Lateritic Surface Research Articles

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.

Geophysical modeling of the manganese deposit for Induced Polarization method in Itapira (Brazil)

This paper presents the results of the investigation of a supergene manganese occurrence in the southeast of Brazil, using the Induced Polarization geophysical method. This study aims to characterize the surface and subsurface morphology of one of these occurrences, named São Roque, based on the contrast of the electrical properties of the ore and the host rocks. The ore, which is composed mainly of manganese oxides and hydroxides and subordinated graphite, occurs in the form of small discontinuous lenses hosted by quartzites and schists of the Itapira Group. The geophysical survey consisted on 10 lines of Induced Polarization Tomography using a Wenner-Schlumberger array, with an along-line electrode separation of 10 m for all lines. The IP data were modeled through commercial inversion software to generate 2D section models of chargeability. In order to create a 3D visualization model of chargeability, the 2D inversion models were then combined and interpolated in order to create a 3D visualization model. The 3D model revealed two independent ore bodies in both surface and subsurface, characterized for high chargeability (up to 20mV/V), instead of a single elongated orebody as suggested by the lateritic surface in the field. In addition, the model showed that the orebodies are elongated perpendicular to the area main structural trend and the regional alignment of the lateritic occurrences, which can bring new ideas in terms of local exploration strategies for manganese in this context.

Nd-isotope evidence for the distal provenance of the historical (c. &lt;3000BP) lateritic surface cover underlying the Equatorial forest in Gabon (Western Africa)

Highlights: • The first Nd isotopic data on the lateritic surface cover (Cover Horizon) of western Equatorial Africa. • Clearly different Nd isotopic signatures between the Cover Horizon and underlying basement. • A consistent model attributing the Cover Horizon to the settling of aeolian particles derived from the Namib desert. Abstract: Surficial formations in Gabon, as well as in other places of western Central Africa include a ubiquitous, homogeneous and 1–3 m-thick clayey to sandy lateritic surface cover known as the ‘Cover Horizon’. From 14C radiometric dating it has been concluded that the emplacement of this unit was correlative with a major environmental crisis which affected Central Africa c. 3000–2000 years ago. 10Be and Nd-isotopic analyses have been performed to provide new constraints on the age and origin of this layer. Six samples from two depth profiles investigated for 10Be exhibit an almost constant concentration consistent with a very recent deposition age. Nd-isotopic analyses performed on the silt to clay fraction of eleven samples from widely spaced locations over Gabon attest for mildly radiogenic signatures (eNd = −23 to −17) in ten of them, and a slightly radiogenic signature (eNd = −9) in one sample. TDM model ages range from 1.6 to 2.6 Ga, and a perfect discrimination is observed between the Nd-isotopic signature of the Cover Horizon and that of the underlying Congo Craton. This makes an aeolian origin as the most probable for the Cover Horizon. The average eNd (c. −20) is however rather unusual for aeolian sediments or aerosols. A possible source of particles is therefore tested by considering the present-day atmospheric flux over Gabon and adjacent regions. Combined atmospheric modeling and Nd-isotopes leads to the conclusion that the fine fraction of the Cover Horizon could have originated from the northern part of the Namib desert.

First 40Ar/39Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India

Lateritic surface processes have shaped large platform and cratons of the tropical belt. Constraining the timing of such processes is crucial to decipher their role in cratonic morphogenesis and their response to long-term climatic change and lithospheric deformation. Weathering histories have been documented for South America, Africa and Australia, but precise time constraints of the lateritic weathering processes in South India are still lacking. We present 40Ar/39Ar ages of supergene cryptomelane (K–Mn oxide) formed in the Sandur Mn ore deposits exposed on the highest lateritic paleolandsurface that once covered the Mysore plateau and the adjacent Deccan Traps. Significant 40Ar/39Ar ages are estimated between ∼36 and ∼26 Ma from well-defined plateaus in step heating 39Ar release spectra and from best-fitted inverse isochrones. These ages constitute firm time constraints that document intense late Eocene–Oligocene lateritic weathering over Peninsular India under the influence of warm and wet climate comparable to that prevailing in tropical humid forests. These results imply that Southern India was weathered between ∼36 and 26 Ma and may have been dissected mostly in the Neogene.

