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Abstract

The Currawong massive sulphide deposit is one of two known deposits at Benambra,
eastern Victoria. The deposit occurs near the base of the Gibsons Folly Formation
which is a deep water, basin centre facies association of the Late Silurian Cowombat
Rift (Allen, 1992).

Coherent volcanic units of the Gibsons Folly Formation comprise andesite and
plagioclase-phyric rhyodacite. These are predominantly shallow sills with
margins of sediment-matrix hyaloclastite. The rhyodacite also forms cryptodome-like
intrusions which have penecontemporaneously deformed the sequence. These and the
sills were emplaced into a relatively unlithified sequence of mudstone (interbedded
with thin, fine sandstone turbidites) and andesitic volcaniclastic units prior to the
mineralising event. Units of strongly flow-banded and/or brecciated rhyodacite in the
footwall sequence may be lavas but textures are equivocal.
Quartz-plagioclase-phyric rhyolite (the Currawong Porphyry) is a sill which
intruded relatively lithified rocks at the base of the Gibsons Folly Formation.
Geochemical evidence indicates that it represents on-going silicic volcanism of the
Middle-Upper Silurian Thorkidaan Volcanics.
A sequence of andesitic scoriaceous breccia and plagioclase-quartz-bearing
altered rocks comprises several depositional units separated by thin mudstone units.
These are ambiguous rocks but several features suggest that they are lava-derived
mass-flow deposits. They are lithologically and geochemically distinctive and host the
mineralisation at Currawong.

Ti, Zr, Nb and Y have behaved essentially in an immobile manner during hydrothermal
alteration and subsequent metamorphism of the volcanic rocks at Currawong. Volcanic
lithologies are best distinguished using the plots Zr/TiO2 vs Nb/Y (after Winchester
and Floyd, 1977) and Nb vs Zr. The coherent volcanic units of this sequence form a
fairly continuous geochemical magmatic evolution trend but coherent andesite and
andesitic volcaniclastic rocks show a broad range of compositions. Some of the
andesitic units contain xenocrystic-quartz and volcaniclastic rocks of andesitic
composition also contain silicic volcanic clasts. Together these suggest that the
andesitic rocks are the result of magmatic differentiation combined with mixing of
andesitic and quartz-phyric silicic magmas.
The Currawong deposit is interpreted as a subsea-floor replacement style volcanic
hosted massive sulphide deposit. Massive pyritic mineralisation is intercalated with,
and laterally equivalent to, strongly altered volcanic units which carry variable
disseminated or vein mineralisation. Alteration and mineralisation show a strong
stratigraphic control related to primary permeability of the host sequence. Quartzxenocrystic,
andesitic scoriaceous volcaniclastic rocks were the locus of the strongest
mineralisation at Currawong, and possibly at the nearby Wilga deposit. These should
be a primary target for future exploration at Benambra.

Item Type:	Thesis - Coursework Master
Authors/Creators:	Ebsworth, Gregory B
Keywords:	Lithofacies, Geochemistry, Sulfide minerals, Geology, Stratigraphic
Copyright Holders:	The Author
Copyright Information:	Copyright 1994 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:	Thesis (M.Econ.Geol.)--University of Tasmania, 1995. Includes bibliographical references (p. 62-67)
Item Statistics:	View statistics for this item

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