Origin and evolution of large Precambrian iron formations

Abstract

Collectively, iron formations represent Earth's preeminent supracrustal repository of iron. The largest iron formations were deposited in the Late Archean and Paleoproterozoic via a unique confluence of atmospheric, hydrospheric, lithospheric, and biospheric conditions. Understanding these conditions better requires a deeper appreciation of the sedimentary features of iron formations. Many researchers refer to them collectively as banded iron formations or the acronym BIF, but banding is not always well developed. Iron formations that lack thin banding consist of sand-sized detritus and are cross-bedded, and should be referred to as granular iron formations or the acronym GIF. The mineralogical and textural heterogeneity of iron formations is also underappreciated. The iron in many iron formations resides in siderite or iron-rich silicates rather than oxides. This implies that iron formations did not all form from local releases of oxygen by photosynthetic microbes. Both the heterogeneity of iron formations and the variety of different rock types with which they are associated indicate that large iron formations are not products of a particular depositional setting, such as evaporites. They owe their existence to a combination of: (1) copious masses of dissolved iron supplied by deep-sea hydrothermal systems, (2) the appearance of large continental shelves to serve as depositional repositories, and (3) a stratified ocean with a chemistry suitable for connecting the two. The mechanisms of precipitation are still unclear, but it probably took place along regional chemoclines. Evidence of microbial involvement is increasing. The largest iron formations of all are those of the Hamersley and Transvaal Basins in western Australia and South Africa, respectively, and they may have originally formed a single huge unit.

Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes

“…All other facies are generally interbedded with variably recrystallized chert (Simonson, 2003). Oxide-facies iron formation consists predominantly of magnetite or hematite, whereas carbonate-facies varieties contain siderite or ankerite as major constituents.…”

“…The granules are considered detrital, with some, both well-rounded and angular, being derived by sedimentary reworking of iron-rich mudstone and arenite (Simonson and Goode, 1989). Many wellrounded granules resemble peloids (Simonson, 2003) and possibly were originally composed of iron-rich clay. Oolitic banding in granules is relatively rare in late Paleoproterozoic GIFs but common in younger deposits (e.g., Harms, 1965).…”

“…gap, at about 1.88 Ga. These are predominantly GIFs and are common in North America, at the margins of the Superior craton (Simonson, 2003), and are coeval with a ca. 1.88 Ga ultramafic LIP (Heaman et al, 1986(Heaman et al, , 2009Hulbert et al, 2005), which is potentially related to a mantle plume breakout event (Hamilton et al, 2009; see for a different view Heaman et al, 2009).…”

“…They comprise iron silicates and oxides without the chert enrichment characteristic of Precambrian deposits but with appreciable phosphorous contents. Significantly, Archean, Paleoproterozoic, and Phanerozoic iron formations and ironstones are temporally associated with organic matter-rich black shales (e.g., van Houten and Arthur, 1989;Simonson, 2003). In contrast, other Phanerozoic examples of Fe-Si−rich rocks, commonly termed umber, generally developed above volcanic rocks and probably formed through diffuse sea-floor hydrothermal venting, although direct precipitation from seawater as hydrothermal plume fallout has also been suggested (Elderfield et al, 1972;Alt, 2003;Little et al, 2004).…”

“…The growth of continents created a crucial shallow-water depository (Simonson, 2003), because before large landmasses developed considerable freeboard, iron formations may have been deposited in close association with volcanic edifices and seemingly were smaller as a result. The Mesoproterozoic and early Paleoproterozoic (ca.…”