Bowen, F.E. 1966 The Parahaki Volcanic Group and its associated clays. : . Report NZGS 6 72 p.
Abstract: The Parahaki Volcanic Group, consisting of the holocrystalline Pukekaroro Dacites, the vitrophyric Maungarei Dacites, and the Putahi Rhyolites, are locally altered to poorly plastic clays that in three localities have been identified as fully hydrated halloysite. The clays form a mixed zone that contains incompletely altered material, and the prime zone that contains mainly white clays with but little incompletely altered material. The clays of the superimposed superficial zone have been degraded by the addition of extraneous material and are frequently iron-stained. Detailed prospecting, involving drilling to depths exceeding the 40 ft maximum known thickness of the superficial zone, has been carried out on only two of the areas known to contain clay and have established reserves of the order of 7 million tons and 10 million tons. Total reserves greatly exceeding these figures may be confidentially inferred. The clays are considered to be the result of hydrothermal activity that closely followed the extrusion of the Miocene to early Pleistocene volcanics.
