PALEOMAGNETISM AND MAGNETIC-ANISOTROPY OF PROTEROZOIC BANDED-IRON FORMATIONS AND IRON-ORES OF THE HAMERSLEY BASIN, WESTERN-AUSTRALIA

Rock magnetic properties and palaeomagnetism of weakly metamorphosed b anded-iron formations (BIFs) of the Palaeoproterozoic Hamersley Group, Western Australia, and Proterozoic PIP-derived iron ores have been in vestigated. The BIF units sampled here are slightly younger than 2500 Ma. At Paraburdoo, Mount Tom Price and Mount Newman iron ore formation was completed before 1850 Ma. Sampling was mainly from the Mount Tom Price and Paraburdoo mining areas and for the first time a palaeomagne tic fold test on fresh (unweathered and unaltered) BIF samples has all owed the nature of the remanence of the BIFs to be defined. The remane nce of the BIFs is carried by late diagenetic/low-grade metamorphic ma gnetite after primary haematite. This remanence is pre-folding and is unlikely to be greatly affected by the high anisotropy because the pal aeofield inclination was demonstrably low. Determination of palaeofiel d directions from measured remanence directions is complicated by self -demagnetization effects in strongly magnetic, highly anisotropic BIF specimens. We present a method for correcting measured directions for the effects of self-demagnetization and anisotropy. For typical BIFs, the effect of magnetic anisotropy on measured remanence inclinations a nd inferred palaeolatitudes is minor for low palaeolatitudes, but can lead to large errors in calculated palaeopoles for intermediate to mod erately steep palaeolatitudes. Anisotropy also causes cones of confide nce to be underestimated, due to compression of the range of inclinati ons. In principle, deflection of post-folding remanence towards the be dding plane by high magnetic anisotropy can produce an apparent syn-fo lding signature, with best agreement between directions from different fold limbs after partial unfolding. Thus high anisotropy cannot only bias estimated palaeofield directions and cause underestimation of err ors, but can also mislead interpretation of the time of remanence acqu isition. The anisotropy of anhysteretic remanent magnetization (ARM) p robably yields an upper limit to the anisotropy of the chemical remane nt magnetization (CRM) carried by the BIFs. Therefore, from the anisot ropy of ARM, a maximum inclination deflection of 9 degrees is suggeste d for the sampled BIFs. This corresponds to less than 5 degrees change of palaeolatitude. The palaeomagnetic pole position calculated for BI Fs at Paraburdoo is 40.9 degrees S, 225.0 degrees E (dp=2.9 degrees, d m=5.8 degrees) after tilt correction, but without correction for aniso tropy. Other pole positions reported include that from flat-lying BIFs from Wittenoom at 36.4 degrees S, 218.9 degrees E (dp=4.6 degrees, dm =9.1 degrees), from Mount Tom Price iron ore at 37.4 degrees S, 220.3 degrees E (dp=5.7 degrees, dm=11.3 degrees) and from Paraburdoo ore at 36.4 degrees S, 209.9 degrees E (dp=4.7 degrees, dm=8.8 degrees). The poles from the BIFs, the Paraburdoo ore and the part of the Tom Price deposit that was sampled in this study are indistinguishable from eac h other and from the Mount Jope Volcanics overprint pole. The magnetiz ation of the BIFs was probably acquired during burial metamorphism of the Hamersley Group, soon before the main folding and uplift event in the southern part of the Hamersley Province. This tectonic event expos ed magnetite-rich BIFs to near-surface oxidizing conditions, producing extensive martite-goethite orebodies and also appears to have produce d the syn-folding overprint magnetization recorded by the Mount Jope V olcanics of the underlying Fortescue Group. Ages of magnetization are tentatively interpreted as similar to 2200 +/- 100 Ma for the BIFs, si milar to 2000 +/- 100 Ma for the supergene enrichment of BIF to martit e-goethite ore, recorded by the Parabudoo and Mount Tom Price orebodie s, and similar to 1950 +/- 100 Ma for the metamorphic martite-micropla ty haematite ore, recorded as an overprint by the Tom Price orebody an d as the only surviving magnetization of the Mount Newman orebody.