Lateritic Cover Research Articles

The Mara Rosa gold district, Central Brazil

In the Mara Rosa District, three different magmatic series have been identified and all of them host gold mineralization. The Mara Rosa Metavolcano-Sedimentary Sequence constitutes a poorly differentiated calc-alkaline magmatic series typical of arc settings. It is arranged into three belts, forming synclinal structures and was affected by high-grade amphibolite facies metamorphism. Early intrusive rocks of the Posse Unit show an alkaline affinity and host the epigenetic Posse Gold Deposit, which is associated with a late hydrothermal event, preferentially where earlier shear zones are present. Early hydrothermal alteration has produced an aluminous, sulphide rich greisen rock, which is barren of gold. Slices of an allochthonous and more magnesian sequence are associated with metachert and graphitic schists that host the syngenetic Au-Ag Zacarias mineralization and extensions. Gold-bearing quartz veins were emplaced before the second main isoclinal folding event. Supergene ore has played an important role in the mining activity of the Mara Rosa Region, since it represents the majority of the ore exploited. It is possible that yet undiscovered deposits lay hidden underneath the extensive lateritic cover.

The effect of organic matter on chemical weathering: study of a small tropical watershed: nsimi-zoétélé site, cameroon

The effect of organic matter during soil/water interaction is still a debated issue on the controls of chemical weathering in a tropical environment. In order to study this effect in detail, we focused on the weathering processes occurring in a small tropical watershed (Nsimi-Zoetélé, South Cameroon). This site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry. The lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones: unsaturated slope soils on the hills and/or elevated areas; and water-saturated soils in the swamp zone which represent 20% of the basin surface. The study present chemical analysis performed on water samples collected monthly from different localities between 1994–1997 and on soil samples taken during a well drilling in December 1997. The results suggest the existence of chemical and spatial heterogeneities of waters in the basin: colored waters flooding the swamp zone have much higher concentrations of both organic matter (i.e., DOC) and inorganic ions (e.g., Ca, Mg, Al, Fe, Th, Zr) than those from springs and groundwater from the hills. Nevertheless, these organic-rich waters present cation concentrations (Na, Ca, Mg, K) which are among the lowest compared to that of most world rivers. The main minerals in the soils are secondary kaolinite, iron oxi-hydroxides, quartz, and accessory minerals (e.g., zircon, rutile). We mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through SEM observations the textural characterization of weathered minerals such as kaolinite, zircon, rutile, and the secondary recrystallization of kaolinite microcrystals within the soil profile. Water chemistry and mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin. Thus, in order to explain the increase of element concentration in the organic-rich waters, we suggest that organic acids enhance dissolution of minerals such as kaolinite, goethite, and zircon and also favors the transport of insoluble elements such as Al, Fe, Ti, Zr, and REE by chemical complexation. SiO 2(aq) concentrations in these waters are above saturation with respect to quartz. Dissolution of phytholithes (amorphous silica) may be responsible for this relatively high SiO 2(aq.) concentration. Al/Mg ratios obtained for the soil and the Mengong river waters show that a significant amount of Al does not leave the system due to kaolinite recrystallisation in the swamp zone soils. Geochemical data obtained for this watershed show the important contribution of vegetation and organic matter on chemical weathering in the swamp zone. Quantitatively we propose that the increasing amount in total dissolved solid (TDS) due to organic matter and vegetation effect is about 35%. In summary, this interaction between soils and waters occurs mostly in soils that are very depleted in soluble elements. Thus, the low concentration of major elements in these water is a direct consequence of the depleted nature of the soils.

The fluvial geochemistry of the rivers of Eastern Siberia: III. Tributaries of the Lena and Anabar draining the basement terrain of the Siberian Craton and the Trans-Baikal Highlands

The conventional view of the climatic influence on weathering is that weathering rates are strongly temperature-dependent due to the near-exponential relationship (Clausius-Clapeyron) between temperature and the saturation vapor pressure of water, and hence precipitation and runoff. This is a central theme in the Earth “thermostat” model, i.e., weathering of aluminosilicate rocks on continents acts through the greenhouse effect as a negative feedback on atmospheric CO 2. However, there is very little direct field evidence to support this hypothesis. To remedy the lack of systematic geochemical data for cold high latitude rivers as compared to the tropics, large, pristine drainages of Eastern Siberia have been studied. Here, data from basement terrains of the Siberian Craton are reported. The low Si to total cation ratios suggest a superficially weathered system. The total dissolved solids flux of 0.39 × 10 6 mol/km 2/yr and the CO 2 uptake flux of 149 × 10 3 mol/km 2/yr are similar to those of the tropical cratonic systems and the collisional/accretionary zone of northeastern Siberia, but about a factor of 3 lower than for the orogenic zones of the western Americas at both low and high latitudes. The lack of systematic climatic effects on the solute and CO 2 fluxes is ascribed to the unique non-glacial frost shattering processes which continuously expose fresh rock surfaces and, thus, overcome the effect of temperature inhibition on high-latitude shields and to the lateritic cover that seals in the weathering front away from the weathering agents on the tropical shields. No primary climatic effects on weathering rates on the present Earth were detected.

