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Laisvall |
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Norbotten, Sweden |
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Pb Zn Ag
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Super Porphyry Cu and Au
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The Laisvall sandstone hosted lead-zinc-silver deposit is located ~480 km north of Stockholm and 250 km WNW of Lulea in northern Sweden (#Location: 66° 8' 56"N, 17° 9' 50"E).
Laisvall is one of several hundred lead-zinc occurrences in a zone of late Neoproterozoic and Cambro-Ordovician sedimentary rocks that form the eastern border of the Caledonian mountain chain from the Arctic Ocean, 2000 km south to the southwest coast of Norway.
The Laisvall deposit is hosted by flat lying uppermost Neoproterozoic to Lower Cambrian sandstones unconformably overlying late Palaeoproterozoic (1625 Ma) crystalline basement of the Baltic Shield. The host rocks are overlain by Caledonian nappes brought eastward on the Caledonian thrust zone, and outcrop as a narrow sliver of shallow water sedimentary rocks, resting on basement to the south east and overlain by thrust plates moved from the north west.
The succession may be summarised as follows (after Saintilan et al., 2015):
Paleoproterozoic basement granitic complex of the Fennoscandian Shield.
Unconformity - weathered palaeosurface with sporadic palaeosol.
Late Neoproterozoic to Lower Cambrian
Ackerselet Formation - comprising a basal conglomerate, possibly a diamictite, overlain by arkose to feldspathic sandstone.
Laisberg Formation - that is subdivided into:
• Pebble shale - that is ~12 m thick, and represents the uppermost Neoproterozoic unit;
• Lower Sandstone-ore horizon - ~25 m thick, comprising well-sorted, medium-grained quartzitic sandstone with numerous thin intercalated shale bands;
• Middle Sandstone - ~6 to 8 m thick, which is similar to the Lower Sandstone but has a clayey matrix. To the south and east, the Lower and Middle sandstones grade laterally into shale and siltstones;
• Upper Sandstone-ore horizon - ~6 up to 11 m thick and consists of well-sorted layers of coarse-grained quartzitic sandstone alternating with fine-grained layers;
Grammajukku Formation - ~40 m of shale and siltstone, with fossiliferous limestone at top.
Middle Cambrian
Alum Shale Formation - highly organic, black shales which acted as the lowermost sole for the thrusting that followed.
Thrust
Parautochthonous and Kaskejaure and Nappe Complexes - the lower Parautochthonous complex is composed of a number of slices of autochthonous sedimentary rocks that have only been transported a short distance from the west. The Kaskejaure complex is tectonically mixed with the Parautochthonous suite, but has contrasting lithologies, principally composed of cataclastic and mylonitised basement and sedimentary rocks transported from further to the west.
Thrust
Yraf Nappe Complex composed of mica quartzites, mica schists, gneisses and amphibolites. Laisvall is located on the easternmost limit of this nappe complex.
The ore deposit is present as two separate, overlapping, NE-SW elongated lenses, the Lower and Upper Sandstones of the Laisberg Formation. The lower lens has dimensions of 5000 x 200 to 600 m and is between 12 and 27 m thick, locally thinning to 4 m, with Pb grades between 0.1 and 10%. Zn grades are generally below 1%. The upper body is 2000 m long and increases from around 300 m wide at one end to 1000 m at the other extremity, averaging 6 to 8 m in thickness, locally up to 11 m. Pb grades are between 1 and 4%, locally up to 12%, while Zn grades are between 1 and 5%. The Zn:Pb ratio is ~8:1 in the Upper Sandstone. Mineralisation is approximately stratabound, although in detail it is transgressive, having a lower limit which cuts across bedding at a low angle.
Mineralisation is bounded in the NE by a basement hill onto which the the Lower Sandstone pinches out. To the NW the mineralisation, now exclusively within the Upper Sandstone, is capped by the overlying siltstones and shales. Lateral mineralisation limits are generally very diffuse with the lead-zinc content grading from ~2% to essentially zero over 100 to 200 m, particularly in the NW. Only in the SE are there sharp lateral contacts, where 10 to 20% Pb is in contact with virtually unmineralised sandstone. Vertical contacts are generally sharp. Layers of sandstone with fairly constant Pb and Zn contents can be traced up to several hundred metres laterally. The geometry of the deposit as a whole is not conformable with the bedding. Mineralisation occurs at a successively higher position in the sandstone sequence toward the NW, where is located in the uppermost Upper Sandstone, from the southeastern extremity where it is concentrated in the lowermost Lower Sandstone. The Middle Sandstone is generally unmineralized, although a small areas of lower grade ore occurs where the Lower Sandstone and Upper Sandstone ore are in contact with the Middle Sandstone. A plane below which there is no ore grade mineralisation can be delineated below the whole deposit. This plane rises by <20 m in 2 km from the SE to NW across the deposit while the sandstones, which dip steadily at about 3°, drop >100 m in the same section (Rickard et al., 1979).
