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The effect of Devonian metamorphism and metasomatism on the mineralogy and geochemistry of the Cambrian VMS deposits in the Rosebery-Hercules district, Western Tasmania


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Zaw, Khin 1991 , 'The effect of Devonian metamorphism and metasomatism on the mineralogy and geochemistry of the Cambrian VMS deposits in the Rosebery-Hercules district, Western Tasmania', PhD thesis, University of Tasmania.

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The Rosebery, Hercules and South Hercules deposits in western Tasmania are composed of
polymetallic massive to disseminated sulphide mineralisation hosted in felsic volcanics of the
Cambrian Mt Read Volcanic belt. The deposits have been affected by regional metamorphism
of upper greenschist facies and associated tectonic deformation related to the Devonian
Tabberabberan Orogeny. The Devonian tectonic movement has resulted in folding, shearing and faulting (thrusting) of the ore lenses which caused significant changes in the overall
configuration of the original strati.form ores and underlying stringer zones. In addition, the
south-end of the Rosebery deposit has undergone metasomatic replacement related to a postorogenic
Devonian granite intrusion.
The VMS mineralisation at Rosebery consists of three primary sulphide-sulphate zones:
lowermost pyrite-chalcopyrite zone (>4% Cu), overlain by a sphalerite-galena ± pyrite ±
chalcopyrite zone, and followed by an uppermost massive barite zone. The south-end of the
orebody has been overprinted by Devonian transgressive metasomatic mineral assemblages.
Three major replacement zones can be distinguished: (1) magnetite-biotite ± chalcopyrite
zone, (2) pyrrhotite-pyrite zone and, (3) tourmaline-quartz± magnetite zone. Othermetasomatic
minerals such as fluorite, garnet and helvite are present.
The main Hercules deposit occurs 10 km south along strike from Rosebery and lies in
a similar tuffaceous shale unit The deposit consists of a number of disconnected stratabound
Zn-Pb-Cu-Ag-Au ore lenses with ~imilar metal distribution and alteration features to the
Rosebery ores. The Hercules deposit shows a zonation from a massiye to disseminated pyritechalcopyrite
zone (>4% Cu) at the footwall through a massive sphalerite-galena ±pyrite zone
and spotty sphalerite-galena ± pyrite zone to a carbonate ± barite zone at the top. The South
Hercules deposit is located about one kilometer south of the main Hercules ore lenses at the
same stratigraphic level. The deposit displays comparable mineralogical zonation with Rosebery
and Hercules but contains low-grade lead and zinc, and has a low copper content ( <1 wt %
Cu) and relatively high (Au+Ag)/(Pb+Zn) ratio. The deposit consists of a disseminated to
stringer sulphide zone with variably altered carbonate assemblages.
Metal zonation studies indicate that zinc occurs dominantly as blanket-like enrichment
in the Rosebery district deposits and silver broadly follows zinc. Gold is largely concentrated
at the top of the ore lenses. In the transgressive pyrrhotite-pyrite replacement zone of the
Rosebery south-end, zinc is conspicuously depleted but gold values of more than 20 g/t Au
are noted. In contrast, the other biotite-, magnetite-, and tourmaline-bearing replacement zones
contain low gold grades (generally less than 5.0 g/t Au). Like zinc, silver is also depleted in
the replacement zones but a significant amount of copper is present Copper values as high as
2% are observed in the biotite-magnetite zone as well as the pyrrhotite-pyrite zone. This
indicates that although zinc and silver were dissolved and removed during the Devonian
replacement event, gold and copper appear to have been redistributed and recrystallised.
A detailed mineralogical study of gold in the F(J) lens indicates that the gold occurs as
electrum inclusions in pyrite, as individual electrum grains in pyrrhotite, and in chalcopyrite
veins cutting pyrrhotite. Similar mineragraphic investigations at Hercules and South Hercules
show that gold occurs as individual electrum grains locked in the pyrite, and as patches or blebs
in the remobilised and recrystallised sphalerite, galena and tetrahedrite. The Devonian
metamorphic and metasomatic processes resulted in significant recrystallisation and
