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Gold. The country's most important gold-bearing regions are mounts Soula and Kangari - two main gold mining locations. Small placers were developed along the Mongo, Mabole, Bafi and Pampana river beds. 10.5 tons of gold were extracted in the time span of 1930-1959. Then gold extraction in these regions declined and almost stopped because of the soon depletion of these easy-to-develop mines.
In the river valleys of Soula region many small alluvial placers are known. The river currents run in the mountains composed of greenstone rocks of Kambui series. The placers are 0.5 to 2.5 kilometers long and around 100 meters wide. Gold content equaled 6 to 20 grams/m3. Almost all of the placers are by now depleted.
In the Kangari regions small placers are known as well. These placers are similar to those of Soula. Baumahun deposit is also here, located in the schists of the Kambui series, in the 1,300 meter long zone of silicification and breakage. Average gold content here is 8 to 9 grams/ton. Average purity - 760. Total reserves (300 meters deep) - 30 tons.
Makong deposit lies 12 kilometers to the north of Baumahun. The mineralization is located in tremolite schists and serpentine rocks of Kambui series in form of 1-1.8 meters thick steep pitching quartz veins of sub-lateral course. The mineralization is uneven. Gold content - 20 grams/ton. The gold is coarse-grained, nuggets of as much as 30 grams of weight were found. Maximum extraction reached 100 kilograms of gold a year. By 1970 the deposit had depleted.
Yurisen deposit is situated in the upper parts of the river Yurisen. Mineralization belongs to the aureole between talc-chlorite-carbonate schists of the Kambui series and granites, and is represented by quartz veins of 2.4 meter thickness. Native gold, native silver and sulfides were discovered in the veins. Average gold content - 12.8 grams/ton. Gold is fine-grained. Extraction lasted until 1956.
In the 1970s geologic department of Sierra-Leone Soula-Kangari region contemplated the idea of recovering the depleted gold deposits, carried out geologic mappings, searched for new veins, executed other prospects. The results of it were quite reassuring. In the southern parts of the Kangari hills of the Baumahun region three ore-bearing zones were discovered, altogether occupying 1.2 kilometers. In the central zone, drilling proved mineralization 210 meters long and 200 meters deep. Gold content - 13.4 grams/ton. On the southern side of the zone, the thickness reaches 48 meters with 4.4 grams/ton gold content.
Platinum. Platinum mineralization belongs to the Freetown gabbro-anorthosite complex, in the alluvium of the rivers making their ways through the complex. Platinum-bearing placers set up. Between 1929 and 1950 extraction of platinum from these placers totaled 149.76 kilograms. Chemical composition: platinum - 87%, osmiridium - 1.3%, palladium - 2%, iron - 9.4%. The placers are of small size and small reserves. The metal content varies between several milligrams to 30 grams per ton.
Diamonds. Sierra-Leone does about 2.5% of the world diamond extraction. Total reserves count 30 mln carats, explored reserves - 5 mln carats. A diamond-bearing region (20 thousand sq. km. of area) is situated in the Eastern province in the riverbeds of Seva, Moya and their tributaries. The diamonds are mainly extracted from the alluvial placers. They are known for their premium quality. Crystal's average weight - 0.5 carats. Probability of extracting a big crystal is quite high here. Diamond content in placers - 1 to 1.5 carats/m3. Some placers were characterized by fabulously high diamond recovery - as much as 250-300 carats/m3.
In the Sefadu region both placers and kimberlite dikes are being developed. The latter form a series of parallel, en echelon bodies stretching from 100 meters to 1.5 kilometers. They are grouped together in zones. The main zone is 19 km long and 3.2 km wide. Dikes are 0.2 to 1.5 km long and around 2 meters thick.
The deposits of the Koidou region are represented by kimberlite pipes and placers. One of the pipes was explored as far deep as 90 meters and has been utilized since 1967. In the region there are alluvial placers of small size and small reserves. Diamond reserve in the pipe is 0.9 carats/m3, in the "yellow ground" of the dikes - 15 carats/m3, in the placers - 1 carat/m3 (maximum - 200 to 300 carats/m3); size of the diamonds - up to 50-100 carats. In 1978 underground development started in the Koidou deposit. The supposed extraction counts as much as 100 thousand carats a year.
The Seva region deposits are represented by numerous alluvial placers of small size. The easy-to-work placers are now all depleted.
The Tongo region deposits in Kenema province are represented by small alluvial placers and a little kimberlite body. Illegal diamond extraction is a pronounced tendency here.
As the result of placer depletion in Sierra-Leone, primary deposits were studied most actively in 1976-1979. It was proved that private development of kimberlite bodies by means of pits is economically profitable.
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