Lateritic Cover Research Articles

Integration of airborne magnetic and geological data for mineral prospectivity mapping in the Ambam-Amvom rainforest area, Archean Ntem Complex, southern Cameroon

The accurate mapping of mineral resources in rainforest areas is challenging due to dense vegetation and thick lateritic cover obscuring rock outcrops. This study utilized airborne magnetic survey data to map geological features and target mineral resources in the Ambam-Amvom area at the NW margin of the Congo craton. Linear and non-linear filtering algorithms were used, including reduction to the equator (RTE), upward continuation (UC) at 1000 m and 2000 m, first vertical derivative (FVD), and analytic signal (AS). Field surveys were carried out for sampling and petrographic investigations. The TMI map showed magnetic signals varying from -4489 nT to 2317 nT, while the RTE map displayed magnetic signals varying from -2984 nT to 3528 nT. The RTE-Reg map exhibited anomaly signal variations ranging between -2704 nT and +3637 nT and between -2829 and +3864 nT, respectively for UC at 1000 m and 2000 m. Geological structures were interpreted from the FVD, AS, and TMI-RTE images, and linear structures of regional extend mainly trend NW-SE, E-W, and NE-SW. Field investigations revealed that the study area comprises iron formations, amphibolite, gneiss, gabbro, biotite granite, and charnockite. The bulk chemical composition of iron formations showed a high content of TFe, averaging 39.80 wt.% for itabirite, 53.79 wt.% for hematite-geothite ore and 59.63 wt.% for quartz-hematite ore. The zones of peak magnetic anomalies coincided with the occurrence of the greenstone belt, and the integration of aeromagnetic and geological data led to the identification of three main target zones for iron ore minimization. High fracture density zones within the research were identified as high prospective targets for gold mineralization. The study's findings highlight the high potential of the Ntem Complex for mineral resources and provide valuable information for future research and development.

Influence of thick lithomargic soil cover on Bouguer gravity low: Imprints from passive continental margin of southwestern India

The Periyar Plateau in central Kerala is characterized by a strong negative Bouguer gravity field around three major islands of a strongly negative field of the value of -150 mGal. Detailed gravity studies reveal that these islands of strong negative field are of a shallow nature and are not traceable in the 1/4° average gravity grids. Spectral analysis of Bouguer anomaly map and 2D gravity modelling of Periyar Plateau region reveal the average depth of the shallow source layer is as shallow as 700-1000 m, which is correlatable with the layer of lateritic/lithomargic soil cover and the underlying weathered country rock having a density of 2.2 g/cc. Such a strong influence on the regional gravity field is due to the wide contrast in the densities of the lateritic cover and the primary rocks of the basement. These results underline the need to recognize the strong influence of such alteration zones on the regional gravity fields, especially the tropics, a factor not always considered.

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.

(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 <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.

Reading the climate signals hidden in bauxite

The dynamics of tropical weathering through time with the formation and long-term evolution of laterite associated with climatic and geodynamic forcing is still a poorly explored issue. To better access this, we focused on lateritic-bauxitic duricrusts from the well-explored, 300 m high Kaw mountain in northeastern French Guiana. Macroscopically homogeneous Fe (oxyhydr)oxides-rich subsamples were separated from 10 bulk samples. Bulk- and subsamples were investigated by mineralogical and geochemical analyses and (U-Th)/He geochronology. The samples show a large heterogeneity on the macro- and microscopic scale reflecting different stages of duricrust formation and evolution through processes of dissolution and (re)precipitation of Fe (oxyhydr)oxides. The 284 (U-Th)/He ages obtained for goethite α-(Fe, Al)OOH and hematite α-Fe2O3 range from 30.5 ± 3.1 to < 0.8 Ma and show large variability within a sample. The oldest hematite and Al-poor goethite subsamples precipitated since 30 Ma and formed while kaolinite was stable. Precipitation of Fe-minerals increased since ∼ 14–12 Ma but still happened under ferruginous lateritic, non-bauxitic conditions. The dissolution and (re)precipitation of Fe minerals, Al-substitution in goethite, and the overall enrichment in Th, as well as gibbsite precipitation indicate an intensification of weathering and a shift towards bauxitic conditions since the end of the Miocene. The Th-, U-, and Al-rich Fe phases formed in this late episode of intense weathering partially replace the oldest, often Th- and U-poor phases leading to a considerable age spread. We claim that this episode of intensified weathering, which had its peak at ca. 6–2 Ma, corresponds to the bauxitization of the lateritic cover of Kaw mountain.Our proposed model of lateritization since at least 30 Ma and bauxitization since the late Miocene provides new constraints on the timescale and intensity of weathering at Kaw mountain. The onset of lateritization in French Guiana (Kaw region) is potentially older than ca. 30 Ma and was possibly synchronous with the development of bauxites in neighboring countries, and differences in the Paleogene weathering intensity might be due to different drainage capacities of the parental material. Late Neogene bauxitization suggests a regional increase in precipitations and drainage potentially linked to increased incision owing to local uplift.Finally, we confirm that a detailed combination of geochronological results coupled to mineralogical and geochemical analyses improves our understanding of tropical weathering processes and duricrust formation by placing mineralogical and geochemical processes into a temporal framework.

