Fluid inclusion and stable isotope studies on some Caledonian sulfide-bearing veins in Sweden

Abstract

Sulfur isotope measurements have been made on sulfides from Caledonian sulfide-bearing calcite veins in the Precambrian basement of the Swedish Caledonides and from sulfide-bearing quartz veins within the Caledonides in Sweden. The calcite veins show a narrow range in delta 34 S of the sulfides, between -7 and +1 per mil vs. CDT, whereas the quartz veins show a much wider range, from -12 to +14 per mil. A more detailed study of sulfide sulfur in one quartz vein deposit, Nasafjll, however, shows little variation within that deposit, with most delta 34 S values in a narrow range between 7 and 10 per mil. No vein deposits with sulfur as heavy as that of the sandstone-hosted impregnation ores of Vassbo and Laisvall (delta 34 S = 17-28) have been found in this study.Fluid inclusion data from the Aakerlandet calcite vein suggest that it formed at temperatures between 75 degrees and 165 degrees C (not corrected for pressure) during mixing between two hot saline brines. Carbon isotope values, with delta 13 C of the calcite close to -15 per mil vs. PDB, point to a significant contribution of reduced carbon to the ore-forming fluid. These data are used to reconstruct the chemical environment in terms of pH and oxygen fugacity for the formation of the Aakerlandet deposit. A similar tentative reconstruction is also made for the Nasafjll quartz vein deposit.The sulfur in the calcite veins is probably derived from leaching of sulfides in the granitic basement, in contrast to the organic reduction of seawater sulfate which has been proposed for the sandstone-hosted impregnation ores. Sulfide sulfur in the quartz veins may be a mixture of sulfide and sulfate sulfur leached both from the granitic basement and the overlying metasediments and metavolcanics of the Caledonian nappes, the sulfate being inorganically reduced through reactions with wall rocks prior to sulfide precipitation. The sulfur isotope data thus suggest different processes to have been involved in the formation of the different types of deposits. Together with lead isotope data, the sulfur data show that the Caledonian quartz and calcite veins in Sweden are not directly related to the Caledonian strata-bound ores but form distinct ore types of their own.