A reassessment of the genetic model for the Paleoproterozoic McArthur River (HYC)
Pb/Zn/Ag deposit of northern Australia based on new molecular and compound
specific isotopic data
K.H. Williford1, K. Grice1, G.A. Logan2, J. Chen2, M.R. Walter3, D. Dawson1
and F. Busetti1
1) WA Organic and Isotope Geochemistry Centre, Curtin University of Technology, Perth, WA 6845,
Australia (E-mail: k.williford@curtin.edu.au)
2) Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
3) Australian Centre for Astrobiology, University of New South Wales, Sydney, NSW 2052, Australia
The Late Paleoproterozoic (1639 ± 2 Ma) “Here’s Your Chance” (HYC) Pb/Zn/Ag deposit
in the Barney Creek Formation of the McArthur Group in the Northern Territory of
Australia is the third largest sedimentary exhalative (SEDEX) deposit currently known [1].
The extraordinary preservation and accessibility of the HYC deposit make it an ideal place
to investigate stratiform, sediment-hosted ore formation processes, in particular the effects
of varying degrees of hydrothermal alteration on sedimentary organic matter.
Prior to 2002, available data pointed to syngenetic or early diagenetic ore formation
models, with metals deposited by basin-derived, oxidised fluids of moderate temperatures
(150-250ºC) at or just below the sediment-water interface [2, and references therein]. Due
to the low solubilities of Pb and Zn, these temperatures imply that the ore fluid was derived
from a large reservoir in the clastic sedimentary rocks of the McArthur River Group at a
depth of 5 to 6 km, fed by recharge zones such as the Tawallah Fault and released along a
major discharge zone such as the Emu Fault. A seismic survey conducted by Geoscience
Australia in 2002 was unable to identify the recharge zones required to establish the
convective flow system invoked in the low temperature models, but revealed that the Emu
Fault zone was much deeper (10 km) than previously thought and penetrates the sediments
of the Tawallah group [3]. This expands the available fluid reservoir and indicates that
deeper, hotter (>300ºC) fluids could have been involved in ore generation, decreasing the
total volume of fluid necessary to form the deposit.
Distributions of polycyclic aromatic hydrocarbons (PAHs) are similar to hydrothermal oils
in the Guymas Basin, also associated with high formation temperatures (250-400ºC) and
the concentrations of PAHs decrease with distance from the Emu Fault (coincident with
decreasing formation temperature) [1]. This decreasing trend in the concentrations of
chrysene, pyrene and especially coronene is more clearly demonstrated with additional
samples collected from the area closest to Emu Fault (Fig. 1).
We are developing LC-MS methods to analyze PAHs of molecular weights above gas
chromatographic detection limits in HYC sediments, which it is anticipated will reveal
further information about formation conditions. D/H ratios of n-alkanes and isoprenoids
extracted from HYC sediment samples taken along the fluid flow path will be determined
in order to establish thermally-induced hydrogen exchange processes in hydrothermally
altered sedimentary organic matter, which may be analagous to processes that occur in the
deep subsurface over millions of years during petroleum generation [5]. These new data
will provide further support for an improved genetic model for the HYC deposit, may
provide insight into petroleum formation processes, and will have significant implications
for mineral exploration strategies worldwide.
Figure 1: Concentrations of PAHs in HYC ore samples taken along the flow path of
hydrothermal fluid at increasing distances from the Emu Fault Zone. Distance from
the Emu fault increases from left to right.
References
[1] Chen J., Walter M.R., Logan G.A., Hinman M.C. & Summons R.E., 2003. The
Paleoproterozoic McArthur River deposit of Northern Australia: organic geochemistry and
ore genesis. Earth and Planetary Science Letters 210, 467–479.
[2] Logan G.A., Hinman M.C., Walter M.R. & Summons R.E., 2001. Biogeochemistry of
the 1640 Ma McArthur River (HYC) lead-zinc ore and host sediments, Northern Territory,
Australia. Geochimica et Cosmochimica Acta 65, 2317–2336.
[3] Rawlings D.J., Korsch R.J., Goleby B.R., Gibson G.M., Johnstone D.W. & Barlow M.,
2004. The 2002 Southern McArthur Basin Seismic Reflection Survey. Geoscience
Australia, Record 2004/17, 78 p.
[4] Symons D.T.A., 2007. Palaeomagnetism of the HYC Zn-Pb SEDEX deposit, Australia:
evidence of an epigenetic origin. Economic Geology 102, 1295–1310.