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Faleme |
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Senegal |
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Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Falémé iron ore deposits are located in far south-eastern Senegal, West Africa, within 20 km west of the border with Mali. The district comprises 9 major and 19 minor orebodies, distributed over a 65 x 15 km north-south elongated belt. The mineralisation has been interpreted to represent an exo-skarn, much of which has been upgraded by lateritic processes.
The district lies within the Kedougou-Kéniéba inlier, the western most exposure of the Paleoproterozoic Birimian Supergroup rocks in West Africa, separated from other blocks of the same supergroup by Neoproterozoic basinal sediments. The inlier is composed of the sediments, with subordinate andesitic (and minor rhyolitic) volcanic rocks of the Dialé-Daléma Series to the east and the volcano-sedimentary Mako Series to the west, as well as gneiss and 2079 to 2081 Ma intrusive granitoids. Rhyolitic flows and pyroclastics within the Dialé-Daléma Series have been dated at 2117 ±9 to 2099 ±4 Ma.
The sedimentary components of the Dialé-Daléma Series comprise a siliciclastic succession of arkose, subarkose, wacke, siltstone and conglomerate, including some manganiferous schists, and a carbonate sequence of dolomitic and calcitic marble. The marbles are the protolith of the iron mineralisation. Both the volcanic and siliciclastic rocks have been modified by thermal events, interpreted to be related to the intrusion of plutonic and sub-volcanic rocks in the area.
Granitoid intrusives include generally north-south elongated plutons of clino-pyroxene-hornblende granodiorite, pyroxene-tonalite and diorite in the belt containing the iron deposits and a larger biotite(-muscovite) granite batholith to the west. Only the diorite, particularly the micro-diorites appear to be related to skarn iron mineralisation and are the only hosts to endo-skarn magnetite accumulations.
The protolith of the endo-skarn iron mineralisation comprises a heterogeneous group of sub-volcanic dioritic rocks, locally termed as "microdiorite" which have been variously described as diorite, gabbro, monzonite, greenstone, "albitophyric greenstone" and feldspar-bearing hornfels.
The "microdiorite" has a fine- to medium-grained relict texture, varying between equigranular, seriate, and porphyritic with major phase plagioclase (40 to 65%) and amphibole (25 to 55%). Albite veinlets commonly cut across larger plagioclase grains. The plagioclase is composed predominantly of 0.3 to 3 mm equidimensional to prismatic grains which are unzoned and have the composition of albite (An 0-9). Epidote-clinozoisite, calcite, and muscovite are common secondary phases. Amphibole is mainly actinolitic-hornblende and actinolite, present as 0.05 to 3 mm subhedral grains, often corroded and altered to chlorite + epidote-clinozoisite ±calcite ±biotite ±titanite ±rutile. Biotite occurs as 10 to 1200 µm flakes, while the <3% K feldspar occurs as 0.1 to 0.5 mm anhedral grains. Quartz is present as veinlets and lenses. Accessory minerals include apatite and pyrite. Magnetite is up to 5% of the microdiorite outside of the endo-skarn concentrations, and is associated with secondary phases such as actinolite, epidote-clinozoisite and muscovite.
The 2 km long north-south striking and vertically to steeply east dipping, up to 240 m thick Goto exo-skarn is magnesian and hosted by graphite bearing dolomitic and calcitic marble. The skarn is bounded to the west by marble and to the east by microdiorite and has been supergene enriched to a depth of 50 m. The marble is composed of 70 to 80 carbonate minerals, mainly dolomite and calcite with subordinate siderite and minor serpentine, serpentinised pyroxene and biotite. Silicate minerals are concentrated in planar and contorted, wavy or irregular 1 to 5 cm thick layers. There are 3 main types of magnetite skarn, namely: i). clinopyroxene + biotite ±amphibole with accessory dolomite, calcite, chlorite, talc, plagioclase and andradite-grossular; ii). serpentine ±calcite with accessory amphibole, biotite, chlorite, talc and traces of graphite; and iii). clinopyroxene ±amphibole ±calcite ±chlorite ±serpentine with accessory apatite, titanite, plagioclase and quartz. Grade variations between the different types is only minor. Magnetite is associated with prograde clinopyroxene (Di 89-100 Hed 0-11) and phlogopite-rich biotite (Mg/(Mg + Fe) ~0.89) as well as retrograde serpentine. Garnet is rare, while sulphur averages 1%. Pyrrhotite and subordinate pyrite ±chalcopyrite ±arsenopyrite ±pentlandite ±cobaltite postdate the majority of the magnetite. Magnetite in the exo-skarn has a similar trace element geochemistry to that in the endo-skarn but is distinguished from igneous magnetite in granodiorite and magnetite in andesite by low concentrations of Cr2O3 and V2O5.
Detailed studies have been undertaken on one of the endo-skarn bodies, the 25 Mt, supergene enriched Karakaene-Ndi deposit which is hosted by microdiorite which has been pervasively albitised. The endo-skarn comprises all stages from accessory magnetite to semi-massive magnetite ±garnet ±clinopyroxene ±muscovite ±biotite. Four types of alteration are recognised: i). garnet (And 56-100 Grs 0-43 Alm 0-2 Sps 0-1 Pyr 0-1), ii). clinopyroxene (Di 58-100 Hed 2-42) + garnet, iii). clinopyroxene + mica, iv). mica. The individual garnet and clinopyroxene grains are commonly inhomogeneous, with Fe being concentrated at their margins rather than in the core. The main phase of 0.02 to 5 mm magnetite is intimately associated with the type ii). and iv). phases and post-dates the garnet and clinopyroxene, occurring in the interstices between these grains. Magnetite also fills interstices between plagioclase and amphibole, replaces plagioclase and amphibole and occurs as veinlets and lenses
The supergene ore has been derived from ~30% Fe endo-skarn and comprises of hematite and hydrous iron oxides with a kaolinised gangue. The largest orebodies are around 100 m thick and are located in the central part of hills rising up to 250 m above the surrounding peneplain. The base of the zone of enrichment is usually is centered deep below the top of the hills. Supergene enrichment is absent from the exo-skarn bodies, probably as a result of the lack of favorable structural conditions. Supergene enrichment of endoskarn is apparent the result of both residual enrichment, in which Ca, Mg, Na, and Si were removed, and the addition of Fe from descending solutions to the site of enrichment. The supergene ore averages around 84% Fe2O3, 5% SiO2, 0.1% P, 5% Al2O3, 0.02% MnO and 0.2% MgO.
Two of the major exo-skarn orebodies, Farangalia and Goto, have total reserves of ~320 Mt (125 and 192 Mt respectively) of magnetite ore @ 42% Fe, while seven supergene-enriched orebodies overlying endo-skarns, have contained 310 Mt @ 59% Fe, individually ranging from 25 to 107 Mt (Schwartz & Melcher, 2004).
More recent reports quote a reserve of ~750 Mt of ore at unspecified grades.
The most recent source geological information used to prepare this decription was dated: 2004.
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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Lambert-Smith, J.S., Allibone, A., Treloar, P.J., Lawrence, D.M., Boyce, A.J. and Fanning, M., 2020 - Stable C, O, and S Isotope Record of Magmatic-Hydrothermal Interactions Between the Faleme Fe Skarn and the Loulo Au Systems in Western Mali: in Econ. Geol. v.115, pp. 1537-1558.
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Schwartz M O and Melcher F, 2004 - The Faleme iron district, Senegal: in Econ. Geol. v99 pp 917-939
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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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