remobilisation of these gold grains as indicated by their texture, grain size, and fineness
The Devonian overprinting processes also resulted in the redistribution of FeS in
sphalerites from the Rosebery district deposits. The sphalerite-hexagonal pyrrhotite-pyrite
assemblages from the Rosebery south-end were used to apply the sphalerite geobarometer to
estimate the pressure conditions during the Devonian replacement process. The estimated
pressure is 3.0 ± 0.5 kb for the requilibration of sphalerite in the F(J) lens which would
correspond to a depth of 8.0 ± 1.0 km. The correlation of the mole % FeS in sphalerite and
the gold grades at the Rosebery, Hercules and South Hercules deposits displays complex
patterns owing either to later Devonian metamorphic and metasomatic recrystallistion or to
variation of the initial depositional conditions.
The VMS carbonates from both north- and south-ends of the Rosebery deposit display
tightly constrained and similar isotopic patterns. The isotopic patterns imply that no later
isotopic changes have taken place although the south-end of the Rosebery deposit was strongly
overprinted by the high temperature (> 300°C), pervasive Devonian replacement process. The
0180 values of biotite and magnetite from the F(J) lens replacement zone were used to calculate
the oxygen isotopic composition of the Devonian hydrothermal fluid. The calculated o1
80CH.z0) values for the overprinting Devonian hydrothermal fluid vary from 8.0%0 to 12.0%0 and are
consistent with a magmatic fluid.
Detailed sulphur isotopic investigations on the hand specimen scale indicate that there
are no obvious o34S variations between the lead-zinc lenses of Cambrian volcanogenic origin
and the pyrrhotite-pyrite assemblages of Devonian origin. The sulphur isotope values of the
sulphide minerals from the South Hercules deposit range from o34S values of 8.2o/oo to 14.1 o/oo
and the calculated 034J\S values in the ore fluids display a permissible correlation with gold
grades. This correlation suggests the possibility of using the sulphur isotopic composition as
a guide to assist in targeting the gold-rich sections of massive sulphide systems.
Fluid inclusion studies indicate that early formed low-temperature (ea 200'C), low-salinity
( <5 NaCl equiv. wt % ) Type I inclusions with no appreciable C02 may be interpreted
as primary, Cambrian exhalative fluids that have survived the Devonian recrystallisation.
Fluid inclusion characteristics of necking down and healed microfractures together with
trapped carbonate and barite daughter minerals identified by laser Raman spectroscopy
suggest an extensive remobilisation and recrystallisation of carbonate minerals during the
Devonian overprinting processes, which corroborates with the redistribution and recrystallisation
of primary Cambrian sulphides (e.g. sphalerite) and gold, during Devonian metamorphic and
metasomatic processes.
Although the VMS deposits in the Rosebery-Hercules area display no evidence for
large-scale chemical remobilisation of ore constituents during metamorphism, the postorogenic
Devonian granite intrusion below the south-end of the Rosebery mine resulted in
chemical remobilisation of the ore lenses. The early replacement assemblages in the F(J) lens
formed from interaction of moderate to high temperature(~ 330°C), saline (~0 NaCl equiv.
wt % ) fluid with the original lead-zinc mineralisation. The later stage tourmaline veining and
associated replacement assemblages resulted from lower temperature (~ 300°C), less saline
(~0 NaCl equiv. wt%) fluid. During the Devonian metasomatic event gold may have been
remobilised and recrystallised either as Au(HS)2 - or AuC12 - complexes but the high
temperature and salinity conditions favour the AuC1i-complex. Copper also appears to have
been recrystallised during the replacement process with minor remobilisation. In comparison
to copper and gold, zinc, lead and silver have been dissolved and removed as the very soluble
chloride complexes.

Item Type: Thesis - PhD
Authors/Creators:Zaw, Khin
Keywords: Metamorphism (Geology), Metasomatism (Mineralogy), Geology, Stratigraphic, Geology
Copyright Holders: The Author
Copyright Information:

Copyright 1991 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).

Additional Information:

Includes bibliographical references. Thesis (Ph.D.)--University of Tasmania, 1992

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