CRC LEME
Open File Report 66
ABSTRACT
Spectral properties of muscovite- and paragonite-bearing rocks
and soils from the Panglo Gold Deposit, Ora Banda Region, Western
Australia
Cudahy, T.J., Scott, K.M. and Gabell, A.R.
Gold exploration in the Yilgarn Craton would be greatly assisted
if minerals, diagnostic of gold-related alteration, were developed
at the surface and detectable using spectral remote sensing methods.
One group of minerals that may satisfy this need are the potassic
micas which are often found in weathered surface materials overlying
zones of potassic, gold-related, hydrothermal alteration. These
minerals exhibit characteristic adsorptions in the 0.4 to 2.5 µm
wavelength region, though it was not clear whether the natural abundances
of these minerals are sufficient to produce these adsorptions. An
answer to this problem was the first objective of this study. The
second objective of the study was to determine whether different
species of mica could be discriminated.
These problems were investigated using a suite of surface and subsurface
samples of weathered mafic rock, shale and soil collected from the
Panglo gold deposit, Western Australia. The Panglo deposit was interesting
as the alteration comprised muscovite (K-bearing mica), associated
with mineralisation, and paragonite (Na-bearing mica), in areas
peripheral to mineralisation. Therefore, spectral discrimination
of these two mica minerals could theoretically better define the
limits of gold mineralisation.
The spectral investigation concentrated on the wavelengths of the
1.4 and 2.2 µm adsorptions (hypothetically provide information
on the composition of micas related to the size and composition
of the mica unit-cell), and the depth of the 2.35 µm absorption
(provides information about the abundance of mica). The geometry
of the 1.4 and 2.2 µm adsorptions were not considered as these
are strongly affected by other clays, such as kaolinite and smectite.
Some consideration was given to the associated spectral mineralogy
(nature of other clays, sulphates, water) and possible relationships
to the character of the regolith.
The results showed the wavelengths of the OH-related absorption
at 1.4 µm and the A1-OH 2.2 µm absorption are approximately
10 nm longer than expected for either muscovite or paragonite. There
is also no pattern between these parameters from paragonite-rich
to muscovite-rich. These negative results can be caused by mixing
with other 2.2 µm absorbing, A1-OH minerals, such as kaolinite.
The spectrum of a kaolinite-poor, muscovite-rich shale sample showed
wavelengths expected for muscovite.
The concentration of K2O was used as a quantitative measure for
the abundance of muscovite and was compared with the depth of the
2.35 µm absorption. The results showed significant correlation,
though a threshold muscovite concentration of approximately 3% K2O
is required, below which the 2.35 µm absorption is no longer
distinguishable in the reflectance spectra. This threshold abundance
equates to approximately 20% muscovite (by weight). The only exceptions
to this relationship were exhibited by the spectra of quartz-rich
shales (>60% SiO2) which showed the threshold abundance can be
as little as 1% K2O. This lower threshold is probably related to
the relatively transparent nature of quartz.
The natural K2O abundances of the weathered mafic subcrop were
sufficient to produce recognisable absorption at 2.35 µm.
Therefore, mica-bearing, mafic outcrop or float (if present), can
be detected using spectral techniques. However, the soils did not
show this absorption apparently because these contained <1% K2O
(by weight). Therefore, spectral sensing of soils derived from mica-rich,
mafic rocks is unlikely to show mica-related absorption at 2.35
µm.
A relationship, analogous to that between the K2O content and the
depth of the 2.35 µm absorption, could not be established
for the Na2O and paragonite abundances. This is because the samples
contained halite.
The reflectance data show different spectral properties for the
mafic subcrop, soils and sedimentary subcrop. The mafic subcrop
are characterised by strong absorption at 1.395, 1.411, 2.165, 2.318
and 2.386 µm, producing absorption doublets at 1.4 and 2.2
µm. These properties are typical of well-crystalline kaolinite.
The soils are characterised by weaker adsorptions at these wavelengths,
producing poor absorption doublets, indicating more poorly crystalline
kaolinite. The soil spectra also show small adsorptions at 1.46
and 2.25 µm, possibly caused by water, Fe- or Si-cations in
the kaolinite structure (typical of more poorly crystalline clay).
The shale subcrop show weak to non-existent development of the kaolinite
absorption doublets.
The results from this study have limited application to gold exploration
strategies. The most important result is the requirement for 3%
K2O in a material (contained in potassic mica) before the mica-related
absorption at 2.35 µm becomes apparent in the reflectance
spectra. If soils dominate the surface materials over mica-bearing
gold mineralisation, as is the case at Panglo, then remote sensing
techniques (at these wavelengths) are not practical, unless the
soils are quartz-rich (quartz-rich soils are not likely to be associated
with mafic-ultramafic rocks). The most appropriate application of
these results is in the field where a spectrometer could be used
to log the presence and abundance of mica in subcrop and drill core.
Further work is required to establish whether mica-bearing, mafic
saprock is spectrally characterised by well-crystalline kaolinite
and K-bearing mica.
Last updated: Thursday, January 06, 2000 11:33 AM
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