POSSIBLE BIOLOGICAL ORIGIN OF PRECAMBRIAN IRON-FORMATIONS

Detailed petrographie examination of approximately 2,000 thin sections of both Early and Middle Precambrian iron-formations from the U. S., Canada, Australia, and South Africa reveals the common presence of spheroidal structures which range from about 5 to 40 μ, in diameter. The structures were observed in every iron-formation studied (albeit not in every sample) which had not undergone appreciable metamorphism and/or shearing. Approximately 60 percent of the samples studied contained structures pigmented by hematite, iron-silicates, or organic matter, and/or contained unpigmented chert whose grain size is within the range of the pigmented structures. Therefore, the 30 μ. structures are considered to be a characteristic feature of iron-formations. The spheroidal structures in the chert, and siderite grains within the same size range, are intimately involved in the depositional fabric of the rock (such as layering and cross-bedding) and they are also present within granules (which are also part of the depositional fabric). The 30 μ, spheroidal structures are the same size as certain microfossils reported from a number of Precambrian occurrences. The common occurrence of the spheroidal structures in iron-formations, their apparent absence in Phane-rozoic cherts, their relationship to depositional features in the rocks, and the fact that at least some are similar to distinctive organic forms, is best explained by the interpretation that the structures are relict microfossils. If the interpretatioon that the structures are relict microfossils is correct, then their widespread occurrence and abundance in iron-formations may suggest that the organisms were involved in the deposition of the materials. It is suggested that at least some of the organisms may have been silica precipitators (perhaps bacteria or algae) and that at least some of the chert in iron-formations was biologically precipitated. © 1973 Society of Economic Geologists, Inc.