TRANSACTIONS OF SECTION C. 197 



The Discovery of Diamonds in the Gold Coast, British West Africa. 

 By A. E. KiTsoN, C.B.E., F.G.S. {Director, Geological Survey, 



Gold Coast).'- 



The site of the discovery of diamonds, -which I made early this year, is at 

 Abomoso, in the district of Akim Abuakwa, about sixty-five miles to the north- 

 west of Accra, the capital of the Gold Coast. The rocks of the district consist 

 of a t?iick series of phyllites, altered mudstones, sandstones, grits, and con- 

 glomerate, with interbedded altere-d dolerites and tuffs. They have a general 

 strike of about N.E.-S.W., and are moderately folded. On their north-western 

 and south-eastern sides, and in their south-westward extension, they are 

 intruded in many places and greatly metamorphosed by granites and pegmatites, 

 and to a less extent by diorite. 



The diamonds occur in detrital gravels resting on the top and side of a 

 low ridge, and in the beds of shallow streams. These gravels consist very 

 largely of quartz, pi'incipally of opaque, semi-opaque, and transparent varieties. 

 The associated minerals are chiefly sisolite, white and brown micas, zircon, 

 pyrite, magnetite, ilmenite and gold, with rarer fragments of rutile, black tour- 

 maline, red garnet, chalcedony, bi'own corundum, kyanite, and sphene. In 

 general character this gravel has unquestionably the appearance of having been 

 derived from granitoid rocks. 



iSmall diamonds have been found also in the beds of four streams up to 

 fourteen miles to the north-west, and in two streams to five miles to the north- 

 east of this place. In addition, one diamond has been found in the quartz 

 pebble gravels of the upper terrace of the Birrim River. 



At the site of the original discovery the bulk of the gravel is distinctly 

 angular in character, but has associated bands of rounded quartz sand, and a 

 few water-worn pebbles of quartz. 



The diamantiferous gravels of the streams to the north-west consist largely 

 of quartz, felspar, mica, and fragments of pegmatite, but have a good deal 

 more chert and chalcedony than those near Abomoso, though still in small 

 proportions. 



The pebbly gravels of the high terrace? of the Birrim River, and of the 

 beds of the streams to the north-east, which are mainly rounded pebbles of 

 quartz, altered sediments, and volcanic rocks, give concentrates consisting 

 largely of chloritic minerals, ilmenite, magnetite, and quartz. 



Some 620 diamonds have been found merely by panning during operations 

 to test the character, extent, and distribution of the diamantiferoas material. 

 All the stones are small, the average being about thirty-two to the metric carat 

 (3.174 grs. Troy). The largest, a clear, colourless, octahedron, weighs a little 

 over one-sixth of a carat. Many of the diamonds are beautiful crystals, colour- 

 less and transparent, the commonest forms being the octahedron and the rhombic 

 dodecahedron, though there are numbers of tetrahedral forms as well. Cleavage 

 plates of octahedra occur in fair numbers. jSeveral of these show that the 

 parent crystals were much larger than the largest stone hitherto found here. 

 Some of the crystals show curved faces ; many others are chipped ; and there 

 are numerous fragments. Many of the stones are clear and colourless; others 

 are of pale yellow, blue, green, grey, and brown tints, principally the last. 



The largest stone is valued at about £1. The largest grade of colourless 

 stones, weighing about eleven to the carat, are valued at 80s. to 100«. per 

 carat ; the medium grade, about twenty-two to the carat, at 60'?. to 70>s\ per 

 carat; and the smallest grade, about sixty-four to the carat, at 2.5«. to 40.?. per 

 carat. The coloured stones of the same groups may be taken at about 40s., 30s., 

 and 15^'. per carat respectively. 



In the Abomo Su area the bed-rock in some ^ilaces proved to be phyllite.s or 

 sandstones; in others, a decomposed altered ba.sic tuff or lava, full of chloritic 

 minerals. A good deal of this material was panned, but no diamonds were 

 found in it, though the overlying detrital material in every case yielded 

 diamonds. 



' See Report published by Government of the Gold Coast. 

 1919. s 



