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Different sources make different estimations of Guinea's total diamond reserves, from 30 to 300-400 mln carats. Known reserves account for 3.3 mln carats. All workable diamond reserves are situated on the south of the country, in the so-called Forest Guinea, along the riverbeds of Baule, Makona and Diani rivers. Around 20 kimberlite pipes, a hundred dikes and veins and numerous small, almost depleted alluvial placers are known in the area. Kimberlites are well-developed in Upper Archean gneiss granites, migmatites and Baule granites. Kimberlite pipes have 10 to 350 meters in the diameter, kimberlite dikes stretch for 0.01 to 1.2 kilometers, their thickness varies from 10 centimeters to several meters. Diamond content in the pipes and dikes ranges from 0.1 to 1.5 carats per every cubic meter of the rock. In Forest Guinea, 4 best diamondiferous deposits are distinguished: Banankoro-Urukoro, Ferugban, Fenaria and Bunudu.
In Banankoro-Urukoro diamonds can be found in kimberlites and placers. It's here that the biggest kimberlite pipe Antoshka (350 by 150 meters) is located. In Antoshka you can run across rhomboid-dodecahedral and octahedral diamonds with irregular and oval-cascade facets. Their average weight is 0.08-0.35 carats. Gem diamonds percentage varies between 20 and 60%. Proved reserves make up 1 mln carats.
Ferugban is a rather complicated kimberlite body consisting of 12 neighboring dikes sized from 2 by 20 meters to 25 by 175 meters. Most of the diamonds have irregular shape and a complex sculptural surface. Average weight-0.08 carats.
On Fenaria formation are installed only those kimberlite dikes that have developed within 25 kilometers of a big fracture zone. The diamonds are either irregular crystals or rhomboid-dodecahedral diamonds with mat surface.
Bunudu is a sizeable kimberlite pipe Druzhba (50 by 170 meters). The pipe is accompanied by numerous dikes containing porphyritic kimberlite. Diamond quality is low, average weight ranges between 0.06 and 0.81 carats. But sometimes you may extract crystals weighing 30-50 and even as high as 275 carats. In the late 1970s - early 1980s diamondiferous areas of the country became the arena for joint companies' activity. In 1982, 25 thousand sq. km. in Kisidougou-Banankoro-Masanta region were allotted to AREDOR joint company (Guinea, Australia, Switzerland). In the mid-1983 a big diamond placer was discovered. The estimated capacity of the enterprise came to 400,000 cubic meters or 140,000 carats a year. 70% of diamonds are gem diamonds. Known reserves total 3.7 mln, with average carat content 0.35 carats/m3, which means there is 1.3 mln carats here. The layer is presently being developed; however, there is no proved information about the company's actual diamond recovery. A.D.J. was founded by Guinea, USA and France in the year 1978. It received the area of 40.5 thousand sq. km. in Firana-Kouroussa-Kankan and Kerouane-Beyla regions. Prospecting of large placers in Bunduku resulted in more than 4.5 mln m3 of diamondiferous rocks with average diamond content coming to 0.4-0.5 carats/m3 or about 2 mln carats of diamonds. 70% of them are industrial diamonds. The company ruled the development of the placer inefficient. In 1982 and 1983 GOLF AFRICAN company conducted exploration and assessment works in the valleys of Diani and Mani rivers. The outcome of this endeavor we don't know. In 1983 other works were planned in two regions where small diamond occurrences had been found. The works never took place.
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