Magmatic-hydrothermal transition in Mo-W granite-pegmatite-greisen systems: trace element chemistry of quartz, Krupka district, eastern Erzgebirge (Czech Republic)
Authors: Peterková Tereza , Dolejš David 
Published: March 2020

&lt;p&gt;Magmatic-hydrothermal transition in highly differentiated silicic igneous systems is responsible for several mineralization styles including pegmatite-, porphyry- or greisen-related ore deposits. The Kru&amp;#353;n&amp;#233; hory/Erzgebirge province in central Europe is characterized by late Variscan, Sn-W greisen mineralization that is spatially and temporally associated with W-Mo pegmatite-greisen mineralization in its eastern part. In the Kn&amp;#246;ttel district at Krupka, a complete sequence of highly differentiated Li- and F-rich granites, aplites, pegmatites, breccias, greisens, hydrothermal quartzites and late quartz veins is exposed and the nature of magmatic-hydrothermal transition has been investigated and interpreted by trace element composition of quartz. Abundance of the siliceous rocks points to sharp chemical gradients, controlling the precipitation and/or mineralization processes that may have been facilitated by involvement of hydrosilicate (silicothermal) fluids. The trace element concentrations of quartz and their correlations suggest that Si&lt;sup&gt;4+&lt;/sup&gt;&amp;#8239;=&amp;#8239;Li&lt;sup&gt;+&lt;/sup&gt;Al&lt;sup&gt;3+&lt;/sup&gt; and Si&lt;sup&gt;4+&lt;/sup&gt;&amp;#8239;=&amp;#8239;H&lt;sup&gt;+&lt;/sup&gt;Al&lt;sup&gt;3+&lt;/sup&gt; are the most important substitution mechanisms. Ratios in Ti vs. Li, Be and Al define several distinct genetic trends: (1) magmatic, high-Li/Ti or Al/Ti trend which involves granites, aplites and K-feldspar pegmatites; (2) late-magmatic or hydrosilicate, medium-Li/Ti trend recorded by quartz megacrysts and pegmatite-textured aggregates in granites and quartz-protolithionite pegmatite; (3) hydrothermal, low-Li/Ti or Al/Ti trend represented by a stockwork of coarse-grained hydrothermal quartzites and quartz veins, and quartz replacement in greisens. The medium-Li/Ti trend plausibly represents a hydrosilicate liquid, an H&lt;sub&gt;2&lt;/sub&gt;O- and SiO&lt;sub&gt;2&lt;/sub&gt;-rich medium that was probably formed by disequilibrium crystallization in front of rapidly propagating solidification front of highly evolved granitic melt. Thermal evolution of the magmatic-hydrothermal system was monitored by Ti-in-quartz thermometry. The calculated rutile activity in the granitic melts was very low (0.3&amp;#8211;0.05) but it increased (up to 1), that is, rutile saturation in the pegmatites and the hydrothermal quartz veins. Magmatic crystallization of the granites and aplites occurred from 700 to 580&amp;#8239;&amp;#176;C, the pegmatite formation between 600 and 500&amp;#8239;&amp;#176;C. The greisenization stage coincided thermally with the pegmatite crystallization, and it was followed by a late hydrothermal stage precipitating distal quartz veins at 500&amp;#8211;390&amp;#8239;&amp;#176;C. The concentrations of Ti, Al, Ge, Li and Rb in quartz reveal that the granite and pegmatite magmas at Kn&amp;#246;ttel &amp;#8211; and in the Erzgebirge in general &amp;#8211; have reached extremely high Al, Li, Rb and Ge enrichment in comparison with igneous rocks worldwide and their composition approaches that of pegmatites. In addition, the Kn&amp;#246;ttel system exhibits Be enrichment in quartz, apparently linked to F enrichment, and this feature marks the Mo-W-mineralized systems globally.&lt;/p&gt;