Lateritic Profiles Research Articles

Evaluation of mechanical strength and physical properties of indurated laterite with regard to in situ characterization of lateritic profile

The majority of the geotechnical studies on lateritic materials in different parts of the world including India was carried out from the viewpoint of soil characterization and was not particularly meant for indurated laterites. With due need, in this study, mechanical strength and physical properties of indurated laterites from an abandoned quarry of Kharagpur (India) were investigated with regard to in situ characterization of the lateritic profile. Based on texture, appearance and relative strength of the lateritic materials, a total of four horizontal/sub-horizontal zones (Zones 1–4) sequentially lying below a thin humus zone existing on the top of the profile were identified. The indurated laterites were found to be confined within Zones 1 and 2 and were used to determine uniaxial compressive, point load and Brazilian tensile strengths; effective porosity; dry density; vug volumetric percentage (a newly introduced index); and slake durability indices by adopting a modified methodology. The pattern of variation of all these parameters with regard to the sample depth was found to be broadly in agreement with the in situ characteristics of the laterite profile. The quality of the indurated laterites for construction purposes was also assessed. The point load strength proved to be the most useful/practical strength parameter in characterizing indurated laterites. Both vug volumetric percentage and slake durability indices were found to be much more efficient than effective porosity and dry density in estimating the point load strength. The reasons behind all important observations are also elucidated in this paper.

Investigation of Fe isotope systematics for the complete sequence of natural and metallurgical processes of Ni lateritic ores: Implications for environmental source tracing

Metal isotopes are versatile pollutant source trackers, but biogeochemical processes can overprint or alter the original source isotopic fingerprint and thus hinder contamination tracing. Here, we explore Fe isotope systematics for the complete range of natural and metallurgical processes related to Ni lateritic ores from Barro Alto, Brazil, to assess its potential as a tracer in polluted lateritic soil contexts developed in an ultramafic system.The homogeneous δ57Fe values from protolith to soil confirmed that no significant Fe isotopic variation occurred during the formation of the deep lateritic profile. In addition, no Fe isotopic fractionation was found during the smelting process. Although the δ57Fe values resulting from mining activities fall within the range of terrestrial sample signatures, the conservation of the δ57Fe values from the ores to the by-products is an advantage for tracing anthropogenic sources when (i) the pyrometallurgical plant uses feeding material with Fe ores imported from other geological formations exhibiting different δ57Fe values and/or (ii) the by-products are transported or dispersed to other locations with different δ57Fe signatures in the topsoil.

Mineralogy and geochemistry of a bauxite-bearing lateritic profile supporting the identification of its parent rocks in the domain of the huge Carajás iron deposits, Brazil

A bauxite-bearing lateritic profile, developed along a borehole within the supergenic Fe-deposits of the Carajás Mineral Province in Brazil, is investigated after its textural aspects, mineralogy, and geochemistry. Petrographic studies, XRD mineralogical analysis, SEM-EDS, and whole-rock chemistry were used to study selected samples. The 156 m depth borehole is composed of basaltic andesite at the substratum and an overlying lateritic profile well-structured in distinct horizons, from base to top composed of: a saprolite, with rock relicts within a clayey matrix of mainly vermiculite and kaolinite; a clayey horizon dominated by kaolinite; a bauxitic horizon with gibbsite as the main phase; a ferruginous crust mainly composed of hematite and goethite. XRD mineralogical quantifications indirectly confirmed the chemistry of Si, Al, Fe, and Ti, as the most abundant elements in the lateritic horizons. Statistical factor analysis of the whole chemistry and mass balance calculations showed groups of elements arranged according to their enrichment or depletion character in the distinct horizons. Si, Ca, Na, and K are leached out to the top of the profile following the destabilization of their carrier mineral phases feldspar, hornblende, and clinochlore, whereas Al, Fe, and Ti are enriched to the upper horizons, respectively in the structure of gibbsite, hematite/goethite, and anatase. The basaltic andesite at the substratum has its parental affinity confirmed after textural changes, REE normalized patterns, and Zr/Ti ratios throughout the profile, nevertheless, parent rock variations are expected. The studied lateritic profile is comparable to most of the bauxite-bearing laterites in the Amazon region, except for its unparalleled 22 m thick Fe-rich bauxitic horizon and absence of a truncating clay cover on the top of the profile. The lateritic sequence was probably formed during the Paleocene-Eocene bauxitization event that reached the Amazon region and survived as a complete unaltered fossil lateritic profile through the time.

