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The Trombetas bauxite deposits are located in the Amazonas region, and Para State of Brazil approximately 200 km to the east of Manaus. The principal mining areas are at Rio do Norte, from where ore is crushed and beneficiated, railed to the terminal 28 km to the north at Porto Trombetas, a port on the Rio Trombetas, a tributary of the Rio Amazon. The bauxite is either exported wet or is first dried by furnace, and is then directly loaded onto up to 250 m long, 60 000 t ocean going bulk carries at the port, which is ~900 km from the mouth of the Amazon River on the Atlantic Ocean coast (#Location: Rio do Norte - 1° 40' 47"S, 56° 27' 10"W).
It lies on the southern margin of the Precambrian Guiana Shield where it is overlain by Phanerozoic sediments of the Amazon Basin and was formed during Pleistocene time from unconsolidated Cainozoic sediments.
The bauxite deposits contain more than 1 Gt of economic bauxite resources which are high in alumina (43 to 45 Al2O3) and low in both silica (6 to 15% SiO2) and iron (6 to 20% Fe2O3), predominantly composed of gibbsite with only traces of boehmite and accompanying hematite and Al-bearing goethite. The bauxite is in turn overlain by 7 to 11 m of unconsolidated sediments.
The Guiana Shield is predominantly composed of flysch- and molasse-like sediments that are 2 Ga and older and have been metamorphised during the 2 to 1.8 Ga Trans-Amazonian orogenic event into low to high-grade schists, gneisses and charnockitic granulites. These are overlain and intruded by 1.8 Ga ultramafic, gabbroic and felsic magmatic rocks and covered by the 1.6 Ga Roraima Sandstones and cut by NW striking dolerite dykes and sills. The east-west trending Amazon Basin developed on the southern margin of the shield and was filled by a thick sequence of consolidated Late Neoproterozoic to Mesozoic succession and unconsolidated Cainozoic sediments, intruded by deep gabbro bodies.
The bauxitic horizon is found at a depth of approximately 10 m in the Pliocene Barreiras Series sediments which formed a sedimentary plain that was eroded during the Pleistocene and Holocene, now represented on mesas surrounded by enscarpments. The sediments overlying the bauxite are highly porous to spongy kaolinitic clay. There is no drainage pattern, with all rainfall being drained through the sediments and exiting at the base of the enscarpment. The transition from the overlying sediments to bauxite is gradational, but defines a consistent horizontal surface.
The lateritic profile may be summarised as follows:
• Massive kaolinitic clay with sparse clay nodules (<4mm in diameter) - 8 to 10 m thick, thinning towards the edge of mesas;
• Pure gibbsite nodules (1 to 10 cm diameter) in cover clay matrix with a gradational upper boundary - 0.01 to 1 m thick;
• Iron rich (up to 45% Fe2O3) >3 mm diameter pisolites in a soft clay matrix, again with gradual contacts and thickness of 0.10 to 0.50 m;
• Hard iron rich horizons, commencing with cemented pisoliths grading down into massive iron-rich bauxite to bauxitic iron laterite, often with columnar structures - 0.5 to 3 m thick;
• Bauxite horizon of fine grained massive to granular-crystalline gibbsite, commonly with columnar structure and horizontal colour and grain size banding - 1 to 4 m thick;
• Gradational change to irregular blocks or concretions of bauxite in kaolinitc clay - 1 to 3 m thick;
• Massive underlying variegated kaolinitic clay of the Barreiras Series with varying quartz grains, grading down into quartz sand to sandstone - up to 50 m thick.
In 2015, three main mining areas were being worked at Rio do Norte, namley Saracá V, Saracá W and Bela Cruz.
The main Trombetas mine is operated by Mineracao Rio do Norte (MRN) which in 2002 produced 14.2 Mt of bauxite.
At the end of 2014, the JORC compliant remaining reserves and resources at Trombetas were (Rio Tinto Annual Report, 2015):
Proved reserve - 73 Mt @ 49.3% Al2O3,
Probable reserve - 22 Mt @ 49.9% Al2O3,
TOTAL reserve - 94 Mt @ 49.5% Al2O3.
Mineral resources (which exclude Ore Reserves)
Measured resource - 293 Mt @ 49.6% Al2O3,
Indicated resource - 40 Mt @ 48.9% Al2O3,
Inferred resource - 134 Mt @ 49.9% Al2O3,
TOTAL resource - 468 Mt @ 49.6% Al2O3,
TOTAL reserve + resource - 562 Mt @ 49.6% Al2O3.
In 2016, the Trombetas operation was managed by Mineração Rio do Norte, a joint venture between Vale (40%), Alcoa subsidiaries World Alumina, Aluminio S.A. and AWA Brasil (18.2%), BHP Billiton (14.8%), Rio Tinto Alcan (12%), Companhia Brasileira de Aluminio (10%), Norsk Hydro (5%).
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1981.
Record last updated: 4/2/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Aleva, G.J.J., 1981 - Essential differences between the bauxite deposits along the southern and northern edges of the Guiana Shield, South America: in Econ. Geol., v.76, pp. 1142-1152.|
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