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Genesis of the Rosebery massive sulphide deposit, Western Tasmania, Australia

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Martin, NK (2004) Genesis of the Rosebery massive sulphide deposit, Western Tasmania, Australia. PhD thesis, University of Tasmania.

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Abstract

The Rosebery massive sulphide deposit, with historical production and current resources of
~32.7Mt @ at 14.5% Zn, 4.4% Pb, 0.58% Cu, 145ppm Ag and 2.2ppm Au, is located in
western Tasmania. It is hosted within a Middle Cambrian post-collisional succession of
predominantly submarine, calc-alkaline volcanic and non-volcanic sedimentary rocks. The ore
occurs as single or stacked, sulphide and barite-rich lenses with podiform to sheet-like
morphologies, distributed over 3km of strike and >1.5km depth below the current surface.
The basal stratigraphic unit of the east-dipping mine sequence (the footwall volcanics)
comprises a syn-eruptive succession of thick rhyolitic pumiceous mass flow units that were
rapidly emplaced on the seafloor, and succeeded by rhyolitic to dacitic turbiditic mass flow and
suspension settled volcaniclastic sediments of the transitional stratified volcaniclastics (TSV).
The quartz and lithic content of the TSV increased with time as sediment provenance became
more distal. The base of the Hangingwall Volcaniclastics was marked by a reduced
volcaniclastic influx and deposition of a carbonaceous mudstone, followed by further
volcaniclastic mass flow units with a distal clastic provenance. Intrusion of rhyolitic to dacitic
peperite sills occurred prior and subsequent to the onset of mineralisation. The current study
has shown that syn-depositional displacement of the sediments occurred as the result of
basement faulting and sill emplacement, with cessation of significant fault movement prior to
deposition of the Hangingwall Volcaniclastics. The mine sequence is truncated at the upper
and lower margins by major Devonian reverse faults. Mineralisation is hosted within sediments
at the top of the footwall volcanics and within the TSV.
Regional low-temperature diagenesis commenced upon deposition of the volcaniclastic
sediments. Hydrothermal circulation through unconsolidated volcaniclastic sediments
commenced prior to sediment compaction and lithification, locally preserving uncompacted
vitriclastic textures. Hydrothermal alteration proximal to ore comprises a halo of quartz ±
sericite ± Mn/Fe-carbonate assemblages, with a thin discontinuous chlorite assemblage
located immediately beneath sulphide ore. The current study has identified a transgressive
zone of intense quartz-rich alteration beneath P lens that now delineates what was a zone of
fluid upflow along a syn-depositional fault. A broad halo of sericite-rich alteration envelops the
ore and proximal alteration assemblages, but does not extend into the Hangingwall
Volcaniclastics, as the hydrothermal system had waned prior to their emplacement.

Isotopic data indicates a hydrothermal fluid derived through the modification of seawater as it
circulated through the Cambrian volcanic succession and underlying Precambrian basement,
from which sulphur and metals were leached. At the mine scale, diffuse hydrothermal upflow
was loosely focussed along syn-sedimentary faults defined during this study, resulting in the
localisation of ore lenses along the margins of a fault-bounded basin. Significant lateral fluid
flow within ~200m of the seafloor was promoted by sediment stratification, thick packages of
fine-grained sediment, peperitic sills and the development of stratiform alteration zones.
Lateral movement of hydrothermal fluids was focussed along coarser, more permeable
horizons within the sediments. This resulted in stratiform zones of alteration and
mineralisation, and in the intense alteration of the lower margins of a peperitic quartz-feldsparphyric
sill and a thick siltstone package now overlying the northern ore lenses.
The current study has identified features within the northern part of the mine that indicate a
sub-seafloor mode of ore formation, including: massive barite and sulphide lodes that are
hosted within rapidly emplaced mass flow and turbidite units; ore lenses that locally transgress
bedding; massive sulphide ore with the same immobile element signature as enclosing mass
flow sediments; and a laterally extensive halo of alteration and disseminated mineralisation
that extends for hundreds of metres laterally away from massive ore, for tens of metres into
the footwall, and for several metres into the hanging wall. The siting of ore lenses
stratigraphically below a peperitic quartz-feldspar phyric sill that was emplaced prior to the
onset of significant hydrothermal circulation further supports a sub-seafloor mode of ore
formation. The ore and the host succession do not display any textures to indicate that
mineralisation occurred on the Cambrian seafloor.

