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

ORES OF TIN AND TUNGSTEN 77 Amer., 1919, pp. 94-100), on the other hand, consider that the ores belong to one period which was later than the vol- canic eruptions, and so of modern date and moderate depth. W. M. Davy (Econ. Geol., xv, 1920, pp. 463-06) accepts the late Kainozoic origin of both types, but claims that the granite and quartz-porphyry ores were formed under deep- uted conditions, and the tin-silver sulphide ores at moderate epths. The sulphide deposits are famous for their richness in silver and yield but little tin; their stannite has been even less productive than that of New South Wales. The economi- cally valuable Bolivian tin ores were formed under pneu- matolytic conditions like those of other important tin-fields. . TIN-FIELDS IN GENERAL OF PNEUMATOLYTIC ORIGIN— Sulphide tin ores occur at Campiglia in Tuscany at the con- fact of augite-porphyry with Jurassic limestone. Tin in small quantities occurs in pegmatites in South Carolina and the Black Hills of South Dakota, and in stringers in granite ear its contact with limestone in Alaska (Fay, Tr. Amer. LM.E. xxxviii, 1908, pp. 664-82); and also in quartz lodes containing lumps 6f coarse cassiterite in the Mt. Cudgewa tin-field in the Mitta-mitta Valley, Victoria (Gregory, Bull. G.S. Vict., 1007, No. 22, p. 107), which, though yielding beautiful museum specimens, were too scattered in barren uartz to be mined profitably. } The essential feature of the chief tin lodes is their formation under deep-seated conditions within or beside intrusive massifs of Upper Palzozoic granites, by the action of boric and fluoric acids with superheated steam. The granite attacked by these pneumatolytic agents has been recon- stituted as bands of pegmatite; the felspars have been de- Stroyed, leaving a rock composed of quartz and mica, which 1 known as greissen and has been described as the parent rock of tin. The rise of the tin-bearing solutions through the granite Mass explains why tin deposits beside small granite intrusions are often richer, as in Burma, than beside large intrusions. A small outcrop (Fig. 27) may be the tip of a projection from a large granite mass, while a broad outcrop shows that the upper part of the granite has been removed by denuda- ton. As the most concentrated deposition of tin would have been at the upper edge of the granite, most of the ore

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