The geochemical and textural transition between the Reef Package and its hanging wall, Stillwater Complex, Montana, USA

Abstract The highest grade Pd-Pt deposit on Earth, the J-M Reef, is hosted in coarse-grained to pegmatoidal cumulates called Reef Package. Decades of mine development have revealed that a distinct discontinuity rock fabric marks top unit hosts economic-grade sulfide mineralization. Mine geologists refer this as hanging wall contact. This contact Package and always locatable—either by change or distinctive textures silicate minerals—even when reef mineralization absent. rather subtle textural feature used reliably follow during exploration development. Although some high tenor sulfides (>1000 ppm Pd 100% sulfide) are found sporadically cumulates, these accumulations too small be economically viable. We present quantitative data for four intersections collected electron back-scattered diffraction (EBSD). Plagioclase fabrics characterized low variance grain sizes strong point maximum concentration (010) perpendicular girdle distribution [100] consistent with an axial B-type fabric. These indicative either compaction crystal mush settling nucleated crystals, bulk magma chamber. Conversely, show higher size distributions weak undeveloped preferred orientation plagioclase crystals did not undergo significant alignment equilibration grains. absence foliation stands contrast both other reported EBSD datasets orientations from Bushveld Complex, Skaergaard Intrusion, Rum Intrusion. Furthermore, flatter slopes convex profiles fewer at fractions indicating dissolution partial melting coarsening (i.e. Ostwald ripening) growth. Crystal growth was favored over nucleation new prolonged interaction hot infiltrating melt into resident resulting cumulates. represents boundary between partially remelted Package, where formed accumulated, overlying essentially barren cumulate pile. following cessation footwall erosion resumption accumulation normal chamber processes.