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The Archean Cu-Zn magnetite-rich Gossan Hill VHMS deposit, Western Australia: Evidence of a structurally-focussed, exhalative and sub-seafloor replacement mineralising system


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Sharpe, R 1999 , 'The Archean Cu-Zn magnetite-rich Gossan Hill VHMS deposit, Western Australia: Evidence of a structurally-focussed, exhalative and sub-seafloor replacement mineralising system', PhD thesis, University of Tasmania.

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The Archean Cu-Zn Gossan Hill volcanic-hosted massive sulphide deposit is situated on
the northeast flank of the Warriedar Fold Belt in the Yilgarn Craton, Western Australia.
The deposit is hosted within re-deposited rhyodacitic tuffaceous volcaniclastics of the
Golden Grove Formation and is overlain by rhyodacite-dacite lavas and intrusive domes
of the Scuddles Formation. The Gossan Hill deposit consists of two discrete subvertical
ore zones situated stratigraphically 150 m apart in the middle and upper Golden Grove
Formation. The stratigraphically lower Cu-rich ore zone (7.0 Mt @ 3.4% Cu) consists of
stratabound, podiform to discordant massive pyrite-chalcopyrite-pyrrhotite-magnetite. In
addition to massive sulphides, the lower ore zone also contains discordant to sheet-W,e
zones of massive magnetite-carbonate-chlorite-talc (-12 }\·ft). The upper Zn-Cu ore zone
(2.2 Mt @ 11.3% Zn, 0.3% Cu, 15 glt Au and 102 glt Ag) is mound-shaped with sheetW,
e, stratabound, massive sphalerite-pyrite-chalcopyrite overlying discordant massive
pyrite-pyrrhotite-chalcopyrite-magnetite. A sulphide-rich vein stockwork connects the
upper and lower ore zones. Metal zonation grades from Cu-Fe (±Au) in the lower ore
zone to Zn-Cu-nch sulphides at the base of the upper ore zone. The upper ore zone
grades upwardS and laterally from Zn-Cu to Zn-Ag-Au (tCu, tPb)-rich sulphides.
Regional preservation of primary tuffaceous volcanic textures within the Golden Grove
Formation is attributed to an early syndepositional, quartz-chlorite alteration. Induration
and differential permeabilityI porosity reduction of the succession during the early
alteration W,ely promoted more-focussed pad1\vays for successive hydrothermal fluids.
Subsequent hydrothermal alteration related to mineralisation at Gossan Hill has a limited
lateral extent, and forms a narrow Fe-chlorite-ankerite-siderite envelope to the massive
magnetite and sulphide of the lower ore zone, and an intense siliceous envelope
surrounding d,e stockwork and upper ore position. Pervasive calcite-muscovite alteration
is recognised Ul d,e hangingwall volcanics of the ScuddIes Formation.
The nature of deformation and metamorphism (greenschist facies: 454 t 4°C at I kbar
based on andalusite-chloritoid-quartz equilibrium) is uniform throughout d,e massive
magnetite, massive sulphide and host succession. Sedllnent-sulphide-magnetite
relationships at Gossan Hill suggest d,e formation of magnetite and sulphide during
deposition of the upper Golden Grove Formation. Massive magnetite formed entirely by
sub-seafloor replacement processes as inferred from gradational upper and lower contacts
and interdigitating volcaniclastics. Replacement occurred along permeable tuffaceous
strata outward from a discordant feeder. Massive magnetite was later veuled, replacedand cut by massive sulphide. The synchronous formation of both upper and lower
sulphide ore zones is indicated by the connecting sulphide stockwork. Both sulphide ore
zones formed by sub-seafloor replacement, although stratiform hydrothermal chertsulphide-
sediment layers in, and adjacent to, the upper sulphide zone attest to some
exhalation of fluids onto the seafloor.
The thickest occurrence of massive magnetite, massive sulphide and stringer stocb.-work
spatially coincide and support a common feeder conduit during massive magnetite and
sulphide mineralisation. The asymmetry of hydrothermal alteration envelopes, massive
magnetite and massive and veins sulphide zones are consistent with synvolcanic structural
controls, with a growth structure occupied and obscured by a younger dacite dome from
the Scuddles Formation.
A systematic increase in sulphide 8;4S values (range of -4.0 to 7.8%0, average 2.1 ± 1.7%0)
stratigraphically upwards through massive and vein sulphide is suggestive of progressive
mixing of upwelling ore fluids with entrained seawater. Homogeneous 8"s values of
-1.5%0 in the lower ore zone have a consistent homogeneous rock sulphur source with
possible magmatic contributions.
The 8180 H20 values of ore fluids responsible for deposition of magnetite in massive
magnetite and disseminated magnetite in the sulphide zones range from 6%0 to 13%0.
This data is inconsistent with the direct input of Archean seawater, and favours
derivation of hydrothermal fluids by rock buffering of circulating fluids, or by direct
magmatic contribution.
Thermodynamic considerations suggest massive magnetite and sulphide formed from high
temperature (300° to 350°C), reduced (low f 0,), slightly acidic hydrothermal fluids. HzSdeficient
fluids formed massive magnetite, whilst HzS-rich fluids formed massive
sulphides. Fluid chemistry differences are attributed to magmatic sulphur contributions
during sulphide mineralisation. Precipitation of sub-seafloor sulphide in the lower ore
zone resulted from chemical entrapment by the interaction of upwelling HzS-rich fluids
with pre-existing massive magnetite. It is suggested that shallow parental magma
chambers to the Scuddles Formation drove hydrothermal convection of seawater and may
have supplied volatiles and HzS to the ascending hydrothermal fluids.
The Gossan Hill sulphide-magnetite deposit represents an evolving hydrothermal system
in an environment characterised by rapid volcaniclastic sedimentation and changing
structural and magmatic processes. An important influence on this hydrothermal system
was the creation and destruction of porosity and permeability in the host succession. The
hydrothermal system initiated as part of a regional seawater convection-alteration system
that led to VHMS mineralisation at Gossan Hill by (1) synsedimentary metasomatism and
progressive heating of convecting fluids, (2) formation of massive magnetite by host rock
replacement above a buried synvolcanic conduit, and (3) structural re-activation and
tapping of deeper HzS-rich and metal-bearing fluids, leading to the sub-seafloor sulphide
replacement and local exhalation of hydrothermal fluids forming sulphide and chert.
Burial by proximal felsic volcanism led to preservation of the deposit.

Item Type: Thesis - PhD
Authors/Creators:Sharpe, R
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