Regional variations in the Pb isotopic compositions of ore galena across the Archaean-Proterozoic border in northern Sweden

Abstract

Galena is a rare component in the Palaeoproterozic mineralizations in northernmost Sweden. In the present work Pb isotope data from galena from nineteen deposits tend to vary as a function of geographical location. Basically, the zone dividing radiogenic and non-radiogenic lead isotopic compositions is consistent with the position of the inferred Proterozoic-Archaean border in northern Sweden. Radiogenic lead is generally found in galena of the 2.0–2.7 Ga old Greenstone group in the northeast along the Finnish border, and in some Cu occurrences in the ∼1.9 Ga old Porphyry group near Malmberget. Non-radiogenic lead is found in the Huornaisenvuoma ZnPb ore in the Greenstone group, in epigenetic CuZnPb ores in the Porphyry group in southern Norrbotten, and in similar deposits near the Caledonides. The lead isotopic compositions of galena in the CuZnPb ores of southern Norrbotten are fairly homogeneous with 206 Pb 204 Pb = 15.34 to 15.50 , 207 Pb 204 Pb = 15.16 to 15.30 , similar to those of the sulphide deposits associated with the Central Finnish batholith and to data from the North-central Skellefte district. The coincidence of data for the areas of southern Norrbotten and the North-central Skellefte district is interpreted such that these terrains formed one crustal unit at the time of ore deposition. Tentatively, this event took place at ∼1.87 Ga or slightly thereafter, and was related to the suturing of the Skellefte arc to the northerly inferred continent. Although Pb model ages should be treated with caution when applied in Proterozoic terrains in Sweden, the present data suggest an age difference between the deposits (and between source rocks) at Tjåmotis and Huornaisenvouma (∼2.0 Ga old) on one hand, and the remaining CuZnPb occurrences (∼1.9 Ga old) on the other hand. Possibly, the indicated depositional event at ∼2.0 Ga is an expression of simultaneous, large-scale crust-mantle interactions, in accordance with the hypothesis of magma-forming processes at that time. In the PbPb diagrams, data for galena in northern Sweden indicate linear relationships. The radiogenic lead is the result of leaching processes in Phanerozoic (Caledonian) times, which released rock lead evolved in the time interval ∼2.0–1.9 Ga to 0.4 Ga. The data furthermore suggest that radiogenic galena was formed by mobilizing lead from U-rich rocks ( 238 U 204 Pb ratios ca. 25–32) of the Greenstone group which are likely to have a similar age, possibly close to 2.0 Ga. Since the deposits with radiogenic lead are distributed over a very large area, and still define linear trends, it is likely that the ∼2.0 Ga age is typical for the bulk of Greenstone rocks in Norrbotten. From the slopes of straight-line relations for the Pb isotope data, it is inferred that only Proterozoic rock lead was mobilized at ∼0.4 Ga and that no significant Archaean lead was involved in such processes. Accordingly, lead mobilization was quite local and only involved the upper portions of the crust. As the radiogenic lead is found at localities far away from the Caledonian front, the indicated Pb mobilization processes were most probably connected with reactivation of old Proterozoic fracture zones.