Africa.
—As will be seen from [Table 60], the principal production of tin in Africa is from British Nigeria. The district was worked by natives in the early days, but no important production was made until 1904, after the subjugation of the Emir of Bauchi. The production of Nigeria has grown steadily till it reached 8,500 tons of concentrates in 1917. Seemingly all of the mines are controlled by British capital and the exports have been largely to the smelters in England.
The alluvial deposits of Nigeria are in the valleys of the Bauchi Plateau. Soda granite and pegmatites, intrusive into older crystalline rocks, seem to be the source of the cassiterite that has been concentrated by the present streams. Sluicing is the principal mining method, though some deposits are suitable only for dredging. Tin is also known in northern Nigeria in the Ningi and Burra hills, and other localities. In southern Nigeria tin has been found near Akwa-Ibami, in the Uwet district.
The tin output of the Union of South Africa is chiefly from the Waterberg-Zaaiplaats district, in the western Transvaal, a little tin being mined in Swaziland and the Cape province. The production has ranged from 2,950 tons to 3,450 tons of concentrate a year, most of which before the war went to England for smelting, but since 1915 to the Straits Settlements. A small smelter, rated at 250 tons a year, was built at Zaaiplaats in 1917; it is expected to supply the tin needed in South Africa.
The Waterberg district contains several tin fields. Tin ores are found in the Red Granite and Waterberg felsites, sandstones, and conglomerates. In the former the ores occur in pipes, in irregular bodies of altered granite, disseminated in the granite, in impregnations along fissures, and in pegmatites and quartz veins. In the Waterberg series the tin ores are in lodes, and in irregular lenses and pockets whose position is determined by fissures or bedding planes.
In the Potgietersrust district the principal mines are largely pipe deposits in the Red Granite. These pipes, which are very erratic in direction, range from a few inches to 20 feet in diameter; some have been followed for 3,000 feet. The filling material varies greatly, ranging from slightly altered granite to a greenish homogeneous rock; the outer zones are tourmaline-quartz rock. In the smaller pipes the cassiterite is fairly evenly distributed but in the larger pipes it occurs near the outer edges.
In the Nylstroom district the principal mines are working ore deposits in felsites and shales of the Lower Waterberg series. The deposits are brecciated country rock cemented by quartz, tourmaline, cassiterite, and fluorite.
The Warmbaths field includes several mines located along the junction of the Red Granite and the felsites. Tin is found in lodes in both types of rocks and some alluvial tin has been mined. In the Rooiberg field, the tin deposits are practically all in fissures in quartzite intruded by Red Granite. Tin occurs in the Red Granite 40 miles north of Pretoria.
In the Cape Province, near Kuils River, cassiterite is found disseminated in granite and in veins at the contact of granite and slates. Most of the small amount of tin won has been obtained from gravels derived from these deposits.
In northwestern Swaziland alluvial deposits have been worked for a number of years, producing around 500 tons of concentrates a year. At Forbes Reef, schists and slates have been intruded by granite and tin lodes are found near the contact of the two formations.