Quartz chemistry of lithium pegmatites and its petrogenetic and economic implications: Examples from Wolfsberg (Austria) and Moylisha (Ireland)

Abstract

Trace element concentrations in quartz were determined from two major LCT pegmatite occurrences in Europe to test the applicability of quartz as a pathfinder mineral for Li mineralized pegmatites. The Wolfsberg spodumene pegmatite deposit and pegmatites throughout the wider Austroalpine Unit (Austria), and the Moylisha spodumene pegmatite deposit (SE Ireland), present two distinctly different geological histories. Spodumene pegmatites at Wolfsberg are associated with late Permian lithospheric extension that formed the Austroalpine Unit Pegmatite Province spanning the Eastern Alps. They were metamorphosed at up to eclogite-facies conditions during the Alpine orogeny. Contrastingly, the simple and spodumene pegmatites at Moylisha form a late Silurian to early Devonian NE-SW-striking pegmatite belt that intruded the East Carlow Deformation Zone along the SE margin of the late Caledonian S-type Leinster Batholith. All analyzed pegmatites contain assemblages including K-feldspar, albite, quartz, muscovite ± spodumene. LA-ICP-MS analysis shows quartz from spodumene pegmatites in both regions is distinguishable from that in simple pegmatites by higher concentrations of Al, Li, Ge and B, whereas simple pegmatite quartz contains higher Ti. Increasing concentrations of Al, Li and Ge and decreasing Ti in quartz from simple pegmatites (to leucogranites in the Austroalpine Unit) to spodumene pegmatites reflects increasing degree of fractionation, resulting from either magmatic differentiation or separately generated but increasingly fractionated melts. Chemical profiles through individual spodumene pegmatite bodies show relatively little chemical variation, consistent with Li saturation through most of their crystallization history. Principal component analysis of quartz data shows that high Ge, Be and B concentrations characterize quartz in pegmatites from the Austroalpine Unit whereas high Al and Li concentrations characterize quartz in pegmatites from Moylisha. Concentrations of >100 μgg−1 Al and >30 μgg−1 Li in pegmatite quartz represent an important threshold for potential spodumene mineralization, which may also be indicated by host rocks (e.g., mica schist) metasomatized by fluids expelled by pegmatites during emplacement and/or crystallization and generating a chemical halo. Retention of pegmatite chemical signatures at Wolfsberg supports the robustness of quartz as an effective pathfinder tool for Li mineralized pegmatites in regions that have been affected by high-pressure metamorphism. LA-ICP-MS of quartz in soil and stream sediments may also be a useful pathfinder in pegmatite provinces where Li-rich quartz is not too diluted by other quartz.