THE CURIOUS CASE OF THE BERGELL INTRUSION (SWITZERLAND): INSIGHTS INTO THE MAGMATIC-HYDROTHERMAL TRANSITION IN I-TYPE PLUTONS

The chemistry of granitic melts is key to the generation of many mineral deposits associated with magmatic-hydrothermal processes. For instance, A-type granitic rocks are commonly related NYF-pegmatites, while S-type plutons are typically associated with LCT-pegmatites. In the context of such deposits, I-type granites are unique: they are only granites that produce large porphyry-Cu systems, but rarely produce the volumes of pegmatites associated with A- and S-type granitoids. Over the last years, suggestions that I-type granites might be colder and wetter than originally assumed (and possibly much colder than their A- and S-type siblings) have opened a debate on whether the granite solidus should be lower than typically accepted haplogranite solidus temperatures (650-680°C). If substantiated, this hypothesis could reshape our understanding of mineral deposit formation mechanisms and help explain the singular character of I-type plutons.

In order to understand the magmatic and fluid processes above and below the solidus in I-type systems, we studied the Bergell Intrusion (SE Switzerland) and its pegmatites through multiple mineral phases (quartz, zircon, apatite, and feldspars). Granite-pegmatite pairs offer an optimal framework for these investigations: while granites primarily crystallise above the conventional solidus, pegmatites are believed to form at significantly lower temperatures. Our preliminary analyses reveal a curious case: quartz in both granitic and pegmatitic contexts exhibited similar trace element signatures, particularly in Ti, which serves as a temperature proxy. Moreover, zircon samples from the granite displayed remarkably low Ti concentrations (1-2 ppm). Together, these findings suggest that granites and pegmatites mostly formed at similar temperatures, likely below typical solidus values. In addition, FTIR analyses reveal that feldspar megacrysts in the granite are exceedingly enriched in water - even more than the pegmatitic feldspars - indicating that fluids were exceedingly abundant in the final stages of the crystallisation of the granite. Following the recent suggestions in the literature, our data signals that significant portions of I-type granites may crystallise at conditions akin to pegmatites, colder and wetter than those observed in A- and S-type systems.