The magmatic-hydrothermal transition record in zircon: implications for zircon texture, composition and rare-metal granite dating (Beauvoir granite, French Massif Central)

During their continuous cooling and differentiation, granitic magmas progressively shift from a medium dominated by crystal-melt interactions to a medium that is dominated by crystal-fluid interactions. We refer to this transition between a purely magmatic to a hydrothermal system as magmatic-hydrothermal transition (MHT), which is often associated with the formation of ore mineralisation. In highly differentiated and volatile-rich magmas (e.g., rare-metal granites and pegmatites), the circulation of hydrothermal fluids often modifies the original rock texture, by dissolving and/or replacing the primary minerals phases by secondary ones. Since most of our petrological interpretations are based on mineral composition (e.g., chemical zoning, geochronology), it is crucial to evaluate and quantify the chemical and mineralogical changes such rocks have undergone during the MHT.

To better understand the MHT in highly evolved magmas, especially how these episodes of fluid circulation impacted the texture and composition of the primary mineral phases, we have investigated the internal texture and chemical composition of heterogeneous zircons from the Beauvoir rare-metal granite (Massif Central, France). By combining µ-Raman spectroscopic, mineralogical and geochronological analyses on these grains, we show that primary (magmatic) zircon was partially replaced by secondary (hydrothermal) porous “zircon” through dissolution-reprecipitation mechanisms. The zircon-fluid interactions were notably facilitated by the primary, high trace element content in zircon (especially for U). This newly formed mineral grains (secondary “zircon”) are extremely enriched in non-stoichiometric elements up to few weight percent of P, U, F, Ca, Fe and Mn while they are depleted in Si and Zr compared to pristine zircon. These drastic compositional changes during the MHT of the Beauvoir granite clearly indicate that altered, secondary pseudomorphs after magmatic zircon can be a good tracer for the MHT in evolved silicic systems.

As a result of these dissolution-reprecipitation processes, these zircon grains are porous and highly metamict from the high degree of decay damage related to percent levels of Uranium, which considerably limits their use for zircon petrochronology. By comparing the ID-TIMS geochronological analyses performed on these zircon (312 ± 7.2 Ma – discordia upper intercept) with those performed on apatite (313.4 ± 1.3 Ma – ²⁰⁶Pb/²³⁸U weighted mean age, 9 analyses), we thus envision that the use of zircon to precisely date the emplacement of highly differentiated magmas is limited, while that of other minerals such as apatite (and potentially columbo-tantalite, cassiterite) may be more appropriate in such systems.