Lateritic Weathering Research Articles

Mineralogy of Lateritic Weathering Profiles Developed on the Rocks under a Sub-equatorial Monsoon Climate: Case of the Bambouto Mountains

ABSTRACT The main objective of this work is to make a comparative assessment of the mineralogy on weathering profiles resulting from the alteration of the different rocks in an equatorial microclimate of altitude. This was carried out in the lower flank of the southern slope of the Bambouto Mountains. Mineralogical analysis and normative restructuring have given very interesting results on the profiles, between the basement rocks and the surface/cover rocks. Given that the alteration pathways of the basement rocks are different, they are similar within the zone of containment and potential leaching. Meanwhile, the cover rocks vary from one domain to another: virtual weathering, potential confinement, potential and virtual weathering, and induration. The alterological activity is very intense in the profile on trachybasalt. The predominant weathering processes in all cases are allitization and monosialitization, which are marked by a singular phenomenon of ferrolysis. The characteristic minerals of this profile are kaolinite and diopside. The weathering profile on biotite and hornblende granitoid is not very differentiated and is particularly rich in quartz (max: 40 weight-%) and microcline (max: 40 weight-%). The soils developed on granitoid, adopt elastic-fragile deformation to elastoplastics with sharp break in relation to the load applied. The alteration profile on orthogneiss is poorly differentiated, and thin, with a low degree of alteration. Hydrolytic alteration by monosiallitization marked this profile. This is particularly rich in quartz (max: 100 weight-%) and kaolinite (max: 57 weight-%). The profile on anatexite is particularly rich in quartz (max: 66 weight-%) and kaolinite (max: 40 weight-%). The normative restructuring confirmed over 98 weight-% of the various minerals obtained and thus made it possible to better characterize the alteration and its application in geotechnics. These soils constitute potential deposits of chromatogenic minerals: kaolinite, hematite, goethite, gibbsite, magnetite, biotite.

Low Mo mobility during the laterization of ultramafic bedrock: Evidence from the East Sulawesi Ophiolite, Indonesia

Mo (isotope) cycling during the chemical weathering of ultramafic bedrock remains poorly quantified, mainly as a result of the analytical challenges caused by low Mo concentration and complex matrix effects in these rock types. Here, we utilize an improved chemical separation protocol that enables the extraction of Mo while reducing Ru and Fe matrix effects. We apply this method to lateritic weathering profiles developed over ultramafic bedrock in a high-intensity tropical weathering regime. The Mo concentrations in the laterite samples are higher (0.022 to 0.58 μg·g−1) than those of the peridotite bedrock (0.006 to 0.021 μg·g−1). The concentration-weighted average δ98Mo of the laterite profiles is −0.05‰ (n = 17), which is slightly higher but very close to the average δ98Mo of the peridotite bedrock (0.17 ± 0.21‰; 2SD; n = 5). Weakly-laterized samples show somewhat low δ98Mo with a minimum of −1.03‰ and Δ98Molaterite-bedrock up to −0.86‰, possibly as a result of preferential adsorption of liberated light Mo onto Fe (oxyhydr)oxides. In contrast, strongly-laterized samples show an overall Mo concentration gain and a slight isotopic shift towards higher bulk δ98Mo, with a maximum δ98Mo of +0.12‰ and Δ98Molaterite-bedrock up to +0.42‰. This likely reflects the re-scavenging of Mo released from weakly-laterized horizons to the ubiquitous Fe (oxyhydr)oxides, with potential superimposition of additional heavy Mo from atmospheric and/or groundwater input. Overall, this suggests a small contribution of dissolved Mo derived from ultramafic bedrock weathering in tropical settings to the aquatic environment.

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.

