Iron isotopes in hydrothermal ore-forming systems

• Main Author: Chapman, John Branson
• Published: Imperial College London 2007
• Subjects: 553.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.582584

Column chemistry element purification methods were optimised for separation of Cu, Fe and Zn from geological samples taken from hydrothermal ore-forming systems. The method presented produces high-purity elemental separates at -100% recovery, ensuring absence of column-induced isotopic fractionation. Variation of Fe isotope ratios within two different mineral deposit types was investigated. In the West Cumbrian hematite, isotopic variability due to intermineral, redox and biological fractionation is likely absent, allowing identification of fluid flow and mineral precipitation effects. Isotopic ratios span >2.4%, with a trend toward lower values in primary mineralisation close to fluid conduits. This is interpreted to reflect Rayleigh-type distillation during kinetically-controlled hematite precipitation, with ∆56Fefluid-hematite ≈ 0.90%0. Secondary hematite, containing Fe sourced by dissolution of primary mineralisation, showed similar light isotope enrichment. Iron sulfide samples from the Lisheen Zn-Pb deposit, Ireland, were studied to assess the potential of Fe isotope analysis for tracing Fe source and mineralisation influences in a complex massive-sulfide deposit. Here, Fe isotope variation is correlated to paragenetic stage. Preore sulfides retain diagenetic compositions, locally overprinted by red sandstone-derived Fe. Main stage sulfides have a silicate-derived Fe signature, modified by kinetic mineral precipitation, reflecting evolution of the hydrothermal system from shallow to deep flow through time. Laboratory experiments were conducted to better understand the behaviour of Fe isotopes during transfer of Fe from silicate rocks into hydrothermal solutions. Basalt and granite powders were leached with HCI or oxalic acid solutions, with supernatant aliquots retrieved over a 7 day period. Initial aliquots showed significant enrichment of light isotopes, commonly ∆56Ferock-fluid >1.50%0. Fractionation magnitudes decreased over time, to apparent steady-state at ∆56Ferock-fluid ≈ 1.0 - 0.5%0. This study demonstrates the importance of kinetic processes during mineral precipitation, and the potential for future Fe isotope studies for gaining fundamental insight into many aspects of hydrothermal ore genesis.