The effect of weathering on reflectance spectra of hydrothermal white micas and chlorites: Implications for alteration mapping

Abstract

Reflectance spectra offresh and weathered rock were compared in order to determine the effect of weathering on hydrothermal white mica and chlorite. A total of 46 rock samples from the Panorama Volcanic Massive Sulphide (VMS) District in Australia were analyzed.The study area lies in an arid area and the degree of weathering is low. The reflectance spectra from fresh and weathered rocks were analysed for mineralogical composition and spectral characteristics.

The results show that the spectral characteristics of the hydrothermally altered rocks were modified during weathering. Chlorite disappears during weathering while white micas persist. Newly formed minerals are halloysite and Fe-(hydr)oxides. The exact wavelength position of the main absorption feature of white micas near 2200 nm, which is a measure of their chemical composition, shifts due to weathering. This wavelength shift depends on the wavelength position in the unweathered rock. Three different wavelength ranges, reflecting different white mica species in unweathered rock, have been identified. In each range, weathering processes have a different effect on the absorption wavelength. During weathering, the absorption wavelength of Al-rich white mica (having an absorption wavelength_<2198 nm) shifts toward a longer wavelength of 2200–2202 nm. That of Al-poor white mica (having an absorption wavelength_>2208 nm) shifts toward a shorter wavelength of 2203–2205nm. This can be explained as a change in composition of the micas or a changing mineralogy, i.e. an increase in halloysite. The absorption wavelength of white mica of intermediate Al-content (having an absorption wavelength between 2198 and 2208 nm) does not change during weathering, indicating that this white mica is unaffected by weathering.

We conclude that white micas are potentially useful indicators for alteration mapping using remote sensing methods since it survives low degrees of weathering. However care has to be taken when interpreting the wavelength position of its main absorption feature since weathering modifies it. Chlorite is potentially less useful for mapping alteration since it disappears during weathering.