Abstract

The Laisvall deposit in Swedish Lappland is one of a series of sandstone lead-zinc deposits at the border of the Caledonian mountains. The total ore reserves were estimated to be 80 million tons of 4 percent lead and zinc.The ore is located mainly in two thin sandstone horizons in a late Precambrian-Cambrian autochthonous sedimentary sequence overlying a Proterozoic crystalline basement and covered by nappe slices related to the Caledonian orogeny. The sandstones were deposited on a stable platform at the shallow, tidal margin of the proto-Atlantic.Nappe translation into the area was accompanied by minimal disturbance of the Laisvall autochthonous sediments. Disruption of the alum shales, which formed the sole of the lowermost nappe, flexure folding, and some faulting are the major tectonic effects. The ore has suffered folding, faulting, and overthrusting, and thus predates the arrival of the Caledonian nappes into the area. An illite crystallinity study reveals that thermal effects of the overthrusting were limited to within 20 m of the lowermost nappe. This is consistent with textural observations on the sandstones and ore minerals suggesting that the major metamorphic effect on the ore was caused by low-temperature loading. Total recrystallization or remobilization of the deposit is precluded.The minerals of the ore association include galena, sphalerite, calcite, fluorite, and barite. They infill the pore spaces in the sandstones. No definitive paragenesis can be unraveled at Laisvall. The minerals are commonly mutually exclusive and several generations of each mineral exist. However, there is a marked tendency for sphalerite to be generally older than galena, which in turn usually precedes the calcite, barite, and fluorite. The deposit is zoned, particularly with respect to galena and sphalerite, with zinc being dominant in the Upper Sandstone to the northwest and very minor in the galena-rich Lower Sandstones.A fluid inclusion study on the sphalerites shows that the ore was precipitated from a concentrated Na-Ca-Cl brine at around 150 degrees C. No evidence for boiling is found. Sulfur isotopic ratios of the sulfides are not in equilibrium, indicating sequential deposition. The sulfide-sulfur is isotopically heavier than the barite sulfate-sulfur. This indicates that two solutions were involved in the formation of the ore, a sulfidic solution and a low sulfide sulfate solution. Thermodynamic and mass balance considerations suggest that the metals were contained in the sulfate solution.Calcite carbon and oxygen isotope ratios reveal the presence of both sedimentary and hydrothermal carbonate, suggesting partial reaction of hydrothermal solutions with the sedimentary carbonate. Oxygen isotopic ratios are consistent with the fluid inclusion data.The distribution of the ore is mainly controlled by permeability variations in the sandstones caused by the distribution of shale layers and sedimentary structures. Sharp ore contacts in some areas of the deposit indicate that the ore was formed after the deposition of the sandstones. The ore formation temperature, similarities of fluid inclusion properties in sphalerite in probable Caledonide-related joints with bulk ore, and increased crystallinity of illites when in contact with the ore suggest that ore formation was nearer to the arrival of Caledonide nappes in the area.Trace hydrocarbons have been detected in the sphalerite and evidence is presented as to their primary origin, rather than from recent contamination. These observations, together with the fluid inclusion data, suggest that the ore solutions were similar to oil field brines.The accumulated data suggest that pulses of metalliferous, low sulfide brines encountered sulfidic waters in the Laisvall area, leading to precipitation of the ore. Sulfur isotopic evidence indicates that the sulfate was derived from oxidative leaching of sedimentary sulfides and the sulfide possibly from oil-field waters. The metalliferous brines varied in gross chemistry, being zinc-rich initially, lead-rich in the main phase of ore formation, and becoming metal-poor in the later stages. The sulfide solution seems to have been more homogeneous in character.Precipitation occurred at around pH 5 at 150 degrees C from a concentrated chloride solution.Subsequent loading by the nappes effectively sealed the Laisvall sandstone aquifer, leading to the preservation of the ore during the remaining, relatively peaceful, 400 million years of its history.

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