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FIG. 19. Schematic model showing the history of mineralization at the southern Roby zone. Magmas formed by a high degree of partial melting in a depleted mantle source (A¹) became enriched in Cu, Pt, and Pd through fractional crystallization of olivine, chromite, and high-temperature PGM (A²), segregated sulfide melt that had low Cu/Pd ratios along the conduit and the base of the magma chamber (A³), and solidified as the early leucocratic gabbros. A second episode of partial melting in the mantle source produced another batch of fertile magma. As with the early magma, this magma was enriched in Cu, Pt, and Pd through fractional crystallization (A²). This magma incorporated the earlier sulfide melt and intruded forcefully into the partially crystallized leucocratic rocks (B¹), causing brecciation and magma mingling, and solidified as fertile melanocratic gabbro. Aqueous fluids that separated from the melanocratic magma percolated through the cumulates, partially dissolving Pd and concentrating it in the High-Grade ore zone adjacent to barren East Gabbro (B²).

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