Geophysical Assessment of Groundwater Vulnerability to Diesel Contamination at a Telecommunication Mast in Adebayo Area, Ado-Ekiti, Southwestern Nigeria

Abstract An appraisal of the groundwater vulnerability to contamination risk at Adebayo in Ado-Ekiti, Southwestern Nigeria, was carried out using the electrical resistivity method of geophysical prospecting. Two traverses of length 100 m each were established for both vertical electrical sounding (VES) and dipole-dipole. Six (6) Vertical Electrical Sounding (VES) and 2-D dipole-dipole profiling techniques of the electrical resistivity method were adopted for the geophysical investigation. The results show variations in resistivity values, depicting four to five geologic layers: the topsoil, laterite, weathered layer, fractured, and the fresh basement. It was observed from the correlation of the geoelectric section and 2-D resistivity image along traverse 1 (TR1) that the groundwater in the study area flows towards the southwestern part, and the fractured basement serves as a migration pathway to the diesel oil in the same direction as groundwater flow. The fractured basement (Well 2) with no lateritic cover has higher resistivity values compare to the fracture zone that was found directly under the mast which was overlain by the laterite. Comparing the resistivity values of Well 1 and 2, Well 2 that was drilled into the fractured basement has more traces of the contaminant than Well 1.

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.

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.

Integrated geological and geophysical studies for delineation of laterite covered chromiferous ultramafic bodies around Bhuban, southwestern part of Sukinda ultramafic complex, Odisha

Chromite and Platinum Group of Elements (PGE) mineralization in the Eastern Indian Shield region is found in the ultramafic rocks distributed in the Baula-Nuasahi and Sukinda massifs intrusive into the Iron Ore Group (IOG) of rocks. In addition to this, recent geological studies have also reported the occurrences of chromiferous ultramafic bodies within the granulite tracts of Eastern Ghat Group around Asurabandha and Maulabhanja areas in the southwestern part of Sukinda Massif. In the present study, detailed gravity and magnetic surveys (1510 stations) were conducted over a laterite covered area of ~0.16 km2 at Bhuban-Asurabandha to locate the anomalous zones/sources, the geometry of the chromite bearing host rocks and its depth continuity in the subsurface. Four distinct coincident positive gravity anomalies (up to ~0.18 to 0.36 mGal) and residual magnetic anomalies (~−200 to 350 nT) with the NE-SW strike indicate the presence of high density mafic/ultramafic bodies under the laterite cover. The results of 3-D Euler deconvolution and two-dimensional compact gravity inversion models in this region further suggest the presence of high-density material (~3100–3600 kg/m3) at a depth of 20–40 m in the subsurface. Exploratory drill hole data in the area confirms the subsurface occurrence of thin chromiferous serpentinite as lensoidal and sandwiched bodies within the granulite rocks of Eastern Ghat Group. Petrological studies of the drill core samples reveal the occurrences of disseminated and thin banded chromite within the serpentinites which is identical to the Sukinda ultramafics in terms of its mineralogy and alteration. On the other hand, the surrounding mafic granulite rocks contain occasional magnetite and sulfides within the granulite rocks. Hence, the chromiferous ultramafic bodies within the granulite rocks can be considered as the tectonically detached portions of the Sukinda ultramafic rocks during the regional thrusting.

