Abstract

Laisvall is a sandstone-hosted lead-zinc deposit on the eastern margin of the Scandinavian Caledonides. It is part of a large ore province stretching over 1,000 km from the south of Norway to north of Sweden. The ore occurs as interstitial filling between sand grains, but textural and fluid inclusion data demonstrate that the ore was formed by open-space filling of open micropores in the sandstone. The textures of the cementing minerals also demonstrate that postore deformation and metamorphism are restricted to some twinning in calcite and fracturing. No major recrystallization has taken place.Textures include growth zoning of sphalerite with systematic color changes traceable as a sphalerite stratigraphy over large areas of the deposit. Both sphalerite and calcite display several stages of mineral growth including dissolution breaks and etched surfaces. Fluid inclusion homogenization temperatures indicate that each stage was deposited in a distinct temperature interval during a generally overall temperature decrease. The environment of ore deposition was characterized by a temperature of 120 degrees to 180 degrees C and a total salinity of 24 equiv. wt percent NaCl. This resulted from mixing a hot, highly saline incoming solution with a cool local ground water. The principal minerals were deposited in the order calcite, barite, fluorite, sphalerite, and galena. This process was repeated several times and sulfide deposition was accompanied by calcite dissolution.Deposition of the minerals was affected by different mechanisms during the mixing process: (1) heating of ground-water-deposited calcite, (2) cooling of brine-deposited barite, and (3) mixing which deposited sulfides and fluorite. The mixing occurred as a result of a double diffusion process where the equalization of densities resulted from salt diffusion along an extended interface between the hot brine and the cool ground water in the porous sandstone.

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