The
portable infrared mineral analyser (PIMA) was trialled to determine the extent
and intensity of any hydrothermal alteration halo associated with gold
mineralisation in the Hyde-Macraes Shear Zone (HMSZ) of the Otago
Schist.

Twelve diamond
drillholes through the HMSZ were analysed with the PIMA at a sample spacing of 1
m. The drillholes were selected on two traverses along the line of strike and
down-dip of the mineralisation in and around Panel 2 of the Frasers underground
development.

The
study showed that the Hangingwall shear of the HMSZ has a more distinctive
infrared signature in intersections with high grades. The Hangingwall shear in
these drill holes is defined by a shift in the wavelength of the Al-OH
absorption feature from ~2220 nm to <2210 nm. this shift is attributed to a>
change in the mineralogy from phengite to muscovite. Overprinting this trend in
some of the drillholes is the presence of the 2210 nm Al-OH absorption feature,
ascribed to kaolinite. Kaolinite also tends to be present in numerous other
regions of the schist above the Hangingwall shear associated with steeply
dipping faults and foliation parallel joints. In drill holes with low grades the
Hangingwall shear does not exhibit any obvious infrared
signature.

At
the scale of study, the PIMA does not provide evidence for an alteration halo
associated with hydrothermal mineralisation above the Hangingwall shear.
Possible tectonic movement may have displaced any alteration halo that once
existed, away from the zone of mineralisation. For this reason, PIMA apparently
cannot be used for predicting the location of the Hangingwall shear during
exploration drilling. However, the study did show that the PIMA could define the
Hangingwall shear when the rocks were sufficiently altered and
mineralised.