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Moanda |
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Gabon |
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Mn
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Moanda manganese deposits are located in south-eastern Gabon, Africa, approximately 50 km west of Franceville and 450 km south-east of the capital Libreville which is on the Atlantic coast.
The Moanda manganese deposits lies within the intracratonic Franceville Basin which contains a 1000 to 4000 m thick Paleoproterozoic sequence bounded to the NE by the 'North Gabon Craton' and to the SW by the associated Chaillu Block. 'North Gabon Craton', and the connected Chaillu Block are composed principally of 2900 Ma charnockites, pyroxene gneisses, granitic gneisses and migmatites. Both sections of the craton were granitised at 2700 Ma, although septa of greenstone and schist belts still remain in places, represented by banded ironstones, amphibolites, pyroxene amphibolites, amphibole quartzites and biotite gneiss.
The overall Franceville Basin depository is subdivided into a number of sub-basins by basement ridges. The succession within the basin comprises, from the base comprises:
Mabinga Sandstone, 0 to 1000m thick - a basal red sandstone which is coarse, feldspathic and conglomeratic along the margin with the Chaillu Block and has associated dolomite, anhydrite and gypsum, passing upwards into a pyritic and asphaltic facies.
Bangombe Shales, 600m thick - calcareous and carbonaceous pelites with alternating jaspers and some sideritic and manganese rich iron formations and lenticular channel sandstones, including a lower mafic volcano-sedimentary unit. The carbonaceous section in the upper Mabinga and Bagombe Shales have also been designated as the FB Formation within southeastern Gabon, which contains one of the greatest accumulations of organic carbon of its age. The unit, which is weakly metamorphosed and dominantly pelitic, ranges in thickness from 400 to 1000 m and covers an area of over 35 000 sq. km. It consists of about 80% marine shale with 0.5 to over 10% total organic carbon;
Djoumou Shales, up to 50m thick - a marker unit of mostly thickly banded chert and massive dolomite which oversteps onto the basement;
Bambai Black Shales with associated ignimbritic tuffs at the top;
Lepaka Sandstones - the uppermost unit comprising alternations of sandstone and shale.
The age of deposition is limited by the 2700 Ma dates on the Chaillu Block basement and 2050 Ma of the contained Franceville uranium mineralisation (see the Franceville record) which is hosted by the upper carbonaceous sections of the Mabinga Sandstone and sections of the overlying basal Bangombe Shale.
The manganese mineralisation at Moanda is hosted by the Bangombe Shale, higher than the Franceville uranium deposits, and is formed from the weathering of manganese carbonates at the top of the sequence which occurs with banded iron formations containing siderite and greenalite. The primary Mn carbonates, prior to upgrading to 45 to 50% Mn by weathering, carry around 15% Mn.
The main deposit occurs as a horizontal, elongate lens that is 3 to 7.6 m thick and covers an area of 19 sq. km.
When compared to other black shales, the average FB Formation black shale is enriched only in Au, Ag, Ba, and Cr. General paucities in U and Mn represent negative anomalies in this area of world class U and Mn deposits. Detailed geochemical profiles of the lower FB Formation show that Fe/Mn ratios increase upwards from the base of the formation to the ores at the top. At least locally, Mn build-up in the sedimentary column is interpretted to be linked to increased oxidation potential caused by photosynthesis. The abundance of decomposing organic matter is believed to have played a key role in further concentrating Mn in carbonate protore during early diagenesis.
The Moanda Mn orebody had reserves in 1964 of 550 Mt @ 51% Mn. Operational startup was in 1962 at a production rate of 2 Mt of Mn conc. per annum. In 2000 production was reduced to 1.74 Mt, although the operation has a capacity of 2.5 Mtpa of ore and 0.6 Mtpa of sintered manganese ore (USGS 2000).
The most recent source geological information used to prepare this decription was dated: 2005.
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.
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Selected References:
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Mossman D J, Gauthier-Lafaye F and Jackson S E, 2005 - Black shales, organic matter, ore genesis and hydrocarbon generation in the Paleoproterozoic Franceville Series, Gabon: in Precambrian Research v.137 pp. 253-272
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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