Magmatic-hydrothermal transition of Mo-W-mineralized granite-pegmatite-greisen system recorded by trace elements in quartz: Krupka district, Eastern Krusne hory/Erzgebirge

摘要

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 compa