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Abstract

The cassiterite-sulphide deposits in Western Tasmania are stratiform
replacement deposits (Mt. Bischoff; Renison Bell; Cleveland; Mt. Lindsay),
fissure deposits with marginal replacement (Federal Lode, Renison Bell;
Razorback-Grand Prize), and fissure lodes (Mt. Bischoff), that consist
essentially of pyrrhotite with subordinate pyrite, arsenopyrite, chalcopyrite,
sphalerite and cassiterite. They are localized in dolomitic horizons of
Upper Proterozoic-Lower Cambrian age in faulted, complex anticlinal structures.
These have been the loci for intrusion of cupolas and associated dyke
swarms of late Devonian granitic rocks, which have been extensively
topazized, tourmalinized and/or greisenized. The major and trace element
chemistry of these granitic rocks combined with isotopic dating indicates
that they probably represent the most highly fractionated portions of
penecontemporaneous, high-level, post-kinematic granitic batholiths
occurring throughout Western Tasmania. These granitic batholiths may
be interconnected at depth. The spatial association of the polyascendant
tin mineralization with these granitic rocks, and their associated prejoint
tin mineralization, suggests that they are derived from a common
source.
Around the cassiterite-sulphide deposits are haloes of dispersed
Pb-Zn-Ag mineralization, which on the basis of field relationships is
considered to be related to the same granitic activity. A uniform
distribution of Co and Ni in Fe-sulphides, Cd in sphalerites, Se in
sulphides, and Mn in both the cassiterite-sulphide and Pb-Zn-Ag deposits
provides empirical support for a single metallogenic province covering
Western Tasmania in the late Devonian.
In the cassiterite~sulphide deposits, cassiterite and wolframite have generally crystallized before the earliest formed sulphides, which are pyrite
and arsenopyrite, followed by pyrrhotite. Chalcopyrite, sphalerite and
stannite were formed penecontemporaneously and were followed by tetrahedrite,
jamesonite, bismuth and finally galena with associated canfieldite, frackeiite
boulangerite and pyrargyrite. At Mt. Bischoff pre-sulphide metasomatism
of dolomite at temperatures in excess of 4000 c resulted in formation of
tourmaline, quartz, wollastonite, chondrodite and garnet. These minerals
were subsequently replaced by sulphides and talc, fluorphlogopite, serpentine,
chlorite, Fe-Mn~g carbonates, fluorite and sellaite. Similar alteration
has occurred at Renison Bell and Cleveland but the pre-Sulphide phase is
limited to the formation of minor tourmaline, topaz and quartz.
A mineralogical and isotopic lateral zonation can be demonstrated at
Mt. Bischoff. The zonal and paragenetic sequences are essentially parallel.
Fluid inclusion studies demonstrate that the zoned sequence is partly a
function of declining temperature, with initial deposition of fluorite
up to 5800
, decreasing to 2000 C in the marginal zone. It is probable that
the temperature, salinity and composition of the ore-forming fluids
were constantly changing during mineralization as an initially hot,
saline fluid was mixed with cooler, less saline meteoric and connate waters,
in conjunction with heat loss to the wall rocks. In general, compositional
changes in sulphide systems within the zonal sequence are consistent with
cooling of the ore-forming fluid from temperatures as high as 7000C to below
3000 C, with a sympathetic decline in fugacity of sulphur and oxygen.
However some anomalies exist between interpretations based on the Fe-S and
Fe-Zn-S systems, and the Fe-As-S system. The other cassiterite-sulphide deposits lack a well defined zonation with respect to both mineralogy and
isotopic composition of sulphur, but their initial deposition temperatures
are comparable or slightly lower than those at Mt. Bischoff. Ore
deposition was probably initiated by increasing alkalinity of the
ore-forming fluids caused by reaction with dolomitic host rocks.

Item Type:	Thesis - PhD
Authors/Creators:	Groves, DI
Additional Information:	Copyright the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
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