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Polymetallic mineralisation at Hornkullen, Western Bergslagen: a metamorphosed Svecofennian formation?

Andersson S.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

454--455

Although the Fe-Pb-Cu-(As-Ag) mineralization at Hornkullen in the classic Bergslagen ore province has been a subject of economical assessments earlier in addition of being mined intermittently during the last 100s of years, the metallogenesis of the deposit is poorly constrained. The region enclosing the deposit has experienced a rather complex geological and tectonic evolution; consequently several conceivable processes that could have influenced the mineralization need to be considered. The mineralization, which is situated in the western part of the Bergslagen province in Sweden, is hosted in ca 1.90-1.88 Ga Svecofennian metavolcanic and metasedimentary rocks. These were deposited in association with an extensional back-arc system inboard an active continental magmatic region (Allen et al. 1996). This succession subsequent underwent deformation and metamorphism during the Svecokarelian orogeny in regional greenschist to amphibolite facies (e.g. Stephens et al. 2008). At the western margin, the area was later intruded by younger granitoid rocks, (referred to as the GSDG intrusive rocks suite (Stephens et al. 2009) or the Trans-scandinavian Igneous Belt (e.g. Hogdahl et al. 2004), locally the so-called Filipstad granite, dated at 1783 ± 10 Ma (Jarl & Johansson, 1988). The metasupracrustal rocks in the Hornkullen area are enclosed as an inlier in this intrusive suite. The region has also locally been affected by the ca 1.0-0.9 Ga Sveconorwegian orogeny (e.g. Stephens et al. 2009), probably in a brittle regime. The present hypothesis is that the mineralization is syngenetic, i.e. formed in conjunction with the Svecofennian volcanic activity. However, as stressed so far, the mineralization has been affected by several thermal and deformational processes following its formation. The aim of this project is therefore to assess whether the deposit comprises of metamorphically overprinted, syn-volcanic assemblages or not. To assess this, comprehensive studies of phase and textural relations on ore minerals were conducted. Additionally, fluid inclusion analysis is planned that would give further constrain on the ore-forming fluid. Tentative results so far suggest overprinting of brittle structures to some extent in the ore mineralogy.