Structural, stratigraphic and ore genetic significance of stromatolites in the Sala stratabound Zn-Pb-Ag deposit, Bergslagen, Sweden
2008 (English)Conference paper, Presentation (Other academic)
Sala is Sweden's most famous silver mine, with production from 1500 AD of about 450 tonnes of silver from 5 MT of ore and waste. The ores comprise vein, skarn and breccia-fill sphalerite-galena within dolomite and show similarities to other stratabound volcanic- and limestone-hosted Zn-Pb-Ag ores in Bergslagen, such as Garpenberg. The Sala dolomite contains several stromatolite occurrences, one being the type-locality for Swedish stromatolites. In this presentation, the first detailed account of stromatolites within the Sala Mine is given along with a discussion on the structural geological, stratigraphic and ore genetic significance of the stromatolites. The stromatolites display a wide range of morphologies, occurring as microbial mats, domal shapes, digitate forms as well as columns. They are best preserved in the southern part of the mine and are less conspicuous in the northern part. Stromatolitic way-up indicators reinforce earlier interpretations that the ore is hosted by a major syncline whose axis is parallel to the plunge of the mineralization. Furthermore, the stromatolites provide evidence that the planar intensely Chl-Phl-Srp-Di-Tr-Cal-Qz altered layers which transect the dolomite are concordant to bedding. These altered rocks drape stromatolitic structures, suggesting they are altered siliceous sedimentary rocks. Stromatolites also allow recognition of discordant, altered layers that are mineralogically similar to the siliceous sedimentary rocks but lack diopside and tremolite. These layers are interpreted as shear zones. The most significant is the Storgruvan Shear Zone, which parallels the strike of the deposit. Previously, these two contrasting geological features were lumped together under the loosely defined Swedish mining term 'sköl'. Way-up determinations from the stromatolites suggest that sphalerite ore mainly occurs stratigraphically below galena ore. The relationships between the shapes of workings, stromatolitic layers and altered siliceous interbeds suggest a stratigraphic control on the hydrothermal plumbing system during ore formation. Sphalerite vein-networks that occur adjacent to well-preserved stromatolitic textures suggest that ore formation was not completely texturally destructive. Skarn and sphalerite locally mimic stromatolitic laminae and form infillings in stromatolitic vugs. Although some ore is tectonically remobilized, ore displaying similar deformation patterns to the host-rock has also been observed. Thus, early to pre-orogenic ore formation is indicated. The ore may initially have formed epigenetically by sub-sea floor infiltration of metalliferous fluids into a buried stromatolite reef with volcaniclastic interbeds. Ore minerals precipitated as massive sulphide lenses and semi-massive vein-networks with long dimensions parallel to bedding. Because of later tectonic modification, ore is now concentrated in the axial plane of the Sala Syncline and in the vicinity of the Storgruvan Shear Zone.
Place, publisher, year, edition, pages
Research subject Ore Geology
IdentifiersURN: urn:nbn:se:ltu:diva-35210Local ID: 9a5c3380-6237-11dd-bb79-000ea68e967bOAI: oai:DiVA.org:ltu-35210DiVA: diva2:1008462
International Geological Congress : 06/08/2008 - 14/08/2008
Godkänd; 2008; 20080804 (niljan)2016-09-302016-09-30Bibliographically approved