Tertiary volcanic rocks
make up a significant proportion of the surficial cover rocks of New
Zealand, especially in the Taupo Volcanic Zone.
Industrial minerals that are associated with or derived from the volcanic rocks
include: amorphous silica, allophane, bentonite, diatomite, ignimbrite building
stone, illite clays, halloysite clay,
obsidian, perlite,
pumice, sulphur, titanomagnetite ironsand and zeolite. Of these: bentonite,
ignimbrite building stone, perlite, pumice and titanomagnetite ironsand have
been mined and processed for 20 to 30 years, but many of the others require
further characterisation of the resource and/or development of markets.

We
have compiled interim mineral deposit models for halloysite clays in rhyolite
and for zeolites in altered vitric tuffs. These models include geological and
mineralogical attributes, which are linked to the underlying volcanic and
geothermal processes that have produced the minerals and their source volcanic
rocks. Halloysite clay deposits at Matauri Bay (Northland) have formed from
hydrothermal alteration and weathering of rhyolite. Clinoptilolite + mordenite
zeolite deposits in the Ngakuru area (Rotorua district) have formed from
hydrothermal alteration of volcanic glass in lacustrine sediments and
ignimbrite. Further refinement and application of these models will result in a
better understanding of the mineralogy, localisation and origin of the deposits,
with consequent applications in exploration and resource
assessments.