, the impact of the lateritic dynamic, and the continental influences as a landscape-shaping agent. The area was studied using SRTM images, swath profiles, and the Roughness Concentration Index, as well as fieldwork control, textures, mineralogy, and geochemistry analysis. Surface 1 marks the regional erosional event, while surfaces 2 and 3 mark two stable well-drained ferruginization weathering episodes and a seasonal drier to humid climate change. Surface 4 sculpts the lowlands. These regolith sequential landforms surfaces were produced in a complex landscape process modelled by an extensive erosional regime as a result of tectonic movements during the Atlantic Ocean evolution evolution and tropical weathering climate since the Neogene, as indicated by the lateritic duricrusts features (textures, mineral, and chemical composition). This landscape is younger than expected for Archean to Paleozoic terrains subjected to tropical weathering, dating back at least to the Paleogene, the age of the majority of older lateritic duricrusts in Brazil and around the world. These younger morphostratigraphic surfaces in cratonic domains, as well as the geological processes involved in tectonic and climate influence, had significant implications for the Brazilian South American Platform regolith.

(U-Th)/He Geochronology Constraints on Lateritic Duricrust Formation on the Guiana Shield

Thick regoliths developed under tropical climate, namely, laterites, resulting from long-term and pronounced geochemical and mineralogical rearrangement of the parent rock in response to environmental changes. Little information is available on the timing of laterite and bauxite formations, especially on the chronology of the main weathering episodes responsible for lateritic cover formation on the Guiana shield. For this purpose, we focused on both lateritic and bauxitic duricrusts developed over the Paleoproterozoic Greenstone Belt in the Brownsberg, Suriname. The duricrust samples have a relatively simple mineralogy (i.e., goethite, gibbsite, hematite, and kaolinite) but reveal, when observed at a microscopic scale, a complex history of formation with multiple episodes of dissolution/reprecipitation. The (U-Th)/He dating of 179 Fe-oxides subsamples shows that duricrusts sampled at the top of the Brownsberg plateau have ages ranging from &amp;lt;0.8 Ma to ∼19 Ma. In contrast, Fe-oxides extracted from detrital duricrust boulders collected downslope indicate formation ages up to 36 Ma. This age discrepancy may indicate that a main episode of physical erosion affected this region between ca. 30 and 20 Ma. Consistently, the bauxite sampled at the mountaintop indicates a younger phase of formation, with Fe-oxides recementing fragments of a preexisting bauxitic material older than ∼15 Ma. Geochronological data also reveal a long-lasting weathering history until the present day, with multiple generations of Fe-oxides in the bauxite and the duricrusts resulting from successive cycles of dissolution and reprecipitation of Fe-oxides associated with redox cycles. This long-lasting weathering history led to geochemical remobilization and apparent enrichment in some relatively immobile elements, such as REE, aluminum, and vanadium, especially in the duricrust sampled at the mountaintop. Our geochronological, mineralogical, and geochemical study of Fe- and Al-crusts from the Brownsberg mountain provide constraints on the evolution of environmental conditions prevailing since the early Oligocene in Suriname.

Nd and Sr Isotopes and REE Investigation in Tropical Weathering Profiles of Amazon Region

The exceptional development of lateritic profiles make the Amazon one of the best places to conduct a refined study on the effects of long-term tropical climate on the Earth's surface rocks. Concentrations of Nd and Sr isotopes, as well as rare earth elements (REE), Sr, and other trace elements were determined in six profiles in order to shed new light into element behavior and into the processes controlling the geochemistry and the transfer of these elements among the different compartments of the critical zone. Our investigation indicates natural isotope regional pulse transformations in the Oxisol at the top of all profiles. The ƐNd(t=0) values of the parent rock, mottled horizon, and lateritic duricrust are radiogenic similar to the Amazon craton signature. The thick Oxisol has ƐNd(t=0) values which are much more radiogenic and less variable, whereas it has Zr, Th, Y, and REE in higher concentrations. These issues highlight a complex process for the Nd system, and the following possibilities are proposed to explain this behavior: formation of authigenic cerianite and/or kaolinite, penetration of low-temperature weathering solutions along zircon fractures, and some vegetation uptake. These possibilities allow keeping the more radiogenic Nd isotopes, whereas the less radiogenic Nd is released especially in the Oxisol after REE mineral(s) weathering. The Sr system contrary to that of Nd became less radiogenic along the profiles and relatively to the parent rocks content, because nearly all Sr was removed after K-Rb feldspar and mica weathering. However, a mixture return of less radiogenic Sr from plant, biogenic aerosol, and rainwater may have helped let the weathering profiles become less radiogenic. These characteristics let the Oxisol even less Sr radiogenic but still permitting to identify the general parent rocks signature. In contrast, Nd is highly fractionated in the Oxisol relative to the parent rock. Thus, the REE and Sr behavior, similar in the six profiles, does not necessarily reflect only the parent rock geochemistry, but it also depends on the multiple surficial processes typical of the critical zone: weathering, pedogenesis, plant root activity, and rainwater composition which change the inherited primary minerals isotope signatures.

