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Noble Metal Concentrations in Shoshonitic Lamprophyres: Analysis of the Weekend Dykes, Eastern Shore, Nova Scotia, Canada |
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| Authors: | GREENOUGH JOHN D; OWEN J VICTOR; RUFFMAN ALAN |
| Institution: | 1Department of Geological Sciences, University of British Columbia, Okanagan University College 3333 College Way, Kelowna, B.C., Canada V1V 1V7 2Department of Geology, Saint Mary's University Halifax, N.S., Canada B3H 3C3 3Geomarine Associates Ltd. Box 41, Station M, 5112 Prince Street, Halifax, N.S., Canada B3J 2L4 |
| Abstract: | Devonian, spessartite dykes, known as the Weekend dykes, onthe Eastern Shore of Nova Scotia contain panidiomorphic texturesand mineral (amphibole, clinopyroxene, and biotite) compositionstypical of shoshonitic lamprophyres. The major element and traceelement geochemistry of the Weekend dykes is also representativeof shoshonitic lamprophyres with high large ion lithophile elementconcentrations (LILE, e.g., Rb, K, and Ba) relative to the lightrare earth elements (LREE, e.g., La) and very low Nb and Ti.Only Ta concentrations are uncharacteristically high relativeto Nb and Ti. The dykes are variably evolved but many samplesshow primitive compositions with high mgnumber >0.70, wheremg-number=Mg/(Mg+09total Fe) atomic] high MgO and Ni concentrations(>10 wt.% and 150 ppm, respectively), and low heavy REE concentrations.All noble metal concentrations (Au, Pd, Pt, Rh, Ru, and Ir)tend to be lower in evolved samples than in primitive rocks,suggesting that evolved magmas were efficiently scavenged byimmiscible sulphide globules and that shoshonitic lamprophyreplutons may hold economic promise for the platinum-group elements(PGE). Noble metal abundances in even the most primitive rocksare low compared with many basaltic rocks excepting mid-oceanridge basalt (MORB). This makes doubtful the idea that Megumagroup mesothermal gold deposits, which are spatially and temporallyassociated with the dykes, derived their gold from the lamprophyres.Models that explain PGE concentrations and Pd/Ir ratios in awide variety of mafic rocks suggest that the low noble metalabundances probably reflect metal retention in mantle sulphides,olivine, and PGE alloys at low percentages of melting. However,noble metal abundances are higher than expected if the oceanicslab was involved in the melting process or if the source regionresembled that for most MORB, which appears to have low PGE.Mantle metasomatism represents an unlikely explanation for thePGE abundances, which are equivalent to those expected in alkalinehot-spot magmas. The high PGE abundances of hot-spot magmas(compared with MORB) may be related to the incorporation ofcore materials in deep mantle plumes. Presumably the hot-spotlikemantle became part of the subcontinental lithospheric mantlebefore the subduction and transpressional event that producedthe shoshonitic magmas. |
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