Berthierine-Rich Bauxites in the Kursk Magnetic Anomaly (KMA)

Studying with precision methods the mineral composition of bauxites in the Kursk Magnetic Anomaly (KMA) made it possible to reevaluate the conditions of their formation. Occurring among Early Proterozoic jaspilites in the Belgorod ore region, they are confined to the upper part of the Early Carboniferous lateritic weathering profile on the biotite–muscovite schists. The northern part of the region is dominated by the pseudopisolitic gibbsite ores; the southern part, by the boehmite varieties with an inherited layered structure of parent rocks. Along with aluminum hydroxides, berthierine is a rock-forming (major) mineral in these bauxites. Previously, berthierine was attributed to the subsequent transformation of bauxites. Our data suggest the existence of two berthierine generations. The first generation represents a natural intermediate mineral related to the formation of bauxites during Fe concentration in the weathered rocks. Therefore, we can substantiate a certain stage in the evolution of bauxite formation in the Earth’s history leading to the formation of Paleozoic deposits.

Mineralogy, geochemistry and parent rock of Décio bauxite-bearing lateritic profile (Rondon do Pará, Eastern Amazon)

Cenozoic world-class bauxite deposits developed on sedimentary sequences of the Parnaíba and Amazon Basins. The Décio pilot mine, excavated in the Rondon do Pará bauxite deposit, exposes a 3 m thick bauxite-bearing lateritic profile on the top of a 350 m high plateau. From the base to the top, this profile is composed of clayey bauxite; porous microcrystalline bauxite; spherulitic ferro-aluminous duricrust, fragmentary on the top; as well as loose ferro-aluminous spherulites and nodules, embedded in clay. The lower horizons represent a classic lateritic succession, while the top of the profile was produced through post-lateritic reworking. During this process, Ga, Cr, Mo, As, Pb, Hg, Sb, Zn, V and Sc were captured by the iron oxi-hydroxides; and REE, Hf, Nb, Ta, Y remained carried by relict zircon. Isocon diagram and mass balance demonstrate the affinity of the bauxite with claystone from Itapecuru Formation.

The Loma de Hierro Ni-laterite deposit (Venezuela): Mineralogical and chemical composition

Nickel laterite deposits developed on ultramafic rocks have traditionally been a significant source of Ni and Co and recently of Sc. Although the Loma de Hierro deposit (Venezuela) has been in operation for more than 50 years, it lacks detailed studies on the mineralogical and geochemical composition of the lateritic profile. In this study, we present a geochemical and mineral description of the main carrier phases of Ni and Co in a complete profile of the deposit. The selected weathering profile has been developed from a partially serpentinized harzburgitic protolith and has a well-developed saprolitic horizon covered by a thin limonitic horizon. The geochemical signature of the profile is characterized by a significant Mg and Si decrease towards the top of the saprolite, with a clearly visible Mg discontinuity. The main Ni-bearing minerals are secondary serpentine (1–4 wt.% NiO) and kerolite-pimelite-dominated garnierite mixtures with serpentine (18–22 wt.% NiO). Limonite is rich in goethite (0–1.85 wt. % NiO), gibbsite, and Mn-oxy-hydroxides. The latter have intermediate compositions between lithiophorite and asbolane (2–13 wt.% CoO). The highest Sc grades (40–68 ppm) were observed in the limonite with amounts positively correlated with Fe content. Rare earth elements are mainly concentrated in the upper part of the saprolite horizon (60–80 ppm), while they have a lower content in the limonite (7–45 ppm). In this horizon, rare earth elements are clearly associated with Fe, indicating adsorption and/or coprecipitation. This association was not observed in the saprolite, suggesting that other minerals (e.g., clay minerals) are controlling their concentration; more information is needed to identify the rare earth element-bearing minerals. The lateritic profile of Loma de Hierro can be classified as representative of hydrated Mg silicate deposits, and was formed in a context of continuous tectonic uplift and a low water table conditions favoring the development of a thick saprolitic horizon and the precipitation of kerolite-pimelite-dominated garnierites.

