Stages and Conditions of Formation of Carbonate-Silicate Veins and Near-Vein Aureoles in the Early Proterozoic Complexes of the Belomorian Mobile Belt, North Karelia

Abstract

On the islands and the coast of the White Sea in North Karelia, the bodies of Early Proterozoic metamorphosed gabbroids are widely distributed in Archean gneiss. Carbonate-silicate veins with sulfide mineralization are confined to these bodies of metabasites, as well as to their contacts with gneiss. The main vein minerals are plagioclase, quartz, carbonates and chlorite. The stages of vein formation correspond to the transition from early quartz-plagioclase to late quartz-carbonate associations with chlorite and sulfides. The early (high-temperature) stage is fixed by the amphibolite aureoles around the vein with temperature estimates of about 550–650°C by the TWQ method. This stage corresponds to the quartz-plagioclase association of the marginal zones of the veins. The transition to the late stage with the formation of veined quartz-carbonate associations (± biotite) occurred at temperatures of 540°C and lower, judging by the calcite-dolomite associations. Further development of quartz–chlorite–carbonate and sulfide associations in veins and wall amphibolites corresponds to a decrease in temperature to 350°C and below, judging by chlorite thermometers. Vein formation and near-vein amphibolitization are presumably associated with the impact of metamorphic fluids at the latest retrograde stage of metamorphism in the Early Proterozoic.