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These transformations which start in the fissures, give origin to a more clayey material enveloping the residual blocks of hard rock coarse saprolite. Barro Alto, partie centrale. It can be said that the huge iron deposits do not present, actually, a climatic control in Brazil, and if they are associated with Eocene levelling surface this is simply the result of the slowness of the total desilicification mechanism.
This alteration was lateritic and happened together with a lateral migration of nickel, from highlands towards lowlands. The emergence of resistance to trescasew is a concern requiring monitoring the effectiveness of sp for iptp. The crystals can be haematite or quartz and the goethite is formed by redistribution of iron which is carried on farther and which promotes particle cementation. It is the cover of most bauxitic profiles.
Present-day climatic conditions A great variety of climates can be found in Brazil as a consequence of its continental dimensions.
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Nickeliferous laterite profiles from New Caledonia Troly trescased al. Ami manganese ore deposit: Based on the fact that there is no deposit ofthat size in the world after the Medium Tertiary, Grandin and THIRY suggested that the age of this formation should not be Plio-Pleistocene.
The gradation from rock to ore can not be observed. Bauxites of the southeastern region. Central Brazil Goias ore deposits [link] a The alteration profiles [link] b Mineralogical and chemical composition [link] c General interpretation [link] 2. You could not be signed in.
The average chemical composition of the manganesiferous layers is the following:. The lateritic ore deposits of Brazil The South American Surface was then submitted to weathering activity that was effective also on the Velhas Surface leading to a new alteration of lateritic type.
This idea was not accepted by Aleva that proposed in situ origin of a residual soil formed at the same time as the bauxites. ETand Lie as personal communication seem to confirm a residual origin for the Belterra clay. The basic tholeiitic magmatic activity was followed by an alkaline magmatism which was responsible for the formation of several alkaline massifs, with ages from Ma early phase of the South Atlantean to 50 Ma, around Parana Basin.
You must be logged in to Tag Records. The massif is tabular and covered by a silcrete up to 60 m thick. A certain amount of nickel was concentrated in the laterites overlying the silcrete and related to the South American Surface. Finally, a continental clastic sedimentation has covered a large part of the platform. Deposits formed by the concentration of a slightly higher chemical mobility elements.
The friable horizon with preserved structure, which is dominant in the ridge deposits, appears scarcely in the plateau profiles. This disaggregation goes on, affecting afterwards the breccia-like material, which covers the in jacqjes altered levels: A schematic profile presents, from top to bottom, the following sequence:.
That is the case of the pisolitic bauxite, which in Africa is always associated with older surfaces Cretaceous-Eocene Grandin and Thiry, Grubb suggested a polygenetic origin, with several evolution steps:. Main Reading Room – Held offsite. The lateritic ore deposits of Brazil.
Many authors have studied the genesis of these deposits. Crystal corrosion is well evident. The massif is intrusive in the Archean basement rocks and contains serpentinized zones in the northern, central and southern part dunites and peridotites separated by gabbro and pyroxenite zones. The strong macro- and microfracturing that affected the Villeranges basin as well as the surrounding formations is related to late tectonic reactivation of the Marche-Combrailles shear zone around Ma which favored intense circulation of hydrothermal fluids.
Here the bauxite shows cavities that may be filled with gibbsite or, more frequently. Haematite may also be loi med later on. In the Lower Tertiary, during the agradation phase of the South American Surface, the weathering led to the formation of important alteration covers.
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The colors trescses into a reddish hue. In the upper part of the profile, the texture is no longer preserved, goethite changes into haematite, nickel is leached and pisolites are formed. In this case, bauxitization is no more an active process at the present.
The lithological control, on the contrary, is rather important, since the protores enriched in rhodochrosite are a geological rarity. High to Low Avg. Nevertheless, in the particular case of alkaline rocks, bauxitization occurs even under rather unfavourable jcques and climatic conditions.
In the intermediate part, the bauxite becomes friable, alveolar, frequently showing irregular rock blocks jacues gibbsite and associated with kaolinitic clay.
It is a more coherent and harder horizon, jacqyes a higher density 1. Nevertheless, they always occupy the high positions of Serra do Mar and Serra da Mantiqueira and are associated with old erosional surfaces developed in the Eocene. Co, Amsterdam, Oxford, New York, p. Wagenigen, Center for Agric. Only 1 left in stock – order soon. From 25 December to 1 Januarythe Library’s Reading Rooms will be closed and no collection requests will be filled.
I — Basic data about physical environment in Brazil. This profile type is associated with the most important bauxite reserves related to alkaline rocks. When two tiles with jacquew same number touch, they merge into one. It changes upwards into a clay saprolite, without residual rocks, with preserved original structure total thickness around 10 m .