НОВЫЕ ДАННЫЕ О МИНЕРАЛОГИИ, ГЕОХИМИИ И ГЕНЕТИЧЕСКИХ ОСОБЕННОСТЯХ БЕЛОГОРСКОГО МАГНЕТИТОВОГО МЕСТОРОЖДЕНИЯ (СИХОТЭ-АЛИНЬ)

New data are in favor of the view previously substantiated by geological, geochemical and isotopic data that orebodies of the Belogorskoe deposit are metamorphosed and partially regenerated in the Late Cretaceous-Paleogene products of erosion (in the Triassic) of laterite weathering crust of gabbroids. The source for the formation of the Belogorskoe deposit was shown to be products of exogenous destruction of rocks, which by their isotopic and geochemical characteristics are close to the Cambrian gabbroids of the Vladimiro-Alexandrovsky massif (the southern part of the Okrainsko-Sergeevsky terrane). It has been found that the Belogorskoe deposit is composed of rocks and ores, the primary (magmatic) distribution of REE in which was changed to varying degrees as a result of interaction between sediments (protoliths) and seawater (presumably during the Late Jurassic-Early Cretaceous accretion), as well as their metamorphosed analogues and hydrothermal solutions (in the Late Cretaceous-Paleogene). The influence of gabbroids (as a source of matter) on the chemical and mineral composition of the Belogorskoe deposit is consistent with the data reported in the article concerning the enrichment of orebodies in such elements as Fe and Mn that are characteristic of ultrabasic rocks and the presence of Au-Ag-Pd-Pt, Ni-Co and Bi mineralization in them. The orebodies of the Belogorskoe deposit contain accessory minerals and mineral varieties that are rarity in nature and little-studied. These include an unusually rich Th variety of zircon, baddeleyite, gudmundite, a large group of bismuth compounds, including Bi2Te, (Ag,Pb)BiS2, as well as coloradoite, lafossaite, sanbornite, perovskite, and the compound InPO4. There is also a large group of rare and unusual compounds of noble metals: copper gold, platinum gold, jonassonite, disordered solid solutions of Cu, Ag and Au (Au-based), Pt-Pd, Pt-Ag intermetallics, and other rare minerals and mineral varieties.

Nickel isotope fractionation during intense weathering of basalt: Implications for Ni output from continental weathering

Nickel isotope fractionation during continental weathering is not well constrained due to the limited research on Ni isotopes during weathering of mafic rocks such as basalt. This study investigated two weathering profiles (strongly and extremely weathered) derived on basalts in tropical China. The strongly weathered profile was dominated by kaolinite and divided into three layers with depth (soil, saprolite and rock). The extremely weathered profile was mainly composed of Fe- and Al-(oxyhydr)oxides and kaolinite, and included a special ferricrust layer, iron-rich duricrust with ferruginous nodules, between the soil and saprolite. We analysed the Ni speciation (e.g. Fe-oxide phases and residual phase) and isotopic composition of soil, ferricrust, saprolite, rock and water samples in the weathering profiles. Pore water and groundwater were collected underlying the extremely weathered profile, and the size-distribution of particulate Ni (1 kDa–450 nm) in water samples was analysed.Nickel in both profiles was mainly hosted in the residual phase (>71% of total Ni) which included primarily secondary silicates and Al-(oxyhydr)oxides. Weathering intensity controlled the mineral compositions and physico-chemical properties of the weathering profiles as well as Ni isotopic variations. Nickel redistribution (∼30%) and isotope fractionation (Δ60Niregolith-rock = −0.08‰ to 0.07‰) were slight in the strongly weathered profile due to the dominance of secondary silicates in Ni speciation. For the extremely weathered profile, significant Ni depletion (∼67%) and isotope fractionation (Δ60Niregolith-rock = −0.25‰ to 0.32‰) were related to the transformation of secondary silicates into Fe- and Al-(oxyhydr)oxides during lateritic weathering. The positive correlation of δ60Ni and Ni concentrations indicated that Ni isotope fractionation was controlled by Ni adsorption and incorporation of secondary minerals. In the lower saprolite, the calculated isotope fractionation between Ni remaining in the profile and leaching to the solution (Δ60Nisolid-solution = −0.47‰) indicated that Ni isotope fractionation was mainly controlled by the preferential adsorption of isotopically light Ni on Fe- and Al-(oxyhydr)oxides. However, in the soil and ferricrust, acidic pH (∼5.0) might reduce Δ60Nisolid-solution to −0.28‰ by releasing the isotopically light adsorbed Ni, leading to the predominance of incorporated Ni in Fe- and Al-(oxyhydr)oxides. Specifically, the highest δ60Ni value (0.27 ± 0.05‰) and Ni concentration in the upper saprolite indicated that the isotopically heavy Ni leached from the soil and ferricrust was scavenged by precipitating secondary silicates. Finally, the estimated isotopic composition of Ni leached from the extremely weathered profile (δ60Nioutput = 0.02 ± 0.15‰) was close to that of pore water (0.06 ± 0.01‰) where Ni was mainly present in the particulate phase. Compared with groundwater (δ60Ni = 0.33 ± 0.04‰) dominated by dissolved Ni, Ni output from the profile likely consisted of isotopically heavy dissolved Ni and light particulate Ni. Overall, this study provides a comprehensive understanding of Ni redistribution and speciation in basalt weathering profiles, highlighting the significance of weathering intensity in controlling Ni isotope fractionation and its implications for Ni outputs from continental weathering.

