Hydrothermal origin of platinum-group mineralization in the Two Duck Lake Intrusion, Coldwell Complex, northwestern Ontario

Abstract

Platinum-group minerals (PGM) occur in chalcopyriterich sulfide assemblages [Cul(Cu + Ni) = 0.94, Ptl(Pt+Pd) = 0,221 from very coarse-grained to pegmatitic gabbroic rocks of the Two Duck Lake intrusion at the eastern contact of the Coldwell alkaline rock complex with Archean metavolcanic rocks, in nonhwestern Ontario. The PGMoccur: l) in sulfides or at their contacts, 2) in postcumulus chalcopyrite, 3) in the rim of plagioclase grains with chalcopyrite, 4) in veins along the contacts of chalcopyrite and other minerals, and 5) in veins and areas of alteration of the primary silicate and oxide assemblage. Some PGM are strikingly zoned (hollingworthite and atokite-zvyagintsevite); others are single homogeneous grains (sperrylite, bismuth-rich kotulskite, Pd5As), and some occur in complex aggregares (such as mertieite II with Pd2(Sb,As), palladiumbearing nickeline, majakite, sperrylite and gold; atokite with zvyagintsevite and lead-rich kotulskite). The relationship of theplatinum-group minerals to: l) interstitial areas in pegmatitic gabbro, 2) F- and Cl-bearing biotite and apatite, 3) chloritic and calcitic veins at mineral contacts, 4) assemblages of iron-rich chlorite, epidote, amphibole, sericite, calcite, especially after magnetite, and 5) chalcopyrite - cubanite replacing primary pyrrhotite, are compatible with enrichment of sulfides by Cu and platinum-group elemenls where hydrous fluid has interacted with primary igneous minerals at low temperature.