Liberia: Geological Mapping
Liberia was mapped by geologic and geophysical methods during the period 1965 to 1972 as part of a program undertaken cooperatively by the Liberian Geological Survey (LGS) and the U.S. Geological Survey (USGS). A series of 1:250,000 geologic maps were compiled for the country from field data gathered by project geologists and private companies, photogeologic maps, interpretation of airborne magnetic and radiometric surveys, field mapping and ground-based radiometric surveys in which hand-held scintillators were used.
Below is a description of the LGS and USGS geology for each project area.
Nimba
The Nimba area comprises Liberian age rocks around 2700 million years in age. Most of the rock types are leucocratic gneisses, which make up about 90% of the surface area. The gneissic suite is made up of:
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Granitic gneiss - coarse to massive in texture, fairly common in outcrop, and exhibiting a low magnetic gradient. They appear to represent older rocks which have been altered by anatexis.
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Leucocratic gneiss – predominantly quartzo-felspathic, these rocks are less resistant and make up areas of low relief. They appear to represent mostly metasediments. They show very few outcrops and contain amphibolite bodies which give rise to small, linear magnetic anomalies.
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Melanocratic gneiss – these include amphibolite and pyroxenehornblende- plagioclase gneiss. They represent mafic intrusives and volcanic rocks.
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Micaschist – Two horizons of micaschist occur in Nimba, lying stratigraphically above and below the iron formations.
- Schist – these are undivided sequences of the above rock types
Iron formation – these are distinguished into the Nimba Itabirite, occurring in both oxide and silicate / carbonate facies. They form strong magnetic anomalies, are resistant so as to form high ridges, and represent metamorphosed sedimentary iron formations.
The geological sequences represent a high grade metamorphic assemblage, being up to amphibolite metamorphic facies. Although the area was well covered with reconnaissance scale mapping, geological information was insufficient to establish a stratigraphy. In terms of structural geology, two types of folding were observed in the Nimba Itabirite – isoclinal and broad open folds, both with a NE axis. Faults of 3 different major trends were observed – being approximately. N 10 E, N 30 E and N 80 E.
Economic Geology
The Nimba iron ore mine to the northwest of the Nimba project area, is a large, high grade, itabirite iron ore deposit of world renown. It was mined by LAMCO (a joint venture between Bethlehem Steel of the US and Granges of Sweden) from 1959 to 1992. The joint venture built a 278 km railway to the port of Buchanan, and exported up to 15 Mt per year of high grade iron ore. In 1992 it was closed down, not through shortage of reserves but because due to the civil war in Liberia. It has now been acquired by Mittal Steel Ltd, who reportedly plan to invest some USD 1 billion to rehabilitate the mine and railway.
Cestos
Geological mapping shows the following at Cestos:
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Leucocratic gneiss – predominantly quartzo-felspathic, these rocks are less resistant and make up areas of low relief. They appear to represent mostly metasediments. They show very few outcrops and contain amphibolite bodies which give rise to small, linear magnetic anomalies. Muscovite bearing pegmatites are abundant.
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Quartz diorite gneiss unit 2 – underlies a large area from the Sino River to the Sehnkwehn River, and northwest to the Cestos batholith. Amphibolite bodies form topographically positive features which are visible on airphotos.
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Schist – these are undivided sequences of the above rock types.
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Diorite – this rock type forms the Cestos batholith, as well as other intrusive bodies. Most of them form low-lying outcrops, with a broad flat magnetic pattern.
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Amphibolite – these form discontinuous tabular bodies, many of which are too thin to map, while others are very thick and show as ridges with negative magnetic anomalies.
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Diabase dykes – these are Jurassic in age and form NW trending dyke swarms. Most are steeply dipping and are intruded along faults, forming linear ridges 10 to 30m high.
The Cestos area lies within the broad NE trending structural province that characterises most of Liberia. Most folds in this area are open in style. Most of the rocks are up to amphibolite metamorphic facies, and the area appears to be transitional between the Liberian and Eburnean geological domains.
Dugbe
Geological mapping shows the following at Dugbe:
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Leucocratic gneiss – predominantly quartzo-felspathic, these rocks are less resistant and make up areas of low relief. They appear to represent mostly metasediments. They show very few outcrops and contain amphibolite bodies which give rise to small, linear magnetic anomalies. The widespread occurrence of itabirite units characterises this as an ‘itabirite province’.
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Quartz diorite gneiss unit 2 – underlies a large area from the Sino River to the Sehnkwehn River, and NW to the Cestos batholith. Amphibolite bodies form topographically positive features which are visible on airphotos.
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Composite gneiss unit 2 – mainly graphite–bearing biotitequartz- diorite gneiss. Small granitic intrusives contained within the unit cause high amplitude radiometric anomalies which cross cut the regional stratigraphy / structure.
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Diabase dykes – these are Jurassic in age and form NW trending dyke swarms. Most are steeply dipping and are intruded along faults, forming linear ridges 10 to 30m high.
Piso
Geological mapping shows the following at Piso:
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Composite granitic gneiss unit 2 – layered biotite rich gneiss, which in many places encloses or is associated with itabirite, schist, quartzite and amphibolite. The unit is characterised by higher magnetic anomalies than those of other gneiss units.
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Melanocratic gneiss – these occupy the coastal belt, and include granodioritic to gabbroic gneisses, with lesser quartose leucocratic gneiss.
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Pleistocene sediments – ‘beach deposit unit 2’ – nearly pure white quartz sand, on average 1m in thickness, forming large areas of savannah near to the present coast line.
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Diabase dykes – these are Jurassic in age and form NW trending dyke swarms. Most are steeply dipping and are intruded along faults.
The NW trending Todi Shear Zone, which runs through the project area, is a regionally prominent shear zone, and separates the Liberian Province rocks to the NE (2700 million years old, amphibolite facies metamorphics) from the Pan African age province on the SW side (550 million years in age, up to granulite facies metamorphics). The individual shears making up the Todi shear are SW dipping (50-700 dip), based on airborne magnetic data. The Todi shear is made up of proto- to ultramylonitic rocks, with distinctive slabby weathering characteristics.
Within the Piso area large synforms and antiforms were observed on the aerial photographs. These have steep axial planes, which are parallel to the regional structural grain, and have fold hinges which are gently pitching. In hand specimen, schistosity was observed which generally parallels the lithological banding. In itabirite, schist and quartzite, secondary foliation can be seen which parallels primary stratification.