Chromatographic separation of the platinum-group elements, gold, base metals and sulfur during degassing of a compacting and solidifying igneous crystal pile

The major platinum-group elements (PGE) concentrations in layered intrusion s are typically associated with zones in which, the sulfide abundance begin s to increase. In a number of layered intrusions, there is also a distinct stratigraphic separation in the peak concentrations of the PGE from those o f the base metals, gold and sulfur through these zones. These stratigraphic "offsets" are characterized by a lower, typically S-poor, Pt- and Pd-enric hed zone overlain by a zone enriched in the base metals, S and Au. The sepa rations amount to a few decimeters to several tens of meters. III some inst ances, the high Pt and Pd concentrations are associated with trivial amount s of sulfide. Theoretical considerations suggest that these offsets can be modeled as chromatographic peaks that develop during an infiltration/reacti on process. Using Pd as a typical PGE and Cu as a typical base metal, a num eric model is developed that illustrates how metal separations can develop in a vapor-refining zone as fluid evolved during solidification of a cumulu s pile leaches sulfide and redeposits it higher in the crystal pile. The so lidification/degassing ore-element transport is coupled with a compaction m odel for the crystal pile. Solidification resulting from conductive cooling through the base of the compacting column leads to an increasing volatile concentration in the intercumulus liquid until it reaches fluid saturation. Separation and upward migration of this fluid lead to an upward-migrating zone of increasingly higher bulk water contents as water degassed from unde rlying cumulates enriches overlying, fluid-undersaturated interstitial liqu ids. Sulfide is resorbed from the degassing regions and is reprecipitated i n these vapor-undersaturated interstitial liquids, producing a zone of rela tively high modal sulfide that also migrates upward with time. Owing to its strong preference for sulfide, Pd is not significantly mobile until all su lfide is resorbed. The result is a zone of increasing PGE enrichment that f ollows the sulfide resorption front as solidification/degassing continues. In detail, the highest Pd concentrations occur stratigraphically below the peak in S and base metals. The high Pd/S ratio mimics values conventionally interpreted as the result of high (silicate liquid)/(sulfide liquid) mass ratios ("R" values). However, in this case, the high Pd/S ratio is the resu lt of a chromatographic/ reaction front enrichment and not a magmatic sulfi de-saturation event.