Aeromagnetic And Aeroradiometric Response To Hydrothermal Alteration

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In Precambrian terrains all regional and most local intensive magnetic anomalies are produced by magnetite. Monoclinic pyrrhotite is responsible for some local, but often intensive, magnetic anomaly patterns. Both magnetite and pyrrhotite are affected by hydrothermal alteration processes in various ways, resulting in changes either in abundance or in grain fabric. These changes are recorded in the magnetic properties of the altered rock units and reflected in their aeromagnetic signatures. Hydrothermal alteration in deformed bedrock zones is commonly controlled by structural or tectonic features. Regional high-resolution aerogeophysical surveys can be utilized, in both re- gional and detailed investigations, to map the overall geological and tectonic setting or to estimate local changes in magnetic mineralogy and the relative abundance of radionuclides. Magnetite is most commonly destroyed in alteration processes, such as biotitization, carbonation, sulfidization and silicification. The progressive destruction of magnetite begins at grain margins and results first in broken and cracked grain texture and smaller grain size, then progresses to total disap- pearance of magnetite. Alteration in magnetite- bearing rock units may be recognized by decreased magnetic intensity and by the broken, disrupted magnetic pattern. The abundance of monoclinic pyrrhotite is enhanced by reducing hydrothermal fluids, and typical crystal anisotropy is developed due to tectonic stress. The relative contents of radioelements are changed in the same hydrothermal processes and partly for the same reasons as the ferrimagnetic minerals. Potassic alteration often results in elevated K radiation particularly for mafic rocks, and then anomalous K/Th ratios along local shear or fracture zones may be indicative of gold-bearing mineralization. On the other hand, high U/Th ratios within metasedimentary units may point out prospects for sulphidization. Although variation of U/Th ra- tios largely reflects the environmental conditions during primary diagenesis or a later deformational phase, mainly the decrease in Th radiation close to sulphide mineralization seems to be responsible for the elevated U/Th ratios.