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 III. Earthy minerals

Collection:: Economics Books

THE MICAS, ASBESTOS, AND GEMS 163 have mostly been obtained where they have been recon- centrated in river gravel; they are associated with quartz, zircon, tourmaline, disthene, and mica; the only igneous rocks known are the pegmatites in the underlying schists and occasional pebbles in the conglomerate, and a kimberlite tuff at Uberaba, which contains no diamonds (Hussak, Z. prakt. G., xiv, 1906, pp. 322-4). The younger Brazilian deposits, the Levras sandstone and conglomerate, of Bahia, may be Carboniferous in age. The associated minerals include tourmaline, zircon, garnet, staurolite, and kyanite. - No igneous rocks occur with the diamonds, which have been attributed to undiscovered dykes by D. Draper (Mining Mag., ix, 1913, p. 435), E. C. Harder and R. T. Chamberlin (Yourn. Geol., xxiii, 1915, P. 418), and Miller and Singewald (Min. Dep. S. Amer., 1919, pp. 213-4). On the other hand, J. C. Branner (Amer. Fourn. Sci., (4), xxxi, 1911, p. 490) regarded the diamonds as formed during the metamorphism of the quartzites, and according to O. A. Derby (Journ. Geol., xx, 1912, p. 455) they were due to pneumatolytic alteration of a fractured ultra- basic rock. The mineral association suggests a pneumatolvtic origin by some ascending boric acid solution. The diamonds discovered in 1908 along the coast of South- western Africa were at first regarded as washed out of sub- marine necks of kimberlite. According to E. Kaiser (Diaman- tenwiiste Sud-West- Africas, 1926, ii, pp. 329, 338) they came from various sources, and were collected into a sheet of sand- stone, whence they were carried into Eocene and later sands, The association of these diamonds with native gold and copper, chalcopyrite and pyrite, iron and manganese oxides, garnets, zircon, sillimanite, tourmaline and topaz, does not suggest an ultra-basic igneous origin ; some were probably formed in contact zones with the sillimanite, and others under pneu- matolytic conditions with tourmaline and topaz. The dia- monds are small ; the largest found weighs only 50 carats. The diamonds of the field on the Gold Coast discovered by Sir A. E. Kitson (Gold Coast G.S., Bull. i, 1925, p. 35) are attributed to the action of igneous rocks on carbonaceous slates. TuEORIES OF FORMATION BASED ON SUPPOSED ARTIFICIAL Diamonps—The current theories as to the formation of

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