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 II. Ore deposits

Collection:: Economics Books

1S 8 n ay 1 ORES OF GOLD 43 Settlement of the country after the volcanic activity. The ores are Middle or Upper Kainozoic in date, and have not the Secondary enrichments common in older lodes. New Zealand, in addition to gold-quartz veins in slates, has two goldfields in volcanic rocks with fracturing of a different pattern from that usual in the plateau eruptions of the Rocky Mountains ; it is however similar to that at Gold Field, Nevada, which as shown by F. L. Ransome (U.5.G.S., Prof, Pap., 66, 1919, p. 196) was torn across by intersecting fissures without fault movements. In the Hauraki Gold- field in the North Island Kainozoic andesites and dacites were traversed by a network of ruptures along which were deposited Quartz, calcite, pyrite, chalcopyrite, blende, galena, and gold. When the fissures were full the solutions were forced into the country and formed replacement lodes 50 feet, and at the 800 feet level, even 100 feet wide. Owing to the saturation of the country by the solutions the ore-shoots were remarkably Persistent to the depths of 1000 feet, and they have been worked to the depth of 1900 feet (cf. J. M. Bell, Tr. Austr. LME, 1911, pp. 548-79; and P. C. Morgan, Bull. Geol. Surv. N.Z, 1924, No. 26). The Thames Goldfield, on a peninsula E. of the Hauraki Gulf, illustrates the formation of a gold- field where the fracturing and ore deposits are shallower. The andesites there have been intensely altered hydro- thermally to the depth of 500 feet, and traversed by veins of quartz with rich pockets of ore where the main veins are joined by small quartz stringers. PNEUMATOLY TIC OrEs—CRrIPPLE CREEK, AND PASSAGEM, BraziL—Cripple Creek, in the red pre-Cambrian granite of Pike's Peak, Colorado, represents the pneumatolytic goldfields. Itisin the pipe of a Middle Kainozoic volcano and Pneumatolytic agents are shown by the characteristic mineral being telluride of gold, by the abundance of fluorite, and the alteration of felspars into kaolinite (cf. p. 169). The volcanic breccia in the pipe has been so silicified that its original nature IS recognizable only under the microscope. The breccia was cut by dykes of basalt, andesite, and phonolite, and has been torn by fractures due to shrinkage of the cooling rocks. The lodes are later than the dykes, which contain chimney-shaped ore-shoots ; the ascent of the solutions was doubtless aided by the reheating due to the intrusion of the dykes.

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