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The Långban deposit in the Paleoproterozoic Bergslagen ore province in southwestern central Sweden is well known for its complex and unique mineralogy. The largest number of exotic species occur in late-stage Pb-Mn-As-Sb-bearing fissures of disputed origin. Fluid inclusions in fissure-hosted allactite, barite, blixite, calcite, sarkinite, tilasite and finnemanite su ggest a low-temperature near-surface formation at, or close to, atmospheric pressure, from aqueous NaCl-CaCl2-MgCl 2 solutions of initially moderate salinity. Mineral formation started at temperatures of ca. 180°C, with the fluid evolving through reactions with Mn-oxide-bearing dolomites and a sequence of boiling events, initiated by repeated fracturing, at temperatures fluctuating betw een ca. 80 and 130°C. This main stage is characterized by precipitation of alternating species ( e.g., barite, calcite, allactite, native lead, pyrochroite), with solutions evolving further to heterogeneous high-salinity compositions at temperatures less than ca. 70°C. Late low-salinity inclusions formed through partial condensation of the expelled vapor phase. Continued formation of minerals (e.g., arsenites, H 2O-rich arsenates) after the major boiling events is characterized by temperatures decreasing well below 70°C. The solutions responsible for the specific fissure-mineralization at Långban were introduced in a very shallow post- Svecokarelian brittle tectonic environment, in conjunction with a ca. 1 Ga tectonothermal episode, or possibly coupled to post- 1.8-Ga granitic magmatism. The absence of similar mineralized systems of brittle fissures on a regional scale suggests that the metals in the Långban fissures were derived very locally from the pre-existing volcanic-exhalative mineralization and not intro- duced from an external source.

FLUID INCLUSIONS IN LATE-STAGE Pb Mn As Sb MINERAL ASSEMBLAGES IN THE LANGBAN DEPOSIT, BERGSLAGEN, SWEDEN