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

ECONOMIC GEOLOGY The exploration of western Brazil was stimulated by re ports of gold. The first successes were in 1699 and in 1718, when the discovery of the rich placers of Cuyaba led to the opening of the remote interior. The Passagem Mine at Ouro Preto, which was opened in 1817, occurs in gently dipping pre-Palzozoic qQuartzites. The main lode is a sheet of white quartz, which ranges up to 36 feet in thickness and is streaked with layers of tourmaline, pyrrhotite, and arseno- pyrite. Below it is a dark tourmaline rock up to 3 feet in thickness, containing pyrite, pyrrhotite, arsenopyrite, and quartz. According to Derby the Passagem Lode is a peg- matite dyke intruded between the overlying Itabirite or banded ironstone, and the underlying quartzite, and subse- quently impregnated with gold and sulphide. The lode is, however, probably a felspathic quartzite, in which pneuma- tolytic solutions “altered the felspar into tourmaline and deposited the metals (cf. O. A. Derby, Amer. Yourn. Sci. (4), Xxxii, 1911, pp. 185-00; E. Hussak, Z. prakt. G., vi, 1898, PP. 345-57). IsoraTED GoLp-QuarTZ VEINS—SADDLE- AND LADDER- Lopes — The earth-movements in some goldfields instead of forming continuous fissures, produced isolated spaces, which have been filled by short lodes. The Bendigo Gold- field was discovered in 1851, and after its rich gravels were worked out search was made for the lodes whence the gold had come. Large quartz-“ blows” were found, and some were so rich that they were broken up and crushed by hand hammers. These bodies of quartz proved to be wedge- shaped and they rapidly tapered out below. The view was therefore held that as the field had no persistent quartz-lodes like those of California the mines would be shallow. The mines are in Ordovician slates which have been corrugated by many parallel folds. As neighbouring quartz-blows sloped in opposite directions, it was suggested that each pair was the remains of a saddle or arch of quartz formed over a fold. This view was established by the Geological Survey of Victoria by E. J. Dunn, H. S. Whitelaw, and H. Herman, and supported by Rickard. The latest report is by H. Herman (Bull. G.S. Vict, No. 47, 1923), who has shown that the field is a geosynclinal. Saddle-shaped sheets of quartz were found one below the

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