Source‐To‐Sink Sedimentary Budget of the African Equatorial Atlantic Rifted Margin

AbstractDespite their very low relief and erosion rates, non‐orogenic (i.e., cratonic) continental domains account for over 60% of the Earth's exposed lands. Therefore, they contribute significantly to the clastic sediments and solutes exported to the ocean and should be accounted for in global studies. Nonetheless, they have been much less studied than orogenic domains. In this study, we establish the source‐to‐sink sedimentary budget of the sub‐saharan West African cratonic domain and its Equatorial Atlantic rifted margin using published low‐temperature thermochronological data to estimate onshore denudation and regional geological cross‐sections to estimate offshore accumulation. We show that during and immediately following rifting (130‐94 Ma), the build‐up and subsequent erosion of rift‐related relief resulted in a transient, 100–200 km wide strip along the margin recording high denudation rates (>50 m/Myr), while the inland domain underwent steady and very low denudation (<10 m/Myr). Afterward, the whole onshore domain underwent very low and steady denudation. Thus, the changes in post‐rift accumulation rates documented along the rifted margin were caused by changes in the climate and/or drainage network. During the Late Cretaceous, we document a regional rise in accumulation rates caused by the enlargement of drainage areas feeding the basins by a hinterlandward migration of the continental divide. During the Paleogene, we document a general drop in accumulation rates in all the basins of the African Atlantic margins caused by the global greenhouse climate, which enhanced the development of lateritic weathering mantles, storing clastic sediments on the continent and favoring solute exports to the ocean.

BAUXITES OF THE TATAR DEPOSIT (YENISEI RIDGE, RUSSIA): THE FIRST EVIDENCE OF CONTACT-KARST GENESIS

The Tatar bauxite deposit on the territory of the Russian Federation was formed as a result of sedimentation in contact-karst depressions of the products of denudation of lateritic weathering crusts of amphibolites. Detailed mineralogical studies of bauxites made it possible to reliably reconstruct the conditions for their formation. As it turned out parent rocks and weathered before rocks occurred due to the close location of the areas of nutrition and accumulation and denudation. At the same time, chemical processes was continuing in karst depressions. For the first time, the presence of nanoparticles of amorphous aluminum oxide was revealed into contact-karst bauxites. This characteristic feature of the form of alumina precipitation is associated with the subsequent cessation of lateritization processes and their attenuation with depth. The presence of amorphous aluminum monohydrate must be taken into account when choosing a scheme for bauxite enrichment.

Mechanisms leading to exceptional niobium concentration during lateritic weathering: The key role of secondary oxides