Multi-criteria-based sub-basin prioritization and its risk assessment of erosion susceptibility in Kansai\u2013Kumari catchment area, India

Environmental processes are interrupted by the water action like soil erosion, mass movement as well as siltation on the dam in the undulating catchment area. Soil erosion is one of them and degraded the basin potentiality. This paper demonstrates that erosion susceptibility status in the 13 sub-basins of Kansai–Kumari catchment area has been determined depending upon its morphometric, lithology, geomorphic, land use/land cover (LULC), slope and soil characteristics used by integrated micro-watershed prioritization rank which is based on susceptible capacity under geographical information system platform. Risk assessment of soil erosion was measured by hypsometric characteristic (Hi) and denudation rate (tu) to assess the geological stage of landform and risk status for conservative practices. The result shows that SB13, 2 have a high risk (tu > 90 t/km2/year) due to the presence of low priority rank of morphometric, geomorphic, slope, soil and high priority rank of lithological set-up and LULC but reached under the late mature geological stage (Hi > 0.35). SB3, 4, 10 have a low risk (tu < 83 t/km2/year) due to the presence of high priority rank of morphometric, geology, geomorphic, slope and soil types under the mature stage of geological setting (Hi > 0.5). But SB7, 8, 9 have the medium risk (tu < 85 t/km2/year) due to the presence of erosion-prone LULC patterns like cropland and laterite cover but having low final priority rank and old geological stage (Hi < 0.35). Therefore, erosion susceptibility does not depend on morphometric aspects but also depends on other determinant themes at the sub-basin level.

Mineralogy and geochemical features of Foumban clay deposits (west Cameroon): genesis and potential applications

ABSTRACTFive clay deposits in Foumban, west Cameroon, were studied for their morphological, mineralogical and geochemical properties to determine their suitability for ceramics. The clays were examined with X-ray diffraction, X-ray fluorescence, thermal gravimetric analysis and Fourier-transform infrared spectroscopy. Field studies showed that a homogeneous clayey layer occurs at the upper part of the laterite cover of the interfluves, while the valleys are occupied by a clayey heterogeneous hydromorphic material. The clays are composed of kaolinite, illite, smectite and chlorite, associated with quartz, K-feldspars, plagioclase, goethite, traces of rutile and hematite. Geochemical analyses of these samples show a relatively large amount of SiO2(45–71%), Al2O3(14–31%) and relatively little Fe2O3(up to 11%), suggesting weathering of mainly granitic and rhyolitic parent rocks. The majority of these clays may be used in the production of structural ceramics such as bricks (refractory or not) and tiles. The relatively high proportion of the alkalis (K2O + Na2O; 6–8%) in some samples from Marom and Njindare areas might be responsible for the low firing temperatures. The abundance of smectite limits the application of some Koutaba and Marom clays for structural ceramics, while the high Fe2O3contents (&gt;8%) in some Bangourain clays indicate that some pre-treatment might be necessary prior to use.

Mapping lithological variations in a river basin of West Bengal, India using electrical resistivity survey: implications for artificial recharge

Groundwater is a treasured earth’s resource and plays an important role in addressing water and environmental sustainability. However, its overexploitation and wide spatial variability within a basin and/or across regions are posing a serious challenge for groundwater sustainability. Some parts of southern West Bengal of India are problematic for groundwater occurrence despite of high rainfall in this region. Characterization of an aquifer in this area is very important for sustainable development of water supply and artificial recharge. Electrical resistivity surveys using 1-D and 2-D arrays were performed at a regular interval from Subarnarekha River at Bhasraghat (south) to Kharagpur (north) to map the lithological variations in this area. Resistivity sounding surveys were carried out at an interval of 2–3 km. Subsurface resistivity variation has been interpreted using very fast simulated annealing (VFSA) global optimization technique. The analysis of the field data indicated that the resistivity variation with depth is suitable in the southern part of the area and corresponds to clayey sand. Interpreted resistivity in the northern part of the area is relatively high and reveals impervious laterite layer. In the southern part of the area resistivity varies between 15 and 40 Ωm at a depth below 30 m. A 2-D resistivity imaging conducted at the most important location in the area is correlated well with the 1-D results. Based on the interpreted resistivity variation with depth at different locations different types of geologic units (laterite, clay, sand, etc.) are classified, and the zone of interests for aquifer has been demarcated. Study reveals that southern part of the area is better for artificial recharge than the northern part. The presence of laterite cover in the northern part of the area restricts the percolation of rainwater to recharge the aquifer at depth. To recharge the aquifer at depth in the northern part of the area, rainwater must be sent artificially at depth by puncturing laterite layers on the top. Such studies in challenging areas will help in understanding the problems and finding its solution.

