Quartz chemistry fingerprints melt evolution and metamorphic modifications in high-purity quartz deposits

Abstract

In this paper, statistical quartz chemistry is combined with petrographic observations to improve our understanding of how metamorphism reworks trace elements of igneous quartz. We elucidate the key processes involved in the formation of the pegmatite-type high-purity quartz deposits in Tysfjord, Northern Norway, where produced quartz has the second-best chemical purity in the world. The Tysfjord pegmatites are the largest known intra-plutonic Nb-Y-F type, comprising Paleoproterozoic and Devonian generations. The Paleoproterozoic pegmatites show characteristics of Caledonian deformation and can be divided into a strongly-deformed type and a less-deformed type: primary internal zonation is replaced by quartz core and K-feldspar-rich intermediate zones surrounded by sheared granitic wall zones; quartz has undergone dynamic recrystallization and contains very few fluid inclusions. The Devonian pegmatites are undeformed and the quartz is rich in healed, fluid-inclusion-bearing microcracks.Trace elements in the pegmatite quartz are dominated by Al, Ti, Li, Ge and B, and total concentrations of the five elements are 29 ± 9 μg/g, 35 ± 10 μg/g and 38 ± 12 μg/g for the strongly-deformed, less-deformed and undeformed types, respectively. The quartz chemistry of the Devonian pegmatites is largely heterogeneous and shows significantly increasing Ge and B contents from early- to late-stage quartz, reflecting the fractionation enrichment of Ge and B in the evolved melts. The quartz chemistry of the Paleoproterozoic pegmatites has become homogeneous and is dominated by either Ti or Li and Al, resulting from the metamorphic modifications during the Caledonian overprint. The modifications on quartz took place via volume diffusion or dynamic recrystallization equilibrium, resulting in a resetting of Ti (at low TiO2 activity) and losses of Li and Al. Therefore, the less-evolved pegmatite-forming melts derived from the Paleoproterozoic magmatism and the moderate metamorphic modifications during the Caledonian orogeny are important for the formation of the high-purity quartz deposits. This study manifests that quartz chemistry has the potential to fingerprint igneous and metamorphic mineralization processes in tectonomagmatic terranes.