Niobium (Nb) is considered as one of the most immobile elements during supergene weathering due to its low solubility and the expected resistance of the pyrochlore mineral group (A2B2X6Y) in which Nb occupies the B site. Although the resistance of pyrochlore is challenged by direct evidence of alteration in lateritic profiles, the geochemical and mineralogical processes controlling its behavior in the critical zone remains elusive. This study uses a multiscale geochemical, mineralogical and spectroscopic approach to monitor Nb solid speciation in a thick weathering profile. The weathering of the pyrochlore-bearing core carbonatite facies from Morro dos Seis Lagos has resulted in a Fe-enriched laterite with exceptional Nb concentrations (2.91 wt% Nb2O5 on average). The preservation of the successive secondary phases (Ti-, Fe-, Ce-oxides) resulting from weathering processes along the horizons offers the opportunity to track Nb during pyrochlore alteration from the fresh parent rock to the final stages of weathering. Intense lateritization processes have led to an enrichment in Fe and Ti in the pyrochlore B-sites, resulting in the weakening of the structure and its ultimate destabilization due to weaker B-O bonds. After the release of Nb from pyrochlore, the fate of Nb is dependent on the secondary phases formed during weathering. In the lower horizons dominated by Fe oxides, goethite is the main host for Nb (1 wt% Nb2O5) due to its capacity to substitute Fe3+ for Nb5+, in contrast to hematite. In the upper horizons, where Ti oxides have formed during the reworking of the laterite, Nb5+ is preferentially incorporated into brookite and rutile (20 wt% Nb2O5), which may have formed owing to slightly acidic pH and high Nb activity during their growth. The presence of Nb in hollandite and cerianite in a vein from the manganiferous horizon involves the transport of Nb with mobile elements at the scale of the laterite. With this study, we describe a new model accounting for the formation of Nb-enriched laterites. The latter is based on the ability of Nb5+ to substitute for Fe3+, Ti4+, Ce4+ in minerals formed by lateritization processes after its release from pyrochlore. The nature of the secondary Nb-bearing oxides is dependent on the chemistry of the parent rock and the conditions of formation of the laterite. This work, evidencing the mobilization of Nb both at the scale of the mineralogical assemblage and of the entire profile questions the pervasive use of Nb as a geochemical invariant in the critical zone.

Bauxites of the Tatarka Deposit (Yenisei Ridge, Russia): The First Evidence of a Contact-Karst Origin

The Tatarka bauxite deposit on the territory of the Russian Federation was formed as a result of sedimentation of the products of denudated lateritic weathering crusts of amphibolites in contact-karst depressions. Detailed mineralogical studies of bauxites made it possible to reconstruct reliably the conditions for their formation. As it turned out, the initial rocks and rocks, weathered before, have been denudated due to the close location of the areas of alimentation and accumulation. At the same time, chemical processes have been continued in karst depressions. For the first time, the presence of nanoparticles of amorphous aluminum oxide was revealed in contact-karst bauxites. This specific feature of the form of alumina precipitation is associated with the subsequent cessation of lateritization and their attenuation with depth. The presence of amorphous aluminum monohydrate must be taken into account when choosing a scheme for bauxite enrichment.

The development of laterite weathering profiles as a function of rainfall and time: A geophysical approach

AbstractLaterite weathering profile (LWP) thicknesses are functions of precipitation rate and time, but their exact dependence on them is uncertain. We investigate LWP development on ground surfaces in Hawai'i that are neither aggrading nor eroding across substrates from 0.01 to 4 Ma and rainfall rates of <250 to >3000 mm/a. The Hawaiian Islands provide an excellent opportunity for LWP studies across climates and over millions of years on a single rock type, basalt. LWP weathering rates are usually determined by geochemical approaches. We present a geophysical method once bedrock ages and precipitation rates are known. We employed multichannel analysis of surface waves and horizontal‐to‐vertical spectral ratio methods. Results indicate that >70% of the variability in LWP thickness is due to precipitation and bedrock age. The remainder is attributed to measurement uncertainty and heterogeneity in the permeability of basalt. LWPs develop by two paths. Dry (<1000 mm/a) areas have a negative water balance with evapotranspiration exceeding rainfall. LWPs thicken until they reach a steady state where the storage capacity of the saprolite precludes the percolation of water into subjacent lava. In wetter areas, downslope interflow produces thick laterite wedges near coastlines that migrate upslope over ~1 Ma. Subsequently, they thicken and reach a steady state where precipitation cannot deliver water through the vadose zone. LWPs' thickness (T) increases as a function of time (t) and precipitation (P) according to the expression (95% confidence): . The first partial derivative, or weathering rate, is . Weathering rates decrease with time and are strong functions of t and P. These expressions add considerable insight into the rates and processes driving weathering at large scales and can also be solved for t, giving a rough estimate of the time of landscape formation of eroded surfaces.

