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The Manantenina bauxite deposits are spread over an interval of approximately 50 km and are some 110 km north of the town of Tolagnaro in south-eastern Madagascar.
Mineralisation is localised on two Coastal Peneplain platforms at approximately 25 and 60 m ASL respectively, and is developed over leptynites of the Bekily Block terrane. These leptynites comprise leucocratic metamorphic rocks derived from arkoses or granites, predominantly composed of fine grained, homogeneous quartz and feldspar (alkaline), which are often garnet and/or cordierite rich but amphibole and mica poor. They are high alumino-silicate rocks, with low ferro-magnesian minerals and high silica and assay 13 to 25% Al2O3, 1 to 17% Fe2O3 and 45 to 75% SiO2. The leptynites exhibit intense fracturing and faulting facilitating drainage and weathering. To the north the leptynites pass into a band of charnockites which appear to be less conducive to lateritisation and the ore terminates.
The deposit is divided into three blocks by four large river valleys, namely the North Zone, the Central Zone and the South Zone. Heavy mineral bearing dune sands are developed on the eastern margin of the deposit which is generally within 2 to 3 km of the shoreline.
Two platforms parallel the coast. The lower, 25 m elevation platform is generally 3 to 9 km in width (east-west, with the eastern limit being defined by a lagoon while the western margin corresponds to the step-up to the higher 50 to 60 m ASL platform that varies from 4 to 20 km in width and is more dissected. To the west of this peneplain, the hills rise to 160 to 1100 m, still composed of leptynite, but with no bauxite.
The bauxite occurs as remnant tabular bodies which may be up to 10 m thick preserved on the ridges, with some transported bauxite in the valleys. The base of erosion is around 20 to 30 m below the crests of the ridges. Ore on the ridges occurs as lenses of around 150 m in length, occurring in clusters as dissected mesas which cover areas of 1 to 2 sq. km each.
The quoted resource in 1973 was 165 Mt of dry bauxite. This included 53 Mt in the Central Zone for which more detailed testing had established a grade of 48.9% Al2O3, 10.2% Fe2O3, 12.1% SiO2, 0.1% CO2, 0.1% anhydrite, 0.04% CaO and 26% LOI.
The average thickness of the mineralised zone is 6.9 and 5.6 m in the Northern and Central Zones respectively.
A typical profile is a follows:
i). Soil and gravel , 0 to 0.3 m thick - pink to brown bauxitic gravel and humic material.
ii). A layer of clay with rounded concretions which have a white or brown platy texture and blocks of granular bedrock which have feldspars completely converted to gibbsite. Gibbsite occurs as nodules which when in sufficient quantity form the ore grade mineralisation.
iii). A lower layer with preserved bedrock texture and gibbsite development from feldspars. The zone contains free quartz and phyllic minerals which have been disaggregated during the weathering process. If the bedrock is gneiss or migmatite it is richer in gibbsite and can be ore, whereas mica schists and phyllite generally only produces low grade.
The most recent source geological information used to prepare this summary was dated: 2004.
This description is a summary from published sources, the chief of which are listed below.
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