IRON FORMATION: THE SEDIMENTARY PRODUCT OF A COMPLEX INTERPLAY AMONG MANTLE, TECTONIC, OCEANIC, AND BIOSPHERIC PROCESSES--A DISCUSSION

Abstract

Sir: Andrey Bekker and co-authors (Bekker et al., 2010) use a holistic approach to the deposition of iron formation (IF) by relating its genesis to a complex interplay of processes within the deep Earth, oceans, and biosphere. They comment (p. 469) that “In contrast to most previous studies, we suggest that no single parameter controlled iron-formation deposi tion.” The word “most” is critical here, since in a Discussion of IF genesis in Economic Geology nearly half a century ago (Trendall, 1965, p. 1069), I noted the need for a multifaceted approach with the words, “It is useless in any considerations of the origin of iron formation to confine attention to the iron formation itself,” and more recently I have discussed, in a paper that has escaped the attention of Bekker et al. (2010), the significance of IF within a wide context of Earth’s evolu tion (Trendall, 2002). Although I disagree with some of their characterization of the Hamersley Group BIFs of Western Australia, my purpose here is only to comment on their use of IF nomenclature and classification. Their adoption of the simple twofold subdivi sion of IF, as a rock type, into banded iron formation (BIF) and granular iron formation (GIF), which I first used in the 2002 paper already cited, is admirable. But I suggest that their parallel use of the terms “Superior-type iron formation” and “Algoma-type iron formation” serves more to obscure than to clarify their message. These names are derived from the classification schemes Gordon Gross developed during the long course of a study, initiated in the 1950s; his focus was the iron deposits of Canada (my italics, here and elsewhere in this paragraph); the contiguous deposits of the United States were also in cluded. Gross (1959, p. 89) initially referred to the “Lake Su perior or Knob Lake type” as one component (of two) within “Group I” (of six) of his classification of iron deposits (i.e., ore deposits); the term “Algoma type” was not used. The criteria for the recognition of the six groups varied widely, but were mainly related to mineralogy and texture. This early scheme underwent substantial transformation, and Gross (1966) later used a fourfold division of iron-formations into Algoma type, Superior type, Clinton type and Minette type. The most ma ture form of Gross’s scheme changed from a classification of Canadian iron deposits into a “classification of iron forma tions based on depositional environments ,” (Gross, 1980), in which the Clinton type and Minette type are grouped as “ironstones,” leaving the “Lake-Superior-type” and the “Al goma-type” as the only two categories of IF. Only generalized criteria were given for the allocation of these labels although, as the title of the paper indicates, the principal criterion was the depositional environment, as deduced from associated rock types. Thus, Gross (1980, p. 215‐216) wrote, “The wellknown Lake-Superior-type iron formations, widely distrib uted in Proterozoic rocks, were deposited in near-shore con tinental environments, and are associated with dolomite, quartzite, black shale, and minor amounts of other volcanic rocks,” while the “Algoma-type iron formations, found in all ages of rock, are consistently associated with greywacke sedi mentary units.” His figure 1, summarizing his scheme in a sin gle octagonal diagram, has the words “Cherty Oolitic” below “SUPERIOR TYPE,” and the single word “Cherty” below “ALGOMA TYPE.” In summary, and as emphasized by the italicized words in the preceding paragraph, Gross’s terminology evolved from a classification of the iron ore deposits of Canada, based largely on mineralogy, to a classification of the IFs of the world, based on their depositional environment. At no stage were di agnostic, as distinct from descriptive, criteria for application of the names “Superior type” and “Algoma type” to specific IF occurrences clearly specified, and it is unsurprising that there was confusion in their practical application: thus Hamersley IFs were Lake Superior type for Gross (1980, Table 1), but Algoma type for Dimroth (1976).