Composition of river sediments from Kerala, southwest India: Inferences on lateritic weathering

Composition of river sediments from Kerala, southwest India: Inferences on lateritic weathering

High-grade iron ores in the laterite weathering crust after the banded iron formation in the Simandou mountain range, Republic of Guinea

In West Africa, in the southeastern Guinea, in the Simandou Range region underlain by the Archean gneiss-granite basement, a north–south synclinorium structure is extended, that is composed of the Lower Proterozoic series of metamorphic rocks. A thick sequence of the banded iron formation (itabirites) with interlayers of phyllites and mica schists is exposed in cores of the synclines. According to their composition, the itabirites are subdivided into quartz-magnetite and amphibole-quartz-magnetite varieties. High-grade iron ores (60–66% Fe) formed in a pseudomorphic manner after the itabirites in the Cenozoic lateritic weathering crust. At the North Simandou, Zogota, and Pic-de-Fon deposits, the rich ores compose mantle-shaped orebodies up to 150–350 m deep. The orebodies possess the following vertical zonality: primary magnetite itabirites – martitized itabirites – quartz-martite friable ore – martite friable ore and martite–limonite friable ore – goethite-hematite friable to solid ore – deluvial limonite ore (kanga and cuirass). All the types of the high-grade ores are dominated by hematite. The rich iron ores were formed from itabirites as a result of the oxidation of magnetite to martite, the almost complete removal of silica, and the subsequent additional supply of iron and its precipitation in the form of hydroxides. During the formation of the iron ores, kaolin clays and bauxites were formed after the phyllite interlayers.

The World’s Largest Bauxite-Bearing Province, Fouta Djallon–Mandingo (West Africa). Part 4: a Zoning Mechanism in Laterite Bauxite-Bearing Weathering Crust

The results of detailed exploration and year-round observations of the hydrogeological regime at large deposits, as well as special hydrochemical studies of various types of water and changes in the gas composition of the underground atmosphere in the lateritic weathering crust, of the very large bauxite-bearing province, Fouta Djallon–Mandingo, thanks to a reliable factual base, made it possible for the first time to draw a number of important conclusions about the conditions and features of zoning formation in the lateritic weathering profile. The weathering profile consists of two metasomatic columns spatially combined in plan, but vertically separated: an upper one, consisting of hydroxides and oxides of iron and aluminum, and a lower one, composed of clayey eluvium and products of hydration and decomposition of aluminosilicates—polymineral clays and kaolinite. In both columns, the processes of change begin with the same rainwater, but differ in the hydrodynamic parameters in each of the columns. Changes in the weathering profile occur in accordance with the main patterns of low-temperature infiltration metasomatism. The modern hydrogeological and gas regime in bauxite-bearing lateritic covers controls the mineralogical and geochemical zoning. Features of biological and soil processes and hydrodynamics and hydrochemistry in the weathering profile lead to a threshold nature of the change in physicochemical and biochemical conditions, providing the action of geochemical (oxidation, reduction, gley, adsorption) barriers, which is the main mechanism of zonal distribution of matter in lateritic weathering crusts.

THE WORLD’S LARGEST FOUTA DJALLON–MANDINGO BAUXITE PROVINCE (WEST AFRICA): PART 4. THE MECHANISM OF ZONING FORMATION IN LATERITE BOXITE-BEARING WEATHERING CRUSTS

Obtained as a result of detailed exploration and year-round observations of the hydrogeological regime at large deposits, as well as special hydrochemical studies of various types of water and changes in the gas composition of the underground atmosphere in the lateritic weathering crust of the largest bauxite-bearing province FDM, thanks to a reliable factual base, allowed us to draw a number of important conclusions on the conditions and features of the formation of zoning in the lateritic weathering profile for the first time. The weathering profile consists of two metasomatic columns spatially combined in plan, but vertically separated: the upper, lateritic cover, composed mainly of the products of the final hydrolysis of lateritic weathering, that is, hydroxides and oxides of iron and aluminum; the lower one, composed of clayey eluvium, products of hydration and decomposition of aluminosilicates - polymineral clays and kaolinite. In both columns, the processes of changing begin with the same rainwater, but differ in hydrodynamic parameters in each of the columns. Changes in the weathering profile occur in accordance with the main patterns of low-temperature infiltration metasomatism. The modern hydrogeological and gas regime in the bauxite-bearing lateritic covers controls the mineralogical and geochemical zoning. Peculiarities of biological and soil processes, hydrodynamics and hydrochemistry in the weathering profile lead to the threshold nature of the change in physicochemical and biochemical conditions, providing the action of geochemical (oxidative, reducing, gley, adsorption) barriers - the main mechanism of zonal distribution of matter in lateritic crusts.

