Mass change during extreme acid-sulphate hydrothermal alteration of a Tertiary latite, Styria, Austria

The Tertiary latitic rock from Gossendorf, in the Gleichenberg Volcanic Area, Styria, Austria, has in places been completely altered to various associations of the secondary minerals opal-C/-CT, alunite, kaolinite, and montmorillonite. The associations occur in a zonal arrangement and correspond to the characteristics of the silicic, advanced argillic and argillic alteration terminology. All elements investigated were mobilized during the hydrothermal acid-sulphate alteration process, except Ti, which was selected as the immobile monitor element to calculate mass changes. Drastic differences in gains and losses of certain elements which refer to the different alteration conditions were found to exist not only between different alteration zones but also within such zones. The gains depend on the newly formed minerals, the losses, however, on the increase of the porosity in the altered rocks. For one altered rock characterizing theoretically the alteration processes in Gossendorf, mass grains were calculated for SO3, H2O, V and Sr, and mass losses for SiO2, Al2O3, Fe2O3, MnO, Mg0, CaO, Na2O, K2O, P2O5, Cr, Co, Ni, Cu, Zn, Rb, Y, Zr, Nb and Ba with respect to an unaltered latite. The net mass balance calculation, i.e. the difference of elemental gains and losses, shows a net mass loss of about 14.60 g. The mass exchange was accompanied by a moderate average decrease in volume of 6.6%. The chemical species added to the rock during alteration, SO3, V2O3, and SrO, probably originated from a volcanic source. H2O could have been of exogenous and/or endogenous origin. The results of the mass change calculations of major, minor and trace elements suggest that the secondary mineral associations and their zonal arrangement at Gossendorf have been formed by a strongly acidic hydrothermal solution rich in SO42−, which underwent chemical variations by reaction with the latite. The calculations indicate that for the formation of the altered rocks and their zonal distribution the elements of the latitic precursor rock, a volcanic source for S, V, and Sr and probably exogenic water were necessarily involved. Based on these results a general chemical characteristic for alteration assemblages in epithermal acid-sulphate districts is presented. Al2O3, Fe2O3, Mg0, CaO, SO3 and H2O% by weight proves to be suitable to characterize silicic, advanced argillic and argillic alteration.