Petrophysical and geochemical constraints on alteration processes in granites

Abstract

The hydrothermal alteration of granites has large influence on their petrophysical properties. To reveal the impact of alteration on magnetic and porosity properties of granites we have conducted a complex study of effects of two largely independent alteration processes, related to chemically different fluids, in granites of the Vysoký Kamen stock (the Krudum granite body, Czech Republic). It includes the whole-rock geochemical, magnetic and pore-space characterization. The alkali feldspathization resulted in decomposition of Li-mica, quartz removal, depletion in mafic cations and growth of new alkali feldspars (albite, K-feldspar), decreasing the overall magnetic susceptibility and disrupting the pore space by its discontinuation. The preservation of the orientation of the principal susceptibility axes is likely related to insignificant influence of the feldspathization process on the paramagnetic and diamagnetic phases orientation acquired during the magma emplacement. The greisenization, on the other hand had considerably more significant effects on microstructure and physical properties of the granite. The microstructure was modified by the growth of large amounts of new phases (Li-mica, quartz and topaz). This changed the mineral density of the rock, the porosity, size and character of pores to larger, flatter and probably more connected. This led also to the complete reworking of the original anisotropy of magnetic susceptibility during the greisenization.