DEEP-MARINE DEPOSITED HAMERSLEY BIFS: SIGNIFICANCE TO ENVIRONMENTAL CHANGE ACROSS THE ARCHEAN-PALEOPROTEROZOIC BOUNDARY

Banded iron formations occur at three levels in the Hamersley Province: (i) in the 2.63-2.59 Ga Marra Mamba Supersequence; (ii) in the 2.50-2.45 Ga Brockman Supersequence; and (iii) in the Boolgeeda Iron Formation of the 2.45-2.41 Ga Turee Creek Supersequence. Most Marra Mamba and Brockman BIFs accompany shelf-derived lowstand turbidites deposited on basin-floor fans, but Weeli Wolli and Woongarra BIFs high in the Brockman Supersequence accompany basalt and rhyolite. Deposition occurred in deep water, below the CCD for Marra Mamba BIFs. An intra-basinal, volcanogenic origin of the precursor sediments to BIF is implied by volcanosedimentary associations. Much layering and lamination in BIF is secondary, with magnetite defining an extensional metamorphic fabric. The chert component is of diagenetic origin: recording precompaction replacement during sea-floor silicification in chert mesobands and compaction-related replacement in BIF microbands. The precompaction source of silica was ambient sea-water diffused across the sediment-water interface but its syncompaction source was connate pore-fluid. Draped lamination, ghost grading, clastic fragments and Tab rhythms indicate that the precursor sediments to most BIFs were deposited from density currents and not by chemogenic pelagic settling, whereas most microbanding is plane lamination. The precursor sediments to BIFs preserving density-current rhythms were resedimented within the basin realm, but Weeli Wolli and Woongarra BIFs lack density-current rhythms and are diffusely to chaotically laminated. Boolgeeda BIFs are different, preserving turbidite rhythms, little evidence of precursor sediments and only minor diagenetic chert but almost complete replacement by metamorphic magnetite. Boolgeeda BIFs may have been sideritic mudrocks but the precursor sediments to other BIFs were in situ or resedimented hydrothermal muds. Precompaction particles of iron oxyhydroxides (now hematite) preserved in Brockman BIFs were precipitated from hydrothermal fluids oxidised by ambient sea-water. Similar particles have yet to be discovered in Marra Mamba BIFs, implying that vent-derived ferrous iron was adsorbed in clay-mineral complexes and that ambient sea-water at that time contained little or no oxygen.