Boolean and fuzzy logic operators and multivariate linear regression applied to airborne gamma-ray spectrometry data for regolith mapping in granite-greenstone terrain in Midwest Brazil

Recent studies have proposed several possibilities of combining K, Th and U airborne gamma-ray spectrometry channels to generate predictive algorithms maps. These algorithms can be used for mapping regolith in deeply weathered terrains with residual, erosion, and deposition surfaces helping to developed strategies to better understand the regolith landscape and to improve geomorphology interpretation, and to identify mineral exploration target sites for primary (bedrock or saprolite) or supergene (hosted in lateritic duricrust) ore deposits. With the goal to easily map the regolith, two mathematical procedures were used on airborne gamma-ray spectrometry data in GIS software, validated by fieldwork on granite-greenstone belts in Midwest Brazil: 1. airborne gamma-ray spectrometry and altimetric data integrated in Boolean and fuzzy logic allowed segregating the areas with ferruginous and manganese residual lateritic duricrusts from erosional surface with rocks and saprolite with 90% of accuracy (κBoolean = 0.69 and κFAPO = 0.66) and 2. airborne gamma-ray spectrometry and altimetric data integrated with weathering stages in multivariate linear regression (basic statistic) helped establish the regional weathering intensity index, with acceptable error (r2 adjusted >0.6 and p-value < 5%). These two modeling techniques provide useful, accurate, rapidly and complementary regolith maps and can be applied in large regions for preliminary interpretations.

Badlands of Gangani in West Bengal, India: An assessment on account of geotourism development

Geotourism is a specialized branch of tourism emphasizing on economic utilization of the geomorphosites under the umbrella of tourism industry. The study focuses on the status of the badlands of Gangani located in the right bank of Silabati river of West Bengal in the context of geotourism development on account of its exceptional scientific, educational and aesthetic values as geoheritages. Formed during Pleistocene period with hard lateritic duricrust layer on the top, Gangani badland is famous as ‘Grand Canyon of Bengal’ for its escarpments, caves, pinnacles, mesa and butte like structures as well as earth pillars, saw-tooth ridges, natural roof like geomorphosites. We have primarily used ethnographic methods for data collection which involves personal interviews, questionnaire surveys and focus group discussion. A SWOT-AHP analysis has been undertaken further for estimating the geotourism potentiality of this wonderful badland topography. TOWS matrix is used to formulate strategies for optimal utilization of its geotourism resources. The result emphasises adaptation of imperative planning strategies for geoconservation in order to attain the sustainable tourism goals.

Advances in geochemical footprint, iron speciation, and Pb isotopes in the tropical regolith of north-northwestern Amazonia, Brazil

