Chemical signature of quartz from S- and A-type rare-metal granites – A summary

Abstract

About 800 new and 1200 already published laser ablation inductively coupled plasma mass spectrometry analyses of quartz from rare-metal granites and related greisens, quartz-tourmaline rocks and quartz veins were evaluated in order to define typical trace-element signatures of quartz of different origin. The studied dataset comprises typical examples of sub-aluminous to slightly peraluminous plutons (A-type granites of the Kimi stock, Finland; Eastern Erzgebirge, Czech Republic; Orlovka, Russia and Ongon Chairchan, Mongolia), and strongly peraluminous plutons (S-type granites of Beauvoir, France; Cornwall, England; Panasqueira and Argemela, Portugal; Western Erzgebirge, Czech Republic). Additional 700, mostly already published, analyses of quartz from barren granites of all geochemical types (I, S, A-type), rhyolites and barren and rare-element pegmatites were evaluated for comparison. Compiled data reveal high diversity in the contents of trace elements in quartz even among rocks of similar geochemical composition. The most common trace elements in magmatic quartz are Al, Li and Ti with medians of 447, 39.6 and 17.4 ppm in S-type rare-metal granites, and 160, 15 and 6.6 ppm in A-type rare-metal granites. The contents of all these elements in greisens and quartz veins are significantly lower than in their parental granites. Al contents above 450 ppm should serve as a reliable indicator of S-affinity of the analyzed granite, while contents <250 ppm Al are typical for A-type rocks. The contents of Al, Ge and Rb generally increase in the course of magmatic fractionation, while the contents of Ti decrease. The Ge/Ti value can be taken as a valuable indicator of fractionation of granitic melt from which quartz crystallized: this value was found to generally range from 0.002 to 0.2 in quartz from common granites. It may, however, reach 1 in most rare-metal granites (Erzgebirge, Panasqueira, Cornwall), 10 at Beauvoir, 35 at Orlovka, and 50 at Argemela. The uptake of Li into quartz is limited by Al contents in quartz and affected by the contents of volatiles in the melt; it does not correlate with Li contents in parental melt and cannot be used as an indicator of Li-rich magmatic systems.