Sulphur isotope data of Proterozoic molybdenites from western Bergslagen, Sweden

Abstract

δ³⁴S values of molybdenites from various types of Proterozoic W-Mo mineralizations in western Bergslagen, Sweden, range from +1.6-+3.1% (in granite) through +1.5-+3.4 (in quartz-feldspar pegmatite veins proximal to the granite) to +2.2% (in HT garnet-pyroxene skarn close to the granite) to +0.4% (in LT actinolite-magnetite skarn) to −0.7–9.7% (in biotite schists distal to the granite). The δ³⁴S values of the intragranitic, pegmatitic, and skarn-hosted molybdenites fall within the normal range of −3.0 to +7.0% for an uncontaminated granitic source of the sulphur. The constant to slightly decreasing average δ³⁴S values from the intragranitic, through the pegmatitic, to the skarn-hosted molybdenites can be explained by a slight cooling of the hydrothermal fluid during its ascent. The significant decrease in δ³⁴S values from the skarn-hosted molybdenite to the biotite schist-hosted molybdenites in higher stratigraphic levels is thought to be the result of rapid cooling due to mixing of the hot hydrothermal fluid with cold sea-water. Contamination of the hydrothermal sulphur with sedimentary sulphur cannot have played a major role in generating the negative δ³⁴S values of the schist-hosted molybdenites, since the sedimentary sulphides show positive δ³⁴S values between +1.3 and + 1.7%.

Key Words:

• Sulphur isotopes
• molybdenites
• granites
• skarns
• biotite schists
• sulphides
• Proterozoic
• Bergslagen
• Sweden