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Geology of the West Coast Range of Tasmania, Part III-Porphyroid Metasomatism

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Bradley, J (1957) Geology of the West Coast Range of Tasmania, Part III-Porphyroid Metasomatism. Papers and Proceedings of the Royal Society of Tasmania, 91. pp. 163-194. ISSN 0080-4703

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Abstract

Quartz and felspar porphyries and associated
greenschists of the West Coast Range are described.Felspar
porphyries are considered to have
originated by the felspathising of spilitic lavas,
lava breccias and related greywackes during a
period of progressive metamorphism. Metamorphic
mineral assemblages of the greenschist and albiteepidote-
amphibolite facies are recognized in the
porphyries and are identified with increasing
degrees of porphyritising of host rocks. Sharp contacts
between chlorite-sericite schists and quartzites
are regarded as metamorphic in origin and
as indicating the replacing action of a chloritising
and sericitising front. Haematite rich rocks and
low grade pyritic ores which respectively occupy
narrow zones above and below the schist quartzite
contact are regarded as representing minor haematlsing
and pyritising fronts.
Sulphide and silicate metasomatism are closely
related to the structure of the range and it appears
that the advance of fronts has been engendered
and controlled by the upward fiow of heated waters
through fault fissures and pore spaces. Metasomatic
replacement was effected by diffusion and
exchange through intergranular films. This
mechanism is thermodynamically efficient and
allows adequate explanations of growth, and also
of corrosion structures, of quartz and felspar
crystals in the porphyries. During the advance of
fronts there was extensive transport of material;
metamorphism was not merely an adjustment to
changed isogradial conditions. In particular,
migrating solutions leached iron, copper and suiphur
from the basic rocks and precipitated them in
specific zones where they now form low grade ores.
After a period of rising temperatures and progressive
metamorphism there followed an interval
of cooling and retreat of thermal zones. Because
of the presence of abundant water there was during
this cooling phase an intense retrograde metamorphism
of the recently formed felspathic rocks.
Hydration of these rocks resulted in the liberation
of silica and thus gave rise to quartz porphyries
and quartz sericite schists. Extreme pneumatolysis
and leaching of silica from the porphyries gave rise
to kaoJinised rocks and to veins of quartz and chert.
A similar kind of leaching of sulphides from the low
grade ores, and of selective precipitation of sulphides
in pipes and veins, gave rise to rich ore
bodies.

Item Type: Article
Keywords: Royal Society of Tasmania, RST, Van Diemens Land, natural history, science, ecology, taxonomy, botany, zoology, geology, geography, papers & proceedings, Australia, UTAS Library
Journal or Publication Title: Papers and Proceedings of the Royal Society of Tasmania
Page Range: pp. 163-194
ISSN: 0080-4703
Collections: Royal Society Collection > Papers & Proceedings of the Royal Society of Tasmania
Additional Information:

Copyright Royal Society of Tasmania

Date Deposited: 24 May 2012 08:15
Last Modified: 18 Nov 2014 04:35
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