Geology, geochemistry, and mineralisation of a spilite-keratophyre association in Cretaceous flysch, East Cape area, New Zealand

Abstract

Geological, geochemical, and mineralogical studies were carried out during a programme of base metal mineral exploration on a band of volcanic rocks intercalated within a flysch sequence of Cretaceous age (Mokoiwi Formation). The volcanics occur in the Tapuaeroa Valley—Te Kumi area, East Cape, New Zealand. These studies led to the conclusion that the characteristics of the volcanic band conform to those of a spilite-keratophyre association. The volcanic rocks, which are represented by dolerite, basalt, and related pyroclastic products are here informally named Rip Volcanics. The pyroclastics have a typical bimodal composition which is exemplified by argillaceous vitric tuff beds and a highly sodic tuff (keratophyre). The Rip Volcanics form a tectonically disrupted lithosome emplaced in its present position by the decolle-ment of the Mokoiwi Formation rock mass to which the volcanics belong, from a provenance somewhere in the north or north-east. Small and economically insignificant pods of sulphide mineralisation consisting of pyrite and chalcopyrite occur at the western end of the volcanic band. This mineralisation is interpreted as being the tectonically broken remnants of a once continuous “distal” volcanogenic massive sulphide deposit.