Numerical modeling unravels differences in greisenization processes

The Zinnwald/Cinovec Sn-W-Li greisen deposit on the border between Germany and Czech Republic in the eastern part of Krušné Hory/Erzgebirge represents a fluorine-rich hydrothermal alteration of a granite-rhyolite association. We investigated the effects of fluid-rock interaction on distal rhyolites of the deposit, using petrological and mineralogical data to constrain the process of greisenization in detail. The samples were selected from the contact between granite and rhyolite. Three distinct zones of high- and low-degree topaz-greisenization and albitization developed with different textures, mineral assemblages and mineral compositions. Beyond the albitization zone, a continuous transition to the least altered rhyolite was observed. In the greisen part, the predominant minerals are quartz (~80 vol%) and topaz (~10 vol%) with minor mica (~5 vol%). We employed a reactive transport model based on mass conservation and local equilibrium to unravel the detailed process of greisenization. We integrated solution models and endmember thermodynamic data for topaz in recent thermodynamic datasets. The model accounts for fluid flow, porosity and density evolution. The model is used to emulated the sequence of observed petrological zones as obtained with automated mineralogy to constrain the original fluid chemistry and reconstruct elements redistribution during greisenization. By comparing fluid-rock interaction models producing topaz greisen and topaz-free mica greisen, we quantify the F-content necessary to form the greisen at Zinnwald/Cínovec. The comparison implies that F content has a great influence on the greisenization types, which may be related to different metallogenic processes and give insights into W-Sn ore deposits.