SYNVOLCANIC MIXING OF ORE LEAD AND THE DEVELOPMENT OF LEAD ISOTOPIC PROVINCES IN THE SKELLEFTE DISTRICT, SWEDEN

Ore lead isotope ratios have been analysed in galenas and sulphosalts from nineteen massive sulphide deposits hosted by Svecofennian (1.9 Ga ) supracrustals in the Skellefte ore district, northern Sweden. The or e lead data can be grouped on the basis of their geographical distribu tion. Most probably, this feature is reflecting a number of lead isoto pic provinces which correspond to crustal blocks. The obtained ore lea d data define linear trends in conventional Pb-Pb plots. The ore lead signatures are due to synvolcanic mixing processes as lead was leached from mafic and acid volcanic sources. The relationships inferred for initial ratios in source rocks at 1.89 Ga require a pre-Svecofennian c rustal history. It is suggested that magmatic processes at c. 2.0 Ga i nvolved recycling of Archean sedimentary material into the mantle and the formation of a crystalline, felsic basement. The metasomatized man tle and the basement melted at the time of Svecofennian magmatism (1.8 9 Ga) which created mafic and acid magma sources. Subsequently, basalt s and rhyolites were extruded onto the sea floor. Ore was formed as ho t solutions penetrated isotopically different levels of the volcanic p ile.