Hydrothermal quartz genesis revealed by the combination of SEM charge contrast maps, FTIR mapping, and LA-ICP-MS trace element geochemistry

Abstract. Magmatic–hydrothermal ore deposits, such as pegmatites, are an increasingly important source of metals and critical elements for the development of green-energy resources. The geochemical processes at the magmatic–hydrothermal transition influence the degree of element enrichment in these ores. Quartz is a mineral that grows throughout the complete crystallization sequence of granitic pegmatites. Tracking the systematics of trace element incorporation into the quartz crystal structure throughout the magmatic–hydrothermal pegmatite evolution may offer unprecedented insights into pegmatite genesis. Quartz crystals from the Rosina pegmatite, Elba, Italy, and from the Misox pegmatite, Ticino, Switzerland, were mapped using scanning electron microscopy (SEM) charge contrast imaging and Fourier transform infrared (FTIR) spectroscopy to determine the distribution of OH coupled to Li, B, and Al. Trace element laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) spot measurements were done on the same quartz crystals, navigated by the spatial distribution of zonation observed via FTIR maps. In Rosina quartz, total Li, B, Al, and Ti mass fractions are higher in the cores of the zoned crystals when compared to the rims, whereas in Misox quartz, the total Li, B, and Al increase from core to rim. In both Rosina and Misox quartz, OH coupled to Li, B, and Al closely follows the zonation of the total trace element contents. Lithium, B, and Al coupled to OH represent 5 %–30 % of the coupled substitutions in quartz. Thus, OH-related point defects provide another tool for provenance and ore body prospecting studies. In Rosina quartz, the LiOH defect is dominant in the FTIR spectra, which is rare for quartz and only characteristic for evolved pegmatitic quartz crystals. Temperature estimates of quartz formation were constrained by Ti-in-quartz geothermometry, linking the observed geochemical processes to the pressure–temperature conditions at which they took place. Rosina quartz formed at pressures of 2.3 kbar and temperatures between 590 (core) and 330 (rim) °C, while Misox quartz formed at pressures of 5–6 kbar and temperatures between 520 (core) and 300 (rim) °C (calculated at TiO2 activity of 0.5). Relative temperatures consistently decrease from core to rim, and absolute temperatures are uncertain due to the difficulty of constraining the activity of Ti during quartz crystallization. The trace element evolution during quartz crystallization was spatially resolved and tracked through the complete quartz crystal growth period of the pegmatites. Trace elements in quartz crystals from Rosina record a prominent change in incorporation at the core–rim transition, while the interpretation of quartz trace element patterns in Misox quartz is further complicated by twinning patterns. These results show that a combination of quartz FTIR and LA-ICP-MS analyses successfully constrains the changes in the geochemical environment during ore body formation. In particular, Li enrichment in combination with H2O contents in quartz might be useful in the study of detrital quartz when prospecting for Li-rich pegmatites or other economically significant magmatic–hydrothermal ore deposits.