ABSTRACT
The crystal chemical properties of muscovite 2M1 in hydrothermally altered volcanic rocks of the alteration halo enveloping the Waterloo VHMS deposit, Australia, is distinctly different to that of muscovite 2M1 contained in background altered wall rocks. The hydrothermal muscovite has cell dimensions and compositions close to that of end-member muscovite whereas muscovite in background altered samples is characterised by elevated a and b cell dimensions and a substantial Fm [= (Fe+Mg)] substitution. Hydrothermal muscovite can also be distinguished from that contained in background altered rocks on the basis of the higher Na* [= 100 Na/(Na+K)] number and the typically lower 1/2csinβ parameter. The observed variations in the composition are interpreted to be primarily a result of chemical inheritance from the precursor materials. It is suggested that the crystal chemical properties of muscovite can be used in base metal exploration to recognise hydrothermal alteration processes in strongly foliated and fine grained volcanic rocks.