GOLD: Geophysical Setting of BIF-Hosted Gold Deposits at Tuckabianna, Western Australia

Tuckabianna Gold Mine is located about 25 km east of Cue, on the Mount Magnet-Meekatharra Shear Zone. The mine sequence consists largely of basalt, dolerite to gabbro, mafic schists, banded iron-formation (BIF), and intrusive quartz-feldspar porphyry dykes and sills. Gold mineralisation is hosted by the NE-striking, E-dipping BlFs with shears localising the ore zones. Laterite derived from the weathering of these BlFs may contain a significant gold resource. Downhole logging has shown that the BlFs typically have lower relative gamma activity and higher densities, resistivities and susceptibilities compared with the mafic units and porphyry. Petrophysical measurements of magnetic susceptibility, anisotropy of magnetic susceptibility and natural remanent magnetisation have characterised two BIF types, although no relationship between gold mineralisation and the BIF types has been conclusively demonstrated. Tuckabianna's regional geophysical dataset consists of Bureau of Mineral Resources (BMR) aeromagnetic and gravity data and high-resolution aeromagnetic and radiometric data. Of these, the most useful have been the high-resolution aeromagnetic data, because they clearly define the BlFs and structures which can localise gold mineralisation. Such lithological and structural detail cannot be recognised in either the BMR regional magnetic and gravity datasets or the high-resolution radiometric dataset. The Geoscan remote-sensing system has been flown over Tuckabianna. However, the laterite cover (1-21 m thick) has blanketed much of the geology and only some rock types and major lineaments can be identified from the data. Ground geophysical surveys have been dominated by the magnetic method, which has been extremely useful in delineating the BlFs and structural features, as has the gravity method. Electrical surveys, including magnetometric resistivity, induced polarisation (frequency and time-domain) and electromagnetic methods have also been used. These electrical techniques have not been completely successful in the direct detection of mineralised zones. However, the resistive nature of the BIF is such that these methods may be successfully used to locate the BIF and porphyry, in addition to delineating some structures.

Redox front geochemistry and weathering: theory with application to the Osamu Utsumi uranium mine, Poços de Caldas, Brazil

Numerical calculations describing weathering of the Poços de Caldas alkaline complex (Minas Gerais, Brazil) by infiltrating groundwater are carried out for time spans up to two million years in the absence of pyrite, and up to 500,000 years with pyrite present. Deposition of uranium resulting from infiltration of oxygenated, uranium bearing groundwater through the hydrothermally altered phonolitic host rock at the Osamu Utsumi uranium mine is also included in the latter calculation. The calculations are based on the quasi-stationary state approximation to mass conservation equations for pure advective transport. This approximation enables the prediction of solute concentrations, mineral abundances and porosity as functions of time and distance over geologic time spans. Mineral reactions are described by kinetic rate laws for both precipitation and dissolution. Homogeneous equilibrium is assumed to be maintained within the aqueous phase. No other constraints are imposed on the calculations other than the initial composition of the unaltered host rock and the composition of the inlet fluid, taken as rainwater modified by percolation through a soil zone. The results are in qualitative agreement with field observations at the Osamu Utsumi uranium mine. They predict a lateritic cover followed by a highly porous saprolitic zone, a zone of oxidized rock with pyrite replaced by iron-hydroxide, a sharp redox front at which uranium is deposited, and the reduced unweathered host rock. Uranium is deposited in a narrow zone located on the reduced side of the redox front in association with pyrite, in agreement with field observations. The calculations predict the formation of a broad dissolution front of primary kaolinite that penetrates deep into the host rock accompanied by the precipitation of secondary illite. Secondary kaolinite occurs in a saprolitic zone near the surface and in the vicinity of the redox front. Gibbsite forms a bi-modal distribution consisting of a maximum near the surface followed by a thin tongue extending downward into the weathered profile in agreement with field observations. The results are found to be insensitive to the kinetic rate constants used to describe mineral reactions.