A dataset with information on samples of gabbro, charnockite, gneiss, granite, basalt, and sandstone-derived iron lateritic duricrusts, concretions, nodules, and soils was analyzed to address the multi-element chemical composition, background values, scavenger trace elements in Fe oxyhydroxides, parent rock inheritance, and weathering effect on Pb isotopes. Tropical weathering causes high mass loss of SiO2, Ba, Rb, and Sr assigned to the mineral rock framework (quartz, feldspar, and mica). The aqua regia leaching indicates that part of these elements was probably accumulated in authigenic Al-phosphates minerals affecting their mobility in the lateritic terrains. There is a generally Sc and V high accumulation in the lateritic duricrusts with Fe oxyhydroxide mineral framework (goethite, hematite, and amorphous). Th, U, and Pb are also accumulated on the gabbro and granite-derived lateritic duricrusts, Cr on the basalt-derived ones, REE on the gabbro-derived ones, as well as Th and U low to high loss on gneiss-derived ones, thus allowing to track inheritance of parent rocks. The more refined geochemical information obtained from the Pb isotopes related to the Fe oxyhydroxides of the lateritic duricrusts, concretions and soils indicated changes according to the parent rocks since the gabbro-derived lateritic duricrust samples are more radiogenic while the gneiss and granite-derived ones are less radiogenic. The basalt and sandstone-derived lateritic duricrust samples maintain the radiogenic signature of the respective parent rocks. Although homogenization was expected, the mineral, the main and trace elements, and Pb isotope compositions produced by leaching and accumulation in the tropical regolith highlight the parent rock signature and a complex weathering process.

A petrographic, mineralogical, and geochemical characterizations of the lateritic hardpans of Bamendjou in the western region of Cameroon

A combined petrographic, mineralogical, and geochemical characterization of the lateritic hardpan deposits of Bamendjou was carried out. The purpose was to better understand the process of laterization in a view to facilitate the broad use of this deposit in building applications. The petrographic analysis reveals the presence of protopisolithic, massive, and nodular facies. On the basis of the results provided by X-ray diffraction (XRD), these facies are composed of such phases as gibbsite, goethite, hematite, anatase, and kaolinite. In addition, boehmite has been found in the massive and protopisolithic facies, whereas gibbsite and magnetite frequently occupy the nodular facies. Whole-rock geochemical data indicates that a alumina represents the most abundant oxide in the protopisolithic and the massive facies (46.78–51.63 wt.% and 42.85–43.45 wt.%, respectively). It is followed by relatively high content of iron 12.93–18.33 wt.% and 23.43–23.78 wt.%, respectively) accompanied by very low contents of silica 1.44–2.75 wt.% and 2.44–3.99 wt.% of SiO2 respectively) and titanium 3.70–47.0 wt.% and 4.00–5.30 wt.%, respectively). Iron is the most abundant chemical element in the nodular facies since it ranges from 49.64 to 58.58 wt.%. The petrographic, mineralogical and geochemical geochemical (especially the high iron content) features suggest that the lateritic duricrust of the Bamendjou area have good potential to be utilized in stonework in buildings (bricks).

Episodic weathering in Southwestern Amazonia based on (U[sbnd]Th)/He dating of Fe and Mn lateritic duricrust

This study presents the results of mineralogical analysis and (UTh)/He dating of micrometric grains of hematite, goethite, cryptomelane and hollandite from six samples of bauxite, Fe, Mn, Mn-Al-Fe, MnAl and Mn-Al-P duricrusts located in southwestern Amazonia (Brazil). The samples were collected at different elevations to constrain temporal epochs of lateritization and to provide insights into the cratonic landscape evolution of Proterozoic and Paleozoic terrains in Brazil. Three main phases of duricrust formation were recognized, indicating a long period of lateritic weathering: The first phase, preserved as a fragment in Fe duricrust on the Paleozoic sedimentary rocks, began in the Permian (~290 Ma) and lasted until the Late Cretaceous (~65 Ma). The second (and best preserved) phase formed Mn duricrusts on the Paleoproterozoic Apuí lateritic surface (150–250 m a.s.l.) and on the Aripuanã lateritic surface (<170 m a.s.l.) between the Late Cretaceous (~90 Ma) and Oligocene (~32 Ma). The third phase of lateritic formation occurred from Late Oligocene to Early Miocene times (~28 to ~19 Ma). A degradation process, indicated by a goethite cortex on hematite fragments, took place in the Early to Late Miocene (~23 to ~13 Ma), affected all previous lateritic duricrusts, and can be correlated to an epoch of extensive bauxite duricrust formation in the region. These Fe, Mn and Al aggradation episodes indicate strong and widespread polyphasic weathering in southwestern Amazonia since ~290 Ma, as expected on stable cratonic terrains exposed to alternating hot, humid, and dry climate conditions over time.

