Phanerozoic history of the Pilbara region: implications for iron mineralisation

The Hamersley Province of Western Australia hosts enormous resources of high-grade iron mineralisation in two deposit types that are rarely found elsewhere in the world. These deposits are the banded iron formation-hosted, supergene, martite-goethite deposits and the fluviatile channel-iron deposits. The West Australian Craton is unusual in that the current land surface is close to the peneplaned land surface that was exhumed at the end of the late Carboniferous-early Permian glaciation. Far from being stable since then, there is good evidence that the craton was largely buried under Triassic-Jurassic sediments before being re-exhumed in the Early Cretaceous. A striking feature of the cratonic geology is the existence of a network of large paleovalleys. The majority are Mesozoic in age, but some clearly mark the position of Permian drainages. Inset valleys were incised within these drainages in the Paleocene to Eocene. While the global climate has cooled overall since the Late Cretaceous, this cooling trend has been punctuated by three climate optima, and these global trends are superimposed on the gradual northward migration of Australia. This resulted in periods of intense lateritic weathering in the Late Cretaceous and mid-Miocene and a shift from a temperate and seasonally wet climate to an increasingly arid climate from the mid-Miocene. The martite-goethite deposits began to form in the Eocene and may have overlapped with the initial accumulation of fluviatile sediments that host the Oligocene to Miocene channel-iron deposits. The detrital component of these latter deposits formed during the period of Late Cretaceous lateritic weathering, and the channel-iron deposits were fully cemented by the end of the Miocene. The key to the unusual Phanerozoic iron endowment of the Hamersley Province lies in the coincidence, in time and space, of an iron-rich source, anomalous ongoing uplift and a seasonally humid climate.