Siljan Impact Structure Sweden

A prime example of an impact-related hydrothermal deposit with inherent economic importance are the large Pb-Zn deposits located in the 377 Ma old (Reimold et al. 2004), ca. 65-km-diameter (Kenkmann and von Dalwigk 2000; others favour ca. 75 km, Henkel and Aaro (this volume)). Siljan impact structure in Central Sweden. A major deposit, estimated at 0.3 million metric tons of ore at grades of 3 % Pb, 1.5 % Zn, and 70 ppm Ag, is located in Ordovician and Silurian rocks of the rim syncline around the central uplift of this complex impact structure and has been exploited near Boda town. According to Johansson (1984), this deposit is the largest one of an entire suite of similar Pb-Zn deposits in the Siljan structure (another notable deposit occurs at Soderon in the southwestern sector of the Siljan rim syncline). Mineralization occurs in the form of veins, as well as fracture and breccia fillings, in Ordovician limestone. It is thought that these traps for ore fluids formed as a consequence of tectonic disturbances resulting from the Siljan impact event. The mineralization at Boda, with regard to geological setting, mineralogy, and isotope geochemistry, seems to compare well with the general characteristics of Mississippi Valley-type deposits of these elements. A similar deposit (though of much smaller extent) is also present at the Kentland impact structure, Indiana, USA. The mineralization at Siljan comprises carbonate-hosted calcite-fluorite-barite-sphalerite-galena-pyrite-marcasite ores that are enriched in radiogenic Pb and heavy sulfide sulfur. Johansson (1984) presented Pb isotopic evidence that suggested that the source of the ore mineralization was the Proterozoic basement, either directly or indirectly, as well as redeposited Proterozoic lead in the Ordovocian limestone, where it was presumably located in the clastic silicate fraction. Textural evidence available suggests that rapid precipitation occurred - repeatedly - from supersaturated ore fluids. The Siljan impact event has been suggested as the cause of ground preparation with regard to leaching (production of hydrothermal fluids) and precipitation (providing suitable open space) of base metals. The Pb isotopes indicate an age for the mineralization similar to that of the impact itself.

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