Genesis of the low sulfide-high-grade PGE mineralization in the W Horizon, Coldwell Complex, Canada: quantitative modeling for PGE reef-style mineralization in syn-magmatic sills

The W Horizon is a thin continuous zone of low sulfide-high-grade platinum group element (PGE) mineralization in the Two Duck Lake gabbro at the Marathon Cu-PGE deposit, Coldwell Complex, Canada, that resembles reef-type PGE deposits found in much larger intrusions. The Marathon deposit was previously shown to have grown by accretion of syn-magmatic, sulfide-bearing sill-like intrusions that formed each of the W Horizon, Main Zone, and Lower Zone mineralization. Each zone can be distinguished by features such as sulfide mineral or platinum group mineral assemblages, or by Cu/Pd, Pd/Se, and Pd/Au values. For example, the W Horizon is defined by the presence of bornite, very high PGE values, low Cu/Pd (< 1000), and high Pd/Au (34 ± 32), whereas the Main Zone and Lower Zone types are characterized by mantle-like Cu/Pd (1000 to 10,000), and Pd/Au of 10.3 ± 4.7. Model calculations based on Kerr-Leitch multistage dissolution upgrading show that PGE enrichment and decoupling of PGE from Au and chalcophile elements in the W Horizon can be related to progressive upgrading and recycling of sulfide liquid by successive pulses of silicate magma. For the W Horizon, extreme PGE enrichment can be explained by the interaction of a sulfide liquid with up to 25 pulses of magma and simultaneous sulfide liquid dissolution resulting in a loss of > 95% of the original sulfide liquid. In some samples, metal and S abundances indicate that PGE were transported as TABS-PGE phases. This work describes an example of thin PGE-rich sill-like injections that could be of importance to PGE exploration elsewhere.