Hydrothermal Processes in the Archean - New Insights from Imaging Spectroscopy

Abstract

The aim of this research was to gain new insights in fossil hydrothermal systems using airborne imaging spectroscopy. Fossil submarine hydrothermal systems in Archean greenstone belts and other geologic terranes are important because of their relationship with volcanic massive sulfide (VMS) mineral deposits and their association with environments that are favorable for early forms of life. Interpretation and reconstruction of these systems is difficult because of their geologic complexity. Airborne imaging spectroscopy provides information about the presence, abundance, and composition of near-infrared active minerals at continues spatial coverage and high spatial resolution, and could therefore be used to obtain new geologic information of the hydrothermal systems. It was applied to the Panorama VMS-district in the Soanesville greenstone belt, Western Australia. Results from the analyses of 189 hand specimen showed that the wavelength position of the main absorption feature of white micas, a proxy for their Al content, varied between 2195 nm and 2225 nm. These wavelength variations and the relative abundance of white micas were used to reconstruct fossil fluid pathways from low-temperature recharge to high-temperature discharge zones. Results also showed that the absorption-wavelength variations of white micas could be mapped from airborne imaging spectroscopy using a stochastic method where the presence of white mica minerals and their absorption wavelengths in field measurements were predicted from band ratios. Analysis of the spatial patterns in segmented images, covering 52 km2, of white mica probability and their absorption wavelengths and their comparison with field data resulted in the identification of a regional-scale K alteration event, previously unmapped fluid pathways, and differences in hydrothermal regime between the northern and southern parts of the test area. A new methodology for the study of hydrothermal processes in the rock record using airborne imaging spectroscopy was synthesized. This methodology involves three steps: (1) creation of maps of surface mineralogy from airborne data, (2) determination of the geologic significance of the near-infrared detectable mineralogy, and (3) evaluation of spectral patterns present in the airborne imagery. The research showed that airborne imaging spectroscopy is a research tool that can be used for discovering new information about fossil hydrothermal processes.