Origin and Paleoenvironmental Significance of Major Iron Formations at the Archean-Paleoproterozoic Boundary
Nicolas J. Beukes, Jens Gutzmer, 2008. "Origin and Paleoenvironmental Significance of Major Iron Formations at the Archean-Paleoproterozoic Boundary", Banded Iron Formation-Related High-Grade Iron Ore, Steffen Hagemann, Carlos Alberto Rosière, Jens Gutzmer, Nicolas J. Beukes
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This paper provides a critical review of advances made in understanding of sedimentary environments, geochemical processes, and biological systems that contributed to the deposition and diagenetic evolution of the exceptionally well-preserved and large iron formations of the late Neoarchean to very early Paleoproterozoic Ghaap-Chuniespoort Group of the Transvaal Supergroup on the Kaapvaal craton (South Africa) and the time equivalent Hamersley Group on the Pilbara craton (Western Australia). These iron formations are commonly assumed to have formed coevally but in separate basins, and they are often used as proxies for global ocean chemistry and paleoenvironmental conditions at ~2.5 Ga. However, lithostratigraphic and paleogeographic reconstructions show that the iron formations formed in a single large partly enclosed oceanic basin along the margins of the ancient continent of Vaalbara. Furthermore, although large relative to other preserved iron formations, the combined Transvaal-Hamersley basin is miniscule compared to marginal basins of the modern ocean system so that the succession probably documents secular changes in depositional environments of that basin rather than of the global ocean at the time.
The iron formations comprise a large variety of textural and mineralogical rock types that display complex lateral and vertical facies variations on basinal scale. Based on detailed analyses of these variations it is concluded that the iron formations were deposited in environments that ranged from very deep-water basinal settings far below storm-wave base and the photic zone to very shallow-platform settings above normal wave base. Precipitation of both iron and silica took place from hydrothermal plumes in a dynamically circulating ocean system that was not permanently stratified. Ferric oxyhydroxide was the primary iron precipitate in virtually all of the iron formation facies. This primary precipitate is now represented by early diagenetic hematite in some of the iron formations. However, in both deep- and shallow-water iron formations most of the original ferric oxyhydroxides have been transformed by dissimilatory iron reduction to early diagenetic siderite and/or magnetite in the presence of organic carbon. Precipitation of ferric oxyhydroxides in very deep water below the photic zone required a downward flux of photosynthetically-derived free oxygen from the shallow photic zone. In these deep-water environments, under microaerobic conditions, chemolithoautotrophic iron-oxidizing bacteria may have played an important role in precipitation of ferric oxyhydroxides and acted as a source of primary organic matter. With basin fill even shallow-shelf embayments were invaded by circulating hydrothermal plume water from which ferric oxyhydroxides could be precipitated in oxygenated environments with high primary organic carbon productivity and thus iron reduction to form hematite-poor siderite- and magnetite-rich clastic-textured iron formations.
Depositional models derived from the study of the iron formations along the Neoarchean-Proterozoic boundary can be applied to iron formations of all ages in both the Archean and later Paleoproterozoic. The facies architecture of the iron formations determines to a large degree the textural attributes, composition, and stratigraphic setting of high-grade iron ores hosted by them. Detailed facies information thus would assist in improving genetic models for high-grade iron ore deposits. Future research should be guided in this direction, especially in some of the very large iron ore districts of Brazil and India where very little is known about the composition and facies variations of the primary iron formation hosts and possible controls on localization of high-grade ores.
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The spark to put together this volume on banded iron formation (BIF)-related high-grade iron ore was born in 2005 during a steamy night in Carajás where the iron research group from the Universidade Federal Minas Gerais, Vale geologists, Carlos Rosière and Steffen Hagemann, were hotly debating the hypogene alteration genesis for the high-grade, jaspilite-hosted Serra Norte iron ore deposits. A couple of caipirinhas later we decided that the time was opportune to put together a volume that captured the new and innovative research that was being conducted on BIF-related high-grade iron ores throughout the world. We had little problem convincing our South African colleagues Jens Gutzmer and Nic Beukes to join the effort and decided that the 2008 biannual Society of Economic Geologists' (SEG) meeting in South Africa would be the perfect place to present this project through a combined field trip and workshop near Sishen.
The enthusiastic support that we received from the research community, SEG, and industry to put this volume together was generated by the significant increase in exploration activity, and with it the need for more detailed information on what exactly controls the location of high-grade iron orebodies, and renewed research interest around the world in models for the genesis of BIF-related high-grade iron ore, and particularly the relative importance of hypogene and supergene processes in formation of high-grade ore.
This volume concentrates on new research on the characteristics and metallogenesis of BIF-related high-grade iron ores. It contains a state of the art series of papers on established and new iron ore districts and deposits, the different components of the BIF iron mineral system, and how to best explore for this ore type. Although the emphasis of many of the contributions to this volume is on the hypogene aspect of high-grade iron ore formation, it is important to note that most BIF-related iron ore districts have a very pronounced supergene overprint due to deep lateritic weathering. The transformation of many hypogene iron orebodies of reasonable grade and size to the giant deposits exploited today can be related to this geologically recent supergene overprint; most of the past and still much of the present mining of high-grade iron ore relates to soft ore interpreted in most cases to be the direct result of supergene processes. Also mentioned here should be the recent resurgence of a syngenetic model that advocates the formation of chert-free BIF