In
Northland, Coromandel, Taupo and Canterbury, weathering and hydrothermal
alteration of Cenozoic volcanic rocks has resulted in the formation of various
types of clay deposits. Subtropical weathering of rhyolite in the Matauri Bay area (Matauri and Mahimahi deposits)
in Northland has formed high quality halloysite deposits. The Matauri Bay halloysite is mined and processed to
produce reputedly the world's whitest clay' that commands a high price in niche
ceramic markets. In the Coromandel, some argillic alteration zones contain
kaolinite clays (at Hahei) and advanced argillic alteration zones contain
kaolinite-gibbsite-pyrophyllite assemblages (at Pumpkin Hill and Lookout Rocks).
In addition to known halloysite and kaolinite clay deposits, there is also
economic potential for smectite clays produced by hydrothermal alteration of
volcanic rocks. Bentonite (mainly ferriferous-beidellite) is mined from
weathered basaltic ash in Miocene lake beds at Coalgate in Canterbury.

We
have compiled interim mineral deposit profiles for bentonite in basaltic ash,
halloysite in rhyolite, kaolinite in altered rhyolitic rocks,
kaolinite-gibbsite-pyrophyllite clays in advanced argillic alteration zones and
for smectite clays in argillic alteration zones. These deposit profiles include
geological and mineralogical attributes, which are linked to the underlying
volcanic, hydrothermal and weathering processes that have produced the clay
minerals from their source volcanic rocks. Further work on refining the deposit
profiles includes characterising the composition of selected deposits to
determine spatial trends in mineralogy and geochemistry.