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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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part III. Earthy minerals

Collection:: Economics Books

162 ECONOMIC GEOLOGY 10 million carats, and from S.W. Africa, 7 million carats. Owing to its hardness and resistance to weathering the dia- mond is widely distributed in alluvial deposits, which were for long the only source. Brazil vielded most of the supply from 1721 till 1870. Soutn Arrica—The first South African diamonds! were discovered in alluvial deposits in 1867. They were found at Kimberley in 1870, in an oval patch of a yellow ground covering the unoxidized * blue-ground (kimberlite) which proved to be an ultra-basic intrusion. After the workings had been amalgamated into one company in 1888 deep mining became possible, and has been carried to 3500 feet. The country consists of the Upper Carboniferous Ecca shale, which rests on pre-Palzozoic quartzites and basic lava; they lie on crystalline schists, which have been intruded by quartz-porphyry and pegmatite. The diamonds. are found in the blue-ground with olivine, pyroxenes, biotite, and garnet, and the largest number of mineral species found in any one rock. The kimberlite includes pseudo-spherulites similar to those formed around geysers; it was probably saturated with superheated steam, and was viscous rather than molten. The diamonds are scattered through the rock, and as some of them are broken, they were formed before its final consolidation. At Newlands, Kimberley, some dia- monds were found in eclogite boulders; but 20 tons of these boulders from the Kimberley Mine did not yield a single diamond. Diamonds have been found in other igneous plugs in South Africa, but they are absent, or practi- cally absent, from nine-tenths of the kimberlite occurrences. The Premier Mine yielded the largest known diamond, the Cullinan, which weighed 22 0z., or 3025 carats, and has been cut into 105 gems. BraziL—Elsewhere diamonds have been derived mostly from pneumatolytic contact rocks and pegmatites, as in Brazil, India, Southern Rhodesia, and West Africa. The Brazilian diamond fields are of two different types. Dia- mantina in Central Minas Geraes, is a belt 250 miles long by 20 miles wide, and consists of pre-Pal®ozoic quartzites, schists, and pegmatites, and the overlying Diamantina Con- glomerate. The diamonds occur in the conglomerate, but LP. A, Wagner, Diamond Fields, Southern Africa, 1014.

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