Evidence for an aeolian origin of the Holocene lateritic surface cover of Gabon (Central Africa)

Field, granulometric and geochemical data are reported for a set of twenty-one samples collected at six sites spread over Gabon in the extensive lateritic clayey to sandy layer (Cover Horizon) which constitutes the upper part of the profiles of surficial formations. Granulometric analyses show that this material of homogeneous aspect is a mixture of two fractions: a clay fraction and a medium- to coarse-grained sandy fraction. Relative proportions of these two constituents show strong variations from one site to another but are almost constant from the bottom to the top of a given profile. The coarse-grained fraction includes lithic elements of local provenance. Irrespective of their underlying basement, the samples display many common geochemical features including a lack of Ca, Na and Mg. Their major element compositions may be accounted by the combination of a discrete number of mineralogical phases, namely quartz, kaolinite, gibbsite, Fe-oxides/hydroxides and subordinate K-minerals (micas). The normative contents of aluminous and ferrous phases regularly increase with the weight-percent of clays. Conversely, the quartz regularly decreases. This inverse relationship is consistent with mixing between an Al- and Fe-bearing clay fraction and a coarse-grained fraction dominated by quartz. Correlations between most immobile trace elements and the weight-percent of clays indicate a control of those elements by Al- and Fe-bearing phases. The main exception is for Zr and Hf, as those elements are probably associated with zircons in the coarse fraction. Because of the control of the trace element distribution by secondary phases, the geochemical signatures of all samples are clearly distinct from those of the upper Continental Crust and any basement rocks. Thus, the trace element characteristics of the Cover Horizon carry no sign of any source-material, being the sole consequence of mineralogical sorting prior to deposit.From field, granulometric and geochemical data, an aeolian origin appears as the most probable for the Cover Horizon. Geochronological constraints, including archaeological data and 14C dating, indicate that this aeolian event took place during the Holocene and probably occurred in less than 3000 y. Additional works will be necessary to assess the global context which made possible this very fast and extended event over large areas of Central Africa.

The development of iron crust lateritic systems in Burkina Faso, West Africa examined with in-situ-produced cosmogenic nuclides

We have investigated the development of iron crust laterites on the stable West African Craton in northern Burkina Faso using cosmogenic radionuclides produced in situ in quartz veins and pebbles. Lateritic soils develop in tectonically stable, slowly eroding, tropical environments and are a major component of the Earth's surface. To examine processes affecting laterite formation, we determined 10Be and 26Al in samples of quartz from three sites representing two sequential and connected iron crust laterite systems. Results from outcropping quartz veins suggest that the mean erosion rate in this region is about 3–8 m Myr −1. In addition, data from quartz cobbles and pebbles incorporated in iron crusts demonstrate that depth-dependent distributions of these nuclides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss. Results from sections of the lowland lateritic system are consistent with mean accumulation rates of a few metres per million years. Quartz cobbles, presently at depths of a few metres in a paleochannel filled with rapidly deposited fluvial-colluvial material, have 10Be distributions that suggest that the lowland lateritic surface may have formed during an erosive episode, presumably associated with a wetter climate, roughly 300 kyr BP. These results illustrate the practicality and the potential of the use of in-situ-produced cosmogenic nuclides for understanding the history of formation of laterites and for differentiating between systems formed through in-situ chemical weathering and mechanical transport.

Composition and origin of lateritic nickel ore at Tagaung Taung, Burma

The recently discovered lateritic nickel ore deposit on the summit area of the Tagaung Taung contains about 40 million DMT nickel-saprolite ore with approximately 2% Ni. The serpentinite massif is covered with a thick weathering mantle consisting of saprolite, limonite, and an allochthonous lateritic surface layer. The thickness, nickel content, and physical properties of the ore are very variable. The main minerals in the nickel saprolite ore are nickeliferous serpentine and smectite. Microprobe analyses show a strong depletion of Mg with increasing nickel content in the serpentine minerals; smectite generally contains less nickel than serpentine. Chemical analyses of samples from more than 100 drill holes are used to calculate the mean chemical compositions, mean accumulation factors, and gain and loss of major and trace elements for the various layers. This permits a quantitative approach to be made to the genetic history of the weathering mantle. Gain and loss determinations on the basis of constant chromium content and of constant volume give nearly identical results for the saprolite layer. Calculations on the basis of Ni/Cr ratios indicate that three-quarters of a former 20-m-thick limonite layer has been eroded away. Only the lower part of the original limonite layer with a relatively high SiO2 content is preserved. The kaolinite- and illite-rich surface layer is a mixture of limonite and weathered clayey material, which was probably derived from adjacent rocks. Part of the autochthonous weathering section is contaminated with infiltrated allochthonous surface material. It is not possible that colluvial-alluvial deposition of clayey material from nonultramafic rocks could have taken place under the present geomorphic regime. Thus, lateritization of the serpentinite and deposition of siallitic material must have occurred before exposure of the Tagaung Taung in its present form.

