THE APPLICATION OF IMAGING SPECTROMETRY DATA TO MAPPING ALTERATION ZONES ASSOCIATED WITH GOLD MINERALIZATION IN SOUTHERN SPAIN

The gold mineralization at the Rodalquilar mine in southern Spain is o f the acid-sulphate type. We have assessed the ability of AVIRIS imagi ng spectrometer data to detect the hydrothermal alteration mineralogy mapped using traditional ground-based techniques at Rodalquilar. Four methods of retrieving apparent surface reflectance have been evaluated , the empirical line method and three methods of radiative transfer mo delling. This study indicates that radiative transfer modelling, using only atmospheric information derived from the imaging spectrometry da ta, can satisfactorily correct the atmospheric effects involved in ret rieving apparent surface reflectance. The alteration at Rodalquilar is characterized by propylitic (vermiculite), illitic, kaolinitic (pyrop hyllitic), alunitic and silicic zones with an increasing degree of alt eration towards the formative hydrothermal cells in whose fossil cores the gold mineralization is found. However, several factors act agains t the usefulness of the AVIRIS data for mapping these mineral zones: t he mine site is heavily disturbed with exposed workings, often of high albedo altered rocks, whilst unworked areas have partial iron-rich so il and dry vegetation cover; the alteration itself is highly discontin uous at the scale of 10m and the shortwave infrared (SWIR) data where much of the diagnostic information for these hydrothermal minerals lie s is very noisy. Nevertheless it has proved possible to map successful ly, at the individual pixel level, the zones of gold-bearing alunitic alteration using the joint presence of alunite absorption features at 1480 and 1760nm. The data from the 2000-2400nm spectral range allow th e areas with general absorption features centred around 2200nm to be m apped but zonation based on individual minerals identification is not possible.