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Textural evolution of the Hellyer massive sulphide deposit.

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McArthur, GJ (1996) Textural evolution of the Hellyer massive sulphide deposit. PhD thesis, University of Tasmania.

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Abstract

The Hellyer zinc-lead-silver deposit of western Tasmania is a well preserved example of a
volcanic-hosted massive sulphide. The deposit is hosted by intermediate-basic lavas and
volcaniclastics of the Que-Hellyer Volcanics, the uppermost volcanic unit of the Cambrian
Mt.Read Volcanics. The complete deposit, including the footwall alteration stringer zone, is
preserved. The current complex morphology of the massive sulphide is due to the combination
of primacy depositional irregularities, ductile Devonian folding and brittle Mesozoic faulting.
Statistical analysis of available mine sample assays and subsequent geostatistical 3D grade
modelling has revealed a classic metal zonation pattern. Whilst Cu and Fe are enriched towards
the footwall, proximal to the central feeder zone recognised by Gemmell and Large (1992); Zn,
Pb, Ag, Au, As and Ba are gradually enriched towards the distal hangingwall. The current
observed metal distribution is interpreted to be substantially the same as the Cambrian
distribution; Devonian deformation resulted in only very local remobilisation.
Spatial analysis of macroscopic textures has shown a clear zonation, similar to the geometry of
the metal distribution. Massive sulphide proximal to the central feeder zone is strongly
recrystallised, but grades upwards and outwards to a featureless, massive texture and fmally to
strongly banded ores at the hangingwall contact.
A very detailed microtextural study of 174 polished thin sections was completed from samples
selected on a 3D grid through the central part of the deposit. Two hundred and twenty-two
different microscopic textures have been recognised, with their spatial occurrence and features
documented in a comprehensive atlas. These textures have been placed into paragenetic groups
ranging through early primitive deposition, in situ recrystallisation, intra-mound veining,
upwards redeposition, thermal retraction, Devonian and Mesozoic deformation-related, and
fmally, surface weathering. These paragenetic groups are zoned, similar to the metal zoning
and macroscopic textures, around the central feeder in the footwall. Various depositional and
recrystallisation processes are postulated in an overall model for textural evolution.
Microprobe analyses of the major minerals from numerous samples have shown variation according to texture and position within the overall orebody zonation. Significantly, pyrite
shows considerable reduction in trace element content as crystallinity increases towards the
proximal base of the sulphide mound. Early sphalerite has a higher Fe content than the late
varieties, early tetrahedrite has a higher Ag content than later generations and carbonates show
increasing CaO content and decreasing FeO content passing from early to late textural types.
Other minerals show more complex compositional variability.
The classic metal and texture zonation patterns, together with evidence from detailed
microprobe analysis lend support to a mound refining genetic model, similar to that proposed
by Eldridge et al. (1983) for the Kuroko volcanic·hosted massive sulphide deposits. The
Hellyer genetic model postulates that a hydrothermal system was focussed at the intersection of
a normal graben fault with a transfer fault on the Cambrian seafloor. These faults tapped a deep
heat source and as temperature increased, rising hot solutions saturated with base and precious
metals and reduced sulphur, began to vent into the cold, oxygenated seawater. Initially,
barite/anhydrite and cherty crusts were deposited on the seafloor overlying the core of the
footwall alteration zone. These crusts, by partly capping the system, allowed higher
temperature deposition of primitive melnikovite pyrite and sphalerite/wurtzite, by replacement
of pre-existing sulphates, and within voids, just below the mound surface. As the mound grew,
these depositional processes moved upwards and outwards, away from the central feeder.
Much higher temperatures in the lower part of the mound, gradually recrystallised and refined
the primitive pyrite, expelling contaminant trace elements to be redeposited in higher, cooler
parts of the mound. The growing mound became unstable, depositing clastic massive sulphide
in adjacent basins that were eventually enveloped by the expanding higher temperature
hydrothermal system, recrystallising and partially destroying the original fragmental
framework. When the mound reached its ultimate extent, rotation of the stress regime closed
off access to the heat and fluid sources and temperatures in the system decreased. As
volcaniclastic mass flows and pillow lavas buried and preserved the deposit, the waning phase
led to further deposition oflower temperature mineralisation, increasingly deeper within the
mound in available voids. Devonian deformation annealed and extended the ductile sphalerite·
galena rich distal hangingwall zones and introduced tensile pull apart fractures in the more
proximal pyritic zones. All minerals, except pyrite, were locally remobilised into newly created
voids. Mesozoic brittle wrench faulting brecciated pyritic areas causing minor remobilisation of
minerals into late narrow cracks that cut across all earlier textural features.

Item Type: Thesis (PhD)
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Date Deposited: 11 Aug 2011 02:06
Last Modified: 15 Sep 2017 01:06
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