Insights into the extreme PGE enrichment of the W Horizon, Marathon Cu-Pd deposit, Coldwell Alkaline Complex, Canada: Platinum-group mineralogy, compositions and genetic implications

Abstract

The W Horizon, Marathon Cu-Pd deposit in the Mesoproterozoic Midcontinent rift is one of the highest grade PGE repositories in magmatic ore deposits world-wide. The textural relationships and compositions of diverse platinum-group mineral (PGM) and sulfide assemblages in the extremely enriched ores (>100ppm Pd-Pt-Au over 2m) of the W Horizon have been investigated in mineral concentrates with ∼10,000PGM grains and in situ using scanning electron microprobe and microprobe analyses.Here we show, from ore samples with concentrations up to 23.1 Pd ppm, 8.9 Pt ppm, 1.4 Au ppm and 0.73 Rh ppm, the diversity of minerals (n=52) including several significant unknown minerals and three new mineral species marathonite (Pd25Ge9; McDonald et al., 2016), palladogermanide (Pd2Ge; IMA 2016-086, McDonald et al., 2017), kravtsovite (PdAg2S, IMA No 2016-092, Vymazalová et al., 2017). The PGM are distributed as PG-, sulfides (52vol%), -arsenides (34vol%), -intermetallics of Au-Ag-Pd-Cu and Pd-Ge(10vol%) and -bismuthides and tellurides (4vol%). The discovery of abundant (>330 grains) large unknown sulfide minerals with Rh allows us to present analyses three significant potentially new minerals (WUK-1, WUK-2, WUK-3) that are all clearly enriched in Rh (averaging 4.2, 8.5 and 28.21wt% Rh respectively). Several examples of paragenetic sequences and mineral chemical changes for enrichment of Cu, Pd and Rh with time are revealed in the PGM and base-metal sulfides. We suggest this enhanced metal enrichment formed in response to increasing fO2 causing the oxidation of Fe2+ to Fe3+ and to a lesser extent, S.Phase relations in the Ag-Pd-S, Rh-Ni-Fe-S, Pd-Ge, Au-Pd-Cu-Ag, Pd-Ag-Te systems help constrain the crystallization temperatures of the majority of ore minerals in the W Horizon at ∼500°C or moderate to high subsolidus temperatures (400–600°C). Local transport by aqueous fluids becomes evident as minerals recrystallize down to <300°C. The PGE-enriched W Horizon ores exhibit a complex post-magmatic history dominated by the effects of oxidation during cooling of a Cu-PGE enriched magma source from a deep reservoir.