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Lateral variability of sedimentology, mineralogy, and geochemistry in the HYC Zn-Pb-Ag deposit, Northern Territory, Australia; implications for ore genesis


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Ireland, T 2001 , 'Lateral variability of sedimentology, mineralogy, and geochemistry in the HYC Zn-Pb-Ag deposit, Northern Territory, Australia; implications for ore genesis', Honours thesis, University of Tasmania.

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An understanding of the lateral variability of sedimentology, mineralogy and geochemistry at HYC
has the potential to contribute to improved ore genesis models, and to facilitate better mine planning
and grade control at the McArthur River Mine. HYC is the largest of the north-Australian
Proterozoic sediment hosted stratiform base metal deposits (230 mT @ 12%Zn), and is hosted by
the reduced sub-wave base marine carbonate-siliclastic Barney Creek Formation.
Macroscopic planar sulphide laminae consist of variable intergrowths of two distinct sphalerite and
pyrite phases, galena, arsenopyrite, quartz, ankerite and dolomite, and disguise significant
microscopic textural complexity. Sphalerite 1 (spl) and pyrite 1 (pyl) are paragenetically early, fine
grained and volumetrically dominant in the deposit, whereas sphalerite 2 (sp2) occurs as late stage,
relatively coarse grained replacement of carbonates, and pyrite 2 (py2) is coarse grained,
volumetrically minor, and is the last sulphide phase to form. The deposit fringe is characterised by
systematic changes of microscopic spl texture with concommitant stratigraphic thinning and
declining base metal grade that is unrelated to the distribution of iron. Characteristic anastamosing
microscopic textures are explained by pelagic fallout of pyrite crystallites from the water column
(pyl) with simultaneous seafloor precipitation ofbasemetal sulphides (spl).
Sheet-like mass flow deposits that separate ore lenses and dilute ore at HYC exhibit rapid lateral
transition from erosive boulder-bearing debris flows to non-erosive normally-graded turbidites.
Turbidites formed by elutriation of fines into a turbid flow that followed and outran the primary
debris flows. Plastically deformed sulphidic intraclasts and sulphidic matrices of these breccias are
texturally and isotopically identical to in-situ laminated sulphide ore, and constrain mineralisation to
the upper few metres of the unconsolidated marine sediments.
Nodular carbonates occur on all preserved fringes of the deposit, and are the direct lateral
equivalent of the mineralised stratigraphy. The SIC ratio of unmineralised siltstone reveals primary
differences in shallow diagenetic processes between the nodular and laminar sulphide ore facies.
Bacterially triggered carbonate precipitation probably took place at shallow depth during base-metal
mineralisation, and resulted in Fe-Mn-calcite nodule formation in a zone concentric about the local
depocentre. Dolomitisation and partial replacement by sphalerite (sp2) occurred shortly after nodule
formation, as modified ore fluids permeated the sediment pile. The ()13C of these carbonates is consistent with that of detrital carbonates in the host unit, which indicates that oxidised organic
carbon (a major product of inorganic sulphate reduction) was not incorporated into the nodular
The o34S values of spl and pyl are heavier in the deposit centre (5-8%) than in the deposit fringe (0
to -2%). This suggests a gradient of 34S fractionation concentric around the focus of mineralisation,
interpreted to be largely due to bacterial processes that were limited spatially by physicochemical
gradients in a stratified water body. Fine grained pyrite textures closely resemble contemporaneous
microfossils and microbially laminated pyritic sediments elsewhere, and confirm the presence of a
prolific benthic microbial fauna spatially related to the mineralising system. Sp2 defines a 835 population (mean= 9.3%) that is distinct from, and always heavier than coexistent spl, interpreted
to arise from partial closed-system sulphate reduction in the sediment pile.
The textural, geochemical and sedimentological data demonstrate that the stratiform HYC Zn-PbAg
deposit at McArthur River is a vent-distal sedimentary exhalative deposit, in which the
contemporaneous benthic microbial fauna played an important role in mineralisation. The HYC
sub-basin was probably not significantly more extensive than the deposit, and restriction of the
water body, including 'plugging' of the northern end by a fanglomerate wedge, were crucial to
establishment of a stratified aqueous environment amenable to extreme concentration of metals.
Metalliferous fluids were introduced into the basin via hydrothermal pluses from a structural
conduit related to the Emu Fault.

Item Type: Thesis - Honours
Authors/Creators:Ireland, T
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