Archean stromatolites in Michipicoten Group siderite ore at Wawa, Ontario

Abstract

Small bioherms with complex conical stromatolites attributable to Thyssagetes occur at the top of the siderite orebody in the late Archean (2.75-2.70 Ga) Helen Iron-Formation at the MacLeod mine 4 km northeast of Wawa. Horizontal sections show round to polygonal outlines 2 to 20 cm across. Longitudinal sections show stacked convex to inflexed, angulate laminae. The lamina shapes are primary and are relatively undeformed; the microstructure is secondary, with dark laminae outlined by coarse secondary pyrite, pyrrhotite, and magnetite, and light laminae by very fine grained siderite.The stromatolites are the first to be reported from siderite. They are relatively well preserved primary biosedimentary structures that provide conclusive morphologic evidence of biologic activity in the shallow-water, photic zone in the late Archean basin in which the carbonate of the Michipicoten Group accumulated. They also indicate that the ore zone was most likely a carbonate originally rather than a tuff, as postulated in some early accounts. These structures now constitute the southeasternmost documented occurrence of Archean stromatolites in North America. They also are the oldest known representatives of the Thyssagetaceae.