The Elements of economic geology

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Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Document type:: Monograph

Structure type:: Chapter

Title:: Part II. Ore deposits

Collection:: Economics Books

76 ECONOMIC GEOLOGY Many of the detrital tin deposits are worked by hydraulic sluicing (cf. p. 53) at costs as low as 3d. per cubic yard (W. R. Jones, Tin Fields, 1925, p. 179). The tin mines of Nigeria work alluvial deposits derived from intrusive granites, The fields are on and near the plateau of Bauchi, S. of the famous city of Kano. The tin has long been worked; it may have supplied some of the metal for the ancient bronzes of Benin. Tin was smelted by workers from Kano who cast it into impressions of straw, and sold it in 9-inch lengths weighing 66 to the Ib. (G.S. Nig., Bull. 4, 1922, pp. 43-5). The age of the Nigerian tin granite is regarded as pre-Palzozoic. The field is also exceptional by the absence of tourmaline (ibid., p. 41). The ores are nevertheless of pneumatolytic origin, for the adjacent granite has been intensely silicified and impregnated with topaz and often fluor-spar, The Nigerian tin was due to fluoric and not to boric acid. Borivia—Bolivia is now one of the chief tin-producing countries, yielding in recent years about 20 per cent. of the total output. The ores have been regarded as different in origin from those of other fields, owing to the reported ab- sence of tourmaline and topaz. The tin-fields trend along the Andes S. of the capital, La Paz. The country is composed largely of Silurian and Devonian slates, which have been invaded by granite and quartz-porphyry, altered by infiltra- tion of silica and crushed into quartz-schists. Vast volcanic eruptions in the Upper Cretaceous and Kainozoic discharged andesites and rhyolites. The tin deposits are of two types. The first are fissure- lodes and brecciated zones, in which quartz and cassiterite have replaced the country rock. The second type includes the famous silver mines of Potosi; the ores are quartz with complex sulphides of copper, lead, tin, and antimony, with a little cassiterite and tourmaline. The reported absence of tourmaline led A. W. Stelzner (Z. 4. g. G., xlix, 1897, pp. 116, 120) to describe the Bolivian tin ores as unique, and as deposited by ordinary mineral springs. According to W. R. Rumbold (Econ. Geol., iv, 1909, pp. 321-64) the ores with the granite and quartz-porphyry are normal and always contain tourmaline; whereas it is rare with the sulphide ores, which he regards as much younger and formed in the volcanic period. Miller and Singewald (Min. Dep. S.

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