Chemical Changes in Quartz and Micas During Greisenization: Examples from European Variscan Plutons

Metals like Li, Sn, W, Nb and Ta accumulate mostly during the magmatic–hydrothermal transition and subsequent hydrothermal alteration of highly fractionated granites, especially greisenization. Evaluation of about 450 bulk-rock analyses, 1500 LA-ICP-MS analyses of quartz and 1600 EPMA and LA-ICP-MS analyses of mica from parental granites and related greisens and quartz–mica veins from four typical areas of European Variscan granite plutons with greisen mineralization (Beauvoir, France; Panasqueira, Portugal; and Cínovec and Nejdek, Erzgebirge, Czech Republic) illustrate diversity in initial magma composition (S- vs. A-types), in style of greisenization (pervasive greisenization in granite cupolas vs. vein-like greisen strings along joints), and in chemical evolution of quartz and micas during magmatic–hydrothermal transition. The contents of all monitored elements in quartz and mica from greisen and veins are of very high variability, with principal differences among studied localities. Generally, very low contents of Al (<100 ppm), Ti (<1 ppm) and Li (<10 ppm) or, on the contrary, extremely high contents of Al (>1000 ppm) or Li (>100 ppm) in quartz may indicate its hydrothermal origin. Contents of Sn, W, Nb, and Ta in micas tend to become depleted during greisenization, this trend is more pronounced in Nb and Ta than in Sn and W. Transition from magmatic to hydrothermal crystallization leads to an increase in the Ta/Nb values in mica: from 0.20 to 0.24 in S-type magmatic systems, and from 0.13 to 0.34 at Cínovec as a representative of A-type granites. Whether granite belongs to the S- or A-type is not essential for the development of greisenization.