P.162 Characteristics, circumstances, and mental health service utilization of adult suicide: psychosocial autopsy study in Lithuania

Reconnaissance whole-rock chemical analyses for Hg and Ce in weathered Mn-oxide-rich rocks from the Quadrilátero Ferrífero, Minas Gerais, Brazil, show a positive linear correlation. In contrast, manganiferous duricrusts from lateritic profiles in south-western Amazonia, Brazil, have no Hg–Ce correlation. Considering the presence fault-related rock fabrics in the former, such as slickenside striae, breccias and veinlets, and the Hg sensitivity to seismic activity, as well as the adsorption of aqueous Ce onto Mn-oxide precipitates, it is proposed that the positive Hg–Ce correlation can be used as an indicator of seismically induced, low-temperature hydrothermal fluid flow in the weathering zone.

Evaluation of selectivity of sequential extraction procedure applied to REE speciation in laterite

Rare earth elements (REE) speciation in laterites are commonly estimated from sequential extraction procedure (SEP). In this study, the selectivity of a 5-steps SEP protocol was evaluated on individual synthetic Ce-material and applied to a well-characterized lateritic profile developing over a granite bedrock (Madagascar). The synthetic Ce-materials are representative of laterite minerals: Ce-doped oxyhydroxides and kaolinite, cerianite, Ce-rhabdophane and Ce-bastnäsite. The morphology, specific surface area, Ce sorption concentrations were thoroughly characterized prior to SEP. X-Ray absorption spectroscopy on synthetic material indicates that Ce(III) dominates apart for cerianite and Mn-oxide. The evaluation of the 5-steps SEP scheme shows that reagent selectivity is generally good, but the use of hydroxylamine hydrochloride is strongly discouraged. First, distinction between Ce sorbed at the surface of Mn- and amorphous or crystalline Fe-oxyhydroxides remains ambiguous. Second, Ce-rhabdophane (phosphate) and Ce-bastnäsite (carbonate) dissolve partially with this reagent. These experimental results combined with previous mineralogical and geochemical characterization were fundamental to interpret SEP results in the laterite profile from Madagascar. In the three laterite samples from the A-, B- and C-horizons, it is proposed that REE mainly distribute in authigenic phosphates and Fe- and (Mn-)oxyhydroxides. In the oxidized B-horizon, there is a significant LREE/HREE and Ce(IV)/Ce(III) fractionation between laterite minerals. While LREE-Ce is mainly concentrated in phosphates (alunite-jarosite supergroup), it is proposed that HREE and Ce(IV) uptake is dominated by oxyhydroxides. In the A- and C-horizons, the LREE and HREE speciations are similar, indicating no significant LREE/HREE fractionation in laterite minerals and organic matter. In all horizons, the REE ion exchangeable is negligible, although kaolinite is a main mineral of the A- and B-horizons.

Chromium occurrence in a nickel laterite profile and its implications to surrounding surface waters

Ultramafic rocks are considered as the most important geogenic sources of Cr whose hexavalent species is of environmental concern. In tropical to subtropical areas (e.g. New Caledonia, Brazil, Philippines), prolonged and pervasive weathering of ultramafic rocks produces Ni laterite ores which are further enriched in Cr. While elevated levels of Cr have been identified in Ni laterites, Cr availability and export from these deposits and its contribution to natural waters remain poorly understood. Therefore, this study examined the speciation and flux of Cr in a Ni laterite profile through sequential extraction and leaching experiments coupled with physical, mineralogical, and geochemical characterization. These were correlated to the behavior and export of Cr in surrounding surface waters studied through hydrochemical investigation. This work demonstrates, through the example of the Rio Tuba Ni laterite deposit, that pervasive weathering of ultramafic rocks in tropical areas generate very important reservoirs of Cr(VI). Although Cr is primarily fixed in insoluble fractions (>89%) and least in bioavailable and easily mobilizable phases (<1%), due to the inherent elevated Cr content (up to 2.9 wt%) of the Ni laterites, the latter still provided water-extractable Cr(VI) contents surpassing drinking water and freshwater standards. By comparison with the silicate-rich saprolite unit, the overlying Fe-(oxyhydr)oxide dominated laterite layer yielded lower Cr(VI) contents due to its acidic nature and higher amounts of organic matter and Fe-(oxyhydr)oxides which promote reduction and readsorption of Cr(VI). During water infiltration, Cr(VI) migrates downward along the profile at a rate of 62 to 3446 t/km2/yr. While the alkaline and oxidizing conditions of the surrounding surface waters favor Cr(VI) mobilization, only a fraction of Cr(VI) from the Ni laterites reaches these water bodies. Cr(VI) concentrations (≤213 μg/L) and fluxes (mostly ≤50 kg/km2/yr) in the surface waters being significantly lower than that of the Ni laterites reflect the action of processes (e.g. dilution, reduction, adsorption) that attenuate the release of this species along water flow paths.

