Paper No. 21
Presentation Time: 8:00 AM-4:45 PM
Shifts in Global Metallogeny at 1850 Ma: A Connection to the Giant Sudbury Impact Event?
Two major changes in mineral deposits linked to global ocean conditions have been independently shown to occur at ~1850 Ma. These include: (1) the end of deposition of Superior-type banded iron-formation (BIF), and (2) presence of abundant hematite and/or magnetite in exhalative sediments related to Cu-rich volcanogenic massive sulfide (VMS) deposits. Also at this time there was a major increase in δ30Si values of chert, possibly reflecting a sudden major cooling of surface ocean water (Robert and Chaussidon, Nature 443:969-972). The Sudbury impact, the second-largest preserved impact structure on Earth, occurred at 1850 Ma, approximately coeval with these changes. Is this a coincidence or did the giant impact trigger global changes manifested in the distribution and character of these mineral deposits? In the case of BIF, the recent tracing of a distal ejecta blanket from Sudbury through the iron-ranges of the Lake Superior region shows that the major BIFs all occur stratigraphically below (mostly immediately below) the ejecta layer, which is overlain by other rock types, commonly black shale. This 1850 Ma age restriction appears to apply world-wide based on U-Pb zircon ages of BIF-hosting sedimentary sequences. In the case of VMS deposits, those >1850 Ma that contain abundant chalcopyrite and that formed at water depths of >850 m, typically have related sulfide-facies, carbonate-facies, or silicate-facies iron-formation, but rarely oxide-facies iron-formation. In contrast, Cu-rich VMS deposits <1850 Ma also locally have jasper, hematite iron-formation, or magnetite iron-formation. Might the Sudbury impact have caused mixing of shallow oxic and deep anoxic seawater, producing mild oxygenation of the deep ocean? Such mixing would oxidize hydrothermally-derived soluble ferrous iron to insoluble ferric oxyhydroxides, preventing transport of the ferrous iron onto continental shelves for deposition of Superior-type BIF. This mixing also could produce suboxic deep ocean water accounting for the changes in VMS-related exhalites.