The tin-ore deposits contain the metal in the form of the oxide, SnO, (cassiterite) which is probably derived from the granite in the form of fluoride of volatile compound which, emitted at a high temperature, decomposes at a lower temperature in the presence of water vapour to a dioxide of tin, as shown by the following equation : =--- Sn02-1-4HF The deposits are characterized by the presence of the fluorine bearing minerals—fluorspar (fluoride of calcium) and topaz (a fluoride and silicate of aluminium) and the boron-bearing minerals —tourmaline and axinite; whilst among metallic compounds are found wolfram (a tungstate of iron and manganese), ilmenite (a titanate of iron) and various sulphides and arsenides of copper, iron, lead and silver. Some of these compounds often constitute valuable ore-deposits of the respective metals. For instance, in Cornwall the upper portions of the lodes, to-day being mined ex clusively for tin, carried copper in such quantities as to have made that county the world's chief producer of that metal in the early part of the 19th century. This depth-change in mineral content is a function of the temperature and pressure of the ascending min eral-bearing vapours and solutions and is sometimes referred to as the zonary distribution of ores. Experience has shown that the order of the zones of ore occurrence in the Cornish lodes from below upwards is tin, tungsten, copper, zinc, lead, silver, and this is the order of increasing solubility of the compounds of these metals in the ascending magmatic waters.
The highly-heated soluble constituents of the intrusions as they pass into the invaded formations profoundly modify their mineral composition. Among the sediments, the aluminous (shales and slates) and the calcareous (limestones) are the most subject to modification. The aluminous rocks develop tourmaline, sillimanite,
staurolite, andalusite and topaz—all silicates of alumina, while the calcareous rocks yield garnet, axinite, wollastonite, diopside, scapolite, vesuvianite and epidote—all lime-bearing silicates.
In places these contact phenomena are accompanied by the development of ores, and thus give rise to an important class of ore-bodies generally described as contact-deposits. As a rule they are metasomatic replacements of limestone. Excellent examples are found in the so-called porphyry-copper deposits of the Western States of America (e.g., in Arizona, New Mexico and Utah), where copper ores occur in limestone at its contact with intrusive monzonite and quartz-porphyry. The principal contact minerals are garnet (andradite) and epidote. The primary ores are cu priferous iron pyrites and zinc-blende to which must be added the high grade copper sulphides—chalcocite and bornite, in the zone of secondary enrichment. Near the surface, subsequent leaching by meteoric waters generally gives rise to a zone which may be barren, but in places contains oxides and carbonates of copper.
Equally important are the iron-ore deposits, which occur in limestone at its contact with igneous intrusions of an acid or inter mediate type. Magnetite is the principal ore mineral, although haematite also occurs but in smaller quantity. Characteristic asso ciates are the iron-garnet (andradite), the iron-pyroxene (heden bergite), olivine and ilvaite. Deposits of this character are found in the Banat province of Hungary, on the island of Elba and in the Oslo district of Norway.