Platinum-Group Mineral Precipitation from Fluids in Pegmatitic Gabbro: Two Duck Lake Intrusion, Coldwell Complex, Ontario, Canada

Abstract

Platinum-group elements (Pt/Pt + Pd = 0.22) occur with very Cu-rich sulphide assemblages (Cu/Cu + Ni = 0.90) in a 46 million tonne deposit within coarsegrained to pegmatitic gabbro-monzonite of the Two Duck Lake Intrusion. This unit invades and has assimilated Archean metavolcanic rocks near its base and the layered succession of the Eastern Coldwell Complex Gabbro near its top. Assimilation and mixing of rheomorphic material is apparently responsible for the abundant xenoliths, granophyric patches, aplitic dykes, K-rich minerals and deuteric alteration that are spatially related to concentrations of the PGE in disseminated Cu-rich sulphides. Much of the disseminated sulphide is interstitial to and intergrown with biotite, plagioclase, amphibole and ilmenomagnetite. Platinum-group minerals (michenerite, mertieite-II, sperrylite, hollingworthite, stannopalladinite, zvyagintsevite, atokite, kotulskite, merenskyite, as well as electrum) occur mainly with chalcopyrite and sometimes bornite. Many grains of PGM (and pentlandite, argentian pentlandite and pyrite) are found at grain boundaries of chalcopyrite and ilmenomagnetite where the magnetite, but not ilmenite lamellae, is partly altered to chlorite or replaced by chalcopyrite. None has been identified in pyrrhotite, which has apparently been partly replaced by chalcopyrite. Late-stage fluid, perhaps of juvenile origin, but more likely from the breakdown of included xenoliths, mobilized elements from the intruded rocks, especially Cu, Ag, Pb, Pd, Pt and Rh.