Fluid and Halide Melt Inclusions of Magmatic Origin in the Ultramafic and Lower Banded Series, Stillwater Complex, Montana, USA

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Fluid and melt inclusions trapped in igneous rocks below the platinum-group element (PGE)-rich J-M reef in the Stillwater Complex, Montana provide a physiochemical record of a continuum of high P–T magmatic–hydrothermal and low P–T metamorphic events. Magmatic–hydrothermal volatiles ranged from NaCl-dominated halide melts (>82 wt % NaClequiv) to more complex Na–Ca–K–Fe–Mn–Ba–Si–Al–Cl brines (28–79 wt % NaClequiv) that were trapped simultaneously with a moderate density carbonic fluid (CO2 ± CH4). Early primary inclusions containing immiscible brine and carbonic fluid were trapped in the granophyric albite–quartz core of a zoned pegmatite body in the Gabbronorite I unit at T of ∼ 700–715°C, and P between 4·3 and 5·6 kbar. The pegmatitic body crystallized from a fluid-saturated residual silicate liquid that was channeled through the cooling igneous stratigraphy. Approximately 500 m stratigraphically below the pegmatite, in the Ultramafic Series, early halide melt inclusions representing samples of formerly molten NaCl were trapped in unaltered primary olivine over a minimum range in temperature of 660–800°C. In the same olivine that hosts the halide melt inclusions, secondary brine inclusions with a composition similar to brines in the pegmatite were trapped over a minimum temperature range of 480–640°C. As hydrothermal activity continued during post-solidus cooling of the intrusion, quartz precipitation in the vuggy core of the pegmatite body trapped post-magmatic, immiscible brine and carbonic fluid inclusion assemblages that record a progressive decrease in fluid salinity, T and confining P from lithostatic to near-hydrostatic conditions. Late secondary inclusions containing regional metamorphic fluids were trapped in quartz in the pegmatite after cooling to zeolite-facies conditions. The late metamorphic fluids were low to moderate salinity, CaCl2–MgCl2–H2O solutions. Hydrous salt melts, magmatic brines, and non aqueous (carbonic) fluids may have coexisted and interacted throughout much of the late crystallization and post-magmatic history of the Stillwater Complex. Hence, the potential for interaction between exsolved magmatic volatiles and grain boundary-hosted sulfide minerals below the J-M reef at near-solidus temperatures, and the post-magmatic modification of the J-M reef PGE ore compositions by hydrothermal fluids are strongly indicated.

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