ABSTRACT

The bismuth contents of 19 rocks from the Skaergaard intrusion, determined by a sub-stoichiometric tracer isotope dilution method, range from 0.01 to 0.60 ppm. The chilled margnal gabbro contains 0.05 ppm Bi; the average bismuth content of the 7 analysed Lower Zone rocks from the Layered Series is 0.09ppm; of the 3 analysed (olivine-free) Middle Zone rocks 0.24ppm; of the 5 analysed ferrodiorites from the Upper Zone 0.26ppm. A melanogranophyre from the Upper Border Group contains 0.23 ppm, and two acid granophyres 0.24 and 0.60 ppm. Analysis for bismuth of separated cumulus minerals from 5 Layered Series rocks shows that about 70% of the total amount of the element found in a rock is present in the cumulus phases, the remainder in the mesostasis derived from the residual magmatic liquids. While bismuth is not particularly sharply distributed between the cumulus phases, titaniferous magnetite and ilmenite provide the most favourable host minerals, followed by olivine, while pyroxene and plagioclase contain lesser amounts. Bi³⁺ would appear mainly to substitute for Fe²⁺ in the oxides and ferromagnesian silicates. There is little evidence either of any Bi³⁺-Ca²⁺ coherence, or of any tendency for bismuth to become enriched in the accessory sulphide minerals of the intrusion.