Geochemical exploration of the Maicuru alkaline-ultramafic-carbonatite complex, northern Brazil

The Serra do Maicuru represents an alkaline-ultramafic-carbonatite complex, emplaced in the Guiana Craton, which developed a thick lateritic profile, enriched in aluminum phosphates and especially titanium mineralization. The most important parent rocks are pyroxenites and syenites, plus some isolated apatitic bodies, glimmerites and carbonatite veins that cut all of these rocks. Above this lateritic cover and on its neighborhood, a geochemical survey was carried out by Docegeo, a subsidiary of Companhia Vale do Rio Doce ( cvrd). From the surficial laterites, divided into a ferruginous crust and lateritic soil, more than 1500 samples were collected, covering all of the Maicuru structure. They were analyzed for Fe 2O 3, P 2O 5, TiO 2, Mn, Ni, Co, Cr, Cu, Pb and Zn, including some analyses for REE and Nb. Strongly anomalous phosphorus in soils and in iron crust, as aluminum phosphates, furnished a good guide to primary apatite mineralizations, and showed the presence of high titanium as anatase, ilmenite and titanomagnetite and interesting anomalous amounts of Nb, Cu and REE. The geochemical data also helped identify and delineate the distribution of the main parent rock types.

10 Be and natural isotopes of U and Th in a laterite cover from Cameroon

10 Be and natural isotopes of U and Th in a laterite cover from Cameroon

The correlation between geochemical data and SPOT satellite imagery of lateritic terrain in southern Mali

Abstract Detailed geochemical mapping of superficial lateritic formations is compared with recent high-resolution SPOT satellite images in the Dagadamou prospect in southern Mali. The two main landscape features standing out in SPOT images are expressed by thematic indices derived from multispectral data. They reflect the distribution of vegetation cover and the distinction between silty clay soils in the valleys and ferruginous duricrust exposed on the plateaus. Geochemical differentiation factors are closely related to the nature of sampled materials and reflect the relative amounts of major constituent minerals estimated by normative calculation. Duricrust samples with high Fe and Al content are also enriched in trace elements, P, V, Cr, As, Mo, Nb and Cu, immobilized in the weathering profile with secondary oxihydroxide minerals. Soils of flats are composed mainly of quartz and kaolinite; they also concentrate heavy minerals characterized by high contents of Zr, Ti, Ce and Y. Accumulation of detrital material at the periphery of duricrust plateaus is marked by a geochemical halo of higher contents in Zr and quartz at their periphery. This feature is clearly visible on SPOT imagery as a zone of high reflectance, devoid of vegetation. The density of vegetation on duricrust is related to its kaolinite content. Its distribution is characterized by a striped pattern, which is probably controlled by bedrock lithological structures preserved in the lateritic cover. This study shows the relationships, both direct and indirect, existing between spectral reflectance and geochemical composition of superficial lateritic formations. Many other landscape features identified with high resolution on SPOT images provide complementary information, which could be very helpful at various stages of geological and mineral exploration in lateritic terrain.

Gold exploration in the callion area, eastern goldfields, western Australia

Abstract The Callion gold-quartz veins are hosted by metabasalts, interflow sediments and acid metatuffs within the Archaean Norseman-Wiluna greenstone belt. The area has been subjected to deep lateritic weathering. Gold prospecting using soil geochemistry led to the discovery of a laterite Au deposit (geological reserves of 103,000 t with 1.86 g/t Au at 0.7 g/t cut-off) and subsequently to the discovery of the underlying quartz vein mineralization (geological reserves of 104,000 t with 7.9 g/t Au at 2 g/t cut-off). The quartz vein mineralization is blind, beneath 2–10 m of laterite cover. The vein, which can be traced along strike for 1 km, is mineralized over a strike length of 300 m. Weathering around the vein extends to a depth of 70 m below surface. Ore microscopy shows that the Au in the weathered vein was partially mobilized and deposited contemporaneously with Fe-hydroxides. Mining of the laterite and the top of the quartz vein enabled a detailed reconstruction to be made of the Au-grade distribution from the top of the laterite down to the saprolite. The grade distribution is mushroom-shaped, widening at the top of the mottled zone or base of the pisolithic zone. From the vein in the saprolite, the “mushroom”, defined by the 2.5 ppm Au contour, widens to 40 m, at least 5 times the original width. This zone represents the transition to the overlying wide soil anomaly which has peak Au values of 2 ppm, which are about 50% of those in the laterite in the vicinity of the vein. Magnetic methods were also used during the course of exploration and led to the discovery of another small mineralized vein.

EVOLUÇÃO MINERALÓGICA DOS JACUPIRANGUITOS DE JACUPIRANGA (SP) DURANTE A ALTERAÇÃO INTEMPÉRICA

Samples were collected from a weathering profile developed above pyroxenites belonging to the alkaline massif of Jacupiranga. The primary minerals titanoaugite, magnetite, perovskite, phlogopite, and olivine - were first partially transformed by a hydrothermal alteration process and the completely altered by weathering. The earlier products of the supergene alteration of the pyroxenites -are smectites; these minerals change into kaolinite and goethite which are the principal components of the lateritic cover. This paper deals with micromorphological aspects of these transformations.