The mineralisation is apparently post sedimentation, but pre thrusting, i.e., Late Neoproterozoic to Late Silurian in age.
Mineralisation occurs as disseminated aggregates and bands of galena and/or sphalerite that occur as interstitial epigenetic cement in the porous sandstone paleoaquifers. Calcite, quartz, K feldspar, fluorite and barite are present from accessory to locally dominant cement phases. Locally, apatite is intergrown with sphalerite while organic compounds are intergrown with sphalerite or barite. Rare steeply dipping galena-sphalerite-calcite veinlets cut through the sulfide-cemented sandstone. Mineralisation is hosted in two distinct sandstone paleoaquifers; the Lower and Upper Sandstones of the Laisberg Formation. In the Lower Sandstone, the orebody is between 12 and 27 m thick, locally thinning to 4 m, with Pb grades between 0.1 and 10%. Zn grades are generally below 1%. In the Upper Sandstone, the orebody is 6 to 8 m thick in average, locally up to 11 m thick. Pb grades are between 1 and 4%, locally up to 12%, while Zn grades are between 1 and 5%. The Zn:Pb ratio is about 8:1 in the Upper Sandstone (Saintilan et al., 2015).
Production and reserve figures include:
80 Mt @ 0.6% Zn, 4.3% Pb, 9 g/t Ag (Production + Reserves 1993; Mine visit, 1993).
64.3 Mt @ 0.6% Zn, 4.0% Pb, 9 g/t Ag (Total production to closure in 2001; Saintilan et al., 2015)
The most recent source geological information used to prepare this decription was dated: 2015.
Record last updated: 29/6/2017
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Laisvall
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Selected References:
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Hallberg, A., Bergman, T., Gonzalez, J., Larsson, D., Morris, G. A., Perdahl, J. A., Ripa, M., Niiranen, T. and Eilu, P., 2012 - Metallogenic areas in Sweden: in Eilu, P., 2012 Mineral deposits and metallogeny of Fennoscandia, Geological Survey of Finland, Special Paper 53, pp. 139-206.
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Kendrick M A, Burgess R, Harrison D and Bjorlykke A, 2005 - Noble gas and halogen evidence for the origin of Scandinavian sandstone-hosted Pb-Zn deposits: in Geochimica et Cosmochimica Acta v69 pp 109-129
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Lindblom S 1986 - Textural and fluid inclusion evidence for ore deposition in the Pb-Zn deposit at Laisvall, Sweden: in Econ. Geol. v81 pp 46-64
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Lindblom S, 1986 - Textural and fluid inclusion evidence for ore deposition in the Pb-Zn deposit at Laisvall, Sweden : in Econ. Geol. v.81 pp. 46-64
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Rickard D T, Coleman M L and Swainbank I, 1981 - Lead and sulfur isotopic compositions of galena from the Laisvall sandstone lead-zinc deposit, Sweden : in Econ. Geol. v.76 pp. 2042-2046,
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Rickard D T, Willden M Y, Marinder N E and Donnelly T H, 1979 - Studies on the genesis of the Laisvall sandstone lead-zinc deposit, Sweden: in Econ. Geol. v.74 pp. 1255-1285
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Saintilan N J, Stephens M B, Lundstam E and Fontbote L, 2015 - Control of Reactivated Proterozoic Basement Structures on Sandstone-Hosted Pb-Zn Deposits along the Caledonian Front, Sweden: Evidence from Airborne Magnetic Data, Structural Analysis, and Ore-Grade Modeling: in Econ. Geol. v. 110 pp. 91-117
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Saintilan, N.J., Spangenberg, J.E., Samankassou, E., Kouzmanov, K., Chiaradia, M., Stephens, M.B. and Fontbote, L., 2016 - A refined genetic model for the Laisvall and Vassbo Mississippi Valley-type sandstone-hosted deposits, Sweden: constraints from paragenetic studies, organic geochemistry, and S, C, N, and Sr isotope data: in Mineralium Deposita v.51, pp. 639-664.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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