Impact-generated hydrothermal systems: Data from Popigai, Kara, and Puchezh-Katunki impact structures

The basic and common features of impact-generated hydrothermal systems are considered, based on studies of the hydrothermal mineralization in large Russian impact structures - Popigai (diameter: 100 km), Kara (65 km), and Puchezh-Katunki (80 km). The dominant hydrothermal assemblages at all craters are clay minerals (smectites, chlorites, and mixed-layered smectites-chlorites), various zeolites, calcite, and pyrite; in addition, cristobalite, quartz, opal, anhydrite, gypsum, apophyllite, prehnite, epidote, andradite, ferrosalite, actinolite, and albite occur locally. In Puchezh-Katunki, the abundant hydrothermal mineralization within the central uplift area shows a distinct vertical distribution due to thermal gradients: an upper smectite-zeolite zone and a lower chlorite-anhydrite zone can be distinguished. In Kara and Popigai, where the hydrothermal alteration affected mainly the impact melt rocks filling the craters, a vertical zonation of clay minerals is only outlined, whereas the distribution of other minerals mainly results from different host lithologies. The mineral assemblages indicate post-impact hydrothermal alteration at temperatures of 50 - 350degreesC, pH of 6 - 8, and Eh > -0.5. The Al-IV content in clay minerals and zeolites decreases from the base to the top of thick impact rock sequences, indicating the decrease of temperature and pH of the mineral-forming solutions. The composition of the hydrothermal mineralization in impact craters is determined by the target composition and by the occurrence of large amounts of shock-derived aluminosilicate materials, which result in alkaline properties and higher silica activities in the solutions.