From the smoking seafloor to metalliferous mountains: Tracing ore formation and metal fractionation by sulfide microanalysis

Document Type
Doctoral Thesis
Granting Institution
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Naturwissenschaftliche Fakultät
Issue Date
Falkenberg, Jan J.

Magmatic-hydrothermal ore deposits occur at varying depths in the Earth´s crust, including deeper-seated porphyry- and shallow epithermal-style mineralization, as well as (active) hydrothermal systems on the seafloor. These ore-forming systems represent important metal anomalies in the Earth’s crust and provide various metallic raw materials, which are crucial for today´s and future´s economy. However, the magmatic and hydrothermal metal fractionation processes, which occur at different crustal levels and essentially control the trace metal content (e.g., Te, Au, Ag, Se, Bi, As and Sb) of ore deposits, are still poorly constrained on the crustal scale. Porphyry deposits form in subduction zone environments, where water- and volatile-rich magmas exsolve S- and metal-rich magmatic-hydrothermal fluids. Fluid-rock interaction, cooling, and phase separation of the fluids trigger the formation of disseminated and vein-style sulfide mineralization at high temperatures (>600 – 350°C) within deep (~1 – 6 km) porphyry stocks. Epithermal mineralization form in shallow (<1.5 km) areas above porphyries at low-temperature (<350°C) conditions from similar magmatic-hydrothermal fluids with possible varying meteoric water/seawater contribution. Active submarine hydrothermal vent systems in subduction zone settings are potential submarine analogues to epithermal systems. These vents form from seawater circulation through the oceanic crust, where fluid-rock interaction transforms seawater into a hot (<450°C), reducing, sulfur- and metal-rich hydrothermal fluid which precipitate “black smoker” sulfide-sulfate chimneys during quenching with cold seawater. This thesis investigates ore-forming processes and the trace metal fractionation during magma degassing as well as during fluid phase separation, which result in precious (e.g., Gold) and critical element (e.g., Rhenium) enrichment in deep to shallow ore-forming systems. Furthermore, it focuses on how these processes are recorded by sulfide trace element composition and their stable S- and radiogenic Pb isotope chemistry. Chapter 3 examines the active submarine hydrothermal systems at Niuatahi volcano in the north Tonga rear-arc systems and shows that the involvement of magmatic fluids and phase separation result in a distinct metal zonation (e.g., Au, Te, Se, Pb) within the caldera. Chapter 4 investigates the influence of magma degassing on the pyrite trace element variance from subduction-related hydrothermal systems. Here, it is revealed for the first time, that Te/As and Te/Sb ratios correlate systematically with the δ34S isotope composition and that these trace element ratios record the contribution of magmatic volatiles between different vent sites. In Chapter 5, it is shown that Co/As, As/Sb, Se/Te, and Se/Ge ratios in pyrite can be used to resolve variations in the physicochemical fluid parameters and ore-forming history in mineralized porphyry-epithermal veins during the porphyry-epithermal transition at the Maronia porphyry-epithermal system, NE Greece. Empirical observations further indicate that the Re enrichment at Maronia is linked to a distinct hydrothermal stage during which oxidized fluids were reduced and cooled to <400°C, inducing extreme precipitation of Re-rich molybdenite. In summary, this thesis provides significant and novel approaches for fingerprinting magmatic-hydrothermal processes through different crustal levels by microanalytical sulfide chemistry and reveals the control of these processes on the metal endowment in porphyry-epithermal and active submarine hydrothermal systems in subduction zone environments. Particularly, it highlights the usefulness of coupled microanalytical methods, and the use of trace element ratios compared to absolute trace element concentrations, as these ratios are mainly controlled by the underlying magmatic-hydrothermal fractionation processes such as phase separation and magmatic degassing.

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