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Location: 0.5 km W from Porto Trombetas, Brazil
Porto Trombetas, Oriximina, PARAPorto TrombetasBrazil68275-000
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Mineracao Rio do Norte (MRN) consists of a series of bauxite tabular deposits.The bauxites of Trombetas are associated with the plateaus, high areas with relatively plan ridges and elevations between 200 and 160 meters, which were formed from lateritization processes acting on sedimentary rocks (which contained aluminous minerals) from the Paleogene period (about 40 million years).The region of Trombetas is situated in the Low Amazon Sedimentary Basin, which is characterised by a clastic sedimentation of continental origin, lying unconformably over Palaeozoic sedimentary rocks.The bedrock is the Alter do Chao Formation (Upper Cretaceous), constituted by clayey sandstone, sometimes arkose, intercalated with conglomerates, clays and siltstones. Locally occurring above this formation is an immature lateritic covering (Paleogene), that is geomorphologically related to the plateaus and holds the bauxite mineralisation.The plateaus are deeply cut, with top elevations varying from 156 m to 217 m, with a gentle dip (1 to 5 degrees southwest) toward the Amazon River. The slopes are convex and can reach up to 30 degrees.The deposit is formed by tabular layers, with varied thickness, originated from weathering processes under equatorial climate; the bauxite layer is enriched in gibbsite formed from the hydration of kaolinite.The profile is rather homogeneous across the plateaus and presents practically the same sequence of horizons from the top to the bottom: (1) upper kaolinitic layer; (2) nodular bauxite layer (non-Resource), (3) ferruginous nodular layer; (4) bauxitic layer (potentially the Mineral Resources), (5) lower kaolinitic layer and (6) basal sedimentary rock (arkosean sandstones).The bauxite layer is normally continuous over the whole plateau area; sometimes, local discontinuities occur and even this layer is absent in significant parts of the plateau. When observed, these discontinuous areas are the object of an infill drilling campaign to detail the geometry.The plateaus Papagaio, Periquito, Saracá, Aviso, Almeidas, Bacaba and Aramã are exhausted. This regards, as Mineral Resources, the plateaus: Monte Branco, Cipó, Teófilo, Bela Cruz, Greigh, Escalante, Rebolado, Jamari and Cruz Alta. A schematic deposit profile and lithotypes modelled is:- Top Clay;- Nodular Bauxite (waste);- Ferruginous Laterite;- Metallurgical Bauxite (ore);- Mottled Clay.Clustering studies were performed for each plateau, with samples from the bauxite layer. Three subdomains within the bauxite lithotype were established based on this: (1) aluminous, (2) silicaceous and (3) ferruginous. The bauxite blocks were subdivided in these three subdomains using indicator kriging.As geology represents a weathered profile, no specific structures such as folders or layers inversions are present.The effectiveness of the geological domaining was ascertained by contact analysis graphs, for the main variables for all plateaus. Lithotypes (hangingwall, bauxite, footwall) were particularly focused on, as these are defined based on grade cut-offs.DimensionsMineralisation extensions are clearly defined, by top and bottom contacts and laterally by the plateau slopes (defined by DTMs). The plateaus have generally high superficial areas, but this varies for each plateau as follows: Monte Branco: 39 km2; Cipó: 13 km2; Teófilo: 8.5k m2; Greigh: 1.5 km2; Jamari: 38 km2; Rebolado: 42 km2; Escalante: 10 km2 and Cruz Alta: 51 km2. Orebodies are characterised by extensive surfaces areas (dozens of squared kilometres) and small thickness (averaging 4 m).The bauxites of Trombetas operate the mines of Saracá V, Saracá W and Bela Cruz, where the ore lies at an average depth of 8 meters and is covered with dense vegetation and a layer of overburden made up by organic soil, clay, nodular bauxite and ferruginous laterite. The mineralization of bauxite occurs at the top of the plateau and is normally covered by a layer of clay, 4 to 10 meters thick. However, near the edges of the plateaus, bauxite might even be outcropping in the soil.Bauxite is a rock made up of different minerals (gibbsite, kaolinite and quartz), the first being the source of aluminum.A typical bauxitic profile consists of the following horizons:- 4 - organic soil with a maximum thickness of 0.5m;- 3 - yellow clay covering most bauxitic profiles, consisting of kaolinite (80%) and quartz, with a maximum thickness at the center of the plateau, decreasing to its edges;- 2 - bauxitic layer with an average thickness of 4 m, subdivided into three subhorizons:• 2c – nodular bauxite;• 2b – ferruginous laterite;• 2a – massive to granular bauxite;- 1 - variegated clay.
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