Evolution of the Cínovec (zinnwald) Granite Cupola, Czech Republic: Composition of Feldspars and Micas, a Clue to the Origin of W, sn Mineralization
Résumé
A 1596-m-deep borehole (CS-1) located in the central part of the Cinovec (Zinnwald) granite cupola mineralized in Sn and W intersected zinnwaldite granite (ZG) followed by lithian annite ("protolithionite") granite (PG). These two types of granite are separated by a transition zone (TZ). We studied the chemical composition of major mineral phases (plagioclase, K-feldspar, micas) by EPMA and LA-ICP-MS analyses. The plagioclase composition corresponds to albite (Ab(99.6-91.1)) containing 0.09 to 0.18 wt.% Rb2O. Albite crystallized from the magma. K-feldspar is perthitic and shows an increase in Rb concentration (up to 0.83 wt.% Rb2O) in the apical part of the cupola. The partition coefficient K-D-Rb(Mi-Kfs) is constant throughout the cupola, indicating a systematic re-equilibration of these two minerals with fluid. Lithian annite occurs below -735 m. Its F content decreases with depth and from -940.5 m, OH exceeds F. Magnesium and Ti concentrations show a remarkable positive correlation with depth. The TZ and adjacent area are characterized by strong variations in the chemical composition of micas, reflecting the fluctuation of saturation-oversaturation of residual liquid by a F-rich fluid phase. The Rb concentration in zinnwaldite (up to 2 wt.% Rb2O) strongly increases in the apical part of the cupola owing to a significant transfer of volatiles. The chemical composition of mica evolves gradually from the TZ to the apical zone. The discovery of lithian annite included in quartz at -97 m indicates the formation of zinnwaldite by interaction of lithian annite with F-rich fluid. The octahedral site of lithian annite allows the incorporation of Sn, Nb, Ta and W, replacing Ti. Because of the lithian annite -> zinnwaldite transformation, these elements with higher ionic charges are expelled from the mica structure, transferred to the fluid phase and concentrated in the apical part of the cupola. The LA-ICP-MS analyses of the micas confirm this process. We envisage the transport of Sn, Nb, Ta and W in the form of fluorides and the precipitation of cassiterite by hydrolysis of SnF4. Tungsten was probably transported as H2WO4, resulting from the reaction of WF6 with H2O. A similar behavior of Nb and Ta is suggested. The hydrolysis of fluorides leads to a strong enrichment of fluid in HF, which induced albite instability and formation of greisens. Calculations show that the amounts of Sn and W released by the transformation lithian annite -> zinnwaldite transformation are close to the estimated reserves of these metals within the Cinovec cupola, corroborating the proposed metallogenic model.