Magmatic-hydrothermal transition of Mo-W-mineralized granite-pegmatite-greisen system recorded by trace elements in quartz: Krupka district, Eastern Krusne hory/Erzgebirge
Abstrakt
Magmatic-hydrothermal transition of highly evolved granitic melts enriched in volatile and incompatible elements can involve disequilibrium intermediate products between aluminosilicate melt and hydrothermal fluid (hydrosilicate liquids) with high ore-forming potential. We investigate evolution from Li-F-rich granites through pegmatites, greisens and quartz veins in a composite stock at Knottel in the Krupka district (eastern Krusne hory/Erzgebirge mountain range) by monitoring trace-element variations in quartz.
Interelemental correlations reveal Si4+ Li+ Al3+ and likely Si4+ H+Al3+ to be the most important substitution mechanisms. Variations in Ti vs.
Li, Be and Al define distinct evolutionary trends: (i) magmatic, high-Li/Ti or Al/Ti trend (granites - aplites - K-feldspar pegmatite); (ii) transitional, medium-Li/Ti trend (quartz megacrysts and pegmatite lenses in granite - quartz-protolithionite pegmatite); (iii) hydrothermal, low-Li/Ti or Al/Ti trend (stockwork of coarse-grained hydrothermal quartzites and quartz veins, quartz replacement in greisens). The medium-Li/Ti trend represents hydrosilicate liquid, an H2O- and SiO2-rich medium with low density and effective viscosity that was probably formed during disequilibrium crystallization in front of rapidly propagating solidification front.
Thermal evolution of the magmatic-hydrothermal system was monitored by Ti-in-quartz thermometry. Calculated rutile activity of the granites is very low (0.3-0.05) but it increases (up to 1, that is, saturation) towards pegmatites and hydrothermal veins.
Magmatic crystallization (granites and aplites) is constrained to 700-580 degrees C, pegmatite and bulk greisenization stage occurred at 600-500 degrees C, followed by a late hydrothermal stage associated with the formation of distal quartz veins at 500-390 degrees C. The granite-pegmatite systems at Knottel and in Erzgebirge in general reach extremely high Al, Li, Rb and Ge and low Ti concentrations in quartz in comparison with igneous rocks worldwide.
The Knottel stock belongs to P-poor A-type granites in the Erzgebirge and is linked with them by similar evolutionary trends in quartz trace elements. Furthermore, the Knottel system exhibits Be enrichment in quartz (probably related to high contents of F) and this appears to be typical feature of Mo-W-mineralized systems in general.
The hydrothermal phase with Mo-W mineralization at Knottel is closely related to the magmatic phase of parental granite body, which seems to be an important condition for Mo-W-mineralization formation.