RARE-EARTH ELEMENT AND NEODYMIUM ISOTOPIC COMPOSITIONS OF THE BANDED IRON-FORMATIONS AND ASSOCIATED SHALES FROM HAMERSLEY, WESTERN-AUSTRALIA

Major, trace element, and Nd isotopic compositions are reported for the early Proterozoic iron-formations from the Hamersley Basin, western Australia. These data enable new constraints to be placed on the relative roles of continental weathering and submarine hydrothermal systems in the REE budget of the Hamersley banded-iron formations (IF). The lower part of the Hamersley Group (Jeerinah shales/cherts, Marra Mamba IF, Wittenoom Dolomite and McRae Shale units) have an average epsilon(Nd) = -0.6 +/- 0.9 at 2.6 Ga. This uniformity suggests that Nd was mostly supplied by erosion products coming from the nearby upper Fortescue Group and also that these sediments were deposited in a restricted basin. The overlying Dales Gorge and Joffre banded iron-formations at Wittenoom record a major metamorphic event at 2.14 +/- 0.03 Ga. Initial epsilon(Nd) values are estimated to be +1.0 +/- 0.5 at the time of deposition (2.5 Ga), after correction for metamorphic effects. This more positive epsilon(Nd) Value is consistent with the increased influence of submarine hydrothermal sources, probably in response to exchange with the open ocean. The Hamersley IF are therefore viewed as shelf deposits in a newly formed ocean, possibly of an evolving rift. The shaley bands of the Dales Gorge-Joffre formations containing around 30-60% detrital material give epsilon(Nd) = -0.9 +/- 0.6. An epsilon(Nd) almost-equal-to -2 is therefore inferred for river waters/aeolian sources in the Hamersley basin. The Nd mass balance obtained from the Dales Gorge-Joffre IF indicates a proportion of Nd derived from hydrothermal sources associated with mid-ocean ridge volcanism of 50 +/- 10%.