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Spatial distribution patterns of sulfur isotopes, nodular carbonate, and ore textures in the McArthur River (HYC) Zn-Pb-Ag deposit, Northern Territory, Australia


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Ireland, T, Large, RR, McGoldrick, PJ and Blake, MD 2004 , 'Spatial distribution patterns of sulfur isotopes, nodular carbonate, and ore textures in the McArthur River (HYC) Zn-Pb-Ag deposit, Northern Territory, Australia' , Economic Geology, vol. 99, no. 8 , pp. 1687-1709 , doi: 10.2113/99.8.1687.

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The HYC Zn-Pb-Ag deposit at McArthur River is the largest known and least deformed Australian sedimenthosted
stratiform base metal deposit. A study of mineralogical, geochemical, and isotopic zonation through the
deposit reveals concentric distribution patterns in (1) the occurrence of nodular carbonate, (2) the δ34S composition
of sphalerite, and (3) microscopic ore textures. The correlation of previously documented lateral metal
zonation with these other zonation patterns precludes exclusive postsedimentation mineralization and provides
some insight into the mechanisms and controls on synsedimentary to early diagenetic mineralization. This mineralization
timing is also supported by new carbon and oxygen isotope analyses, recognition of two stages of texturally
and isotopically distinct sphalerite, and reinterpretation of complex sulfide textures.
Dolomitic carbonate nodules occur in the south, southwest, northwest, and northern fringes of the deposit
and are interpreted to have surrounded the high-grade ore lenses prior to structural and erosive truncation.
The carbon and oxygen isotope composition of these carbonate nodules is similar to the sedimentary dolomite
within the ore lenses. There is no evidence for incorporation of light carbon derived from oxidation of organic
material under closed-system conditions and the nodular carbonate probably formed under open-system conditions
in communication with seawater. These nodules displace siltstone laminae and, therefore, likely formed
in the very shallow subsea-floor environment.
In ore lens 3, the δ34S of laminated early sphalerite (sp1) changes from a mean value of 5 per mil in the deposit
center to a mean of 1.7 per mil in the extreme fringes. We interpret this to be the result of sulfide precipitation
in a restricted marine basin. Later sphalerite (sp2) associated with nodular carbonate has a mean δ34S
value of 9.8 per mil, whereas laminated early sphalerite (sp1) has a mean δ34S value of 3.8 per mil. This isotopic
separation of the two paragenetic stages of sphalerite is found in immediately adjacent aggregates. Concentration
of 34S in late sphalerite (sp2) is likely the result of closed-system conditions in the sediment pore fluid.
Textures of laminated sphalerite change from strongly anastomosing in the central part of the deposit to
plane laminar and patchy in the deposit fringes. Mass-balance calculations preclude substantial carbonate dissolution
and a stylolitic origin for these textures. Instead we propose that rapidly precipitated sphalerite coagulated
and trapped pelagic silt and early pyrite (py1) as it was deposited on the sea floor. In contrast, paragenetically
late sphalerite (sp2) and pyrite (py2) must be diagenetic as they overprint and are pseudomorphous
after the carbonate nodules. We propose a repeating paragenetic sequence of galena/sphalerite (sp1) → pyrite
(py1) → nodular carbonate → sphalerite (sp2) → pyrite (py2), which accounts for all the textural complexity and
isotopic disequilibrium between sulfide phases.
The data presented in this paper suggest that base metal sulfides formed both in the water column and in
the uppermost sediment pile. Biological and thermochemical sulfate reduction probably occurred simultaneously
in different parts of a complex physicochemical system in which stratification of the marine environment
is seen as the primary control on the lateral distribution of the mineralized facies. We propose a stratified water
body in which sharp internal chemical gradients separate a surficial oxic layer, an anoxic layer, and a basal hypersaline
brine pool. Asymmetric metal zonation across the deposit reflects individual pulses of metalliferous
fluid that were introduced into the basin as a bottom-hugging dense current.

Item Type: Article
Authors/Creators:Ireland, T and Large, RR and McGoldrick, PJ and Blake, MD
Keywords: SEDEX, Seafloor massive sulfide, sediment-hosted,zinc, lead, Mt Isa, McArthur Basin, syngenetic, replacement
Journal or Publication Title: Economic Geology
ISSN: 0361-0128
DOI / ID Number: 10.2113/99.8.1687
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