The use of mercury vs. cerium in recognising seismically induced fluid overprints on weathered profiles

Reconnaissance whole-rock chemical analyses for Hg and Ce in weathered Mn-oxide-rich rocks from the Quadrilátero Ferrífero, Minas Gerais, Brazil, show a positive linear correlation. In contrast, manganiferous duricrusts from lateritic profiles in south-western Amazonia, Brazil, have no Hg–Ce correlation. Considering the presence fault-related rock fabrics in the former, such as slickenside striae, breccias and veinlets, and the Hg sensitivity to seismic activity, as well as the adsorption of aqueous Ce onto Mn-oxide precipitates, it is proposed that the positive Hg–Ce correlation can be used as an indicator of seismically induced, low-temperature hydrothermal fluid flow in the weathering zone.

Behavior of major, trace, and rare earth elements in an atypical lateritic profile overlying micaceous quartzites, Centre Cameroon: imprint of the parent rock structure

An atypical lateritic weathering profile overlying micaceous quartzites in central Cameroon was studied to understand its morphology and the behavior of major and trace elements (including rare earth elements; REE). Mineralogical analyses were performed by X-ray powder diffraction (XRPD). Major and trace element geochemistry were determined using X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), respectively. The studied micaceous quartzites consist of 1 to 2 cm thick quartzitic platelets separated by millimeter to multi-millimeter beds of ferromagnesian minerals (biotite, hornblende, garnet and, epidote), muscovite, graphite, and magnetite. According to chemical index of alteration, mafic index of alteration, index of lateritization, the A – Fe – Mg – Ca – Na – K diagrams, and the Weathering index of granite, the intense hydrolysis of ferromagnesian beds resulted in the genesis of soil unit consisting of quartz, muscovite, illite, kaolinite, anatase, and hematite which occur within rotten quartzites or coarse saprolites (CS). Thus, the weathering profile presents the following macromorphological organization: micaceous quartzite/CS-soil/CS-soil/CS-soil/CS-soil/ CS-soil/nodular materials/loose stony clayey soil. The pH-Eh diagrams indicate that formation of these soils within the saprolitic zone is the result of differential weathering that occurred under acidic-reductant conditions, which enhanced the rapid weathering of ferromagnesian mineral beds. The main pedogenetic processes along the weathering profile are translocation of REE, the transformation of primary minerals into neoformed-clays and Fe-oxide-hydroxides formation (associated mainly with light REE losses), and the addition of organic matter in surface horizons (associated with an accumulation of transition metals, high field strength elements, and heavy REE).

The Cenozoic deposits of the ancient landscapes of Quadrilátero Ferrífero highlands, Southeastern Brazil: Sedimentation, pedogenesis and landscape evolution

The Quadrilátero Ferrífero (QF), Southeastern Brazil, is a very important tropical highland region in the world’s geological context for its large and diverse Archean and Proterozoic rocks with great reserves of gold, iron, manganese, aluminum and industrial rocks, on a apparently stable geological structure. There, Cenozoic deposits perched on highland valleys show unclear genetic relationships with the underlying bedrock, with no apparent regional correlation. We studied five representative Cenozoic deposits (BR356, Água Limpa, Padre Domingos, Pau Branco and Casa de Pedra) on the highlands of the Serra da Moeda syncline, Western QF, to answer the question of their sedimentological origin, and investigate their pedological evolution. Field sampling was complemented by macromorphological, mineralogical and micromorphological analysis supported by X-ray diffraction (XRD), differential thermal analysis (DTA), infrared analysis (IR), scanning electron microscopy (SEM), micropobre and transmission electron microscopy (TEM). The evolution of these isolated highland deposits comprises a deep-weathered source area, a tectonic activity, besides sedimentological and pedological processes, during and after the deposition. The deposits overlie deep saprolites of Precambrian rocks (Piracicaba and Itabira Groups), representing unconformable contact. The onset of the deposition was marked by torrential, coarse colluvial and large blocks landslides into the small basins generated by reactivating tectonic events during the Oligocene, producing local grabens. These tectonic basins were filled by cohesive debris and mudflow from the adjacent and previously laterized cover, developed under the hot and humid climate in the Eocene. The cover reveals an upside-down lateritic profile where morphology and kaolinite crystal properties (values of size of coherent scattering thickness ranging from 135 Å to 162 Å) in the bottom is related to the pedolith horizons of the former lateritic cover. Later, due to climate changes (during and after the Miocene), renewed weathering on these pre-weathered sediments occurred, characterized by alternating ferruginization and Fe-losses, with the superimposed generation of new pedogenic features such as nodules, ferruginous duricrusts and mottling (redox features). In addition to demonstrating that the Brazilian platform was not stable during the Cenozoic, these deposits reveal the role of polygenetic tropical pedological processes in their formation and transformation.

