NOBLE-METAL CONCENTRATIONS IN SHOSHONITIC LAMPROPHYRES - ANALYSIS OF THE WEEKEND DYKES, EASTERN SHORE, NOVA-SCOTIA, CANADA

Devonian, spessartite dykes, known as the Weekend dykes, on the Easter n Shore of Nova Scotia contain panidiomorphic textures and mineral (am phibole, clinopyroxene, and biotite) compositions typical of shoshonit ic lamprophyres. The major element and trace element geochemistry of t he Weekend dykes is also representative of shoshonitic lamprophyres wi th high large ion lithophile element concentrations (LILE, e.g., Rb, K , and Ba) relative to the light rare earth elements (LREE, e.g., La) a nd very low Nb and Ti. Only Ta concentrations are uncharacteristically high relative to Nb and Ti. The dykes are variably evolved but many s amples show primitive compositions with high mg-number [> 0.70, where mg-number = Mg/(Mg + 0.9 x total Fe) atomic] high MgO and Ni concentra tions (> 10 wt.% and 150 ppm, respectively), and low heavy REE concent rations. All noble metal concentrations (Au, Pd, Pt, Rh, Ru, and Ir) t end to be lower in evolved samples than in primitive rocks, suggesting that evolved magmas were efficiently scavenged by immiscible sulphide globules and that shoshonitic lamprophyre plutons may hold economic p romise for the platinum-group elements (PGE). Noble metal abundances i n even the most primitive rocks are low compared with many basaltic ro cks excepting mid-ocean ridge basalt (MORB). This makes doubtful the i dea that Meguma group mesothermal gold deposits, which are spatially a nd temporally associated with the dykes, derived their gold from the l amprophyres. Models that explain PGE concentrations and Pd/Ir ratios i n a wide variety of mafic rocks suggest that the low noble metal abund ances probably reflect metal retention in mantle sulphides, olivine, a nd PGE alloys at low percentages of melting. However, noble metal abun dances are higher than expected if the oceanic slab was involved in th e melting process or if the source region resembled that for most MORB , which appears to have low PGE. Mantle metasomatism represents an unl ikely explanation for the PGE abundances, which are equivalent to thos e expected in alkaline hot-spot magmas. The high PGE abundances of hot -spot magmas (compared with MORB) may be related to the incorporation of core materials in deep mantle plumes. Presumably the hot-spot-like mantle became part of the subcontinental lithospheric mantle before th e subduction and transpressional event that produced the shoshonitic m agmas.