A number of features of platinum group element (PGE)-rich layers in layered intrusions provide evidence for the involvement of late magmatic fluids. The presence of pegmatitic textures, intergrowth of sulfides with intercumulus volatile-rich phases, high Cl content in volatile-bearing phases, and association with graphite and fluid inclusions have been taken by some authors as evidence for postcumulus transport and deposition of PGE by a fluid phase of late magmatic derivation. We argue that these apparently hydrothermal features are the result of postcumulus processes superimposed on layers whose extensive, uniform PGE concentrations were formed by magmatic cumulus processes.
The association with pegmatites and volatiles arises from the relatively low solidus temperatures of orthocumulate layers, which cause these layers to act as traps for fluids migrating through the crystal pile. These fluids are mostly derived by vapor exsolution from fractionated intercumulus melt and are consequently enriched in CI. Redissolution of these fluids into vapor-undersaturated trapped liquid in orthocumulate layers causes recrystallization and formation of gabbro pegmatites.
Immiscible sulfide-oxide liquid solidifies at near-solidus temperatures in the interstitial pore space between early-crystallizing silicate minerals, and it is forced into areas occupied by late-crystallizing CI-bearing silicates. This accounts for intergrowths of Cu and PGE-rich sulfides with amphiboles and micas, a ubiquitous feature of sulfide-bearing gabbros.
Further exsolution of a vapor phase during late stages of crystallization causes deuteric alteration, formation of fluid inclusions, and possible local redistribution of sulfides and PGE. Cooling of this vapor phase may lead to precipitation of graphite at or below the orthocumulate solidus. Graphite deposition may be catalyzed by sulfides and PGE.