The advent of Advanced Spaceborne Thermal Emission and Reflection (ASTER) has enormously advanced the use of remote sensing in mineralogical and lithological studies, making possible the identification of specific alteration mineral assemblages. Movement of various fluids (gases, brine, water and ore metals in solution) may change rock compositions by adding or removing chemical components to or from them. Alteration zones, indicating pathways of hydrothermal fluids through the crust, may represent useful guides for the exploration of many types of ore deposits.
Altered minerals and rock units in Zara gold prospect in Eritria, which is part of the Neoproterozoic Arabian Nubian Shield (ANS), were mapped by employing dark-object-corrected (haze-corrected) ASTER spectral band ratios, ratio codes, and spectradiometric analysis. Bands 5 and 6 detected the 2.17μm absorption feature of alunite and the 2.20μm absorption feature of Al-OH in muscovite and clay minerals respectively. Epidote, chlorite and calcite were spectrally identified by band 8, centered at 2.32μm. Band 14 was also a characterstic spectral signiture for calcite, in addition to band 8. Band 7 and band 3 were also significant for identification of jarosite, a ferric sulfate, but band 3 absorption might be more sensitive to hematite, a ferric oxide that is widespread in the study area and may indirectly indicate the presence of pyrite, which is usually oxidized to jarosite and ferric oxides. Sepiolite, magnetite, illite, buddingtonite and montmorillonite were also mapped using ASTER spectral ratios. Moreover, the TIR ratio of Bands 13/12 identified quartz (silica). The spectra of the collected rock samples also showed results similar to the above ratios results.
The localization of these hydrothermal alteration minerals into zones, plus the presence of the mapped intrusions and tectonic structures, may indicate the likely presence of disseminated epithermal gold deposits, which may possibly be the first to be identified in the country, intrusion-related type and porphyry type ore deposits.