BEHAVIOR OF THE PLATINUM-GROUP ELEMENTS DURING DIFFERENTIATION OF THENORTH-MOUNTAIN BASALT, NOVA-SCOTIA

Differentiated rocks in thick flows of the Jurassic North Mountain Bas alt, Nova Scotia, display evidence for fractionation of noble metals ( Au, Pd, Pt, Ph. Ru, and Ir). Meter-thick layers of mafic pegmatite and vesicular basalt high in the flows are enriched in Au and Pd but depl eted in Pt, Ph, Ru and Ir relative to undifferentiated basalt. Mineral precipitation (e.g., chromite removal) cannot explain Au and Pd enric hment and Pt depletion in the mafic pegmatites. The fractionation patt ern may reflect the early movement and concentration of chloride-beari ng hydrothermal solutions associated with rising plumes of vesicles on ly months after extrusion. On average, basalts with orthopyroxene show Pd and Au depletions and Pt, Ph, Ru and Ir enrichments relative to un differentiated basalt, but there are lame differences in concentration of individual noble metals between samples. This intersample variabil ity, resulting in Pt/Pd fractionation, could reflect noble-metal-beari ng microinclusions in orthopyroxene. ''Rhyolite'' bands derived from t he mafic pegmatites, possibly through silicate liquid immiscibility, c ontain the lowest concentrations of noble metals in the flows. Some '' rhyolite'' samples show Ir enrichment, potentially reflecting selectiv e carrying capacity for magmatic fluids segregated from the ''rhyolite '' or, if the ''rhyolites'' formed through silicate liquid immiscibili ty, preferential partitioning of Ir into the siliceous liquid. These f indings, and previous studies of other Mesozoic mafic rocks from the e astern 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.