Spectral enhancement of selected pixels in Thematic Mapper images of the Guanajuato district (Mexico) to identify hydrothermally altered rocks

Abstract

The use of satellite Thematic Mapper (TM) images in mineral exploration has been very successful in pointing out the presence of key minerals. Here we present the results of the spectral enhancement of a TM image to identify oxides and hydroxyl-bearing minerals in the Guanajuato district, using the two methods that yielded the best results in this area: band subtraction and principal component analysis on an image that included only selected pixels and excluded areas that did not present any interest for mineral exploration. Different types of vegetation cover the study area: crops, bushes and forest. Previous application of conventional methods was used to identify areas where the spectral signatures suggested the presence of hydrothermal alteration, and further detailed spectral enhancement in selected pixels allowed the identification of alteration minerals in areas covered with different vegetation types that do not have a similar spectral response. A field check performed using a PIMA (Portable Infrared Mineral Analyser) spectrometer yielded a good correlation of the marked areas with the presence of hydrothermally altered rock. New areas with argillic alteration were discovered in the Sierra de Guanajuato and the San Anton areas. Also, the main structures were determined by applying several filters to the first principal component. The occurrence of hydrothermal alteration and the presence of main faults were used to define areas favourable for further detailed exploration programmes.

Acknowledgements

Funding for this research was received from CONACyT project 3140PT. The Landsat TM image was provided by the Instituto de Geografia, UNAM. Processing of the spectral data obtained with the PIMA field spectrometer was performed with the Specwin program of Spectral International Inc.