Short wavelength infrared (SWIR) spectral analysis of hydrothermal alteration zones associated with base metal sulfide deposits at Rosebery and Western Tharsis, Tasmania, and Highway-Reward, Queensland

Spectral analysis of rock samples by short wavelength infrared (SWIR) analy sis, using the portable infrared mineral analyzer (PIMA) spectrometer, is a relatively new field method with applications in mineral exploration. The technique allows rapid field-based identification of hydrothermal alteratio n minerals and delineation of alteration zones. Variations in mineral compo sition, crystallinity, and relative abundance may also be determined. Phyll osilicates, including white mica and chlorite, are particularly amenable to short wavelength infrared analysis. These are prominent alteration mineral s in the felsic volcanic rocks in the Mount Read Volcanics, Tasmania, and t he Mount Windsor subprovince, North Queensland. They have regional distribu tions as diagenetic and metamorphic products and exist locally in alteratio n zones associated with volcanic-hosted sulfide deposits. A geochemical and spectral investigation of white micas in the least altere d rocks in the Mount Read Volcanics has shown considerable background compo sitional variations. The white micas range from phengite to moderately sodi c muscovite and SWIR spectral measurements provide reliable estimates of th eir compositions. Chlorites also have a broad compositional range in these rocks. However, SWIR spectral analysis has proven to be unreliable in deter mining chlorite composition in samples where chlorite exists in low proport ions relative to white mica or in mixed assemblages with epidote. White micas in footwall alteration zones of the Rosebery stratiform Zn + Pb deposit (Tasmania) are variably phengitic, similar to those in regional ba ckground volcanics. A unit of relatively unaltered volcaniclastic sandstone in the Rosebery hanging-wall sequence contains anomalous sodic muscovite i n proximity to ore lenses. White micas associated with the disseminated to massive pyritic Cu + Au deposits at Western Tharsis (Tasmania) and Highway- Reward (North Queensland) range in composition from phengite to slightly so dic muscovite. In these symmetrically zoned alteration systems, white micas grade in composition from phengite, in peripheral zones, to slightly sodic muscovite, in proximal zones near ore. In all three cases, the white mica compositional variation is a vector to ore that is effectively measured by SWIR spectrometry, with particular application in prospect-scale exploratio n. Chlorite compositions at Rosebery are not strictly spatially related to ore but tend to be most magnesian in zones of high chlorite abundance, in the outer alteration zone at Western Tharsis, there is a faint proximal trend t o iron-rich chlorites, However, it does not represent a well-defined vector to ore in either case. Attempts at quantitative estimation of the relative abundance of white mica and chlorite in mixed samples, by spectral analysis, have produced results that are imprecise in comparison to geochemically derived estimates. The d ata scatter relates to the low proportions of chlorite in the majority of t he samples analyzed, resulting in weak chlorite absorptions. Better results may be obtainable, in specific alteration or lithostratigraphic domains, b y use of customized spectral unmixing software. Thus investigation has shown that PIMA-based SWIR spectrometry is a potent field method of mapping compositional variations in white micas that repres ent empirical vectors to at least some types of volcanic-hosted sulfide dep osits. It has also highlighted its effectiveness in field identification of megascopically difficult or indeterminate alteration minerals, such as pyr ophyllite, zunyite, and topaz, which may have genetic implications and expl oration significance.