PLATINUM-GROUP MINERALS FROM THE MECHANIC SETTLEMENT PLUTON, SOUTHERN NEW BRUNSWICK, CANADA

Abstract

The upper Neoproterozoic Mechanic Settlement Pluton, located in the Avalon terrane, southern New Brunswick, Canada, consists of alternating lenses of ultramafic through mafic to intermediate rocks. Known platinum-group element (PGE) mineralization occurs mostly in the ultramafic parts of the intrusion in association with Cu–Ni sulfides. The platinum-group minerals (PGM) are mainly Pd-dominated Pd–Te–Bi minerals; sobolevskite (PdBi) is most abundant, with rarer kotulskite, michenerite, moncheite, and froodite. Additional minerals identified are braggite, vysotskite, sperrylite, stillwaterite, hollingworthite, plumbo-palladinite, Pt–Fe alloy, sopcheite, and paolovite. Sperrylite, Pt–Fe alloy, and braggite are the main carriers of Pt; Ir-, Os-, or Ru-based PGM were not encountered. Gold and Au–Ag alloy are also associated with PGM. Most of the PGM occur as inclusions in silicate minerals (olivine, clinopyroxene and, less commonly, amphibole and phlogopite), or in sulfides (chalcopyrite, pentlandite, pyrrhotite and, less commonly, bornite and millerite), but also occur interstitially to the silicate and sulfide minerals. The size of the PGM ranges from <1 μm to 100 × 60 μm. Textural and mineralogical evidence suggests that some PGM are magmatic, but the majority are of subsolidus origin, as indicated also by the low thermal stability of the bismuthotellurides. The Cu–Ni–PGM mineralization is attributed to the formation of an immiscible sulfide liquid and subsequent sulfide and PGM crystallization.