PALAEOBIOLOGY OF A 1640 MA HYDROTHERMAL SYSTEM

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 150°C 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 d¹³C and d³⁴S isotopic analyses to suggest that the hydrocarbon signal was derived from mats that contained sulfide-oxidising bacteria. d³⁴S analyses indicate bacterial sulfate reduction.