HYDRATION MELTING IN OLIVINE-BEARING ZONE I OF THE STILLWATER COMPLEX, MONTANA

Olivine-Bearing zone I (OB-1) in the Stillwater Complex, Montana, hosts the highest-grade platinum-group element deposit in the world, the J-M reef. Several lines of evidence, including hydrous melt inclusions in the olivine and chromite, eroded texture of plagioclase in olivine-rich rocks, excess of pegmatoids beneath OB-1, and unusually Cl-rich apatite all have been used to suggest that the reappearance of olivine and PGE-sulfide is due to hydration melting rather than orthomagmatic processes (influx of a different magma body). This study builds upon the hydration melting model using the programs MELTS and PELE with Stillwater bulk rock compositions for the original protolith. Fluid inclusion evidence suggests that real fluids range from a solute-rich mixture of alkalis (Na and K), Ca, Fe and SiO₂ in solutions ranging from aqueous brines to CO₂-CH₄-rich fluids. The alkalis in particular are strong fluxing agents. Cl-bearing species are not currently well-modeled by MELTS or PELE. Thus, in our modelling, simple oxide mixtures (H₂O or a mixture of H₂O+Na₂O) is added to gabbronorite to melanorite at 1000-1050°C at 2 kbar pressure, conditions for which the nominally “dry” protolith is >95% solid. Incongruent hydration melting produces the most olivine in the melanorite protolith. The amount of clinopyroxene is largely unaffected by the degree of partial melting and is presumably stabilized by Ca liberated by melting of plagioclase. The Mg# of the olivine is a function of the partial melt retained on cooling, and ranges between 66 and 84, overlapping the natural range of olivine compositions observed in the rocks. The amount of partial melting is limited when H₂O alone is the fluxing agent owing to eventual saturation in a fluid. In contrast, the addition of the H₂O-Na₂O mixture can lead to assemblages composed almost entirely of olivine + liquid. Analysis of plagioclase An # indicates that OB-1 plagioclase is best modeled when the protolith is saturated with a 50% H₂O 50% Na₂O fluid as opposed to pure H₂O. Finally, Cl-rich fluids can contain substantial PGE and base metals, and fluids with a few percent S can lead to sulfide precipitation during the hydration melting event. Sulfide is readily remobilized, which is consistent with heterogeneous distribution of sulfide in ore-bearing rocks.