Political and commercial geology and the world's mineral resources

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GEOLOGICAL DISTRIBUTION

Over 70 per cent. of the tin produced in the world is won from placer deposits, although in the last few years the exploitation of tin-bearing lodes has become of considerable importance. Tin ores are intimately connected with siliceous igneous rocks. Practically all of the known lode deposits are either in or lie near siliceous igneous rocks such as granite, granite-porphyry, quartz-porphyry or monzonitic types. In Mexico and the United States unimportant tin deposits have been found in rhyolite. In only one or two places in the world are tin lodes known where siliceous igneous rocks do not show on the surface, and in these places geologic evidence points to the presence of granitic rocks at no great depth. In the world’s chief centers of tin production—the Malay Peninsula, Bolivia, Australia, Nigeria, Cornwall, and South Africa—the granitic rocks are everywhere in evidence, and the tin lodes are so closely related to these granites that there is no question of their origin.

Fluorine-bearing minerals such as fluorite and topaz, tourmaline, and the tungsten mineral, wolframite, are found in practically all tin deposits. Molybdenite and bismuth minerals are present in many tin deposits, though their distribution is not so general as that of the former minerals. Copper, lead, zinc, and iron sulphides, the latter often arsenical, are common in tin lodes, and quartz and feldspar are the chief gangue minerals.

It is generally accepted that the tin lodes were formed near the close of intrusive activity by the final differentiates of the acid magmas. These final solutions are notable for their pneumatolitic action and their ability to cause the profound change of granite to greisen and the formation of stanniferous pegmatite and quartz veins. Greisen, an alteration product of granite, consists of quartz, mica and varying amounts of topaz and tourmaline. It is commonly developed along fractures, and in favorable places large masses of rock may be greisenized.

Tin deposits are most often found as lodes, both fissure and pegmatitic, or stockworks, but some segregations are known. A peculiar pipe-like form of deposit is found at places in the Transvaal and Tasmania.

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.