Visual and geochemical characterisation of Late Cretaceous-Eocene cherts from eastern New Zealand: a preliminary study

Abstract

Sedimentary chert from the eastern North Island and Marlborough region of the South Island, New Zealand (collectively referred to here as the Eastern Chert Province, ECP), was utilised by indigenous Maori communities from the 14^th to 18^th century. The chert is associated with Late Cretaceous-Paleocene siliceous shale of the Whangai Formation (Whangai chert) in the North Island, and the Late Cretaceous-Eocene Mead Hill Formation and overlying Amuri Limestone in Marlborough (Kaikoura chert). There is also an isolated occurrence at Tora, in south-eastern Wairarapa (Tora chert). Visual/petrographic attributes of these chert types are very similar, though some samples from Marlborough are calcareous and contain dolomite rhombs. Chemical analysis of 59 geological samples, by wavelength-dispersive XRF and portable XRF (pXRF), shows that the Whangai and Kaikoura cherts can be largely differentiated on the basis of Zr, Sr and Rb concentrations, but that the Tora chert is chemically indistinguishable from the Whangai chert. A separate group is recognised within the Kaikoura chert, characterised by higher Sr, Ca and Ba values, but there seems to be limited potential for the identification of specific sources within the ECP.