Airborne radiometric data for identifying lateritic surfaces in southwestern Amazonia, Brazil

Airborne gamma-ray spectrometry and altimetric data have been used to recognize lateritic surfaces in southwestern Amazonia, Brazil. Three altimetric surfaces, marked by three main types of lateritic duricrust, comprise the landscape of the studied area. Surface 1 (SP1 ≤ 134 m a.s.l.) is dominated by a deep dissecting U-shaped valley, lowlands, steep slopes, and hills gathering Mn to Mn-Al-Fe duricrusts. Surface 2 (SP2–134 to 186 m a.s.l.) has rounded hills supported by Fe and FeAl duricrusts. Surface 3 (SP3–186 to 290 m a.s.l.) consists of strongly dissected plateaus to rounded hills sustained by bauxites, massive Fe duricrust, rare Mn-Al-Fe duricrusts, and Mn colluviums. Specifically, all lateritic duricrusts are associated with high values of eTh, and these data were employed in conjunction with SRTM data for generating favorability maps for lateritic surfaces using Boolean and fuzzy techniques. Altimetric data is an important input data in the boolean and fuzzy models since the boolean model overestimated the lateritic surfaces in surface 1, while not considering regional stratigraphic stacking when gathering ironstones as lateritic duricrust (lateritic duricrusts are older than ironstones). Both boolean and fuzzy models successfully recognized the lateritic duricrusts in surfaces 2 and 3 (134–290 m a.s.l.), even though the fuzzy product overestimated the lateritic surfaces above 134 m a.s.l.A lateritic index (LI = (eTh/eU) x K + (eTh x eU)) was also applied as complementary method in relation to the fuzzy and boolean models. LI divided the studied area into three main domains according to weathering intensity (lowest LI, low to intermediate LI, and high to extremely high LI associated with duricrust). Although the LI does not consider the altitude, the obtained model describes areas favorable for lateritic surface occurrences, being a useful and rapid method for preliminary large-scale mapping of lateritic surfaces. Mn duricrusts with K-bearing minerals were also identified by the intersection between F-factor (FK x (eU/eTh)) and eTh fuzzy membership maps. Nevertheless, geological control is important, since protolith influences the F-factor responses. The combination among the eTh fuzzy membership map, ternary map of K, eTh and eU on the RGB channels, the geological and soil map database and some chemical analysis allowed improving future research on supergene deposits as bauxite and manganese duricrusts, phosphorous and gold+sulphides.

Characterising lithium host minerals within the lateritic duricrust, Greenbushes, Western Australia

Whilst detailed geochemical analyses have been conducted to mitigate the complications regolith cover poses to modern exploration efforts, less attention has been directed to the mineralogical asso...

Supergene gold characterization by geochemistry, grain morphology and Au-Ag-Cu-Te classification

Primary characteristics and transformations during laterization processes were investigated in lateritic profiles integrating geochemical analysis and gold grain morphology provided by ICP-ES, electron microprobe and electron microscope in nine sites in three distinct gold provinces in Midwest Brazil. The acquired chemical data indicate a supergene concentration of Al, Fe, Au, As, Co, Cr, Cu and V allowing to detect higher leaching rates of the lateritic profiles in Area I where lateritic duricrust is well developed, relative the lower leaching related to truncated lateritic profiles in Area II. The totally free Au grains of veins, colluvium and alluvium deposits in area II and III show sub-rounded to rounded, irregular, elongated, delicate and polygonal-shaped grains with corrosion cavities, indicating primary vein source, feeble dissolution related to subtler lateritization effects and little mechanical reworking in the colluvium and alluvium deposits. These results allowed to identify subtler weathering effects and small distance transport for the Au grains collected in colluvium and alluvium samples. The quantification of Au-Ag-Cu-Te chemical composition of the free Au grains offered important preliminary discrimination, proving to be a fast, easy and useful approach. It allows classifying the studied deposits in five groups, strongly related to their primary sources (gold-rich porphyry deposits and epithermal deposits).

