Astrobiology Workshop, Macquarie University July 12-13 2001
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The anatomy of an ancient hydrothermal system
Malcolm Walter, Junhong Chen (Australian Centre for Astrobiology Macquarie University Sydney, NSW 2109)
Graham Logan, Roger E. Summons (Australian Geological Survey Organisation)
The 1640 Ma McArthur River lead-zinc-silver deposit of northern Australia formed in a hydrothermal system. It has been intersected by more than 100 drill holes, and is exposed in an underground mine. It represents a superb opportunity to examine this class of ore deposits, and has been intensively studied.
Our work and previous studies show that the ore formed 10-20 m below the sediment-water interface in a marine environment below wave-base. It is possible to reconstruct the original fluid flow patterns and to determine thermal gradients. Absolute temperatures are likely to range from ambient seafloor temperatures up to 1500C or more.
Microfossils occur in chert that formed at the sediment-water interface, and in the ore and interbedded sediments. Previous studies have proposed that there are two populations in the chert: allochthonous cyanobacteria and possible microalgae, and benthic non-photosynthetic bacteria. The ore and interbedded sediments contain filamentous microfossils up to 60 mm wide, and spheroidal microfossils 7-14 mm wide with a single aperture.
We have analysed hydrocarbon biomarkers from the ore and associated sediments sampled on a centimetre to millimetre scale, and the d13C composition of selected hydrocarbons. The interpretation of some of the microfossils as cyanobacteria is supported by the presence of 2 methyl-hopanes. C30 24-n-propyl steranes occur in all samples, and their parent sterols are known only from marine Chrysophyte algae. All samples contain a C25 isoprenoid derived from Archaea.
An unusual biomarker distribution found in some samples is similar to that of bitumen described from Neoproterozoic sediments in a previous study that combined sedimentological, micropaleontological, biomarker, and delta 13C and delta 34S isotopic analyses to suggest that the hydrocarbon signal was derived from mats that contained sulfide-oxidising bacteria. delta 34S analyses indicate bacterial sulfate reduction.