Fate of particulate copper and zinc isotopes at the Solimões-Negro river confluence, Amazon Basin, Brazil

The behaviour and fate of copper (Cu) and zinc (Zn) at river confluences is poorly understood, although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean. To identify and quantify the effect of such processes in river mixing zones, particulate Cu and Zn isotope signatures (δ65CuSPM and δ66ZnSPM) were measured along cross sections and vertical profiles at the largest river confluence in the world, the “Encontro das Aguas” mixing zone of the Amazon River, where the organic-rich Negro River meets the sediment-rich Solimões River.The Negro River suspended sediments, with highly variable Cu and Zn concentrations as well as δ65CuSPM and δ66ZnSPM, are mostly influenced by organic matter and by the lateritic cover of the watershed. The Solimões River suspended sediments, more homogeneous in Cu and Zn concentration and isotope composition across the river section, reflect the signature of weathered silicate-rich sediments derived from the Andes. The Solimões River supplies the majority of the suspended Cu and Zn to the Amazon River, and despite important flux losses across the confluence (−35% for Cu and −27% for Zn), δ65CuSPM and δ66ZnSPM show a conservative behaviour during the mixing. In the dissolved load, Cu concentrations and δ65Cudiss, mostly supplied by the Solimões River, behave conservatively whereas Zn, derived mainly from the Negro River, suffers an important loss (−58%) that can be attributed to Zn adsorption onto the suspended sediments from the Solimões River. This transfer does not induce a significant δ66ZnSPM shift in the Amazon River suspended sediments. Therefore, Cu and Zn isotope ratios in the suspended sediments behave conservatively through this confluence, which mixes two very chemically-contrasted rivers. Our findings thus suggest that the riverine isotopic information on the sources of particulate Cu and Zn is preserved during tributary mixing.

Petrographic characteristic of bauxites of the Koulore ore field

As a result of studying geological structure of deposits located in the Koulore ore field, basic cross-section layers have been established: bedrock, transitional horizon and laterite cover. The laterite cover consists of the lower part presented by polymineral and kaolin clays and the top - the bauxite horizon - laterite cover itself, which generally contains hydroxides and oxides of iron and aluminum. The bauxite horizon has a zone structure: in the lower part of the bauxite horizon - zone of light bauxites, and in the top part of the bauxite horizon - zone of the red bauxites. As a result of microscopic study of various samples of residuum, several kinds of rocks which are characterized by similar structural and textural parameters have been identified. The rocks have been grouped by classes: gelified patterned bauxites on silt-rich mudstone, gelified bauxites on dolerites, gel-morphous bauxites with crystalloids on silt-rich mudstone, laterite bauxites (low-quality bauxites) on silt-rich mudstone, laterite bauxites (low-quality bauxites) on dolerites, ferruginous laterites - cuirass on silt-rich mudstone, ferruginous laterites of the top part of the transitional horizon on silt-rich mudstone. Lithological and genetic types of bauxites control the type and composition of aluminiferous minerals. Mineral composition of classical lateritic bauxites mainly contains gibbsite, goethite-alumogoethite, and hematite with less 1% of boehmite. Gel-morphous and gelified bauxites consist of gibbsite, boehmite, goethite-alumogoethite, hematite. Content of boehmite reaches 3% that is more than in classic lateritic bauxites. The composition of the alumogoethite depends not only on lithology of bauxites, but also on position in the residuum section.

New Geological and Structural Facts under the Lateritic Cover in Garga Sarali, Ndokayo (East Cameroon) Area, from Audiomagnetotellurics Soundings

New geological and structural facts have been identified under the auriferous lateritic cover in Garga Sarali, Ndokayo area. Data were collected using AMT receiver system with frequencies ranging from 20 Hz to 50000 Hz. It consists of 16 AMT stations along 03 profiles, over Pan-African formations of East Cameroon. The wide frequency range enabled us to probe deep into the subsurface to obtain necessary information. Using Imagem software, coherency of data has been evaluated and only the data with a coherency below or equal to 0.7 have been considered. Two programs were used to map the subsurface. The pseudosections were obtained using IPI2WIN-MT, while geoelectrical sections were obtained using Stratagem Resistivity Plotter. Analysis of the curves of dimensionality tests shows that there is not always a complete superposition between the two telluric directions, translating the fact that the variation of the resistivity is not 1D, but rather 2D or 3D. Major features of 2D resistivity model from the respective profiles were identified. These features include a set of lower resistive formations going from the surface to 1000 m depth, lying on a set of resistive formations that appears at the surface and below the lower resistive formations. However, a very conductive layer was observed in depth in the three profiles. These facts show that the study area is made up of mixture of both conductive and resistive materials, suggesting a prolongation of the overlap between the Congo Craton and the Pan-African in depth to the north and the location of the CC/Pan-African limit above 4°N parallel accordingly. Deeper electrical discontinuities, interpreted as faults following a NE-SW trend, were highlighted. All these new data suggest that the study area underwent an intense tectonic activity with ductile to brittle deformations due to the presence of the BOSZ.

