Boron isotopic and mineral chemical composition in greisen-related Li-Fe micas: Pathways and mechanisms for hydrothermal lithium enrichment

Abstract

We present electron probe micro analyses (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mineral chemistry and B-isotope data for magmatic and hydrothermal Li-Fe micas of the granite-related Sn-W-Li Sadisdorf greisen system in the Erzgebirge, Germany. These mineral-specific data were combined with novel in-situ mica and whole-rock powder-pellet LA-ICP multi-collector mass spectrometry (MC-MS) B-isotope data to understand the magmatic and hydrothermal processes controlling the distribution and local enrichment of Li, Sn, and W. Igneous Li-Fe micas range from Li-bearing annite to zinnwaldite in composition. They are enriched in Li, as well as Rb, F, and Cs, with contents increasing with progressive magmatic fractionation. The concurrent decrease of concentrations of Nb, Ti, W, and Sn in Li-Fe micas is attributed to the efficient partitioning of these metals into coeval minor and trace minerals. Greisen- and vein-hosted hydrothermal micas of the main and waning hydrothermal stages have zinnwaldite and Li-phengite compositions, respectively. Hydrothermal zinnwaldite is enriched in fluid mobile elements such as Li, Zn and Mn, whereas Sn and W are slightly depleted relative to magmatic Li-Fe micas. Higher concentrations of Mg, Ba and V are observed in micas in a distal position, compared to those occurring in veins more proximally or within the Sadisdorf intrusion. Since the metasedimentary host rocks have higher concentrations of these elements as compared to the granites, their increased abundance is tentatively attributed to fluid-rock interaction processes. Similarly, estimated relative fluid activities of most elements systematically decrease towards paragenetically younger generations of hydrothermal micas, indicating that significant dilution or element depletion occurred during the waning stage of the magmatic-hydrothermal mineral system.

In addition to mica trace element data, decreasing average whole-rock δ¹¹B from −12.4 ‰ in the older syenogranite to −15.0 ‰ in the younger microgranite at the Sadisdorf prospect suggest degassing occurred between consecutive magmatic stages, concomitantly to magmatic fractionation. The boron isotope composition of hydrothermal micas ranges from −27.0 to −9.5 ‰ δ¹¹B in the proximal greisen to –23.6 and −14.6 ‰ δ¹¹B in distal oxide-sulfide veins. Quantitative isotopic modelling indicates that the observed variations in mineral chemistry are mainly controlled by magmatic fractionation as well as cooling of the magmatic-hydrothermal fluid, whereas intra-sample variations are likely a result of local fluid-mineral equilibria and Rayleigh fractionation. Our results show that Li enrichment in metasomatic greisen systems is a result of magmatic and hydrothermal processes. During the early stages of greisenization, local changes between fluid- and rock-buffered conditions and cooling prevailed, whereas progressive mixing with meteoric fluids and fluid-rock interaction, resulted in the formation of oxide and sulfide veins and eventually removal of Li during the latest stage. This study demonstrates that the combined approach of isotopic and chemical in-situ mica analyses provides novel insights into metasomatic processes and associated metal enrichment in magmatic-hydrothermal systems.