EVOLUÇÃO MINERALÓGICA DOS JACUPIRANGUITOS DE JACUPIRANGA (SP) DURANTE A ALTERAÇÃO INTEMPÉRICA

Samples were collected from a weathering profile developed above pyroxenites belonging to the alkaline massif of Jacupiranga. The primary minerals titanoaugite, magnetite, perovskite, phlogopite, and olivine - were first partially transformed by a hydrothermal alteration process and the completely altered by weathering. The earlier products of the supergene alteration of the pyroxenites -are smectites; these minerals change into kaolinite and goethite which are the principal components of the lateritic cover. This paper deals with micromorphological aspects of these transformations.

AS MINERALIZAÇÕES AURÍFERAS DA REGIÃO DE MARA ROSA (GO)

ln Mara Rosa region there is a number of gold mineralized areas that are being mined by prospectors. The main purpose of this work is to describe each mineralized area in order to define types and controls of gold mineralization. A generalized model for gold metallogenesis in the area is also proposed. The stratigraphy of the area comprises: Mara Rosa Supracrustals and Granite-Gneiss Complex, both of Archean to Early Proterozoic Age; Tertiary/Quaternary Lateritic Cover; and Quaternary Alluvial Deposits. Nine of the 14 gold occurrences studied are stratabound and occur in Mara Rosa Supracrustals; two are vein and pipe like quartz bodies in Mara Rosa Supracrustals and Granite-Gneiss Complex; and three are alluvial gold deposits overlying Mara Rosa Supracrustals. Although gold placer deposits have not been studied here, they have been mined in the area. It is proposed that syngenetic and low-grade gold mineralization are associated with interflow graphitic sediments, intermediate tuffs, and chert belonging to Mara Rosa Supracrustals. Folding and metamorphism resulted in local enrichments inside these source beds originating stratabound gold mineralization. Epigenetic and structurally controlled vein and pipe like ore bodies originated at least in part by remobilization of pre-existing singenetic mineralization. Elluvial gold mineralization was originated by weathering of weakly mineralized Mara Rosa Supracrustals rocks, whereas alluvial placer deposits were originated by reworking of all three pre-existing mineralization types.The studied area illustrates the permanence of gold mineralization in a given area by successive reworking of an early gold concentration both by endogenous and exogenic processes. ln fact, a NNE trending belt contains most of known gold mineralization of the area.

Paleodrainage-Unconformity Model as Guide to Uranium Deposits: ABSTRACT

This paper considers a uranium occurrence model that shows how early Eocene and Oligocene depositional patterns and paleogeography can be used to identify favorable host rocks and to suggest where uraniferous ground water passed through these rocks. The uranium in the ground water was derived mostly from volcanic ash of the Oligocene White River Group. This model accounts for most known uranium deposits and occurrences in eastern Wyoming, western South Dakota, and western Nebraska. All major deposits in Eocene sandstones are in rocks of the fan-channel facies that were identified by sand grain size and shape studies, and most deposits are basinward of present-day major mountain valleys. Deposits occur only where rocks of this facies are less than 300 m (980 ft) below the reconstructed basal Oligocene surface--a distance calculated from roll-front migration and erosion rates. Uranium deposits in other than Eocene rocks also are related to the configuration of the pre-Oligocene surface. White River channel sandstones have deposits and occurrences along a 200-km (125-mi) section of a major Oligocene river in eastern Wyoming and Nebraska. Oligocene trans-mountain drainages localized uranium occurrences in Precambrian granitic rocks in the Laramie Mountains. Deposits in Cretaceous rocks in northern Colorado and along the flanks of the Black Hills lie beneath the axes of Oligocene channels. The channels were the major conduits that localized the movement of the uranium-bearing solutions. Rocks underlying the divides between the channels are unfavorable for uranium deposits where the channels are parallel to the regional dip, because the divides have a thick imp rvious lateritic soil cover. End_of_Article - Last_Page 526------------

Clay-mineral studies on some carboniferous sediments in Scotland

The clay mineralogy of 129 samples from the Scottish Carboniferous sequence, representing a variety of depositional environments, has been studied mainly by X-ray diffraction, electron microscopy and differential thermal analysis. Kaolinite and illite are the only clay minerals present in any quantity. The Lower Carboniferous sediments are the most illitic, especially the cementstones (argillaceous dolomites), but in the Upper Carboniferous kaolinite is dominant. Crystallinity of the kaolinite varies according to the rock type in which the clay occurs. Thus, sandstones almost invariably contain a highly crystalline, morphologically perfect kaolinite which formed during diagenesis; in shales and limestones the kaolinite is moderately ordered and apparently of detrital origin, and in underclays a poorly ordered form occurs. The last named is thought to have been degraded in an acid swamp environment. The abundance of detrital kaolinite, espcially in the Upper Carboniferous, is consistent with the concept that the sediments derive from the erosion of a tropically weathered lateritic cover.