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

Abstract

Iron formations are economically important sedimentary rocks that are most common in Precambrian sedi-mentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that sec-ular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Al-goma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and gener-ally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleopro-terozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic suc-cessions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have