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Marampa - Matukia, Masaboin Hill, Gafal, Mafuri, Rotret |
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Sierra Leone |
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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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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Marampa iron deposits are located on the outskirts of the town of Lunsar, ~90 km ENE of Freetown and 75 km by rail from the Atlantic port of Pepel, in Sierra Leone (#Location: 8° 40' 37"N, 12° 30' 40"W).
Regional Setting
The Marampa deposits lie within the northern half of the NNW-SSE elongated, 1000 x 600 km Man-Kenema shield, the southern of the two exposed inliers of the Archaean West African craton.
The Man-Kenema shield is characterised by relict zones of supracrustal rocks occurring as greenstone belts, generally synclinal structures, surrounded by granitic gneisses and autochthonous and paraautochthonous granitoids. In western Sierra Leone, the greenstone belts may be up to 140 km long and contain sequences of up to 6500 m in thickness, a minor component of which are banded iron formations. In contrast, in the southeastern part of the country, these greenstone belts are more restricted (~40 km long), with thinner stratigraphic successions, but dominant banded iron formations (Schluter, 2006).
The country has been divided into two main domains. The eastern domain is a stable Archaean to Proterozoic craton of high grade metamorphic rocks and granite gneisses. The western orogenic Rockelides, or Rokel-Kasila Belt, was deformed during the late-Neoproterozoic Pan-African tectonothermal event at ~550 Ma. The Rokel-Kasila Belt comprises four belt-parallel zones, from west to east the granulite-facies Kasila Group, the amphibolite-facies Marampa Group, reworked Archaean basement known as the Kenema Assemblage and the sub-greenschist-facies Rokel River Group. The Marampa iron deposits are hosted within the Marampa Group (Chan and de Waele, 2010).
The type section of the Kenema Assemblage comprises granites and acid gneisses, granulite facies rocks and greenstone belts of schistose sedimentary and volcanic rocks. These rocks return U-Pb ages of 3.12 Ga representing the oldest zircon, 2.90 Ga from zircons in migmatic gneiss, and 2.85 Ga from porphyritic granite inferred to intrude the Kenema Assemblage (Thieblemont et al., 2001 and references therein; Chan and De Waele, 2010). To the NE, two separate greenstone suites are recognised. The older Loko Group, composed of amphibolites with subordinate serpentinites, quartzites and banded iron formations, has been deformed by the 2.96 Ga Leonian tectonothermal event. The younger suite, the Kambui Supergroup, comprises a lower volcanic unit which includes massive mafic (amphibolitic) to ultranamafic (serpentinitic) pillow lavas, overlain by tuffs, psammites, pelites and banded ironstones, deformed during the 2.75 Ga Liberian tectonothermal event (Schluter, 2006; Chan and De Waele, 2010). The available information indicate that accretion began in the south western part of the West African Craton at ~3.1 Ga and experienced significant growth and plutonism during the Leonian and Liberian events (Chan and De Waele, 2010).
The Kasila Group represents a linear belt of high grade Archaean supracrustal rocks that have been reworked during the Pan-African Orogeny, mainly composed of acid gneisses in the granulite facies, charnockites, garnet hornblende gneiss and garnet plagioclase gneiss, and locally hornblendite and pyroxenite (Schluter, 2006).
The low grade, supracrustal rocks of the recumbenty folded Marampa Group overlie the older greenstone-granitoid terranes, generally separated by thrust faults. It comprises itonstone, mafic to felsic volcabic rocks and volcanogenic sourced sedimentary rocks that are similar to greenstone belt rocks of the Kenema Assemblage. These rocks are believed to have been deposited at around 2.1 Ga, and deformed at ~560 Ma (Schluter, 2006).
The Rokel River Group, to the east, occupies a 30 km wide and 225 km long belt, subdivided into 10 different groups, generally composed of marls, quartzites, sandstones and volcanoc rocks, overlying a basal glaciogene sequence correlated with similar Neoproterozoic successions in Senegal and Mauritania (Schluter, 2006).
To the west, a 20 to 40 km strip of Cenozoic sedimetary rocks occupies the coastal strip, while Mesozoic mafic intrusive rocks are found on the coast at Freetown (Schluter, 2006).
