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

Abstract

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 suggest a low-temperature near-surface formation at, or close to, atmospheric pressure, from aqueous NaCl-CaCl 2 -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 between 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 2 O-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 postSvecokarelian 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 introduced from an external source.Keywords: Långban, ore deposit, fluid inclusions, allactite, barite, blixite, calcite, finnemanite, sarkinite, tilasite, Bergslagen, Sweden. SOMMAIRELe gisement de Långban, dans la province métallogénique paléoprotérozoïque de Bergslagen, dans le secteur sud-ouest central de la Suède, est reconnu pour ses assemblages complexes et uniques de minéraux. La majorité des espèces exotiques se trouvent dans les fissures tardives à Pb-Mn-As-Sb, d'origine controversée. Les inclusions fluides des minéraux déposés dans ces fissures, par exemple allactite, barite, blixite, calcite, sarkinite, tilasite et finnemanite, indiquent une faible température et une déposition à pression atmosphérique, ou presque, à partir de solutions contenant NaCl-CaCl 2 -MgCl 2 , de salinité intermédiaire, au moins au début. La formation de ces assemblages à débuté à environ 180°C, et les fluides ont évolué par réaction avec une séquence de dolomies manganifères et une série d'événements d'ébullition initiés par ouverture répétée de fractures à une température fluctuant entre 80 et 130°C. L'étape principale est marquée par la précipitation d'une alternance d'espèces (e.g., barite, calcite, allactite, plomb natif, pyrochroïte), et par l'évolution progressive des...

Lead isotope systematics in the Långban deposit and adjacent sulphide mineralisations in western Bergslagen, Sweden

“…2) is characterised by metamorphosed iron and manganese oxide-rich ore layers with associated skarn units, sulphide impregnations and veins, as well as abundant vein and fissure-hosted Ba -Mn -As -Pb -SbBe-bearing mineral assemblages (e.g. Flink 1923;Magnusson 1930;Moore 1970;Jonsson & Broman 2002;Jonsson 2004). This unique occurrence hosts some 270 different mineral species, of which more than 60 have their type locality here (e.g.…”

“…Finally, period D corresponds to mineral formation at low P and T, mainly in (more or less open) fissures. These mineralised fracture systems often cross-cut all previous structures, and temperatures of mineral formation for this period range from around 1808C down to significantly below 708C (Jonsson & Broman 2002). The majority of exotic species containing, e.g.…”

“…The majority of these minerals are hosted by epigenetic veins and fissures that occur both concordantly and distinctly discordant to the major fabrics in host rocks and associated stratabound oxide ores. Recent studies of fluid inclusions hosted by characteristic fissure-bound species, indicate that this type of mineralisation formed significantly later than regional metamorphism and major magmatic events (Jonsson & Broman 2002). However, the processes and source(s) of metals involved in forming these unique assemblages remain ambiguous.…”

Late hydrothermal events at LÅngban – how many and how alkaline?

“…Subsequently much progress has been made thanks to Jonsson and Broman (2002), which indicate quite low temperatures of crystallization for many late vein minerals, 80-180°C. Their interesting study of the late mineral forming processes indicates a far more complicated process than anticipated.…”

“…Hausmannite is probably fairly inert at low temperatures as is the case with magnetite. However, high salinites of the solutions (Jonsson & Broman 2002) may have reduced the activation energies needed.…”