Experimental Insights of the Pyrite to Magnetite Transformation at hydrothermal conditions

Hydrothermal deposits are formed by the precipitation of minerals from hot aqueous fluids circulating within the Earth's crust and are often associated with magmatic activity, e.g. VMS deposits on mid-oceanic-ridges. These deposits typically contain a variable amount of pyrite along with other metal-bearing sulfides. During hydrothermal alteration, interactions between sulfides and fluids can cause the formation of magnetite, thus indicating changes in fluid chemistry during the reaction. However, the conditions and replacement mechanism of this reaction remain ambiguous and need further evaluation. Therefore, we conducted experiments to analyse the conditions of this alteration and to constrain the lost of trace-elements into the hydrothermal fluid during pyrite replacement in a temporal evolution.

As starting material, equal mass amounts of natural pyrite (Panasqueira, Portugal) and deionised-water were filled into gold capsules and were exposed to hydrothermal conditions in an autoclave at 400 °C and 500 bar. We conducted the experiments for 3, 5, 7, 11, 14 and 21 days, followed by EDX, EMPA, ICP-OES and XRD. The first indications of magnetite formation were detected after 14 days characterised by a decrease of Fe-content in the fluid compared to the 7 days run and the detection of Fe-oxides by EDX. We assume that the Fe-concentration in the fluid stabilises at thermodynamic equilibrium. Analysis of the trace-element content in the fluid and on pristine and altered pyrites will constrain the trace-element release or partitioning at all time-steps.

Thanks for support to Maria Kokh, Wolfgang Morgenroth, Antje Musiol, Hans-Peter Nablein, Valby van Schijndel, Christian Schmidt.