Small bioherms with complex conical stromatolites attributable to Thyssagetes occur at the top of the siderite orebody in the late Archcan (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 infiexed, 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 Archcan 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 ofArchean stromatolites in North America. They also are the oldest known representatives of the Thyssagetaceae. Introduction STROMATOLITES are biosedimentary structures that are the most widespread and the most readily recognized fossils in Precambrian rocks, particularly in Proterozoic carbonate sequences. Archcan stromatolites are much less common and are very localized. The number of occurrences known globally from the Archcan has more than doubled from the 11 listed seven years ago in Walter (1983). At that time, only three localities had been reported from North America (Steeprock Group, Yellowknife Supergroup, and the Woman Lake Marble in the Uchi greenstone belt). Since then, they have been found at an additional seven localities in the Slave, Superior, and Wyoming structural provinces (Tella et al., 1984; Hofmann and Snyder, 1985; Hofmann et al., 1985; Donaldson and deKemp, 1985; Arias et al., 1986; Thurston et al., 1987; Patterson, 1989). Two additional localities were reported by Roscoe and Donaldson (1988) and Burbidge and Lambert (1990), but their Archcan ages were considered questionable.
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