The Evolution of Magmatic-Hydrothermal Fluids as Recorded by Stable Cl Isotopes and Cl/Br Elemental Ratios in Geothermal Systems, Porphyry Copper and Iron-Oxide-Copper-Gold Deposits

  • Author / Creator
    Bernal, Nelson F
  • This research was conducted with the purpose of identifying the processes that affect the halogen distribution and the Cl isotope composition of hydrothermal fluids under different pressure and temperature conditions in the crust. In order to attain this goal, three different geological settings were investigated; active geothermal systems in the Taupo Volcanic Zone (TVZ), North Island, New Zealand, the Porphyry Copper Gold Deposit (PCD) at Butte, Montana, USA, and selected Iron Oxide Copper Gold Deposits (IOCG) in Norrbotten, Sweden. This research also focussed on identifying analytical issues related to sample preparation procedures and stable Cl isotope analysis. Deep geothermal fluids representing 14 active geothermal systems in the TVZ, North Island New Zealand, were analyzed in this study to identify changes in the halogen (Cl and Br) and isotope compositions (δ37Cl, δ7Li, δ18O and δD) of hydrothermal fluids associated with rhyolitic and andesitic magmas. Low δ7Li values were measured in geothermal water samples indicating that most water-rock interactions occur with rhyolitic host rocks. Two different halogen compositions were identified in the geothermal waters, one related to andesitic magmas in the east and the other associated with predominantly rhyolitic magmatism in the west. Comparisons of the Cl and Br concentrations of geothermal waters from the andesitic geothermal systems and MORB samples from Macquarie Island, located some 1,200 km south of New Zealand, indicate a common origin for these two sets of samples; a high-alumina basalt, which evolved to andesitic and rhyolitic compositions. The geothermal samples related to rhyolitic magmatism have Cl and Br compositions comparable to acidic volcanic rocks reported in previous studies. This study contributes the first data on the effect of magmatic segregation on the δ37Cl composition of geothermal fluids, forming different, and characteristic, isotopic signatures for rhyolitic and andesitic sources. This work concludes that Cl/Br molar ratios and stable Cl isotopes have the potential to identify andesitic and rhyolitic magmatic sources of halogens in geothermal systems. The Butte Porphyry Copper Deposit (PCD) is arguable the deepest PCD in the world, formed at depths around 9 km and maximum temperatures of 600 °C. Data obtained from Cl/Br molar ratios measured in fluid inclusion leachates in the deep pre-Main Stage veins have high Cl/Br molar ratios, which could be derived from the interaction between felsic and mafic magmas. At shallower levels in the deposit the precipitation of Cl-bearing minerals results in distinctive Cl/Br molar ratios, lower than those measured in the deepest veins. The stable Cl isotopes appear affected by kinetic fractionation, the result of phase separation, brine condensation, vapor formation and remixing, occurring locally in cracks and fractures. However, similar δ37Cl values and comparable chemical compositions of the deep pre-Main Stage and the shallower and economically important Main Stage (MS) mineralisations, indicate a common magmatic-hydrothermal source that persisted during the formation of the Butte PCD. The IOCG-type deposits of the Norrbotten County, Sweden, were formed during the Paleoproterozoic by Ca-rich salinity brines at temperature above 500 °C and depths of 8 to10 km. Scapolites were formed during an earlier regional metasomatic event (RM), and during a later IOCG mineralization. These scapolites were found to be of marialitic composition and most of them had Cl/Br molar ratios that reflect mixed sources related to halite dissolution, seawater and/or magmatic contributions. However, some scapolites from the Pahtohavare and Sarkivaara deposits had Cl/Br molar ratios that were lower than seawater, which suggests the chlorinity in these samples may have been derived from evaporative brines. Measured δ37Cl values in fluid inclusions (Gleeson and Smith, 2009) and spatially related scapolite in one of the samples allowed the calculation of an empiric isotope fractionation of +1.9‰ between the mineral and the fluid. This value contradicts predictions for isotope fractionation between a silicate mineral such as scapolite and a monovalent hydrothermal fluid with high NaCl activities. This research shows that the Cl/Br molar ratios and δ37Cl values are decoupled, possibly owing to the presence of recycled Br in IOCG scapolites and different fluid-rock interaction histories.

  • Subjects / Keywords
  • Graduation date
    Fall 2015
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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