Cooling the Kärdla impact crater: I. The mineral paragenetic sequence observation

The impact-induced hydrothermal system in the well-preserved, 4 km-diameter Kärdla impact crater on Hiiumaa Island, western Estonia, was investigated by means of mineralogical, chemical, and stable C and O isotope studies. The mineralization paragenetic sequence, with gradually decreasing temperature, reveals at least three evolutionary stages in the development of the post-impact hydrothermal system: 1) an early vapor-dominated stage (>300 °C) with precipitation of submicroscopic adularia type K-feldspar; 2) the main stage (300 to 150/100 °C) with the development of a two-phase (vapor to liquid) zone leading to precipitation of chlorite/corrensite, (idiomorphic) euhedral K-feldspar, and quartz; and 3) a late liquid-dominated stage (<100 °C) with calcite I, dolomite, quartz, calcite II, chalcopyrite/pyrite, Fe-oxyhydrate, and calcite III precipitation.