Impact-Generated Hydrothermal System — Constraints from the Large Paleoproterozoic Sudbury Crater, Canada

The 1848 Ma impact-generated hydrothermal system in the ∼200-km-diameter Sudbury structure in Canada is sexceptionally well preserved and provides the opportunity to study potential fossil ecosystems associated with impact craters. The hydrothermal alteration fingerprint at the Sudbury impact site is preserved for ∼1 km below the melt sheet and ∼2 km above. The system was capable of producing sufficient heat and fluid flow to form sinter deposits on the crater-floor. Fluid-rock interaction and resultant alteration mineral products record the waxing and waning phases of a complex hydrothermal system within the impact crater with temperatures in the basin ranging from 250–300°C down to ambient. Below the melt sheet fluid-rock interaction took place at <420°C. The exceptional preservation of the Sudbury impact structure including fractured and shocked basement rocks, melt sheet, impact-related crater-fill breccias, chemical sediments on the crater-floor and post impact sedimentation, yields significant new insights into the physical, chemical and potentially the biological framework of impact-generated hydrothermal systems in large craters. Significant to the development of microbial niches is defining the lower temperature regimes (<120°C) of the habitable zone. In the Sudbury basin from base to top, lies a 1.4-km-thick sequence of suevite (Onaping Formation) that has undergone extensive water-rock interaction manifested as regionally extensive semiconformable alteration zones, a thin ∼ 14-m-thick exhalative-sedimentary sequence (Vermilion Formation) and in a metal-enriched hydrothermal plume extending another <1 km into the post-impact basin sediments (Onwatin Formation). The hydrothermal signature includes basin-wide semiconformable alteration zones defined by silicification, albitization, carbonatechlorite alteration in the Onaping Formation. Also present are discordant alteration zones with focussed fluid flow which produced local higher temperature perturbations imposed on the more extensive lower temperature (<250°C) alteration zones within the crater-fill sequence. The Vermilion Formation represents a subaqueous hydrothermal vent complex with a proximal hydrothermal Ca-Fe-Mg-Mn carbonate mound facies containing replacement type Zn-Pb-Cu-Fe mineralization, a distal finely laminated carbonate facies, or “carbonate-facies iron formation”, buried by distal turbidite sediments. Prolonged post-mineralization diffuse fluid flow and unfocussed low temperature emanation of hydrothermal plumes and the Fe-Mn-rich distal carbonates produce favourable habitats for thermophilic microorganisms.