Tourmaline composition and boron isotopes record lateritic weathering during the Great Oxidation Event

AbstractIdentifying evidence of oxidative weathering in the geological record is essential to trace the evolution of Earth's atmosphere oxygenation. Metamorphosed residues of lateritic weathering have been identified as two rock types in the 2.1‐Ga‐old Cercadinho Formation, Piracicaba Group, Quadrilátero Ferrífero of Minas Gerais. One is tourmaline–hematite–sillimanite–kyanite quartzite; the other is rutile–tourmaline–hematite–muscovite phyllite. Both rocks have abundant tourmaline with δ11B values between about −17‰ and −13‰. The Cercadinho tourmaline is roughly parallel to the povondraite–“oxy‐dravite” join of meta‐evaporitic tourmaline, in its more aluminous segment, offset to higher contents of iron. These compositional and isotopic characteristics of the Cercadinho tourmaline indicate that continental evaporitic brines interacted with aluminium‐ and iron‐rich residues of lateritic weathering. The abundance of disseminated tourmaline, a mineral poorly reported from palaeosols worldwide, implies a boron‐rich brine overprint on the lateritic profile before the onset of metamorphism, reflecting a climatic change from humid to arid conditions in a continental setting. The recognition of lateritic weathering in the Cercadinho Formation contributes to the amount of evidence for increased levels of atmospheric oxygen between 2.22 and 2.06 Ga ago.

Mineralogy and geochemistry of the Berong Ni-Co laterite deposit, Palawan, Philippines

Intense weathering of the serpentinized peridotite resulted in the formation of the Berong Ni-Co laterite deposit in Palawan (Philippines). The studied laterite profiles are divided from base to top into: (i) serpentinized bedrock, (ii) saprolite horizon, and (iii) limonite horizon. Olivine in the peridotite contains a significant amount of Ni (average 0.38 wt% NiO), thus representing the main source of the ore in the laterite horizons. Six types of Ni-bearing serpentines (up to 6.65 wt% NiO) in the saprock are discriminated based on their textures and Raman signatures. Garnierite (a mixture of serpentine-, kerolite- and sepiolite-like clays) typically occurs as coatings or fracture fillings in the saprock and is characterized by strikingly high NiO content up to 34.86 wt%. Goethite and Mn-oxy-hydroxides (i.e. lithiophorite-asbolane intermediates) are the principal hosts of Ni in the limonite (up to 2.15 wt% NiO and 23.89 wt% NiO, respectively) with the latter also being the main Co carriers (up to 12.59 wt% CoO). The whole-rock XRF data show typical trends observed in other Ni laterite deposits, where Mg and Si are depleted toward the surface, and the transition metals (i.e. Fe, Al, Ni, Co, and Mn) are retained in the weathered horizons. The saprolite horizon is particularly enriched in Ni, while the lower section of the limonite horizon is enriched in Co. The mildly acidic environmental condition (pH ≤ 6.57) in the limonite horizon was found to enhance the mobilization and re-distribution of Ni from the limonite towards the lower section of the profiles. Geological observations, coupled with mineralogical and geochemical data, show that the Berong Ni-Co laterite deposit formed in two weathering processes: (i) development of the saprolite and the limonite horizons, and (ii) late precipitation of secondary phyllosilicates (i.e. garnierite), Mn-oxy-hydroxides and silica.

