THE MAGMATIC-HYDROTHERMAL EVOLUTION OF A WATER-, B- AND F-RICH GRANITE-PEGMATITE SYSTEM: A MELT/FLUID INCLUSION STUDY

The chemical evolution of the hydrous, B- and F- rich granite-pegmatite system related to the important hydrothermal tin-tungsten mineralization of the Variscan Ehrenfriedersdorf Complex was reconstructed using melt and fluid inclusions in quartz from granites and granite pegmatites. The sequence of the inclusions was determined by a combination of melt rehomogenization experiments, microthermometry and optical observations. Major and trace element compositions were obtained following the procedure of [1] allowing non-destructive simultaneous quantitative analysis of elements with Z between 20 and 92 of unopened fluid and melt inclusions by using SR XRF analysis, by EMPA and Raman-spectroscopy.

At temperatures above ca. 710 °C one water-rich silicate melt exists. The coexistence of two different types of silicate melts preserved as inclusions at lower temperatures - namely type A and B melt inclusions - is interpreted as resulting from simultaneous entrapment on both sides of a two melt solvus [2]. Aqueous fluids coexist at all evolutionary stages partly displaying boiling features [3].

The preserved granite melt as well as type A melts are peraluminous whereas type B melts are strongly peralkaline. During exsolution, water, B, F and Cs and Rb and the trace elements Ag, Ta, Sb, Sn, Fe, W, Nb, As and Zn preferentionally partition into type B melts. Both melt types display a positive correlation of Cs and Rb with Nb, Ta and Sn indicating fractional crystallization. The differentiation grade of the granitic melts is extreme as indicated by Nb/Ta ratios below 0.25 and the general absence of REE. Comparison of elemental compositions of pegmatitic fluids and melts indicate higher concentrations for all elements except Cu in type B melts and continuously lower concentrations in type A melts.

Ore-forming processes take place at the hydrothermal stage: low-salinity fluids of the initial hydrothermal stage transported mainly Cu and Sn, along with As, Nb and minor W. Element enrichment of Zn, Fe and Mn in later hydrothermal brines, is positively correlated with salinity and Cs contents whereas Cu and Sn concentrations are low.

[1] Rickers, K., Thomas, R. and Heinrich, W. (2004) EJM 16, 23-35. [2] Thomas, R., Webster, J.D. and Heinrich, W. (2000) CMP 139, 394-401. [3] Thomas, R., Förster, H.-J. and Heinrich, W. (2003) CMP 144, 457-472.