In
the Ngakuru area zeolite deposits occur in late Quaternary lacustrine vitric
tuffs of the Ngakuru Formation and two of the deposits are being mined. The
Ngakuru Formation overlies the Ohakuri Ignimbrite dated at c. 240 ka, and is
overlain by younger alluvial sands and gravels. Individual zeolite deposits
contain 50 - 80 per cent zeolite over a thickness of up to 45 m in thinly
stratified vitric tuffs and are localised along NE-trending normal faults. Glass
shards in the tuffs are replaced by the silica-rich zeolites mordenite and
clinoptilolite. Amorphous silica (opal A), opal-CT, Kfeldspar and smectite are
also present. The zeolites and other minerals are very fine grained (<10 m),>
with open meshes of crystals that result in high porosities (52 - 70 per cent)
in mordenite-rich tuffs. Zeolitic tuff samples typically have cation exchange
capacities of 80 to 110 meq/100 g and internal surface areas of 25 - 58
m2/g, which combine to give high liquid and odour absorption
capacities. The first zeolite deposit (Mangatete) was found in 1992, and this
and the Twist Road deposit
are worked by quarry benching into the side of hills. The zeolitic tuffs are
dried and crushed to produce a variety of products including adsorbents for
soaking up oil/chemical spills and animal wastes, animal feed supplements, water
treatment, and sports turf and slow release fertiliser. The Ngakuru zeolite
deposits formed at shallow depth (<100 m) along fault zones from reaction of>
glass-rich tuff beds with groundwater-diluted geothermal water during the late
Quaternary, and are as young as 8500 yr BP from 14C dating of plant material in
sinter associated with the Mangatete deposit.