Tectonic and climate control on allochthonous bauxite deposition. Example from the mid-Cretaceous Villeveyrac basin, southern France

Karst bauxite deposits represent 11% of the global quantity of bauxite (Bardossy and Aleva, 1990). Karst Bauxite is defined as bauxite deposits overlying carbonate rocks above a more or less karstified surface. Allochtonous karst bauxites result from two successive processes: (1) intense weathering and formation of a lateritic profile, involving the development of bauxitic and duricrust horizons; and (2) erosion of the lateritic profile and deposition above a carbonate unit located downstream. In the Villeveyrac basin (southern France), Lower to Mid-Albian allochtonous karstic bauxites are deposited on a Jurassic karstified bedrock, and are overlain by a Late Albian marine siliciclastic cover. A drillcore, sampling the substratum, the bauxites and their cover, is analyzed and offers the opportunity to investigate the depositional conditions and interactions of climate and tectonic during the mid- to late Cretaceous, through the analysis of mechanical and chemical weathering evidence. Sedimentological study allowed the definition of four bauxite facies and three siliciclastic cover facies, overprinted by four different pedofacies. Mineralogical and geochemical analyses allowed the distinction of four mineralogical units controlled by climate-related weathering processes. The bauxite deposits emplaced by mud-flow/debris-flow gravity currents are composed by boehmite-hematite and goethite. The top of the bauxite presents a progressive upward decrease in Al2O3 and TiO2 associated with the occurrence of kaolinite. The base of the cover that overlies the bauxite interval corresponds to a paralic environment, dominated by kaolinite, with a sub-ferricrete paleosol. The following paralic sequence presents a clay-rich suite dominated by smectite, associated with mica/illite and quartz, which suggests aridification of climate around the latest Albian. We interpret this sedimentary succession as the result of erosion of an autochthonous laterite cover, induced by major tectonic uplift in the upstream drainage area, which led to a significant increase in slope gradient, under a wet tropical climate. Mechanical erosion of autochthonous laterite bauxite in the upstream drainage led to a reverse stacking of the reworked autochthonous lateritic profile in the depositional basin. A continuous transgressive trend was responsible for trapping of the allochtonous karst bauxite. This was followed by stacking of siliciclastic paralic sequences, which record progressive erosion of the bedrock in the upstream drainage area, under dry tropical conditions. Our results thus suggest that dismantling of the autochthonous laterite cover was initiated by Albian uplift and this ended with a change towards drier and more seasonal climate at the Albian-Cenomanian transition.

On detailed field-based observations of laterite and laterization: a study in the Paschim Medinipur lateritic upland of India

The term ‘laterite’ and the process ‘laterization’—both have raised perpetual scientific debates. The present study reviews the relevant scientific pieces of literature associated with tropical red lateritic and ferralitic soils and tries to justify them with the observations in the Paschim Medinipur Lateritic Upland. This particular area is practically the natural laboratory for observing the laterization process. The area exhibits a distinctive geomorphologic, hydrological, and climatic characteristic over an evolutionary geological setting. The favorable tropical climate, intense weathering, groundwater movement with seasons, and neo-tectonic upliftment exerts complex influences on the pedo-geomorphological distinctiveness of this area. Presence of planation surfaces, typical scarp development along the surface margins and exposure of deep lateritic profiles by basal sapping, accelerated erosion of the margin, and formation of rills and gullies with waxing slope development are worthy of being synced with available scientific understandings. The study is based on extensive field observations, profile studies, and soil sample analysis from different geomorphic units. The study reveals that the role of tectono-climatic evolution has had a massive influence on characteristics and development of the present lateritic formations. Transportation and deposition of Neogene ferruginous sediments over basin shelf region as paradeltaic formation, alternating wet (June–October), and dry (November–May) monsoonal climate with adequate rainfall and iron segregation and relative accumulation along with neo-tectonic modification are the important considerations regarding ferrugination process in the area under study. The justifications of the observed attributes of this particular tropical lateritic landscape may be used as a reference work for future scientific studies and model building processes on the same in this area as well as the other parts of the world.

Gibbsite as low-temperature hydrothermal overprint on lateritised Cenozoic sediments, Quadrilátero Ferrífero of Minas Gerais, Brazil

ABSTRACT Gibbsite, typically a clay-sized mineral in lateritic profiles, occurs as crystals between 50 and 150 µm across in druse-like cavities that characterise veins hosted in lateritised Cenozoic sediments, exposed along a road cut in the Quadrilátero Ferrífero of Minas Gerais, Brazil. Both the veins and the host rock, a red diamictite, contain gibbsite, but the drusy gibbsite in the former is remarkably larger than the matrix gibbsite in the latter. It is suggested that the coarser gibbsite is a low-temperature hydrothermal mineral. The coarse grain size of gibbsite and its occurrence in veins comprise important criteria to recognise hydrothermal overprint of low temperature on lateritic profiles.

