Genesis of basalt-hosted massive sulphide deposits from the Trondheim and Sulitjelma districts,Norway: ore lead isotopic considerations

Lead isotopic ratios of bulk sulphides from eleven stratigraphically equivalent deposits from the Köli Nappe sequence in the Trondheim district, and eleven from the Köli sequence at Sulitjelma Norway, have been determined. When plotted on ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagrams, the data define a linear trend extending from the mantle to the upper crustal model growth curves of Doe and Zartman (1979). Moreover, the data from both districts lie on the same trend. This isotopic trend is interpreted as resulting from the mixing of lead from a mantle source (probably the host basalts) with that of an upper-crustal end member (either sialic basement or the terrigenous sediments surrounding the host basalts). It is also concluded that the deposits in both camps formed more or less contemporaneously. The isotopic mixing line is comparable with that obtained from Besshi ore pyrites in Japan, for which an aulacogenic depositional environment, similar to that found today in the Gulf of California, has been proposed (Fox 1984). It is concluded that a similar depositional environment was responsible for the Trondheim and Sulitjelma ores, although an ensialic back-arc basin, or other possible environments, cannot be entirely ruled out.