Hickman Crater, Ophthalmia Range, Western Australia: evidence supporting a meteorite impact origin

A newly discovered, morphologically well-preserved crater with a mean diameter of 260m is reported from the Ophthalmia Range, Western Australia. The crater is located in hilly terrain 36km north of Newman, and is situated in the Paleoproterozoic Woongarra Rhyolite and the overlying Boolgeeda Iron Formation. The morphometry of the crater is consistent with features characteristic of small meteorite impact craters. The rhyolite of the crater's rim exhibits widespread shatter features injected by veins of goethite bound by sharply defined zones of hydrous alteration. The alteration zones contain micro-fractures injected by goethite, which also fills cavities in the rhyolite. The goethite veins are interpreted in terms of forceful injection of aqueous iron-rich solutions, probably reflecting high-pressure hydrothermal activity by heated iron-rich ground water. None of these features are present in the Woongarra Rhyolite outside the immediate area of the crater. Petrography of the rhyolite indicates possible incipient intracrystalline dislocations in quartz. The Boolgeeda Iron Formation, which crops out only on the southern rim of the crater, displays brecciation and mega-brecciation superposed on fold structures typical of the banded iron-formations in the region. Geochemical analysis of two goethite veins discloses no siderophile element (Ni and PGE) anomalies, negating any contribution of material from an exploding meteorite. Instead, the strong iron-enrichment of the fractured rhyolite is attributed to a hydrothermal system affecting both the Boolgeeda Iron Formation and the Woongarra Rhyolite, and localised to the area of the crater. An absence of young fragmental volcanic material younger than the Woongarra Rhyolite is inconsistent with an explosive diatreme, leading us to a preferred interpretation in terms of an original impact crater about 80m deep excavated by a 10m-diameter projectile and accompanied by hydrothermal activity. A minor north-south asymmetry of the crater, and an abundance of ejecta north, up to about 300m northwest and northeast of the crater, suggest high-angle impact from the south. A youthful age of the structure, probably Late Pleistocene (104-105 years old), is indicated by damming of the drainage of a south-southeast-flowing creek by the southern crater rim.