BEHAVIOR OF THE PLATINUM.GROUP ELEMENTS DURING DIFFERENTIATION OF THE NORTH MOUNTAIN BASALT, NOVA SCOTIA

Differentiated rocks in thick flows of the Jurassic North Mountain Basalt, Nova Scotia, display evidence for fractionation of noble metals (Au, Pd, Pt, Rh, Ru, and Ir). Meter-thick layers of mafic pegmatite and vesicular basalt high in the flows are enriched in Au and Pd but depleted in Pt, Rh, Ru and Ir relative to undifferentiated basalt. Mineral precipitation (e.g., chromite removal) cannot explain Au and Pd enrichment and Pt depletion in the mafic pegmatites. The fractionation pattern may reflect the early movement and concentration of chloride-bearing hydrothermal solutions associated with rising plumes of vesicles only months after extrusion. On average, basalts with orthopyroxene show Pd and Au depletions and Pt, Rh, Ru and Ir enrichments relative to undifferentiated basalt, but there are large differences in concentration of individual noble metals between samples. This intersample variability, resulting in Pt/Pd fractionation, could reflect noble-metal-bearing micro-inclusions in orthopyroxene. "Rhyolite" bands derived from the mafic pegmatites, possibly through silicate liquid immiscibility, contain the lowest concentrations of noble metals in the flows. Some "rhyolite" samples show Ir enrichment, potentially reflecting selective carrying capacity for magmatic fluids segregated from the "rhyolite" or, if the "rhyolites" formed through silicate liquid immiscibility, preferential partitioning of Ir into the siliceous liquid. These findings, and previous studies of other Mesozoic mafic rocks from the eastern U.S., indicate that the incompatible behavior of Pd and Au can result in substantial increases in concentration of these noble metals in the upper reaches of thick flows and intrusions.

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