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IDENTIFICATION OF HYDROTHERMAL ALTERATION ZONES IN GRANITIC AREAS USING MAGNETIC SUSCEPTIBILITY. AN EXAMPLE IN NW PORTUGAL
Abstract
The aim of this study is to contribute to the detection and identification of hydrothermal alteration zones in Variscan granites using magnetic susceptibility. The studied area is located in the North-western of Portugal and is characterised by the occurrence of Au mineralizations in quartz veins hosted in Variscan granites. Three different granite facies were studied, using two methodologies: (i) measurements, in situ, of magnetic susceptibility using a portable susceptibility meter (TerraPlus KT-10), allowing the characterization of the magnetic behaviour and; (ii) petrographic studies to observe mineralogical features which characterize hydrothermal alterations. The in situ magnetic susceptibility measurements allowed the identification of a NE-SW zoning sub-parallel to quartz veins hosted in the granites. It was possible to identify distinct zones characterised by different values of magnetic susceptibility corresponding to three different degrees of hydrothermal alteration: 30x10-6 to 80x10-6 SI, 80x10-6 to 130x10-6 SI and >130x10-6 SI. This susceptibility zoning is mainly related with the degree of hydrothermal alteration characterised by the occurrence of muscovitization, associated with quartz veins, which implies differences in the magnetic signal. In the petrographic studies it was possible to distinguish two distinct generations of muscovite, a first one represented by well-developed crystals (>500 ?m) and a second generation, more abundant, characterised by agglomerates of small crystals (<100 ?m). Only the first generation of muscovite exhibits some deformation patterns, represented by folded or kinked crystals. The magnetic susceptibility values were similar in all unaltered granites indicating a paramagnetic behaviour induced by the presence of biotite and ilmenite typical of ilmenite-type granites. This study showed that the magnetic susceptibility is a fast and inexpensive methodology that can be also useful to identify different degrees of hydrothermal alteration associated with mineralizations.
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