A minimum Thanetian (Paleocene) age for the African Surface in the Eritrean highlands, Northeast Africa

Supergene alunite from a supergene leached capping zone in the Daero Paulos area near Asmara, Eritrea yields Thanetian (late Paleocene) K–Ar ages of 58.6 ± 2.3 and 59.4 ± 2.3 Ma. The leached capping is exposed in an erosional window through a laterite profile, with the supergene alunite forming by oxidation of pyrite contained in quartz veinlet stockworks hosted by porphyry copper-related, felsic porphyry and adjacent volcano-sedimentary country rocks. Geologic relationships show that the leached capping and contained supergene alunite formed simultaneously with the lateritic weathering event that accompanied regional planation of equatorial and subequatorial Afro-Arabia to form the African Surface. Landscape modification of the central Ethiopian-Eritrean plateau by incision of transverse valleys and canyons commenced in the Oligocene, with a principal exhumation event occurring in the Miocene in response to swell-induced rift-flank uplift during Red Sea opening.

Ferricretes at Burringurrah (Mount Augustus), Western Australia: Proof of lateral derivation

Surficial iron-rich deposits (ferricretes) occur on and around Burringurrah (Mount Augustus), a huge, remote inselberg in the Gascoyne district of Western Australia. Oriented NW-SE, Burringurrah is approximately 14 km long, 5 km wide, rises to 1105 m above sea level and stands some 700 m above the surrounding plain. The majority of the ferricretes occupy bedrock valley bottom floors on the southern side of the inselberg at Edney Springs, Flintstone Creek, and on the Edney Trail where they have formed on unweathered quartzose Mount Augustus Sandstone, which only contains a few percent Fe2O3. At ‘The Pound’, a boulder ferricrete overlies deeply weathered igneous bedrock, unroofed by erosion of the overlying Mount Augustus Sandstone, exposing the unconformity between the weathered granitic bedrock and the Mount Augustus Sandstone in the core of a broad, asymmetrical and doubly plunging anticline. On the northern, steeper, side of the mountain, vermiform and conglomeratic ferricretes are sparsely exposed in drainage lines. Ferricrete mineralogy is dominated by goethite with only small amounts of hematite. Oxygen isotope inferred ages of weathered monzogranite produced a top-down weathering front extending up to the Neogene at an elevation coincident with the ferricretes within bedrock valley bottoms. Beyond the mountain, the age of ferricreted sandy alluvium was established by thermoluminescence polymineral fine grain dating. These produced a minimum depositional age of >37.6 ± 2.6 ka and a second finite age of 139 ± 44 ka, suggesting ferricrete formation here may have begun during the last interglacial period and continued throughout the late Pleistocene. Formation of ferricretes on top of iron-poor quartzites is unusual. It reflects not only the absolute accumulation of iron oxides derived from the Mount Augustus Sandstone but possibly the additional contributions of iron-bearing aeolian dust from the surrounding plains subsequently washed down drainage lines to accumulate in swampy depressions at the foot of the inselberg. Ferricrete formation was initially simple, with iron in solution impregnating sandy sediment and organic host materials, sporadically impacted by bioturbation by termites. Thin-section petrography reveals an interesting form of in-situ, self-brecciation of quartz and feldspar clasts, which are fractured and infilled with iron oxides and newly precipitated clay which further promotes fragmentation. The mode of ferricrete formation described from Burringurrah is relevant to the laterite-ferricrete debate, for here it is most unlikely that the zones of iron enrichment developed in situ and vertically above the iron-poor quartzite basement as in the sense of the regional ‘classic laterite profile’.

Unique role of pore water in lateritic bauxite formation, Republic of Guinea

Results of the hydrogeochemical study of bauxite-bearing lateritic mantles of the Futa Djallon-Mandingo Province, West Africa, have allowed us to first characterize the pore water of vadose and infiltration hydrogeological zone. Abundances of major components (aluminum and iron) of the water are shown to be few orders higher than are those typical of the other waters of lateritic mantles. The pore water is proved to be a major factor of matter redistribution over any bauxite-bearing lateritic profile.