To compare this setting of the Man-Kenema shield in Sierra Leone, with that of the Kalia iron deposits, ~240 km to the NE, in south-central Guinea, close to the border with Sierra Leone, see the Kalia record.
District Geology
The Marampa Group occurs as a thrust-fault bounded north south trending belt, which has a relatively narrow width of 1 to 2 km in the northern half of the Marampa district. In the vicinity of the main deposits it has a syncline-anticline pair shape up to 10 km wide, wrapping around, and structurally overlying, a broad nose of Kenema Assemblage muscovite gneisses to the NW, that have a core of biotite gneiss. To the southwest the Kenema Assemblage is represented by amphibolites which are in fault contact with the Marampa Group, separated by a broad mylonite zone. To the east, the Marampa Group has been thrust over hornblende gneisses of the Kasila Group. The Kasila Group is in turn fault bounded to the east, across a mylonite zone, by the NNW trending Rokel River Group (Cape Lambert Resources, 2010).
The Marampa Group is composed of the lower mafic and ultramafic volcanic units (greenstones) of the Matola formation, which occupies a 3 to 5 km wide zone on the western half of the Marampa Group exposure to the south of the deposits. These are overlain by the Rokotolon formation, dominated by metapelitic units and subordinate metapsammite, exposed in the northern part of the Marampa Group belt, and on its eastern margin to the south. The Rokotolon formation is composed of two main facies, a quartz-mica schist and the overlying quartz-albite-hematite schists. The Marampa deposits are hosted by quartz-albite-hematite schist of the Rokotolon Formation, occurring as mineralised specularite-hematite units, which in places are several hundred metres thick (Chan and De Waele, 2010).
Chan and de Waele (2010) report that U-Pb geochronological studies of garnet-bearing quartz-muscovite-biotite-albite schist of the Marampa Group produced at least seven clusters of concordant/sub-concordant ages at ~2040, 1960, 1500, 1110, 580, 400 and 350 Ma. They regard these rocks to have a deposition age of at least ~580 Ma with apparent younger ages related to Pb-loss. However, the origin of the Marampa Group has been variously interpreted as a metamorphic equivalent of the Neoproterozoic Rokel River Group, of Palaeoproterozoic 2.2 to 2.1 Ga age (Snowden, 2013), or klippen of reworked Archaean Kasila Group (Chan and de Waele, 2010).
Chan and de Waele (2010) argue that the absence of Archaean detrital zircons in the Marampa schist suggests that the Marampa Group was deposited in an ocean basin, distant from the Archaean West African Craton and Kenema Assemblage, with iron- rich sedimentary rocks derived from seafloor mafic volcanism. This, together with the presence of ultramafic and mafic units in the Matoto Formation, appears to support the suture model of Lytwyn et al. (2006) where the Marampa Group was subsequently thrust over the Kenema Assemblage from the west. Imprecise age dates, support a metamorphic event which occurred in the latest Neoproterozoic or earliest Cambrian.
All of the rocks of the district are intruded by ENE-WSW trending dolerite dykes.
Deposit Geology
The iron deposits consist of banded quartz-hematite schists surrounded by quartz-mica schists. The mineralisation takes the form of a ~30 to 50 m, to a maximum of 65 m thick layer of hematite-quartz-mica schist, hematite-schist or specularite-schist, consisting of thin bands rich in hematite, alternating with bands free of hematite and rich in quartz. The dominant ore mineral is specularite, with magnetite and martite found locally, but in small quantities (Snowden, 2013).
The mineralised specularite-hematite schists have undergone at least four deformational phases, and are isoclinally folded and thrust faulted, and plunges 45 to 85°ESE with steep contacts with the quartz mica schist wallrocks. It is a fine- to medium-grained rock containing a peak metamorphic assemblage of quartz, hematite, albite, white mica, epidote, magnetite, apatite, tourmaline ± calcite ± garnet ± hornblende. Iron oxide minerals occur as up to 60 modal % specularite-hematite and subordinate (usually <2 to 3 modal %) magnetite, formed as part of the prograde metamorphism. The iron mineralisation appears to be accompanied by manganese-rich units (Chan and De Waele, 2010; Snowden, 2013).
