The Elements of economic geology

Bibliographic data

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 II. Ore deposits

Collection:: Economics Books

36 ECONOMIC GEOLOGY it (e.g. K. E. Andrée, Geol. Meeresbodens, ii, 1920, p. 582). Such gold as there may be in sea-water is probably secondary and does not explain the source of gold in lodes. GoLp Lopes anp IgnEous Rocks—Most of the chief goldfields of the world have been found by their alluvial deposits, in which the gold is conspicuous owing to its bright colour and the beating of small grains into large flakes. Primary gold is generally found in quartz-veins in the older rocks, though younger lodes are found in areas of great earth-movements and volcanic activity. The gold-quartz veins were at first regarded as igneous, owing to their fre- quent association with igneous rocks and their resemblance to dykes by their branching and cutting across bedding planes. Some quartz-veins contain felspar and have been regarded as pegmatites in which nearly all the felspar has been deposited in some lower position, leaving only molten quartz to solidify at the upper end of the intrusion. Secondary felspars, however, occur in limestones, and the absence of the typical igneous minerals from the quartz-veins and of contact metamorphism along the edges and the fact that the quartz in the lodes is the low temperature variety (i.e. B-quartz), show that the veins were formed under aqueous conditions at a moderate temperature, and were introduced in solution and not as a molten intrusion. The vein-quartz often passes indefinitely into the country rock. The white quartz passes gradually into dark quartz, and through silicified into normal slate. In quartz-veins in granodiorite all stages can be seen between silicified frag- ments of that rock and pure quartz. Quartz-veins often include fragments of the country rock that have resisted silicification ; and as such blocks are often in their original position the quartz around them grew by replacement, Most primary gold-quartz lodes are due to deep-seated hydrothermal “action. Pneumatolytic conditions are in places shown by axinite, tourmaline, and kaolinite, The deep-seated origin of some lode gold is indicated by its association with tellurides, as at Kalgoorlie and Cripple Creek. In other cases the action was propylitic, for super- heated steam and carbonic acid altered the felspars into a mosaic of secondary quartz and felspar, and the ferromag- nesian minerals into chlorite, epidote, and zoisite.

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