Abstract

Platinum-group elements (PGE) are typically hosted in base metal sulfides and by platinum-group minerals (PGM) in Ni-Cu-PGE sulfide deposits. At Sudbury, it appears that the majority of PGE are hosted in PGM. In order to understand why this is the case we have investigated the origin of PGM from the 402 trough ore-bodies of the Creighton deposit located on the South Range of Sudbury. These predominantly pyrrhotite-rich sulfides, with low (Pt + Pd)/(Os + Ir + Ru + Rh) whole-rock ratios, represent cumulates of monosulfide solid solution (MSS) that crystallized early from the sulfide melt, collected in troughs and embayments at the base of the Sudbury Igneous Complex, and formed small pendants of ore in the footwall country rock. The majority of PGE (Ir, Rh, Pt ± Os, Ru) show a stronger affinity for the sulfarsenide phases than the cocrystallizing sulfide phases which are strongly depleted in these PGE. The precious metal mineralogy is dominated by PGE sulfarsenides (86%) with subordinate sperrylite (PtAs2: 9%), michenerite (PdBiTe: 5%), and electrum (AgAu2: 0.1%). These discrete minerals are predominantly hosted within pyrrhotite and pentlandite except, however, a large proportion of michenerite is hosted either entirely by silicates and/or juxtaposed against silicates. The PGE sulfarsenides are euhedrally zoned with an irarsite (IrAsS) core, an outer layer of hollingworthite (RhAsS), and a PGE-rich Ni cobaltite rim (CoAsS). Rhenium sulfides, some of which are Os bearing, are documented for the first time at Sudbury.

Platinum-group minerals may crystallize directly from sulfide melt, form by exsolution during cooling of the base metal sulfides or recrystallize from them during metamorphism. We propose that zoned PGE sulfarsenides and sperrylite crystallized from a sulfide melt at high temperatures (1,200°–900°C) and were subsequently surrounded by MSS cumulates, even by disseminated sulfides, that crystallized from the now Ir, Rh, Pt ± Os, Ru-depleted immiscible sulfide liquid. The base metal sulfides recrystallized with secondary hydrosilicates at a late magmatic and/or hydrothermal stage (<540°C) at which time michenerite formed. The magmatic zoning of the PGE sulfarsenides was preserved during later deformation in shear zones but these PGM were corroded, fractured, and juxtaposed against silicates.

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