BORON ISOTOPE MUSCOVITE-TOURMALINE GEOTHERMOMETRY INDICATES FLUID COOLING DURING MAGMATIC-HYDROTHERMAL W-Sn ORE FORMATION

摘要

Reconstructing the thermal evolution of hydrothermal ore deposits mainly relies on their fluid inclusion record, which is limited by favorable trapping conditions, calling for alternative temperature constraints. Muscovite and tourmaline coexist in many hydrothermal ore deposits and in the granites or pegmatites related with them. Whereas in situ analyses of boron isotopes in tourmaline are widely applied to constrain fluid sources and evolution, muscovite has seldom been used in this way, and the potential for isotope exchange thermometry with this mineral pair is unexplored. The different boron coordination in muscovite and tourmaline causes a significant, temperature-dependent isotopic fractionation between them, which has been determined experimentally. We used this relationship to study mineralization conditions and fluid evolution at the Panasqueira W-Sn-Cu deposit in Portugal, where the source and evolution of the mineralizing fluids are still debated. The difference in B-11/B-10 ratios of coexisting muscovite and tourmaline, expressed as Delta B-11(mica-tourmaline) = delta B-11(mica) - delta B-11(tourmaline), yields median temperatures for vein selvages from 400 degrees to 460 degrees C within a total range of 350 degrees to 600 degrees C, which agrees with published Thin-quartz temperatures. Mineral pairs from a late fault zone yield a lower median temperature of about 250 degrees C (range 220 degrees-320 degrees C), which fits with published homogenization temperatures of quartz-hosted fluid inclusions from the veins. Taking these temperatures into account, the calculated fluid composition of the early and late muscovite generations is about delta B-11(fluid) = -6 +/- 2 parts per thousand, which indicates that the recurrent fluid pulses had a uniform composition and a magmatic-hydrothermal origin.