Age and palaeoenvironmental constraints on the genesis of the Yandi channel iron deposits, Marillana Formation Pilbara, northwestern Australia

Fossil pollen and spores preserved in organic-rich claystones near the base of the palaeochannel hosting the Tertiary Yandi channel iron deposits (CID) provide an in situ constraint on the age of this world-class orebody. This assemblage also aids understanding of the palaeoenvironment leading to the deposition and preservation of the host Marillana Formation. The provisional Early Oligocene age of claystones links the genesis of the Yandi CID to the profound disruption of global climates and oceanography at the Eocene-Oligocene transition (Terminal Eocene Event) and provides circumstantial evidence that ore formation was linked to the development of a warm south-flowing proto-Leeuwin Current along the Pilbara coast. It is likely that rainfall increased but remained strongly seasonal (monsoonal?) as global climates wormed during the Late Oligocene - Early Miocene. The same fossil evidence indicates that wood fragments, the iron-oxide-replaced remains of which are a significant component of cemented goethitic gravel making up CID, came from Casuarinaceae- and Myrtaceae-dominated sclerophyll communities lining the channel banks. Increasingly dry conditions since the late Early Miocene Climatic Optimum are likely to have contributed to the preservation of CID at Yandi.