Aeromagnetic And Aeroradiometric Response To Hydrothermal Alteration

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• 作者：M.-L. Airo
• 关键词：airborne methods ; Finland ; gamma ; ray methods ; geophysical surveys ; gold ores ; hydrothermal alteration ; magnetic methods ; massive sulphide deposits ; mineral exploration ; petrophysics
• 刊名：Surveys in Geophysics
• 出版年：2002
• 出版时间：2002
• 年：2002
• 卷：23
• 期：4
• 页码：273-302
• 全文大小：3.58 MB
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Geophysics and Geodesy
  Geosciences
  Astronomy
  
• 出版者：Springer Netherlands
• ISSN：1573-0956

文摘

In Precambrian terrains all regional and most localintensive magnetic anomalies areproduced by magnetite. Monoclinic pyrrhotite isresponsible for some local, but oftenintensive, magnetic anomaly patterns. Both magnetiteand pyrrhotite are affected byhydrothermal alteration processes in various ways,resulting in changes either inabundance or in grain fabric. These changes arerecorded in the magnetic properties ofthe altered rock units and reflected in theiraeromagnetic signatures. Hydrothermalalteration in deformed bedrock zones is commonlycontrolled by structural or tectonicfeatures. Regional high-resolution aerogeophysicalsurveys can be utilized, in bothregional and detailed investigations, to map theoverall geological and tectonic settingor to estimate local changes in magnetic mineralogyand the relative abundance ofradionuclides.Magnetite is most commonly destroyed in alterationprocesses, such as biotitization,carbonation, sulfidization and silicification. Theprogressive destruction of magnetitebegins at grain margins and results first in broken and cracked grain texture and smallergrain size, then progresses to total disappearanceof magnetite. Alteration in magnetite-bearing rock units may be recognized by decreasedmagnetic intensity and by thebroken, disrupted magnetic pattern. The abundance ofmonoclinic pyrrhotite isenhanced by reducing hydrothermal fluids, and typicalcrystal anisotropy is developeddue to tectonic stress.