Lea isotope abundance studies on galena occurrences in Norway.

Lead Isotope Abundances are reported for thirty-five Norwegian and two Swedish galena specimens. The analyses were made with a solidsource mass spectrometer.

The Holmes-Houtermans model was chosen for calculation of ages from the isotopic composition. This model permits calculation of the parameters U238 / Pb204 and Th232 / U238 in the source of the ores.

The significance of the isotope data is discussed regionally in terms of the mineralisation as well as the correlation and origin of different deposits. Of the thirty-seven leads investigated, thirteen are assumed to be normal and to obey the conditions of the model. These normal leads occur (i) in the Precambrian basement, (ii) along the central axis of the Caledonides, (iii) in the Permian Oslo graben. Twenty-one leads with very variable isotope compositions yield anomalously young or negative model ages (' J -type' anomalies). These occur in a broad area extending from the inner part of the Caledonides east- and southeastwards to the outer marginal zone of overthrusting along the Swedish border, as well as along the margins of the Oslo province. They are interpreted respectively as normal Caledonian and normal Permian lead which has become progressively contaminated with different amounts of a single radiogenic lead during passage through the country rocks to the site of deposition. The maximum age limits for the rocks which supplied the excess radiogenic component to the Caledonian and Permian leads are 1340 ± 100 m.y. and 1110 ± 200 m.y. respectively, whilst the respective Th/U ratios of these rocks are 1.9 and 1.5.

The remaining three leads are 'B-type' leads, i.e. the model ages are demonstrably older than the true age of mineralisation. They are interpreted as older leads remobilised and redeposited at a much later date. It is suggested that neither the mode of occurrence in the field nor the immediate geological environment is necessarily diagnostic of the type of isotope composition (i.e. Normal, ]-type of B-type), or of the type and degree of anomaly. The determining factors appear to be large-scale structural and tectonic features within the earth's crust as well as the regional (or local) geochemical character of the basement rocks underlying the area in which a given deposit, or group of deposits, occurs.