IMPACT-RELATED HYDROTHERMAL ACTIVITY IN THE LOCKNE IMPACT STRUCTURE, JAMTLAND, SWEDEN

The Lockne impact structure in central Sweden was formed ca. 455 Ma ag o (Middle Ordovician) in a marine environment, at a seawater depth of more than 200 m. The water depth in the center of the resulting crater was at least 500 m. The impact structure is a concentric crater with a total diameter of 13.5 km, filled with brecciated and fractured rock s. The Lockne area has been affected by Caledonian overthrusting and a ssociated low thermal metamorphism. The maximum metamorphic temperatur e did not exceed 300 degrees C as shown by degree of crystallinity of the black-shale graphite, conodont-alteration color, and the presence of laumontite. The clasts in the impact breccia and the fractured base ment were initially rich in open cavities that became partly or totall y filled with calcite during an impact-related phase of hydrothermal a ctivity. Other cavity-grown minerals are quartz, chalcopyrite, pyrite and minor galena. Fluid trapped as fluid inclusions in minerals from c avities in the impact breccia are mainly composed of hydrocarbons and/ or a brine. The earliest fluid, that consists of methane, ethane and p ossibly aromatic hydrocarbons, was probably derived from an organic-ri ch dark Cambrian clay flowing into the crater during the resurge phase , and thermally altered by the residual heat of the impact. The heat a lso generated a hydrothermal convection system. delta(34)S values of 1 to +5.5 parts per thousand (CDT) for chalcopyrite and pyrite suggest that sulfur was leached from the shattered basement rocks below the i mpact breccia. Calcite and sulfides were deposited in cavities between the breccia clasts from a brine with a salinity around 20 eq. wt. par ts per thousand CaCl2 at a temperature that reached 210 degrees C. del ta(13)C for calcite is between -2 and -14 parts per thousand (PDB), wh ich indicates mixing with carbon from a marine and an organic source. delta(18)O for calcite is homogeneous, varying from +10 to +14 parts p er thousand (SMOW). Using the temperature obtained for the mineralizat ion process and assuming that water was in equilibrium with calcite du ring deposition, the oxygen composition may indicate a meteoric or sea water source. The impact structure acted as a trap for the hydrocarbon s and aqueous solution precipitating calcite and sulfides. The heat pr oduced as a result of the impact only generated a low-temperature hydr othermal system. There is no indication of a heat source such as a lar ge melt sheet as a result of the impact, which is remarkable for an im pact structure of this size.