Sediment routing systems to the Atlantic rifted margin of the Guiana Shield

Abstract Sediment routing systems of cratonic domains have not been studied extensively because their relief and erosion rates are very low, although their vast dimensions allowed them to contribute to a significant proportion of the sediments exported to the global ocean. To gain further insights into the behavior of cratonic sediment routing systems at geological time scales, we investigated the Guiana Shield and its Atlantic rifted margin (i.e., the Guiana-Suriname and Foz do Amazonas Basins, northern South America) over the Meso-Cenozoic with an emphasis on paleoenvironment and accumulation histories of the offshore sediments. We show that the basins of the Guiana Shield rifted margin record (1) periods of very low siliciclastic supply concomitant with the development of carbonate platforms, alternating with (2) phases of higher siliciclastic supply associated with sand-dominated clastic deposits and turbidites. Low siliciclastic supplies reflect either very limited rift-related relief growth and erosion such as during the Central Atlantic rifting in the Late Jurassic or intense lateritic weathering of the cratonic source area during Paleogene–Miocene climate optima. Higher siliciclastic supplies correspond either to (1) periods of rapid rift-related relief growth and erosion such as during the Equatorial Atlantic rifting (Early Cretaceous), (2) periods of drainage reorganization over a steadily eroding cratonic domain (Late Cretaceous), or (3) periods of tapping of sediments stored in the Andean retro-foreland basins via the presentday Orinoco and Amazon Rivers (Plio-Pleistocene).

Pseudo-Karst Silicification Related to Late Ni Reworking in New Caledonia

Silicification in New Caledonian pseudo-karsts developed on peridotite was assessed using δ18O and δ30Si pairs on quartz cements. The objective was to document the chronology of pseudo-karst development and cementation relative to geomorphic evolution. The latter began at the end of the Eocene with the supergene alteration of peridotites and the subsequent formation of extended lateritic weathering profiles. Neogene uplift favoured the dismantling of these early lateritic profiles and valley deepening. The river incision resulted in (i) the stepping of a series of lateritic paleo-landforms and (ii) the development of a pseudo-karst system with subvertical dissolution pipes preferentially along pre-existing serpentine faults. The local collapse of the pipes formed breccias, which were then cemented by white quartz and Ni-rich talc-like (pimelite). The δ30Si of quartz, ranging between −5‰ and −7‰, are typical of silcretes and close to the minimum values recorded worldwide. The estimated δ18O of −6 to −12‰ for the fluids are lower than those of tropical rainfall typical of present-day and Eocene–Oligocene climates. Evaporation during drier climatic episodes is the main driving force for quartz and pimelite precipitation. The silicification presents similarities with silcretes from Australia, which are considered predominantly middle Miocene in age.

Erosion mode and history of Eastern Adamaoua landscapes (Cameroon): Superimposed lateritic weathering of granites and basalts

Landscapes of eastern Adamaoua highlands have been shaped by successive weathering and erosion processes of Pan-African crystalline rocks and Neogene volcanics cover. Basalts have outpoured through major inherited structural discontinuities mostly in shallow incisions of lateritic pediplains previously shaped on the granitic basement. That has resulted in formation of a singular composite lateritic weathering profile on basalt superimposed to a truncated profile composed of mottled clays and saprolite on granite. The composite profile studied here is little evolved, mostly kaolinised on granite while basalt is weakly to moderately lateritised owing to differences in silica and iron oxide contents of the two contrasted parent rocks. During lateritic weathering, Cu, Ni, and Co are enriched in profile on granite but depleted on basalt, while Zr/Ti is relatively constant. Behavior and fractionation of REE are comparable except higher Eu* and Ce* anomaly on granite profile than on weathered basalt, owing to parent minerals differences, mostly feldspars in granite versus plagioclase and Fe-Mg minerals in basalt. The contrast in Eu* is also linked to differences in the Index Of Laterization (IOL) in weathered horizons of the two parent rocks. Beyond quite classical litho-dependent geochemical differences in the composite profile, persistence of per-humid climate and good drainage over Neogene have sustained rock-weathering and local surface erosion processes upon volcanic outpours and their contact surface with pediplains formed on Pan-African granitoids of Adamaoua highland.