Identifying the phyllosilicate minerals of hypogene ore deposits in lateritic saprolites using the near-IR spectroscopy second derivative methodology

Infrared field-based reflectance spectroscopy in the Visible - Near-Infrared - Short-wave Infrared (VIS-NIR-SWIR) domain is a useful tool in mining geology particularly efficient for investigating the clay mineralogy of alteration haloes around ore deposits. It is used as a routine technique for the basic identification, mapping and semi-quantification of clay mineral species. However, the use of this technique for prospecting in hypogene deposits at depth in intertropical areas is strongly limited because of the presence of a thick, kaolinite-rich lateritic cover. Due to the strong IR absorption of kaolinite and the overlapping of its IR bands with those of most of the phyllosilicates in the SWIR domain, the use of field based near-infrared spectroscopy does not permit efficient identification and mapping of the phyllosilicates inherited from hypogene alteration that persist in the saprolite of lateritic profiles.In this paper, we propose a methodology to enhance the detection and semi-quantification of hypogene phyllosilicate minerals in kaolinite-rich lateritic saprolites using calibration curves. Those curves are built from the NIR spectra of binary admixtures of kaolinite or smectite with muscovite, Fe-Mg chlorite, clinochlore or talc in different known proportions.For each admixture series, calibration curves were established, based on investigation of two regions of interest within the NIR domain (1350–1470nm and 2080–2500nm) using a field-based spectrometer. For each binary mixture series of phyllosilicates, the second derivative of the NIR spectra was used to enhance the detection of the diagnostic absorption bands of each type of phyllosilicate, and hence to optimize the calculation of the intensity ratios between the diagnostic bands of the phyllosilicate components as a function of their percentage in the mixture. In presence of large amounts of lateritic kaolinite, the detection limit of the major types of hypogene phyllosilicates has been found at ranges from 5 to 10wt% of the total clay content using the second derivative of the NIR spectra acquired with a field-based spectrometer. Above these aforementioned limits of detection, the semi-quantitative data obtained by comparing the NIR reflectance spectra of natural samples with those of the calibration curves could permit to map hypogene alteration haloes directly from the lateritic saprolite.Finally the described approach has been successfully tested on natural samples from the skarn deposits of the Ity gold mine (Ivory Coast).

Differential weathering in ultramafic rocks of New Caledonia: The role of infiltration instability

Ultramafic rocks of New Caledonia present extensive pseudokarstic features: undulations of the base of the weathering profile, fractures enlarged by dissolution, dolines. Possible influence of the infiltrative instability on differential weathering of ultramafic rocks in tropical climate is investigated here, starting from the Tiébaghi case for which a groundwater model is available. Infiltration instability is governed by non-dimensional numbers, i.e. the Damköhler and Peclet numbers, and by the permeability contrasts between the bedrock and the different layers of the weathering profile. These numbers are firstly estimated for the Tiébaghi case, and secondly a sensitivity study of the weathering style is achieved through numerical modeling. Computations are firstly carried out without the impervious laterite cover, and then the effect of this cover is considered. The characteristic time constant for weathering of peridotites is directly estimated at around 7yrs from the thickness of the weathering front and the fluid velocity. The results show that a permeability contrast of at least 100 between the most pervious layer in the weathering front and bedrock and a diffusivity of chemical species lower than 2.5×10−8m2/s are required to allow instability. The Tiébaghi case lies at the limit of instability and dissolution fingers develop in a time lapse of 1Ma. However, this requires the infiltration of nearly 800mm of rainfall yearly, which is not the case presently. The development of dissolution fingers is promoted by any initial pervious zone such as fracture concentration around a faulted zone and the characteristic time for instability development drops to 0.1–0.3Ma. Extension of these results to other peridotite massifs in the tropical zone submitted to different temperatures and rainfalls is finally discussed.

