Abstract

The Stollberg ore field 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 semimassive polymetallic sulfides and iron oxide in a semiregional 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 premetamorphic replacement of volcaniclastic rocks and limestone that grades into iron formation. The development of skarn assemblages is the result of low-temperature replacement of limestone and volcaniclastic rocks rather than formation by high-temperature metasomatism or synmetamorphic or late hydrothermal replacement of marble. Metamorphosed, hydrothermally altered rocks on the eastern limb are dominated by the assemblages garnet-biotite and gedrite-albite. Silica-altered rocks are generally subordinate in the Stollberg ore field; however, sulfides at Gränsgruvan, on the western limb of the syncline, are located in a silicified zone along with metamorphosed, altered rocks dominated by sericite and the assemblage quartz-garnet-pyroxene. Although the Tvistbo and Norrgruvan prospects along the northern end of the syncline are small, they show geologic 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 spatially associated with quartz-garnet-pyroxene rocks, whereas sulfides at Norrgruvan are hosted by quartz-fluorite 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 sourced 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 are enriched in light REEs, depleted in heavy REEs, and have 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. Garnet and pyroxene enriched in either Ca or Mn are also considered to be pathfinders to ore. Cooling of an acidic, reduced hydrothermal fluid that carried sulfur and metals, which became neutralized as it reacted with limestone, is likely responsible for the formation of sulfides in the Stollberg ore field. The nature of the host rock types, the style of the alteration spatially associated with sulfide mineralization, and the spatial association with iron formation bear some resemblance to volcanogenic massive sulfide and Broken Hill-type deposits. However, the stratabound replacement of limestone by sulfides distinguishes it from these deposit types and is a so-called SVALS-type ore system, which is a class of stratabound, volcanic-hosted, limestone-skarn deposits restricted to the Bergslagen district.

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