Geological setting, petrography and mineralogy of laterites from Swarnagadde plateau of Western Ghats, Karnataka

The word “laterite” has been applied to such a diverse range of geomorphic features that it no longer has value as a precise descriptive term (Paton and Williams, 1972) [19]. The present paper gives an account of Geological Setting, Petrography and Mineralogy of Laterites from Swarnagadde Plateau of Western Ghats, Karnataka. The most widespread occurrences of laterites within the Western India have been recognized in a series of different geological-geomorphological settings, and are found in two distinct geomorphological zones. Mineralogical characterization of laterite and parent rocks could help in understanding the process of Lateritisation and bauxitization. While genesis of laterite is subjected to the process of alteration and it mainly decipher by the composition of preexisting rocks and also processes involved in lateralization/bauxitization. Mineralogy of lithomarge zone is predominated by kaolinite that of aluminous laterite zone by gibbsite. But, laterite and ferruginous laterite zones are predominated by both gibbsite and goethite with good amount of heamatite. In the entire lateritic weathering profile quartz occurs ubiquitously as the main resistant relic mineral. The occurrence of anatase and ilmenite is restricted to aluminous laterite zone.

SEDIMENTARY MANGANESE DEPOSITS IN CARAJÁS, BRAZIL

The Carajás Mineral Province in Brazil is actually one of the most important in the world because it contains world-class mineral deposits (Fe, Cu, Au, Ni and Mn), partly enriched by lateritic weathering (Fe, Mn, Ni). It carries at least four manganese ore deposits: Azul, Buritirama, Sereno-Conquista and Buriti, of these Azul and Buritirama are productive mines. The Azul deposit is the earliest known and was initially considered as typically lateritic. Further studies show that much of the reserves are classically associated with carbonaceous gray to black mineralized shales and enriched in manganese oxyhydroxides, where cryptomelane is the major ore mineral, enveloped by thick packets of red siltstones and sandstones of lower age Proterozoic (around 2.1 Ga). The mineralogical composition of rocks and ore, isotopic data, abundance of carbonaceous organic matter and the presence of stromatolite structures show deposition in a shallow platform, where the source of manganese and associated metals were mainly of ocean hydrothermal origin, while the sediments of the rocks would have a main continental source from Archean rocks. Rhodochrosite is restricted and diagenetic, likely pyrite. Tectonic deformations reached the whole package and provided environment and structures for remobilization and reprecipitation of Mn-oxyhydroxides of high content and crystallinity. These two types of ores were the source of lateritic ore and colluvionary, already practically depleted

Iron quartzites of the Simandou mountain (Republic of Guinea)

In this article, we for the first time fully describe the geology and petrography of the iron quartzite formation, localized in Southeast Guinea in the Simandou mountain range. In this region, a meridional distribution belt of banded magnetite quartzites (itabirite) of the Lower Proterozoic Simandou metamorphic series has been mapped. The rocks form an extended (210 km) and narrow (0.4-7 km) complex synclinal structure in the background of the Archean granite-gneiss basement. The structure of the Simandou metamorphic series includes the lower sequence of metaterrigenous rocks up to 400-500 m thick (metasandstone, quartzite, phyllite, mica schist) and the upper itabiritic sequence up to 220-450 m thick. The metamorphism of the rocks of the series increases from north to south from the greenschist to epidoteamphibolite facies. Members of two varieties of banded iron rocks are distinguished in the itabiritic stratum - itabirite and amphibole itabirite. The composition of itabirite is (wt %): magnetite 51-55, quartz 48-36, and the rest is cummingtonite and mica. The composition of amphibole itabirite is (wt %): magnetite 39-47, quartz 31- 30, amphibole (cummingtonite) 24-15, the rest is hedenbergite, mica and calcite. Rich martite and hematitegoethite ores (60-66% Fe) are formed in the lateritic weathering crust after itabirite, forming manto deposits with a depth of 9 to 350 m.