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The lateritic ore deposits of Brazil. The haematite is still fresh, loosing only its characteristic brightness. The emergence of resistance to sp is a concern requiring monitoring the effectiveness of sp for iptp. Two main mechanisms were important: The previously formed smectites may also change into a more ferruginous and less nickeliferous smectite Paquet et al.
To learn more about how to request items watch this short online video. On the tops, the profiles are not well developed, the silcrete accumulation of silicified blocks covers. Amazon Renewed Refurbished products with a warranty.
The serpentine is changed into smectite with progressive leaching of Mg and conservation of silica, as opposed to the lateritic evolution. The pisolitic material contains centimetric plate fragments, badly rounded, with a very thin cortex up to spheric pisolites 5 mm diameterwhere the nuclei are entirely taken by the cortex.
In the intermediate part, the bauxite becomes friable, alveolar, frequently showing irregular rock blocks in gibbsite and associated with kaolinitic clay. They are metamorphic rocks of amphibolite facies and are formed by a millimetric to centimetric alternation of quartz layers and iron minerals, being haematite the most common.
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These basins, having up to m of sedimentary and associated magmatic rocks, have received a marine sedimentation during Ordovician, Silurian and Devonian times ; a mixed sedimentation in the Carboniferous ; and they have evolved to a continental environment from the Permian to the Jurassic times.
It is a more coherent and harder horizon, with a higher density 1.
When serpentine disappears, it leaves behind a goethitic ghost, epigenic on the previous crystal lattice, and the texture is compacted ferruginous saprolite. Its cover materials have peculiar characteristics. In Goias State, as in most of the regions in Brazil, the ultramafic massifs comprise hills that look jacsues on broad levelled plains corresponding to the country rock fig. It can be noticed that on the steep slopes or underlying the silcrete, only the first stages of the lateritic profile are present hard rock and eventually saprolite.
You do not currently have access to this article. Nevertheless, the exploitable ore is generally a result of a protore lateritic alteration carbonatic, silico-carbonatic or silicatic protore, according to the metamorphic processes. This massif forms a small elongated body with 2.
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Haematites, with polished and bright faces in the fresh rock, loose their brightness in the first stage of alteration and give support to the development of goethite MELFI et al. On some depressed zones in highlandswithout silcrete in Barro Alto convex slopes and in Niquelandia suspended valleysthe profiles are 15 to 30 m thick and show the following sequence of horizons: How do I find a book?
This disaggregation goes on, affecting afterwards the breccia-like material, which covers the in situ altered levels: In Moanda GabonWeber shows that the alteration of a protore similar to that of Azul deposit starts with the formation of manganite. The pyroxenites are not good protores, because of the low Ni content of the pyroxenes, but on the contrary, these rocks may be good Ni reservoirs. Some crust of a white and very hard bauxite with colloidal aspect is found in this level ; — nodular or pisolitic bauxite, consisting of bauxite nodules, very hard and with irregular shapes.
Table II Bauxite deposits of Brazil. The migration of Mn, from surface to bottom, where it takes part as first-generation cryptomelane which pseudo-morphoses rhodochrosite, is the origin of the high Mn content of the exploitable ore deposits.
Seminaron Lateritisation Processes, p. An important group of rocks associated with Archean units is the large ultramafic complex with gabbros, pyroxenites and peridotites of Goias State Niquelandia, Barro Alto and Cana Brava.
Metamorphosed silico-carbonatic protores alteration. Mottled dayforming the lower horizon, directly in contact with the Barreiras sediments and characterized by a strong clay enrichment with decrease of quartz that starts to be dissolved.
The main heavy minerals are rutile, ilmenite, tourmaline, zircon and anatase.
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Get to Know Us. Browse titles authors subjects uniform titles series callnumbers dewey numbers starting from optional. There are no important variations in the deposits characteristic of the more humid coastal zones Esson and Surcan, ; Trescases and Oliveira, and those of Central Brazil tropical zone with contrasting seasons. The later evolution to Mn02 phases nsutite and pyrolusite and afterwards the retrometamorphism to cryptomelane are identical to those of Lafaiete and Azul.
Besides the small deposits of Mn and Ni of the States of Bahia, Ceara and Paraiba, the most important in the semi-arid zone is the nickel ore deposit of Sao Joao do Piaui. When olivine is completely dissolved, the solutions of alteration become more diluted, pyroxenes change totally into smectites, that are more ferruginous and less magnesian than the former.