Fissure-hosted mineral formation and metallogenesis in the Långban Fe-Mn-(Ba-As-Pb-Sb...) deposit, Bergslagen, Sweden

Sammanfattning: The Långban deposit in the Bergslagen ore province is one of the most mineral-rich and complex localities on Earth. Yet, despite more than 100 years of research, much about its origin and evolution has remained unknown. In this study, new data from studies of field geology, mineral assemblages, fluid inclusions, and stable as well as radiogenic isotopes enable interpretations of the origins and evolution of this deposit in more detail.The initial stage of mineral formation took place in association with ca. 1.89 Ga (Svecofennian) felsic volcanism. C and O isotope data suggest that the stratabound Fe-Mn oxide ores were formed from volcanic-hydrothermal processes, indicating protore precipitation from a seawater-derived fluid modified through interaction with magmatic rocks. These most likely comprised the then unconsolidated pile of mainly felsic volcanic material. Possibly, the relative abundance of stratabound mineralisations in this area could be an effect of the nearby, ca. 1.89-1.88 Ga Horrsjö subvolcanic complex, which is likely to have created extensive and long-lived hydrothermal activity coeval with the formation of the local volcanic and carbonate rocks. Pb isotope systematics in minerals from different paragenetic stages implicate volcanogenic Bergslagen-type ore Pb as the dominant source of Pb in the deposit, with a minor input of a more radiogenic component that may have been derived from epiclastic sediments. S isotope data exhibit a very large range, which is interpreted as being largely the result of post-Svecofennian remobilisation of magmatic sulphur that had originally been precipitated as syn-volcanic sulphides. This remobilisation was initiated during Svecokarelian regional amphibolite facies metamorphism. Calcites hosted by skarn only exhibit isotope compositions (C, O) indicative of decarbonation and calc-silicate formation during regional metamorphism, and no evidence for overprinting by granite-derived fluids. Thus, C, O, and Pb isotope data refute previous hypotheses about influence from post-Svecokarelian granites during the evolution of the deposit. Typical vein assemblages formed during regional metamorphism and the subsequent retrograde evolution. A suggested small increase in T during the late or post-Svecokarelian stage may have been related to the TIB magmatism.The unique fissure-controlled Ba-As-Mn-Pb…mineralisation formed in a brittle-tectonic setting. O, S and Pb isotope data together with mineralogical and fluid inclusion evidence suggest that these exotic assemblages formed in a shallow crustal setting (P~atmospheric, T~180°C to room-temperature) from surface water (possibly seawater) modified through water-rock interaction. Metals were transported by a moderately saline Ca-Na-Cl-dominated aqueous fluid that had probably leached these metals from the pre-existing mineralisation at some depth. Although Cl was present, e.g. as early Pb oxychlorides, in locally significant amounts prior to fissure mineralisation, it was probably introduced extensively in the active fluid of the fissure stage. The leached components were reprecipitated in the fissure system during repeated fracturing, where sporadic open-system conditions led to fluid boiling and massive precipitation of baryte-bearing assemblages.It is suggested that the large mineralogical diversity within the fissure system mainly reflects variations in fissure substrate, combined with variable lateral position in respect to the boiling zone. The former governed paragenetic variation through reactions between wallrocks and fluid. That a majority of necessary components for the formation of fissure assemblages was sequestered from the local host rocks and pre-existing mineralisation is not least suggested by the complete absence of mineralised fissures outside the mining area. The most reasonable candidates for the tectonothermal regime leading to fissure mineralisation in the Långban deposit are; 1) the post-1.78 Ga evolution of the TIB; 2) late-Proterozoic extension and basic magmatism; 3) the ca. 1.0 Ga Sveconorwegian orogeny. A concluding stage of calcite precipitation from low-temperature meteoric waters enriched in organic carbon probably took place during the Phanerozoic.