Generally tin deposits he close to the contact of intrusive and intruded rocks and are mainly found near the top of the intruding mass. It therefore follows that in deeply eroded granite masses the chance of finding lode tin deposits is smaller than where search is made in the tops of granitic intrusions. It has also been noted that deposits in intruded rocks generally lie where the dip of the intrusive contact is low and are rarely present along a steeply dipping intrusive contact.
Practically the sole ore mineral of tin is cassiterite (tin oxide), which carries 78.6 per cent. of the metal. Cassiterite is known commercially by various names, such as tinstone, black-oxide of tin, black tin, or, where it occurs in placers, stream tin. The tin concentrates from placer mining normally carry 60 to 75 per cent. metallic tin, 70 per cent. being a fair average. The concentrates from the mills treating Bolivian lode tin make a product called barilla that averages about 62 per cent. tin; the concentrates produced from lodes in Cornwall average about 65 per cent.; and from the lodes and placers of the Malay Peninsula carry about 72 per cent. tin.
In many parts of the world the lodes do not carry sufficient tin to be worked profitably. In Cornwall and in Tasmania, lodes carrying about 1 per cent. of tin are being mined; but in general a content of 1 to 2 per cent. tin is the lower limit for commercial lode mining. In Bolivia the tin lodes average 5 to 8 per cent. tin and some bodies of ore carrying as much as 40 per cent. tin have been opened. In the places where low-grade tin ores have been mined the by-products, principally arsenic and wolfram, have helped to pay expenses, and most of these mines are advantageously situated with respect to transportation and supplies. In the placers of the Malay Peninsula, including Banca and Billiton, and those of Australia, which are worked by dredges, the tin content ranges from one-half pound to as high as 3 pounds, but averages less than a pound of cassiterite to the cubic yard. Advantageous location and cheap labor make profitable exploitation possible.
GEOGRAPHICAL DISTRIBUTION
As will be seen from the map, [Plate IX], tin deposits are found in every part of the world, though an inspection of [Table 60] and [figure 9] will show that the deposits within the British Empire are the most important sources of the world supply. Bolivia and the Dutch East Indies have been the chief producers of tin outside of the British Empire, though China and Siam are steadily gaining importance as tin producers.
British Empire.
—The British Empire has tin deposits in England, Asia, Australia, and Africa. The largest production is from the deposits in the Malay Peninsula. The African deposits, those in Nigeria and the South African Union, yield the second largest output of the empire, the Australian deposits rank third, and the Cornwall deposits, formerly the largest producer of tin in the world, now rank fourth.
Malay Peninsula.
—The Federated Malay States and the British Protected Malay States occupy the southern end of the Malay Peninsula. This region, which is entirely British controlled, produced for many years one-half of the world’s output of tin, but in the last few years the output has declined steadily. The decline seems to be due to the exhaustion of the easily worked placer deposits, though in 1917 and 1918 an additional cause was the scarcity of labor.
