US EPA

4287535 

Book/Book Chapter 

Impact-generated hydrothermal system - Constraints from the large paleoproterozoic Sudbury crater, Canada 

Ames, DE; Jonasson, IR; Gibson, HL; Pope, KO 

2006 

IMPACT STUDIES 

55-+ 

10.1007/3-540-25736-5_4 

WOS:000237271800004 

The 1848 Ma impact-generated hydrothermal system in the similar to 200km-diameter Sudbury structure in Canada is exceptionally 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 similar to 1 km below the melt sheet and similar to 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 degrees C down to ambient. Below the melt sheet fluid-rock interaction took place at < 420 degrees 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 degrees 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 similar to 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 semi-conformable alteration zones defined by silicification, albitization, carbonate-chlorite 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 degrees 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.