Impact-related hydrothermal activity in the Lockne impact structure, Jamtland, Sweden

The Lockne impact structure in central Sweden was formed ca. 455 Ma ago (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 rocks. The Lockne area has been affected by Caledonian overthrusting and associated low thermal metamorphism. The maximum metamorphic temperature 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 basement were initially rich in open cavities that became partly or totally filled with calcite during an impact-related phase of hydrothermal activity. Other cavity-grown minerals are quartz, chalcopyrite, pyrite and minor galena. Fluid trapped as fluid inclusions in minerals from cavities in the impact breccia are mainly composed of hydrocarbons and/or a brine. The earliest fluid, that consists of methane, ethane and possibly aromatic hydrocarbons, was probably derived from an organic-rich dark Cambrian clay flowing into the crater during the resurge phase, and thermally altered by the residual heat of the impact. The heat also 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 impact breccia. Calcite and sulfides were deposited in cavities between the breccia clasts from a brine with a salinity around 20 eq. wt. parts per thousand CaCl2 at a temperature that reached 210 degrees C. delta(13)C for calcite is between -2 and -14 parts per thousand (PDB), which indicates mixing with carbon from a marine and an organic source. delta(18)O for calcite is homogeneous, varying from +10 to +14 parts per thousand (SMOW). Using the temperature obtained for the mineralization process and assuming that water was in equilibrium with calcite during deposition, the oxygen composition may indicate a meteoric or seawater source. The impact structure acted as a trap for the hydrocarbons and aqueous solution precipitating calcite and sulfides. The heat produced as a result of the impact only generated a low-temperature hydrothermal system. There is no indication of a heat source such as a large melt sheet as a result of the impact, which is remarkable for an impact structure of this size.