File:Transactions (1895) (14782828562).jpg

Identifier: transactions14cana (find matches)
Title: Transactions
Year: 1895 (1890s)
Authors: Canadian Institute of Mining and Metallurgy
Subjects: Mineral industries Mines and mineral resources
Publisher: Montreal (etc.)
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: University of Toronto

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country rock and are rarely oflarge dimensions unless some auxiliary fissure or opening in therock has determined their form and extent. Fig. 11 shows bothclasses of detail at the edges of a single ore body. Variations in Chemical Character and StructuralArraxge.me.nt of Enclosing Rocks. Fully as important in determining the shape of replacementmasses is the chemical character and structural arrangement ofthe enclosing rocks. Fissures do not, of course, occur exclusively in homogeneousrocks, or in rocks compo.<;e(l of grains of one mineral only. Theyfrequently pass through rocks of widely varying lilhologiccharacteror rocks that, although as a whole homogeneous, are made up ofaggregates of different minerals which have widely difTert»ntsusceptibilities to replacement processes. Forms ynxiuced in Rocks composed of Different MmeraU.—When rocks are mechanical mixtures of difTen-nt minerals some• Se< page 4 Ml for definition of Urm free face. 414 Replacement Ore-Bodies—Irving
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1 o i-H Kll -4-> /; • ll • 1-^ o o Uh d c5 3 % oj: Replacement Ore-Bodies—Irving 415 minerals will be more resistant to chemical alteration orreplacement than others. Thus, in a calcareous sandstone com-posed of grains of sand cemented together by calcite, the calcitewill be readily replaced; but the more resistant quartz left withoutalteration. In a granite which is composed of feldspars, muscoviteand quartz, the feldspars may suffer replacement and the quartzand mica remain unaffected. It therefore happens that rocks ofthis kind when affected by any given solution will often be butpartially replaced and that the ore resulting from the processwill be disseminated through the mass in much the same attitudeas the original susceptible mineral. A calcareous quartzite of this kind has been transformed intoore in some of the silicious ore districts in the Black Hills,the cementing calcite only having yielded to mineralizingprocesses. This disseminated type of replacement deposit w

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