The TerraSpec is a compact field portable mineral analyser
that uses non-destructive reflectance spectroscopy for mineral identification.
The instrument uses and measures infrared and visible light that is absorbed by
certain covalent and molecular bonds (AlOH, FeOH, MgOH, CO32-, OH,
H2O and NH4) within the lattice of a mineral at specific
wavelengths producing an absorption spectral profile. A variety of minerals can
be detected including clays, micas, carbonates, and hydroxides as well as some
sulfates and silicates. However, not all minerals are detectable, such as quartz
and feldspar. Alteration minerals replacing volcanic rocks have been studied for
several epithermal prospects in the lower Waitekauri Valley (Sovereign, Jubilee, Scotia and Jasper Creek) and a geothermal field in the
Taupo Volcanic Zone. At the Sovereign, Jubilee, Scotia and Jasper Creek
prospects previous XRD investigations determined well developed clay mineral
zonation with illite in the west (Sovereign and Jubilee) grading into
interstratified illite-smectite (Scotia) and
smectite (Jasper Creek) to the east. Although the percent of illite within
interstratified illite-smectite could not be determined and some illite-rich
illite-smectite (>90 per cent illite) and some smectite-rich illite-smectite
(>90 per cent smectite) could be misidentified as illite or smectite
respectively, the TerraSpec survey determined the same zonation of illite to
illite-smectite and smectite. Common chlorite, rare calcite and local kaolinite
were also detected. At the geothermal field, the TerraSpec was primarily used to
determine the occurrence of kaolinite, dickite and alunite, which form in acid
conditions. Widely spaced (every 50 to 100 m) XRD results are in excellent
agreement with closely spaced (every 5 to 10 m) TerraSpec results that further
determined the occurrence of these minerals in previously unrecognised
intervals. It also determined the occurrence of zoned smectite, illite-smectite,
illite, and chlorite alteration. Overall, the TerraSpec is a rapid mineral
identification technique that is very useful in delineating the occurrence and
distribution of certain minerals in both epithermal deposits and geothermal
fields. Although the TerraSpec cannot detect as many minerals as XRD, the
TerraSpec allows data collection at a sampling scale and speed (real time) that
is not practical by XRD.