The central Masaboin Hill deposit comprises a series of repeated synclinal mineralised bodies that strike NNE-SSW over a strike length of 750 m. These repeated synclinal limbs, designated A to E, have been mined to within approximately 90 m of the water table, which seasonally fluctuates, but is considered to be at ~100 m above sea level. The mineralised bodies have an undulating plunge producing limb thickening and becoming more tightly folded to the north of Masaboin Hill to form the Masaboin NE before merging to form the north-south trending Matukia Ridge deposit to the NE. To the south, limb D in the Masaboin Hill continues southeast to the Campbell Town Ridge. Limb E continues southwest into the Hospital Ridge and to the Gafal Hill (or Ghafal Hill) deposits. Gafal Hill trends east-west, comprising a near vertically plunging synform and includes a lower grade central core. Beyond Gafal, the Gafal SW extension leads to the WNW-ESE trending Mafuri deposit, 5 km to the east of Gafal Hill (Snowden, 2013). This string of deposits is distributed over a strike length of 13 km in an arcuate fashion around, and thrust over, the nose of Kenema Assemblage muscovite gniesses to the NW (Cape Lambert Resources, 2010).
At the Gafal West deposit, on the western half of the deposit cluster, the principal folds are a series of tight antiforms and synforms with steeply-dipping F3 fold axes striking NNE with overall wavelengths of ~1 km. These have been refolded by open F4 folds with WNW striking axes, resulting in an overall basin structure to the west of Gafal Hill, where a central core of weakly schistose quartz-albite rock is bounded and underlain by quartz-albite-mica schists with specular hematite-bearing layers. The best iron grades are located at the base of this unit, which is underlain in turn by a piemontite-bearing quartz-mica schist. The Gafal West deposit lies on the northern edge of the basin structure, where the specular hematite schists are interpreted to plunge 20 to 30° to the south, re-emerging at surface at the Rotret prospect in the south. The western part of the basin structure is the Mafuri deposit (Cape Lambert Resources, 2010).
Published resource figures include:
Masaboin and Galfal Hill (London Mining website, 2013)
Indicated resources - 864 Mt @ 32% Fe
Inferred resources - 208 Mt @ 31% Fe
TOTAL resources - 1.072 Gt @ 31.8% Fe
Galfal West (Cape Lambert Resources, 2013)
Indicated resources - 55 Mt @ 29.6% Fe, 41.5% SiO2, 5.1% Al2O3, 0.130% P, 0.002% S, 3.0% LOI;
Inferred resources - 178 Mt @ 26.1% Fe, 47.0% SiO2, 6.7% Al2O3, 0.191% P, 0.005% S, 2.2% LOI;
Matukia (Cape Lambert Resources, 2013)
Indicated resources - 27 Mt @ 30.1% Fe, 40.6% SiO2, 4.9% Al2O3, 0.140% P, 0.004% S, 3.1% LOI;
Inferred resources - 115 Mt @ 30.2% Fe, 40.3% SiO2, 5.2% Al2O3, 0.132% P, 0.005% S, 3.2% LOI;
Mafuri (Cape Lambert Resources, 2013)
Indicated resources - 130 Mt @ 27.5% Fe, 45.0% SiO2, 5.8% Al2O3, 0.150% P, 0.002% S, 2.3% LOI;
Inferred resources - 59 Mt @ 27.4% Fe, 45.2% SiO2, 7.8% Al2O3, 0.100% P, 0.009% S, 2.9% LOI;
Rotret (Cape Lambert Resources, 2013)
Inferred resources - 67 Mt @ 29.2% Fe, 44.1% SiO2, 6.3% Al2O3, 0.140% P, 0.008% S, 2.4% LOI;
TOTAL resources - 681 Mt @ 28.2% Fe, 43.9% SiO2, 6.0% Al2O3, 0.149% P, 0.005% S, 2.7% LOI;
TOTAL resources both groupings - 1.753 Gt Mt @ 30.4% Fe.
The most recent source geological information used to prepare this decription was dated: 2013.
This description is a summary from published sources, the chief of which are listed below.
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