Mineralogy and Geochemistry of the Banded Iron-Formation in the Svartliden Gold Deposit, Northern Sweden

Abstract: The Svartliden gold deposit is situated in the Gold Line, South West of the Skellefte District, northern Sweden. It is considered an epigenetic lode style gold deposit dominated by a pyrrhotite-arsenopyrite assemblage. The gold mineralization, dominantly hosted by a volcano-sedimentary sequence, is structurally controlled and occurs along a ENE-trending steeply dipping shear zone. The deposit has been metamorphosed in low- to mid-amphibolite facies and is hydrothermally altered. The mineralization is hosted by a banded iron-formation (BIF) which is present along the host rock sequence. The boudinaged BIF units extend over 1 km and are from a few centimeters to 5 meters thick. The small size of Svartliden BIF is characteristic of Algoma-type BIF. Two scales of layering occur in the BIF: mesobanding, the most prominent structure because of metamorphism and microbanding. At lowest metamorphic grade, quartz-grunerite-magnetite is the dominant assemblage whereas clinopyroxene-fayalite-pyrrhotite are present in addition to those minerals in the highly metamorphosed BIF. Those mineral assemblages are characteristic of low- to mid-amphibolite facies. Gold occurs as inclusions in arsenopyrite, which is dominantly associated with pyrrhotite. The Svartliden BIF is closely associated with volcanic rocks such as amphibolites and ultramafic units. Metamorphism mobilized elements characteristic of hydrothermal input (Cu and Co), whereas Al2O3, TiO2, Zr, REE and Y stayed immobile. The (REE+Y)NASC pattern of the BIF shows evidence of both primary hydrothermal solutions and seawater whereas LREE enrichment and the positive Eu anomaly are a signature inherited from hydrothermal solutions. This indicates that the Svartliden BIF was formed by a mixture of seawater and hydrothermal fluids. The tectonic setting of the Gold Line and surrounding areas, interpreted as a volcanic back-arc environment, suggests that the Svartliden BIF is of Algoma-type.