The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden, part I: geology |
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| Authors: | Hans Årebäck Timothy J Barrett Stig Abrahamsson Pia Fagerström |
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| Institution: | (1) Exploration Department, Boliden Mineral AB, 936 81 Boliden, Sweden;(2) Ore Systems Consulting, 29 Toronto St. S., Markdale, ON, Canada, N0C 1H0 |
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| Abstract: | The Kristineberg volcanic-hosted massive sulphide (VMS) deposit, located in the westernmost part of the Palaeoproterozoic
Skellefte district, northern Sweden, has yielded 22.4 Mt of ore, grading 1.0% Cu, 3.64% Zn, 0.24% Pb, 1.24 g/t Au, 36 g/t
Ag and 25.9% S, since the mine opened in 1941, and is the largest past and present VMS mine in the district. The deposit is
hosted in a thick pile of felsic to intermediate and minor mafic metavolcanic rocks of the Skellefte Group, which forms the
lowest stratigraphic unit in the district and hosts more than 85 known massive sulphide deposits. The Kristineberg deposit
is situated lower in the Skellefte Group than most other deposits. It comprises three main ore zones: (1) massive sulphide
lenses of the A-ore (historically the main ore), having a strike length of about 1,400 m, and extending from surface to about
1,200 m depth, (2) massive sulphide lenses of the B-ore, situated 100–150 m structurally above the A-ore, and extending from
surface to about 1,000 m depth, (3) the recently discovered Einarsson zone, which occurs in the vicinity of the B-ore at about
1,000 m depth, and consists mainly of Au–Cu-rich veins and heavily disseminated sulphides, together with massive sulphide
lenses. On a regional scale the Kristineberg deposit is flanked by two major felsic rock units: massive rhyolite A to the
south and the mine porphyry to the north. The three main ore zones lie within a schistose, deformed and metamorphosed package
of hydrothermally altered, dominantly felsic volcanic rocks, which contain varying proportions of quartz, muscovite, chlorite,
phlogopite, pyrite, cordierite and andalusite. The strongest alteration occurs within 5–10 m of the ore lenses. Stratigraphic
younging within the mine area is uncertain as primary bedding and volcanic textures are absent due to strong alteration, and
tectonic folding and shearing. In the vicinity of the ore lenses, hydrothermal alteration has produced both Mg-rich assemblages
(Mg-chlorite, cordierite, phlogopite and locally talc) and quartz–muscovite–andalusite assemblages. Both types of assemblages
commonly contain disseminated pyrite. The sequence of volcanic and ore-forming events at Kristineberg is poorly constrained,
as the ages of the massive rhyolite and mine porphyry are unknown, and younging indicators are absent apart from local metal
zoning in the A-ores. Regional structural trends, however, suggest that the sequence youngs to the south. The A- and B-ores
are interpreted to have formed as synvolcanic sulphide sheets that were originally separated by some 100–150 m of volcanic
rocks. The Einarsson zone, which is developed close to the 1,000 m level, is interpreted to have resulted in part from folding
and dislocation of the B-ore sulphide sheet, and in part from remobilisation of sulphides into small Zn-rich massive sulphide
lenses and late Au–Cu-rich veins. However, the abundance of strongly altered, andalusite-bearing rocks in the Einarsson zone,
coupled with the occurrence of Au–Cu-rich disseminated sulphides in these rocks, suggests that some of the mineralisation
was synvolcanic and formed from strongly acidic hydrothermal fluids.
Editorial handling: P. Weihed |
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| Keywords: | Volcanic-hosted massive sulphide deposits Alteration Kristineberg Palaeoproterozoic Skellefte district Sweden |
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