Author: Stefan G. Müller

Publisher:

Published: 2005

Total Pages: 156

ISBN-13:

[Truncated abstract] Banded iron formations of the ~27702405 Ma Hamersley Province of Western Australia were locally upgraded to high-grade hematite ore during the Early Palaeoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between Lower and Upper Wyloo Groups of the Ashburton Province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small spot PbPb dating of in situ baddeleyite by SHRIMP (sensitive highresolution ion-microprobe) has resolved the ages of two key suites of mafic intrusions constraining for the first time the tectonic evolution of the Ashburton Province and the age and setting of iron-ore formation. Mafic sills dated at 2208 ± 10 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dyke swarm that intrudes the Lower Wyloo Group and has close genetic relationship to iron ore is 2008 ± 16 Ma, slightly younger than a new syneruptive 2031 ± 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period 2210 to 2030 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ~2010 Ma dykes present a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ~21002000 Ma period of continental extension similarly recorded by Palaeoproterozoic terrains worldwide well after the initial oxidation of the atmosphere at ~2320 Ma. The Lower Wyloo Group contains, in ascending order, the fluvial to shallow-marine Beasley River Quartzite, the predominantly subaqueously emplaced Cheela Springs flood basalt and the Wooly Dolomite, a shelf-ramp carbonate succession. Field observations point to high subsidence of the sequence, rather than the mainly subaerial to shallow marine depositional environment-interpretation described by earlier workers. Abundant hydro-volcanic breccias, including hyaloclastite, peperite and fluidal-clast breccia all indicate quench-fragmentation processes caused by interaction of lava with water, and support the mainly subaqueous emplacement of the flood basalt which is also indicated by interlayered BIF-like chert/mudstones and below-wave-base turbiditic mass-flows.