Geological, mineralogical, and geochemical studies of the Paleoproterozoic base metal Stollberg ore field, Bergslagen, Sweden
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The Stollberg ore field (~12 Mt) occurs in the Bergslagen region of south-central Sweden, a polydeformed ca. 1.9 Ga igneous province dominated by bimodal felsic and mafic rocks. Sulfide mineralization is hosted by metavolcanic rocks, marble, and skarn, and consists of massive to semi-massive polymetallic sulfides and iron oxide in a semi-regional F2 syncline termed the “Stollberg syncline.” The dominant country rocks are rhyolitic pumice breccia and rhyolitic ash-siltstone with minor mafic sills metamorphosed to the amphibolite facies. On the eastern limb of the Stollberg syncline, sulfide mineralization occurs as stratabound replacement of marble/skarn that grades into iron formation spatially related to metamorphosed, hydrothermally altered rocks dominated by garnet-biotite and gedrite-albite. Although silicified rocks are generally subordinate in the Stollberg ore field, sulfides at GrÃ Â¤nsgruvan, on the western limb of the syncline, are located in a silicified zone along with metamorphosed, altered rocks dominated by sericite and quartz-garnet-pyroxene. Although the Tvistbo and Norrgruvan prospects along the northern end of the syncline are small, they show geological characteristics that are transitional to deposits found on the western and eastern limbs of the syncline. Ore at Tvistbo is hosted by skarn and is also spatially associated with quartz-garnet-pyroxene rocks, whereas sulfides at Norrgruvan are hosted by quartz-fluorite altered rocks that are similar to those hosting the Brusgruvan deposit on the eastern limb of the syncline.
Whole-rock analyses of variably altered host rocks in the Stollberg ore field suggest that most components were derived from felsic volcaniclastic rocks and that Zr, Ti, Al, Hf, Nb, Sc, Th, Ga, U, and rare-earth elements (REEs) were immobile during alteration. These rocks (including altered rocks in the stratigraphic footwall) are light REE enriched, heavy REE depleted, and show negative Eu anomalies, whereas sulfide-bearing rocks (Fe- and base metal-rich) and altered rocks in the ore zone show the same REE pattern but with positive Eu anomalies.
Indicators of proximity to sulfides in altered rocks in the Stollberg ore field include positive Eu anomalies, an increase in the concentration of Pb, Sb, As, Tl, Ba, Ba/Sr and K2O, as well as an increase in a modified version of the Ishikawa alteration index, which accounts for the presence of primary Ca in an original limestone component. In addition, garnets enriched in either Ca or Mn as well as principal component analyses of magnetite in sulfide mineralization are also considered to be pathfinders to ore.
Magnetite occurs in sulfides, skarn, amphibolite, and altered metamorphosed rhyolitic ash-siltstone that consists of garnet-biotite, quartz-garnet-pyroxene, gedrite-albite, and sericitic rocks. Magnetite was derived from hydrothermal fluids (~250˚ – 400˚ C) that replaced limestone and rhyolitic ash-siltstone, and subsequently recrystallized during metamorphism. Utilization of discrimination plots (Ca+Al+Mn vs. Ti+V, Ni/(Cr+Mn) vs. Ti+V, Al/(Zn+Ca) vs. Cu/(Si+Ca)) and spider diagrams (median concentration of Mg, Al, Ti, V, Co, Mn, Zn and Ga) suggest that magnetite compositions in sulfides from the Stollberg ore field more closely resemble those from skarns found elsewhere rather than from metamorphosed volcanogenic massive sulfide deposits. Although spider diagrams show that magnetite compositions from various rocks types have similar patterns, suggesting that its formation was associated with a high water to rock ratio, principal component analyses indicate that the composition of magnetite from the same rock type in different sulfide deposits can be distinguished. This suggests that bulk rock composition also has a strong influence on magnetite chemistry.