Abstract

In the Taupo Volcanic Zone, extensive zeolite deposits occur in late Quaternary rhyolitic vitric tuffs of the Ngakuru Formation that were deposited in a lake formed by caldera collapse following the c. 290ka Ohakuri ignimbrite eruptions. The zeolite mineralization is associated with silica sinters and hydrothermal eruption breccias that were formed in recently extinct (c. 39 to 1.5ka) geothermal systems. Individual zeolite deposits contain 50-80 wt% of zeolite minerals (mainly mordenite) over a thickness of up to 45m in stratified vitric tuff. Glass shards in the tuffs are replaced by the silica-rich zeolite mordenite, along with cryptocrystalline silica (opal-CT), amorphous silica (opal A), K-feldspar, and smectite. The ion exchange and molecular sieve properties of these zeolites make them useful in a wide range of agricultural and environmental applications. A deposit of mordenite-rich vitric tuff at Twist Road near Ngakuru is being worked by Blue Pacific Minerals and uses include: 1) absorbents for oil/chemical spills and animal wastes, 2) water treatment and filters, 3) conditioners for sports turf and slow-release fertilizer, 4) stock feed additives, and 5) stock rock for farm races. These uses are engendered by the high porosity (50–70%), high odour absorption, and moderate cation exchange capacity (90-120cmol+/kg) of the mordenite-rich tuffs.

Keywords:Zeolite; Mordenite; Rhyolitic vitric tuffs; Lake sediments; Adsorbents for oil spills and animal waste; Water treatment; Turf conditioning; Slow-release fertilizer