Mineral evolution and element mobility during episyenitization (dequartzification) and albitization in the postkinematic Bohus granite, southwest Sweden

Abstract

The petrography and geochemistry of two subsurface occurrences of pervasively albitized and episyenitized (i.e. dequartzified) granite in the central part of the postkinematic Sveconorwegian Bohus granite, in southwestern Sweden, were studied. The altered granite is whitened and consists of an albite-microcline framework with a very fine-grained interstitial assemblage of illite + hematite + ankerite ± anatase. Textural relations indicate that the initial dissolution of magmatic quartz, accompanied by albitization of plagioclase, left a vuggy reservoir rock, which facilitated fluid penetration and thereby further alteration. The subsequent stage is characterized by infilling of vugs by the very fine-grained authigenic assemblage and by concomitant alteration of igneous minerals such as magnetite, ilmenite and biotite. Deuteric alteration products of magmatic minerals are affected by these two alteration stages, implying that episyenitization occurred widely after emplacement and cooling of the host granite. Mass balance calculations, with reference to adjacent unaffected granite, evince mainly Si loss and Na gain, reflecting the observed mineralogical changes. The volume decrease is estimated to be ~ 9%, owing to compaction accompanying the dequartzification and a slight porosity increase. The calculations revealed a slight enrichment in HREEs ± Y, essentially hosted by xenotime and a thorite-xenotime solid solution, both exsolved during recrystallization of monazite. Oxidation of ferrous iron and partial loss of chalcophile elements reflect oxidizing conditions. The alteration episode is Permo-Carboniferous and is possibly associated with circulation of oxidizing fluids through the Bohus granite.