Opaque mineralogy and magnetic properties of selected banded iron-formations, Hamersley Basin, Western Australia

The oxide mineralogy and rock magnetic properties of unmineralised banded i ron-formations in selected portions of four drillholes in the Hamersley Bas in, Western Australia are reviewed. In all four drillholes, petrographic st udies indicate that primary euhedral to subhedral hematite is partially rep laced by magnetite as a result of subsolidus reduction. All drillholes show partial recrystallisation of the secondary magnetite. suggesting that earl y subsolidus reduction was probably a regional event occurring during progr ade metamorphism, Incomplete replacement of primary hematite by magnetite w ithin and between sedimentary band structures indicates that equilibration in the magnetite stability field was not reached even at the mesoband scale . Subsequent subsolidus oxidation of magnetite and the formation of a secon d-generation hematite are documented in only two of the drillholes. Goethit e-filled veins and thick selvages of goethite around some veins reflect mov ement of circulating oxidising fluids. The absence of goethite and second-g eneration hematite in two of the drillholes indicates that subsolidus oxida tion is not a regional event, but very much localised. Rapid changes in dow n-hole susceptibility measurements correlate directly with detailed petrogr aphic results as susceptibility readings change with the hematite/magnetite ratio on a mesoband scale. Acquisition of the main remanence correlates wi th the formation of hematite as the primary oxide phase followed by partial replacement by magnetite as a result of subsolidus reduction, supporting r egional models requiring pre-folding remanence. The strong orientation of t he primary hematite parent parallel to band structures in the banded iron-f ormations has influenced the direction of crystallisation remanent magnetis ation during subsolidus reduction to the magnetite daughter. The strong pla nar alignment has also produced a planar magnetic fabric and marked anisotr opy of magnetic susceptibility. A natural remanent magnetisation overprint and reduction in anisotropy of magnetic susceptibility are only recorded in samples that have undergone subsolidus oxidation and the recognition of lo calised post-metamorphic oxidation overprinting can also explain ore deposi t models requiring post-folding remanence. The relative timing of and betwe en oxidising fluid events is not known, but both petrographic and rock magn etic evidence to date suggests that there was at least one and probably two post-folding oxidising events in the area of study.