Application of persistent scatterers interferometry for surface displacements monitoring in N5E open pit iron mine using TerraSAR-X data, in Carajás Province, Amazon region

ABSTRACT: Carajás Mineral Province, Amazon region, is the most important one in Brazil. Vale S.A. Company has the right to operate in the area of the N5E mine. The work is conducted on rock alteration products of low geomechanical quality related to sandstones, siltstones, and a lateritic cover. In order to monitor ground deformation, 33 TerraSAR-X images covering the period of March 2012-April 2013 were used in the investigation. An interferometric synthetic aperture radar (InSAR) approach based on permanent scatterer interferometry (PSI) using an interferometric point target analysis algorithm was applied. Results demonstrated that most of the area was considered stable during the time span of the image acquisition. However, persistent scatterers (PSs) with high deformation rates were mapped over a landfill probably related to settlements. To validate the PSI data, graphs were generated with the displaced information based on topographic measurements in the field. The graphs showed that the surface deformations during TSX-1 runway coverage are within the miner’s safety threshold and do not present a risk of major problems. The PSI data provided a synoptic and detailed view of the deformation process that affects the mining complex without the need of field campaign or instrumentation.

3D modeling of the laterites on top of the Koniambo Massif, New Caledonia: refinement of the per descensum lateritic model for nickel mineralization

Resulting from the weathering of the Peridotite Nappe, laterites are abundant in New Caledonia and host one of the largest nickel deposits worldwide. This work presents a 3D model of the Koniambo nickel laterite ore deposit. It shows that the laterites are located along the ridges of the massif and organized as hectometric-sized patches obliquely cut by the topography and distributed at various elevations. Three kinds of geometry were observed: (i) a thick laterite cover (between 20 and 40 m) overlying saprolite and mainly localized on topographic highs, (ii) a thin laterite cover (from a few meters to 20 m) mainly localized on areas with gentle slopes, and (iii) exposure of saprolite without laterite cover. Our data show that Ni-rich and Ni-poor areas are organized as hectometric-sized patches which broadly correlate with the distribution of the laterite thickness. The highest Ni areas are localized on slopes where laterite cover is thin or absent. The areas with lowest Ni are located in topographic highs under the thickest laterite cover. The vertical Ni mass balance for each borehole shows that, in areas with thick laterite cover, Ni is sub-equilibrated to slightly depleted whereas in areas with thin laterite cover, Ni is enriched. This suggests the existence of lateral infiltration of water rich in dissolved Ni, from areas such as topographic highs to downstream slope areas, in a process leading to enrichment of saprolite in Ni in slope areas. Mechanical transport and leaching of laterite material on slopes, including Ni-bearing material, could also contribute to local enrichment of Ni in the saprolite.

Geochemistry of some ferruginous soils of Kerala, India

The four representative ferruginous soils on lateritic cover developed over Precambrian rocks in parts of Kottayam of Kerala were studied to understand the weathering pattern and genesis through geochemistry. These soils are strongly to moderately acidic, reddish brown with low Cation exchange capacity CEC and base saturation having SiO2 - 33 to 57%, Al2O3 - 16-31%, Fe2O3 - 8 to 15% and TiO2 -0.7 to 1.4%. Kanjirapalli (P3) and Athirampuzha (P4) soil series were more intensely weathered as compared to the Kinalur (P1) and Chingavanam series (P2) with silica to alumina -iron ratio less than 2 and had a significant negative relationship with Chemical index of alteration CIA ( -0.75**), Harnois index ( -0.678**), Richie index (-0.953**) and Plagioclase Index of Weathering (-0.705**). The trace elemental concentration ranges were above the values of world soils having an enrichment index more than 1 in Kanjirapalli series (P3) and Ni contamination in genetic horizons (Ni &gt; 200µgg-1 ). The cluster analysis showed similar major oxide concentration pattern in Group -1 and Group - 2 but varied in trace elemental pattern with Cr &gt; Ba &gt; Cu in Group - 1 and Cu &gt; Cr &gt; Ba in Group- 2 soils whereas Zr &gt; Ni &gt; Mn in Group - 3 to Ni &gt; Mn &gt; Zr in Group - 4 soils. The study further showed that differential rate of weathering in soils under tropical climate was further accelerated due to anthropogenic activities such as improper land use practices and deforestation on sleep slopes.