Exploration and assessment of kaolin clays formed from acid volcanic rocks on the Coromandel Peninsula, North Island, New Zealand

Acid volcanic rocks of Pliocene age on the Coromandel Peninsula east of Auckland, New Zealand have been argillized by subtropical weathering and hydrothermal activity. The volcanic rocks range from unconsolidated ash beds through welded ash-flows to rhyolite flows and domes. An exploration program located kaolinite and halloysitic clays of potential commercial significance. Three distinct clay mineral assemblages were recognized. In situ weathering produced halloysite as the dominant clay mineral and was best developed on the lower flanks of the rhyolite domes. Hydrothermal alteration was recognized by surficial resistant outcrops of siliceous sinter beneath which kaolinite and alunite were identified. In the valleys between the domes, Quaternary stream terraces contain well-sorted lensoid deposits of fine plastic clays which are composed of a mixture of halloysite, kaolinite and minor gibbsite.