Cu and Zn isotope fractionation during extreme chemical weathering

Copper and Zn are trace metal micronutrients whose stable isotope systematics are receiving increasing attention as possible paleoenvironmental tracers. However, to realise this potential, their behaviour during chemical weathering must be better constrained. We present coupled Cu and Zn isotope data for a well-characterised Indian laterite weathering profile, which includes a full suite of samples from unaltered greywacke bedrock to indurated lateritic duricrust. This sample set provides an exceptional opportunity to interrogate Cu and Zn isotope compositions during an extreme example of chemical weathering. Despite their occurrence in different host phases within the parent greywacke, Cu and Zn isotopes behave coherently during weathering. We observe preferential loss of heavy isotopes at increasing degrees of alteration, with 0.6‰ total variability in δ66Zn and 0.9‰ in δ65Cu. Given the absence of evidence for CuS or ZnS phases in the parent lithology, we attribute the liberation of heavy isotopes to organic complexation in the aqueous phase and/or incorporation of light isotopes in secondary aluminous Fe-oxides. Strong enrichment of both metals is also associated with a peak in Mn at a previously identified paleo-water table horizon. This dataset confirms that weathering under oxygenated conditions releases isotopically heavy Cu, regardless of the host phase. Meanwhile, Zn isotopes are only fractionated to any significant extent at the most extreme degrees of chemical weathering reached during lateritization. We conclude that the isotopic composition of the weathering-derived input of Zn to rivers should be largely insensitive to climate change on geological timescales.

Factors driving mercury variability and background values in a tropical region: The case of western Amazonia

This study provides additional information about mercury sources and their distribution in the Amazônia regolith using a large dataset of samples collected from distinct geology, mineralogy, and tropical weathering effects. Furthermore, the Hg distribution allowed discussing the superficial dynamics and controversial question on the Hg natural or anthropogenic source. The dataset composed of 341 samples, allowed assigning Hg background values for western Amazônia of 305, 125 and 132 ppb, respectively, for lateritic duricrust and oxisol developed on Archean-Proterozoic Amazon Craton and Phanerozoic sedimentary rocks and for sediment along the Solimões and Madeira rivers compartments. The data show that natural weathering, leading to the formation of Fe/Al lateritic duricrusts and oxisols, promoted a very high Hg enrichment (up to 800%). Considering this feature, although Hg is naturally accumulated by weathering processes relative to rocks, the large enrichment may be potentially harmful and confirms studies that pointed out the key role of weathering as an efficient scavenger for metals like Hg in Amazônia. Moreover, it highlights the importance of tropical regions as a natural sink that should be better considered in the Hg cycle and as a possible natural Hg source.

Australian laterites reveal mechanisms governing scandium dynamics in the critical zone

Abstract Scandium is often considered as immobile during chemical weathering, based on its low solubility. In contrast to other conservative ( i.e. relatively immobile) elements incorporated into accessory minerals resistant to weathering ( e.g. zirconium, thorium or niobium), the scarcity of scandium minerals indicates that the processes accounting for scandium’s immobilisation are distinctive. However, the evolution of scandium speciation during weathering is unknown, limiting the understanding of the processes controlling its dynamics in the critical zone. Exceptional scandium concentrations in east Australian laterites provide the possibility of unravelling these mechanisms. We follow scandium speciation through thick lateritic profiles ( > 30 m) using a multiscale mineralogical and spectroscopic approach involving electron microprobe, laser-ablation–inductively coupled plasma mass spectrometry, selective leaching and X-ray absorption near-edge structure spectroscopy, complemented by mass-transfer calculations. We show that the initial reservoir of scandium contained in the parent rock is preserved under reducing conditions occurring in the lowest horizons of the profiles. The dissolution of scandium-bearing clinopyroxene generates smectitic clays that immobilise and concentrate scandium. It is subsequently trapped in the lateritic duricrust by goethite. Scandium mobilisation appears in this horizon and increases upward as a result of the dissolution of goethite, possibly assisted by dissolved organic matter, and the precipitation of hematite. Molecular-scale analyses demonstrate that changes in speciation govern scandium dynamics, with substitution in smectitic clays and adsorption on iron oxyhydroxides playing a crucial role in scandium immobility in the saprolite and lower lateritic duricrust. The higher affinity of scandium for goethite relative to hematite drives scandium mobilisation in the upper lateritic duricrust, leading to its concentration downward in the lower lateritic duricrust. These successive mechanisms illustrate how the unique complexity of the critical zone leads to scandium concentrations that may form new types of world-class scandium deposits. Comparison with conservative elements and with rare-earth elements, expected to have similar geochemical properties, emphasizes the unique behaviour of scandium in the critical zone. While scandium remains immobile during the early stages of weathering, intense and long-term alteration processes, observed in lateritic contexts, lead to scandium mobilisation. This study highlights the dependence of scandium mobility on weathering conditions.

