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Open File Report 66

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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