ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉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 〈sup〉11〈/sup〉B/〈sup〉10〈/sup〉B ratios of coexisting muscovite and tourmaline, expressed as Δ〈sup〉11〈/sup〉B〈sub〉mica-tourmaline〈/sub〉 = 〈span〉δ〈/span〉〈sup〉11〈/sup〉B〈sub〉mica –〈/sub〉〈span〉δ〈/span〉〈sup〉11〈/sup〉B〈sub〉tourmaline〈/sub〉, yields median temperatures for vein selvages from 400° to 460°C within a total range of 350° to 600°C, which agrees with published Ti-in-quartz temperatures. Mineral pairs from a late fault zone yield a lower median temperature of about 250°C (range 220°–320°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 〈span〉δ〈/span〉〈sup〉11〈/sup〉B〈sub〉fluid〈/sub〉 = −6 ± 2‰, which indicates that the recurrent fluid pulses had a uniform composition and a magmatic-hydrothermal origin.〈/span〉

Description: Numerical, multiphase pure-water simulations were performed to study the first-order geologic and physical parameters controlling the style and distribution of hydrothermal venting at Brothers volcano, southern Kermadec arc, New Zealand. By comparing the results for different permeability scenarios, we can show that the location of venting on the inner slopes of the caldera (e.g., at the NW caldera site) requires the presence of higher permeability faults. Venting at the top of the Upper cone develops naturally by hydrothermal flow in porous rocks above an underlying magma body. However, this magmatic reservoir cannot alone account for present-day hydrothermal venting at the NW caldera site, which implies that a larger magma chamber, which was responsible for caldera collapse, is still active. Modeled venting temperatures for scenarios with homogeneous host-rock permeability correspond well with formation temperatures determined by sulfate-sulfide mineral pairs from different vent sites at Brothers volcano. Direct measurements of vent fluids at the NW caldera site today, however, show higher temperatures than modeled. This may be due to rapid ascent of hot fluids in individual fractures that are not resolved in the simulations. At the cone sites, measured temperatures are lower than modeled, likely the result of mixing with ambient seawater in near-surface permeable rocks. The inferred presence of a constant magmatic fluid source underneath the volcanic edifice leads to a more rapid development of the hydrothermal circulation and stabilizes the system at higher temperatures. We suggest that the hydrothermal evolution and fluid-flow patterns at Brothers volcano are controlled by episodes of varying magmatic fluid input into a seawater-dominated convection system.