Origin of Euhedral Silicate Megacrysts in Magmatic Sulfide Ore at the Nova-Bollinger Ni-Cu-PGE Deposit

Silicate megacrysts of orthopyroxene, garnet, and plagioclase up to 10 cm in size are observed within massive Ni-Cu-PGE sulfides and emulsion-textured rocks at the Nova-Bollinger deposit (Western Australia). Petrographic, chemical, and cathodoluminescence analyses reveal that the megacrysts differ significantly from their counterpart mineral phases in the host intrusion and country rock in terms of size, morphology, major and trace element compositions, and luminescence properties. Notably, garnet and orthopyroxene megacrysts show pronounced negative Eu anomalies and depleted light REE patterns, suggesting a magmatic origin. This is particularly interesting for garnet, as it is otherwise only found in the metamorphic country rock. Additionally, the megacrysts commonly exhibit skeletal crystal morphologies and contain both sulfide and silicate melt inclusions. These features, along with their frequent intergrowth with emulsion textures, strongly suggest crystallization from a sulfide-silicate melt emulsion. These megacrysts and their hosting emulsion-textured rocks are restricted to sulfide infiltration fronts, where hot sulfide melt percolated into metamorphic country rock, causing partial melting. The resulting anatectic silicate melt, immiscible and less dense than the infiltrating sulfide melt, rose and mingled with the sulfide to form an emulsion. This process could have promoted megacryst formation within the silicate melt portion of the emulsion, as the large grain size could be the result of the combination of early, most likely seeded, nucleation and the elevated volatile and flux content of the anatectic silicate melt. Additionally, the constant movement of the emulsion could continuously supply fresh material onto the growing crystals, thus allowing them to grow to large sizes. Once the movement of the melts decreases, the immiscible melts can separate, leaving the megacrysts behind in massive sulfides. Furthermore, while plagioclase coexists with garnet and orthopyroxene, orthopyroxene and garnet never coexist as megacrysts. This is likely to be a temperature effect, as orthopyroxene is mostly associated with pyrrhotite-dominated sulfides (after monosulfide solid solution), which records temperatures up to 1100 °C, whereas garnet is associated with chalcopyrite-dominated sulfides (after intermediate solid solution), which start to crystallize at temperatures around 880 °C.