The Taharoa ironsand deposit covers approximately 16 km2 and is located 144
km south of Auckland on the west coast of the North Island, New Zealand. The
heavy mineral-rich sands predominantly contain titanomagnetite, ilmenite and
hematite, along with lesser amounts of amphiboles, pyroxenes, and feldspars.
Throughout the Taharoa mine site 142 samples were collected of which
approximately 25 samples were used in mineralogical analyses of the magnetic,
heavy non-magnetic and light non-magnetic fractions. Mineralogical and
petrographic analyses confirmed the high abundance of homogeneous magnetite and
magnetite containing ilmenite exsolution lamellae which have oxidised to
hematite.
A limited number of sand samples were collected from Taranaki beaches.
Petrography reveals exsolution textures of magnetite and ilmenite (now
hematite), and homogenous magnetite grains similar to those observed at Taharoa.
Petrographic analyses from previous research projects were also closely
investigated in order to make mineralogical comparisons from along the west
coast of the North Island and with other potential source areas.
The origin of
detrital magnetite grains along the west coast of the North Island is generally
presumed to be the eroded volcanic cones of the Egmont volcanoes and active and
dormant volcanoes of the central North Island. Electron microprobe analyses of
magnetite grains from Taranaki beach samples reveal TiO2 values of between 1.5 and 120
per cent. MgO and A12O3
contents also vary widely from 0.75 to 6.5 per cent. This large variability may
reflect different volcanic eruptions, the presence of exsolution textures or
alternative source areas for these magnetite grains. Geochemical results from
the Taharoa mine site correlate with the Taranaki beach samples suggesting that
the Egmont volcanoes are certainly one source area. Variability in XRF results,
especially Fe and Ti contents (9.04 - 59.22 and 0.80 - 4.64 respectively), from
the magnetic and heavy non-magnetic fractions likely reflect the detrital nature
of this deposit.