Spectral discrimination of contact metamorphic zones and its potential for mineral exploration, province of Salamanca, Spain

Abstract

In the mineralized contact aureole of Los Santos, Spain, contact metamorphic grade of rocks and the mineralogical composition of overlying soils correlate systematically. From the exocontact towards the granite intrusion the proportions of kaolinite, illite and free iron in soils increase, while the proportions of chlorite and smectite decrease. Laboratory reflectance spectra of soil samples demonstrate that with the increase of the proportion of kaolinite, illite, and free iron and the decrease of chlorite, ratios of Band 2 / Band 3, Band 2 / Band 5, and Band 2 / Band 7 decrease while ratios of Band 3 / Band 4, Band 4 / Band 7, and Band 5 / Band 7 increase. Soil reflectance is significantly affected if more than 1.5 % organic carbon is present. A procedure is presented that enables mapping of soils overlying contact metamorphic rocks in Thematic Mapper (TM) imagery, using these variations. In the TM image, pixels that correspond with kaolinitic soils are identified by elimination of other pixels during a process of stepwise masking various band ratio images. The first step was to eliminate the effect of vegetation and organic material, by masking high values of the ratio of ((TM5 / TM7) / (TM3 /TM4)]. The second step was to eliminate chloritic soils by masking the ratio of TM2 / TM7. The third step was to isolate the most kaolinitic and limonitic soils using the ratio of TM3 / TM4. Finally, a maximum likelihood classification was performed on original band ratio images, using the result of the masking as training set, in order to optimize the classification results.