The Works of Living Social Insects as Pseudofossils and the Age of the Oldest Known Metazoa

Living organisms are known to create structures in ancient rocks that are indigenous but not primary and that have been mistaken for fossils. Examination of burrows recently reported as fossils from 10(9)-year-old sedimentary rocks indicates that they are not the same age as the rocks but were probably made by termites working down after water. The burrows are partially filled with material from a modern lateritic surface from which they descend into steeply dipping, decomposed silt-stones of the Zambian Copperbelt. In fact, no authentic record of Metazoa that are demonstrably coeval with rocks older than 680 million years is known.

Geomorphic mapping in parts of Kerala state

The paper discusses the advantages aerial photographs offer for a quick appraisal of geomorphic surfaces in a terrain of low to moderate relief, high vegetative cover with considerable human activities. The area studied comprises the coastal tracts between Calicut and Ernakulam and the midlands immediately to the east. Two morphological units, namely the costal plain and uplands constitute the terrain between the Western Ghats and the sea. The costal plain from west to east comprise a narrow beach, a younger beach ridge plain, an older beach ridge plain and lateritic surface, generally plateau shaped with gentle to moderate slopes, terraced, dendritically dissected and with insignificant development of pediment. Further eastwards, the areas of high relief consists of hard crystalline rocks laterised at the top. A filled lagoon near Trichur and the flood plains of major rivers which cut through different geomorphic units are significant. Recognition of isolated, discontinuous patches of highly lateritised Tertiary sedimentaries from other lateritic surfaces is often difficult.

Genesis of low iron bauxite, northeastern Cape York, Queensland, Australia

Several small deposits of bauxite with an iron content of less than 2.0 percent were discovered recently on the remote northeast tip of Cape York Peninsula, Queensland, Australia. The low iron bauxite occurs in the low parts of a gently undulating pre-Pliocene lateritic surface which formed on Mesozoic sediments. The low iron bauxite grades into low-grade high iron (>15.0% Fe 2 O 3 ) bauxite on the ridges of the pre-Pliocene surface through a thin transition zone. In the transition zone the high iron bauxite changes downward into red pisolitic bauxite with white (low iron) cement, through bauxite containing pisolites with red cores and white rims in white cement, to all white low iron bauxite. In places boulder-size blocks of dense, red high iron bauxite occur as scattered remnants in the white low iron bauxite. The low iron bauxite is overlain by brown carbonaceous sandstone, whereas the high iron bauxite is exposed or covered by sand containing no carbonaceous material. The abundance of carbon and plant remains indicates that the Eh and pH of water draining through the brown sandstone since its deposition would have been moderately low. The bauxite is highly permeable except for the dense blocks. The low iron bauxite was formed by leaching of iron from preexisting high iron bauxite. The leaching of iron is thought to have been accompanied by both precipitation of alumina and removal of free quartz. In the transition zone between the high and low iron bauxites leaching is incomplete.

Genesis of bauxite deposits in the Boolarra‐Mirboo area of Gippsland, victoria

Abstract Bauxite deposits in Gippsland are small, low in grade, and buried under a thick cover of Tertiary sands. Occurring at various stratigraphic levels in the Thorpdale volcanic sequence, they are also texturally and compositionally primitive and hence now presumably fossilised. Although broadly comparable with deposits in Tasmania and New England, they show few obvious analogies with other Australian bauxites. Profile studies reveal a somewhat erratic vertical mineralogical zonation which can, at least in part, be accounted for by assuming a degree of post‐bauxitisation planation so that some profiles are now truncated. Some profiles show gibbsite increasing with depth and at least in the Boolarra profile this has significant genetic implications. Here, during submergence of the lateritic surface, rising carbonated connate waters precipitated scarbroite and this, with subsequent renewed leaching, decomposed initially to form nordstrandite, and finally gibbsite.