Magmatic volatiles and platinum-group element mineralization in the Stillwater layered intrusion, U.S.A.

Abstract

The activity of volatile-rich fluids may be important in the evolution of basaltic magmatic systems and associated precious metal ore formation. There is evidence for Cl-rich fluids within the Stillwater Complex (Montana, USA), which have been linked to platinum and palladium mineralisation in the economically important J-M Reef ore body. We present the first dataset for heavy halogens (Cl, Br and I) and natural noble gas isotopes in bulk rock and mineral separates from the Peridotite Zone and the Olivine-Bearing Zone I of the Stillwater Complex, including samples from the J-M Reef and G Chromitite bodies. Our data reveal concentrations of 4 to 13500 ppm for Cl, 26 ppb to 360 ppm for Br and <1 ppb to 9 ppm I over This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

The petrology of the J-M Reef Pd-Pt deposit and the Lower Banded series of the Stillwater Complex, Montana, USA

“…shown by the presence of Cl-rich apatite and phlogopite in the Reef Package (Boudreau et al, 1986;Boudreau and McCallum, 1989), volatile-rich fluid inclusions and halide melt inclusions from pegmatoids in the Gabbronorite zone I and the Bronzitite zone (Hanley et al, 2008), high temperature carbonate minerals associated with sulfides underlying the reef (Aird and Boudreau, 2013), a wide range of halogen concentrations in bulk rocks (e.g., 4 to 13,500 ppm Cl) from the Peridotite zone and Reef Package (Parker et al, 2021). It has been proposed that the addition of dissolved water breaks tectosilicate bonds in silicate melts, decreasing the number of nucleation sites for tectosilicate minerals like feldspars (Fenn, 1977) while also decreasing melt viscosity and increasing diffusion rates in silicate melts (Mungall, 2002a).…”