Impact crater lake evolution and concomitant hydrothermal mineralization recorded by the Wörnitzostheim drill core at the Ries impact structure, Germany

Impact-generated hydrothermal systems and postimpact crater lake systems are well-documented geological phenomena; however, evidence of hydrothermal venting into impact crater lake systems has rarely been reported. We investigated the well-preserved contact between hydrothermally altered impact melt-bearing breccia (outer/surficial suevite) and postimpact crater lake deposits sampled by the Wörnitzostheim drill core at the Ries impact structure, Germany. We logged the upper 32 m of core, describing sedimentary structures and general lithological and mineralogical variations. Mineralogy was studied in detail using X-ray diffraction, optical microscopy, backscattered electron imagery, secondary electron imagery, and wavelength-dispersive spectroscopy analyses. Twelve different units were identified in the logged section of drill core, which we broadly separated into four distinct groups: (1) marlstones and limestones, (2) sand/siltstones, (3) conglomerates, and (4) impact melt-bearing breccias. The sedimentary deposits (groups 1–3) likely represent a transition from a back-stepping alluvial fan to a transgressing, shallow lake shoreline. Secondary dolomite, smectitic clay minerals and clinoptilolite occur as void-filling phases in the conglomerates—the earliest sedimentary deposits of the Wörnitzostheim drill core. A potential temperature range of 50–130°C was estimated for these void-filling minerals based on previous mineral synthesis experiments, and the typical mineral assemblages reported for the principal sequence of hydrothermal mineralization in impact craters and argillic alteration. Early postimpact sedimentary deposits likely host limited hydrothermal mineralization, potentially indicating ideal conditions for some microbial life forms during initial crater lake formation.