Mineralization and element redistribution in the Chah-Gheib Ni-laterite ore zone, Bavanat, Zagros Belt, Iran

Nickel laterites are supergene deposits derived from the weathering of ± serpentinized ultramafic rocks and they commonly accounts for over 70% of world land-based Ni resources. Serpentinized ultramafic rocks outcrop extensively in Iran as result of collision between Afro-Arabian and Iranian micro-plate during the Late Cretaceous. Two Ni-bearing laterite belts developed on the ultramafic rocks of the Neyriz ophiolite have been recently discovered in the Bavanat region (South Iran). The Chah-Gheib deposit, as the biggest Ni-bearing laterite in the Bavanat area, hosts a total estimated reserve mineralization of about 102,000 tons of ore at 0.92% Ni and 180,000 tons at 1.63% Cr.This study focuses on elemental fractionation and mineralogical control of major, trace and rare earth elements in a selected profile from the Chah-Gheib Ni-laterite, in order to assess the conditions of ore formation and its paleo-environmental significance.Four distinct horizons, including saprolite, lower limonite, upper limonite, and lateritic duricrust, have been recognized from the bottom to the top of the studied profile. Iron oxy-hydroxides are the dominant phases found in the laterite profile. Overall, Fe and other first raw transition metals (i.e. Cr, Ni, V, Co, Cu, and Zn) show abundances higher than those of the bedrock. The main Fe and Cr carriers in the laterite deposit are Fe oxy-hydroxides and chromite, respectively, although authigenic Ni- and Cr-bearing palygorskite is also common throughout the profile.Elemental mobility, estimated using Th as immobile element, showed that the major oxides are largely depleted with the exception of Fe2O3 and CaO. Among trace elements, Cr is the element resulting strongly enriched in those levels where large chromite crystals occurs while V, Co, Ni, and Zn are enriched only in some levels of the upper limonite horizon and in the lateritic duricrust, likely as adsorbed cations onto iron oxy-hydroxides and as a consequence of an alkaline pH environment. Alkaline pH conditions can also explain the Cu depletion observed throughout the profile. Such a condition during lateritization appears further supported by both the occurrence of carbonates and palygorskite and by the distribution of the Ba/Sr ratio. Carbonates and palygorskite, as well as secondary quartz, require dry climate to form and alternating wet-dry climate may allow lateritization. This study suggests that the Chah-Gheib laterite formed in a subtropical climate, possibly during the Late Cretaceous-Late Paleocene period, in a roughly 25°–35° latitude range, promoting Si and Mg mobilization during the wet season and carbonates, quartz, and Mg-rich silicate precipitation during the drier periods.

Exotic sulphate and phosphate speleothems in caves from eastern Amazonia (Carajás, Brazil): Crystallographic and chemical insights

This study investigates the mineral, crystallographic and chemical characteristics of sulfate and phosphate speleothems and the physicochemical conditions that enabled their development in caves in the banded iron formations and lateritic duricrusts of the Carajás region, in eastern Amazonia, Brazil. The X-ray powder diffraction (XRD), Rietveld refinement, scanning electron microscopy (SEM), electron microprobe (EMPA), thermogravimetry, as well as Raman, infrared, and Mössbauer spectroscopy techniques were used in this investigation. The chemical composition analyses of the guano and cave dripping water were performed by atomic absorption spectroscopy (AAS), spectrophotometry and multi-parameter probe. The results of the analyses revealed aluminite, felsőbányaite, jarosite, spheniscidite, phosphosiderite and amorphous phosphatic material in coralloid type speleothems while spheniscidite, phosphosiderite and strengite were identified in stalactite type speleothems. These minerals are the product of the direct chemical reaction of the bat guano, urine and bones accumulation (with K+, Na+, Ca2+, Mg2+, NH4+) and the iron and aluminum supplied by the lateritic duricrusts from the walls, ceilings and floor of the caves, into a mixture of sulfates and phosphates.