Initial primary sphalerite-galena-rich mineralisation formed under relatively low-temperature
(~200-250°C) and near neutral conditions, as a hot hydrothermal fluid mixed with cooler
seawater and reacted with host sediments through which it passed. This mineralisation was
characterised by primitive sulphide textures, including spongiform, atoll, colloform and skeletal
morphologies. Utilising the laser ablation-ICP-MS analytical technique, this phase of
mineralisation was found to be characterised by a low temperature trace element signature
within the sulphide minerals that includes: elevated levels of Mn-Ni-As-Ag-Sb-Au-Tl-Pb and
low levels of Co-Bi in pyrite; low Co:Ni and Bi:Pb ratio values in pyrite; low levels of Cu-Fe and
elevated levels of Zn-Ag-Sb-Pb in sphalerite; and low levels of Sb-Ag in galena. The low
temperature sulphide and barite mineralisation locally preserved an albitised volcaniclastic
plagioclase component. The low temperature sphalerite-galena mineralisation was locally
overprinted by and interspersed with mineralisation formed at higher temperatures, as hot
hydrothermal fluids passed through permeable horizons within the ore and adjacent
sediments. This phase of mineralisation produced more coarsely crystalline pyrite textures,
including anhedral to euhedral grains, aggregates, and overgrowths on earlier sulphide
phases. This phase of mineralisation was also characterised by a high temperature trace
element signature within sulphide minerals that included: elevated levels of Bi-Sn, moderate
levels of Ni-Ag-Sb-Tl-Pb-Co, and low levels of Mn-As-Au in pyrite; elevated Bi:Pb ratio values
and moderate Co:Ni ratio values in pyrite; an increased Fe content in sphalerite; and galena
with elevated levels of Sb-Ag. The lateral flow of high temperature (~300°C) hydrothermal fluid
within the immediate footwall to sphalerite-galena ore produced discontinuous disseminated to
massive pyrite-chalcopyrite mineralisation and chlorite alteration. The footwall mineralisation was characterised by euhedral pyrite morphologies, and a high temperature sulphide mineral
trace element signature comprising: elevated Co-Bi in pyrite; elevated Co:Ni and Bi:Pb ratio
values in pyrite; and high levels of Fe-Mn-Cu-Sb-Bi in sphalerite.
Sphalerite-galena ore lenses are vertically and laterally zoned, with Fe-Cu enrichment at the
base and proximal to interpreted zones of hydrothermal upflow, and Au-Ag-Pb-barite
enrichment at the upper and lateral margins. The zonation was the due to the influence of
physicochemical gradients present at the time of initial ore mineral precipitation, in combination
with subsequent zone refinement processes. The vertical zonation is locally inverted.
In comparison with the northern ore lenses, a cluster of sulphide ore lenses at the southern
end of the mine exhibits evidence for higher temperatures of ore formation that includes:
significantly elevated average Fe and Cu contents; low average Au and Ag contents; low
average h
34S values (<12‰); and elevated Co, Ni and Co:Ni ratio values within footwall
pyrites. This suggests that the southern ore lenses were located proximal to a major site of
hydrothermal upflow at Rosebery, at the intersection of two northeast and northwest trending
Cambrian faults. The peripheral ore lenses, including those at the northern end of the mine,
formed at lower temperatures after mixing with a greater volume of locally circulating seawater.
Devonian deformation and metamorphism produced a pervasive cleavage, brittle-ductile
faulting and folding, and syn-tectonic quartz-carbonate veining containing locally remobilised
sulphides variably enriched in Au±Ag. Sulphide ore exhibits deformation fabrics at the mesoand
micro-scale. Late syn- to post-deformation Devonian granite intrusion produced minor
faulting and veining, and metasomatic fluids that resulted in replacement of sphalerite-galena
ore with massive pyrite-pyrrhotite and magnetite-biotite ± chalcopyrite assemblages. Sulphide
annealing occurred syn- to post-metasomatism. The host sequence was later subjected to
brittle-ductile faulting and minor dolerite intrusion.

Item Type: Thesis (PhD)
Keywords: Sulfide minerals; Rosebury
Copyright Holders: The Author
Copyright Information:

Copyright 2004 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).

Date Deposited: 20 Feb 2015 01:14
Last Modified: 11 Mar 2016 05:54
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