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Abstract:
Boron isotope variations in minerals are useful to constrain the sources and evolution of ore-bearing fluids in
hydrothermal ore deposits and in the granites or pegmatites related with them. Because of the complexity
of B-distribution in natural ores, in-situ microanalysis is the method of choice, but so far in-situ studies are
almost exclusively limited to tourmaline. Muscovite coexists with tourmaline in many ore deposits and it is also common in deposits lacking tourmaline. In those cases, muscovite is commonly the main mineral host for boron, with concentrations of several hundred μg/g, easily measured by techniques like secondary-ion
mass spectrometry (SIMS). Furthermore, muscovite-fluid fractionation of B-isotopes is experimentally established, so the lack of studies using this mineral is a missed opportunity. One reason why muscovite has been neglected in B-isotope studies was the lack of homogeneous reference materials. The Potsdam SIMS lab has now tested and confirmed the B-isotope homogeneity of two widely-available muscovite reference materials (Harvard 11271 and 98973), and is in the process of establishing them as international reference materials. This contribution gives the results of a test application of in-situ SIMS analysis of muscovite based on the granite-related Panasqueira W-deposit in Portugal. Muscovite in this deposit occurs together with tourmaline in ore-vein selvages and alteration zones, and one goal of the study was to test the utility of B-isotope exchange between muscovite and tourmaline as a geothermometer. Applying the SIMS 11B/10B ratios in muscovite-tourmaline pairs with experimental fractionation factors yielded median temperatures of
400 to 460°C from vein selvages and 250°C from a cross-cutting, mineralized fault zone. These values agree with independent temperature estimates from fluid inclusions and Ti-in-quartz thermometry. We conclude that the B-isotope composition of muscovite can be reliably measured by SIMS, and thus it can be used like tourmaline as a fluid monitor and for geothermometry if both minerals coexist.
